Method and system for providing battery balancing for an electrical series string of lithium-ion batteries

Abstract

A battery balancing method and system for charging multiple batteries connected together in an electrical series string with a single electrical charger. For example, the battery balancing method and system utilizes the battery management system (BMS) of the individual batteries to provide battery balancing of multiple batteries connected together in an electrical series string when charging the series string with a single electrical charger.

Description

[Technical Field]

[0001] The present invention relates to a method and system for battery balancing the state of charge (SoC) of a string of lithium-ion batteries connected in an electrical series configuration. [Background technology]

[0002] Lithium-ion batteries are often designed using multiple lithium-ion cells arranged electrically in series to reach the higher voltages required for certain applications.

[0003] No two lithium-ion battery cells are identical, even though battery manufacturers use sophisticated procedures to select and match cells during battery assembly. There will always be small differences between lithium-ion batteries in SoC, capacity, internal resistance, self-discharge rate, and temperature characteristics. If these small differences in lithium-ion cells are not addressed, they will result in variations in cell voltage over time. Cell voltage variations can lead to reduced battery performance and potentially safety issues. For this reason, many batteries incorporate cell balancing into their battery management systems (BMS).

[0004] Two commonly used methods for cell balancing of lithium-ion batteries electrically connected in series include passive balancing and active cell balancing.

[0005] Passive cell balancing is the most common. It is low-cost, easy to implement, and effective. In passive cell balancing, a load is switched across the cells during charging to bypass the charging current in order to equalize the cells with other battery cells. Each battery cell in electrical series has its own bypass resistor.

[0006] Active cell balancing uses a more complex method of redistributing charge between cells to maintain balance. Active cell balancing typically uses capacitors or inductors to store energy and redistribute energy between battery cells. This is a low-loss process that eliminates the power losses inherent in the resistive load of passive cell balancing.

[0007] Cell balancing can be very effective in maintaining balance between cells within a single battery pack. However, there is also a need to balance batteries (e.g., rechargeable batteries, Li-ion rechargeable batteries) when connected together in an electrical series string.

[0008] Many applications require two or more batteries to be electrically connected in series to reach higher applied voltages. This string of batteries electrically connected in series is often charged and discharged as a single unit. This can lead to imbalances between batteries, just as it can lead to imbalances between individual cells within a battery.

[0009] Within a battery, individual cells are matched during manufacturing and then the battery is sealed. In contrast, batteries in an electrical series string are not necessarily matched, and because they are separate and removable, they may be exposed to different conditions that can affect their SoC. For these reasons, it is desirable to implement some method for balancing series-connected batteries. Several common battery balancing methods are described below.

[0010] (1) No balancing: The most common situation is not performing any form of battery balancing. Without balancing, the batteries can become unbalanced over time, causing the BMS to trigger overcharge or overdischarge protection on one of the batteries. This can open the electrical series string of batteries and cause an application fault.

[0011] (2) Charging individual batteries: A simple solution for balancing series-connected batteries is to charge each battery separately within the series-connected string. For example, a 48V system consisting of four series-connected 12V batteries would utilize four individual 12V chargers (i.e., one 12V charger for each battery). While this is an effective balancing strategy, it is cost and space prohibitive for many applications. Most system designers prefer a single charger for the entire electrically series-connected battery string.

[0012] (3) Periodic Checks and Rebalancing: Another strategy used to balance batteries connected in series is to periodically check the balance and manually rebalance the batteries as needed. In this method, the batteries are disconnected from the load and / or charger and measured. If the batteries are out of balance, each one is individually charged with a charger to restore balance. While effective, this method has drawbacks, including the need to interrupt battery use to check the balance, determining the optimal time interval for checking the balance, and the fact that it is a manual operation that must be scheduled and performed.

[0013] (4) External Balancing: Battery charge balancing systems are available that are designed to monitor each battery in an electrically series-connected string and use balancing techniques to maintain a balance between the batteries. These systems are external to the batteries. While effective, they require additional hardware external to the batteries and external wiring to each battery in the series string. Summary of the Invention

[0014] The present invention is directed to providing battery balancing for multiple batteries connected together in an electrical series string when charging multiple batteries with a single charger. The present invention provides battery balancing among multiple batteries electrically connected together in an electrical series string without additional hardware or cabling external to the batteries. No communication between the batteries is required. Battery balancing occurs within each individual battery in the electrical series string.

[0015] The present invention involves performing passive balancing at the individual battery level (e.g., pack level). Each individual battery has a balancing resistor that bypasses a portion of the charging current near the end of charging. In an electrically series-connected string of batteries, this has the effect of reducing the charging current for batteries with higher voltages, while batteries with lower voltages receive full charging current. As a result, the lower voltage batteries catch up with the higher voltage batteries. Depending on the amount of imbalance, the batteries may require one or more charging cycles to reach a balanced state.

[0016] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries, wherein a single electrical charger is used to charge the plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries.

[0017] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries, the battery voltage threshold at which the balancing resistor begins to bypass the charging current being set to a voltage that is near the full-charge voltage of the particular lithium-ion battery chemistry being used.

[0018] The subject matter described herein is directed to a method of performing cell balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries; the battery voltage threshold at which the balancing resistor begins to bypass the charging current is set to a voltage that is near the full-charge voltage of the particular lithium-ion battery chemistry being used; and as the electrical series string of batteries is charged, cell balancing is turned on on each battery as it reaches the balance threshold, until cell balancing is turned on for all of the batteries in the electrical series string.

[0019] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, wherein this sequence shunts a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries, and wherein balancing is performed only during charging.

[0020] The subject matter described herein is directed to a method of performing battery balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence having the effect of shunting a portion of the charging current flowing through the balancing resistor, bypassing the battery cells of each of the plurality of lithium-ion batteries and slowing the charging of each of the lithium-ion batteries, wherein balancing is performed only during charging, requiring the balancing circuitry to detect when the batteries are being charged.

[0021] The subject matter described herein is directed to a method of performing battery balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries; balancing is performed only during charging, balancing requires the balancing circuitry to detect when the batteries are charging, and balancing is performed by setting a voltage threshold for balancing to be near a full-charge voltage, and when the batteries are charging and this threshold is reached, balancing is enabled.

[0022] The subject matter described herein is directed to a method of performing battery balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries; balancing is performed only during charging; balancing requires the balancing circuit to detect when the batteries are charging; balancing is performed by setting a balancing voltage threshold to be near a full charge voltage; when the batteries are charging and this threshold is reached, balancing is enabled; and after charging is terminated, the battery voltage naturally relaxes to a voltage below the threshold, disabling balancing.

[0023] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each of the plurality of lithium-ion batteries having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries; and control electronics in the battery management system (BMS) can provide a signal to the balancing circuitry indicating whether the batteries are charging.

[0024] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries, the balancing current should be high enough to effectively balance the batteries but low enough so as not to interfere with a charge termination scheme used in a battery charger.

[0025] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor; and using the comparator to detect when the battery voltage of each of the lithium-ion batteries is near end-of-charge and then turning on each respective MOSFET of each of the lithium-ion batteries, the sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries and providing proper balancing, the value of the balancing current being between 100 mA and 500 mA to work with most battery chargers.

[0026] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium ion batteries, comprising the steps of providing a plurality of lithium ion batteries electrically connected together in series, each having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium ion batteries, which has the effect of slowing the charging of each of the lithium ion batteries; and wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries.

[0027] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, comprising the steps of providing a plurality of lithium-ion batteries electrically connected together in series, each of the plurality of lithium-ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries, wherein the battery voltage threshold at which the balancing resistors begin to bypass the charging current is set to a voltage that is near the full-charge voltage of the particular lithium-ion battery chemistry being used.

[0028] The subject matter described herein is directed to a method of performing cell balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries electrically connected together in series, each of the plurality of lithium-ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages and detect when the battery voltages are near end-of-charge, then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries; the battery voltage threshold at which the balancing resistors begin to bypass the charging current is set to a voltage that is near the full-charge voltage of the particular lithium-ion battery chemistry being used; and as the electrical series string of batteries is charged, cell balancing is turned on on each battery as it reaches the balance threshold, until cell balancing is turned on for all of the batteries in the electrical series string.

[0029] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium ion batteries, comprising the steps of providing a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium ion batteries, which has the effect of slowing down the charging of each of the lithium ion batteries, and wherein balancing is performed only during charging.

[0030] The subject matter described herein is directed to a method of performing battery balancing on an electrical series string of lithium ion batteries, the method including the steps of providing a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium ion batteries, which has the effect of slowing the charging of each of the lithium ion batteries; and wherein balancing is performed only during charging, and balancing requires the balancing circuitry to detect when the batteries are being charged.

[0031] The subject matter described herein is directed to a method of performing battery balancing on an electrical series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries electrically connected together in series, each of the plurality of lithium-ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries; balancing is performed only during charging, balancing requires the balancing circuitry to detect when the batteries are charging, and balancing is performed by setting a voltage threshold for balancing to be near a full-charge voltage, and when the batteries are charging and this threshold is reached, balancing is enabled.

[0032] The subject matter described herein is a method of battery balancing an electrical series string of lithium ion batteries, comprising the steps of providing a plurality of lithium ion batteries electrically connected together in series, each having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; and using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, The present invention is directed to a method in which the sequencing shunts a portion of the charging current through a balancing resistor to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries, the balancing is performed only during charging, the balancing requires a balancing circuit to detect when the batteries are charging, the balancing is performed by setting a voltage threshold for balancing to be near the full charge voltage, when the batteries are charging and this threshold is reached balancing is enabled, and after charging is terminated the battery voltage naturally relaxes to a voltage below the threshold which disables balancing.

[0033] The subject matter described herein is directed to a method of battery balancing an electrically series string of lithium-ion batteries, the method including the steps of providing a plurality of lithium-ion batteries electrically connected together in series, each of the plurality of lithium-ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries; and control electronics in the battery management system (BMS) can provide a signal to the balancing circuitry indicating whether the batteries are charging.

[0034] The subject matter described herein is directed to a method of battery balancing an electrically series string of lithium ion batteries, comprising the steps of providing a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium ion batteries, which has the effect of slowing the charging of each of the lithium ion batteries, and the balancing current should be high enough to effectively balance the batteries but low enough so as not to interfere with a charge termination scheme used in a battery charger.

[0035] The subject matter described herein is directed to a method of battery balancing an electrical series string of lithium-ion batteries, comprising the steps of providing a plurality of lithium-ion batteries electrically connected together in series, each having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors; using the microcontroller to read the battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltages are near end-of-charge and then turning on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium-ion batteries, which has the effect of slowing the charging of each of the lithium-ion batteries and provides proper balancing, the value of the balancing current being between 100 mA and 500 mA to work with most battery chargers.

[0036] The subject matter described herein is directed to a battery balancing system for charging an electrical series string of lithium-ion batteries, the battery balancing system including a plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries, each of the plurality of lithium-ion batteries having a battery management system (BMS) including a comparator, a MOSFET, and a balancing resistor, the comparator configured to detect when the battery voltage of each of the lithium-ion batteries is near the end of charge and then turn on each respective MOSFET of each of the lithium-ion batteries, this sequence shunting a portion of the charging current flowing through the balancing resistor to bypass the battery cells of each of the plurality of lithium-ion batteries, which has the effect of slowing down the charging of each of the lithium-ion batteries, and wherein a single electrical charger is used to charge the plurality of lithium-ion batteries connected together in the electrical series string of lithium-ion batteries.

[0037] The subject matter described herein is directed to a battery balancing system for charging an electrical series string of lithium ion batteries, the battery balancing system including a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, MOSFETs, and balancing resistors, the microcontroller configured to read the battery voltage using the analog-to-digital converter (ADC) to detect when the battery voltage is near end-of-charge and then turn on the MOSFETs, this sequence shunting a portion of the charging current through the balancing resistors to bypass the cells of each of the plurality of lithium ion batteries, which has the effect of slowing down the charging of each of the lithium ion batteries, and wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries. [Brief explanation of the drawings]

[0038] [Figure 1] Figure 1, for example, is a schematic diagram of four batteries connected together in an electrical series string, with all four batteries charging and discharging together. [Figure 2] Figure 2 is a schematic diagram showing a battery balancing scheme implemented with four battery cells. All components shown are internal to the battery. The control method, as shown, uses a comparator. Other similar or functionally equivalent control methods can be used. [Figure 3] Figure 3 is a schematic diagram showing a balancing scheme implemented on a four-cell battery. All components shown are internal to the battery. A microcontroller-based control method is shown. Again, other similar or functionally equivalent control methods can be used. DETAILED DESCRIPTION OF THE INVENTION

[0039] A system or circuit 1 for charging four batteries 2A, 2B, 2C, 2D (Battery 1, Battery 2, Battery 3, Battery 4) connected together in an electrical series string is shown in Figure 1. The system or circuit 1 comprises the four batteries 2A, 2B, 2C, 2D, a single battery charger 3, and a balancing resistor 4 (i.e., load).

[0040] A system or circuit 10 for balancing is shown in FIG. 2. System or circuit 10 includes battery cells 12A, 12B, 12C, and 12D (cell #1, cell #2, cell #3, and cell #4), a comparator 16, a MOSFET 22, and a balancing resistor 24 (i.e., a load). Comparator 16 is used to detect when the battery voltage is near end-of-charge and then turn on MOSFET 22. This sequence shunts a portion of charging current 30 (i.e., balancing current 32) through balancing resistor 24, bypassing battery cells 12A, 12B, 12C, and 12D, which has the effect of slowing the charging of that particular battery.

[0041] A battery balancing system according to the present invention comprises multiple batteries connected together in an electrical series string, each having a system or circuit 10. The multiple batteries are charged using a single charger.

[0042] Another system or circuit 110 for balancing according to the present invention is shown in FIG. 3. The system or circuit 110 includes battery cells 112A, 112B, 112C, and 112D (cell #1, cell #2, cell #3, and cell #4), an analog-to-digital converter (ADC) 118, a microcontroller 120, a MOSFET 122, and a balancing resistor 124 (i.e., a load). The microcontroller 120 reads the battery voltage using the analog-to-digital converter (ADC) 118, detects when the battery voltage is near the end of charging, and turns on the MOSFET 122. This has the effect of shunting a portion of the charging current 132 (i.e., the balancing current 132) through the balancing resistor 124, bypassing the cells and slowing down the charging.

[0043] Another battery balancing system according to the present invention includes multiple batteries, each with a system or circuit 110, connected together in an electrical series string.

[0044] The battery voltage threshold at which the balancing resistor begins to bypass the charging current is set to a voltage close to the full charge voltage for the particular lithium-ion battery chemistry being used. For example, reasonable values ​​for an exemplary battery are shown below. Other values ​​can be chosen to optimize balancing for a particular battery and chemistry.

[0045] example: 1) (3.60V / cell) x (4 cells) = 14.4V (4-cell LiFePO4 with a fully charged voltage of 3.65V / cell); and 2) (4.18V / cell) x (3 cells) = 12.54V (3-cell cobalt battery with a fully charged voltage of 4.20V / cell)

[0046] In the systems and methods according to the present invention, when an electrical series string of batteries is being charged, cell balancing is turned on for each battery as it reaches the balance threshold described above, eventually turning on cell balancing for all batteries in the electrical series string.

[0047] Balancing is only performed during charging. This requires the balancing system or circuitry to detect when the battery is being charged. This is done by setting a voltage threshold for balancing near the full charge voltage. When the battery is charging and this threshold is reached, balancing is enabled. After charging is complete, the battery voltage naturally relaxes to a voltage below the threshold, disabling balancing.

[0048] Alternatively, control electronics within a battery management system (BMS) can provide a signal to a balancing system or circuitry indicating whether the batteries are being charged.

[0049] A design consideration is the selection of the balancing current: it should be high enough to effectively balance the batteries, but low enough so as not to interfere with the charge termination scheme used in the battery charger.

[0050] Chargers utilizing constant current / constant voltage (CC / CV) terminate charging when the current falls below a certain level during the constant voltage portion of the charging cycle. A typical threshold used by chargers is 5-10% of the bulk charge current. To ensure proper charge termination, the balancing current should be below this level.

[0051] Many general-purpose battery chargers use only constant current (CC) charging. These chargers reduce the charging current as the battery voltage approaches the termination voltage. To ensure proper charge termination, the balancing current should be below the minimum charging current level.

[0052] A reasonable value for the balancing current to provide adequate balancing and work with most battery chargers is between 100 mA and 500 mA.

[0053] The present invention described above utilizes passive balancing to achieve balance between cells electrically connected in series, although active balancing may alternatively be used as well.

Claims

1. 1. A method of performing cell balancing on an electrical series string of lithium ion cells, comprising: providing a plurality of lithium ion cells connected together in the electrical series string of lithium ion cells, each of the plurality of lithium ion cells having a battery management system (BMS) comprising a comparator, a MOSFET, and a balancing resistor; using the comparator to detect when the battery voltage of each of the lithium ion batteries is near end-of-charge, and then turning on each respective MOSFET of each of the lithium ion batteries; a portion of the charging current is shunted through the balancing resistor to bypass all of the battery cells of each of the plurality of lithium ion batteries, effectively slowing the charging of each of the lithium ion batteries; The method, wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries.

2. 1. A method of performing cell balancing on an electrical series string of lithium ion cells, comprising: providing a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, a MOSFET, and a balancing resistor; using a microcontroller to read the battery voltage using the analog-to-digital converter (ADC) to detect when the battery voltage is near end-of-charge, and then turning on the MOSFET; a portion of the charging current is shunted through the balancing resistor, bypassing all of the cells of each of the plurality of lithium ion batteries, thereby providing the effect of slowing the charging of each of the lithium ion batteries; The method, wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries.

3. 10. The method of claim 1, wherein the battery voltage threshold at which the balancing resistor begins to bypass charging current is set to a voltage near the full charge voltage of the particular lithium ion battery chemistry being used.

4. 4. The method of claim 3, wherein as the electrical series string of batteries is charged, cell balancing is turned on on each battery as it reaches a balance threshold, until cell balancing is turned on for all of the batteries in the electrical series string.

5. The method of claim 1 , wherein balancing is performed only during charging.

6. 6. The method of claim 5, wherein said balancing requires a balancing circuit to detect when a battery is being charged.

7. 7. The method of claim 6, wherein the balancing is performed by setting a voltage threshold for balancing near a fully charged voltage, and when the battery is charging and the voltage threshold is reached, balancing is enabled.

8. 8. The method of claim 7, wherein after charging is terminated, the battery voltage naturally relaxes to a voltage below the voltage threshold, which disables balancing.

9. 10. The method of claim 1, wherein control electronics within the battery management system (BMS) can provide a signal to a balancing circuit indicating whether a battery is being charged.

10. 10. The method of claim 1, wherein the balancing current should be high enough to effectively balance the batteries, but low enough not to interfere with the charge termination scheme used in the battery charger.

11. 10. The method of claim 1, wherein the value of the balancing current is between 100 mA and 500 mA to provide proper balancing and work with most battery chargers.

12. The method of claim 2, wherein the battery voltage threshold at which the balancing resistor begins to bypass charging current is set to a voltage that is close to the full charge voltage of the particular lithium-ion battery chemistry being used.

13. 13. The method of claim 12, wherein as the electrical series string of batteries is charged, cell balancing is turned on on each battery as it reaches a balance threshold, until cell balancing is turned on for all of the batteries in the electrical series string.

14. The method of claim 2 , wherein balancing is performed only during charging.

15. 15. The method of claim 14, wherein the balancing requires a balancing circuit to detect when a battery is being charged.

16. 16. The method of claim 15, wherein the balancing is performed by setting a voltage threshold for balancing near a fully charged voltage, and when the battery is charging and the voltage threshold is reached, balancing is enabled.

17. 17. The method of claim 16, wherein after charging is terminated, the battery voltage naturally relaxes to a voltage below the voltage threshold that disables balancing.

18. 3. The method of claim 2, wherein control electronics within the battery management system (BMS) can provide a signal to a balancing circuit indicating whether a battery is being charged.

19. 3. The method of claim 2, wherein the balancing current should be high enough to effectively balance the batteries, but low enough not to interfere with the charge termination scheme used in the battery charger.

20. 3. The method of claim 2, wherein the value of the balancing current is between 100 mA and 500 mA to provide proper balancing and work with most battery chargers.

21. 1. A battery balancing system for charging an electrical series string of lithium ion batteries, comprising: a plurality of lithium ion batteries connected together in the electrical series string of the lithium ion batteries, each of the plurality of lithium ion batteries having a battery management system (BMS) comprising a comparator, a MOSFET, and a balancing resistor, the comparator configured to detect when a battery voltage of each of the lithium ion batteries is near end-of-charge and then turn on each respective MOSFET of each of the lithium ion batteries; a portion of the charging current is shunted through the balancing resistor to bypass all of the battery cells of each of the plurality of lithium ion batteries, effectively slowing the charging of each of the lithium ion batteries; A battery balancing system wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries.

22. 1. A battery balancing system for charging an electrical series string of lithium ion batteries, comprising: a plurality of lithium ion batteries electrically connected together in series, each of the plurality of lithium ion batteries having a battery management system (BMS) including an analog-to-digital converter (ADC), a microcontroller, a MOSFET, and a balancing resistor, the microcontroller configured to read battery voltages using the analog-to-digital converter (ADC) to detect when the battery voltage is near end-of-charge and then turn on the MOSFET; a portion of the charging current is shunted through the balancing resistor, bypassing all of the cells of each of the plurality of lithium ion batteries, thereby providing the effect of slowing the charging of each of the lithium ion batteries; A battery balancing system wherein a single electrical charger is used to charge the plurality of lithium ion batteries connected together in the electrical series string of lithium ion batteries.

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