Devices and methods for protection of rechargeable elements

Abstract

A protection circuit for use with a charger and a chargeable element, such as a rechargeable lithium ion battery, comprises a shunt regulator having a threshold ON voltage coupled in parallel across the chargeable element, and a temperature-dependent resistor, e.g., a positive temperature coefficient device, coupled in series between the charger and the chargeable element. The temperature dependent resistor is thermally and electrically coupled to the shunt regulator, wherein the first variable resistor limits current flowing through the shunt regulator if the current reaches a predetermined level less than that which would cause failure of the regulator, due to ohmic heating of the regulator.

Description

RELATED APPLICATION DATA [0001] This application is a continuation-in-part of application Ser. No. 09 / 060,863, filed Apr. 15, 1998, and a continuation-in-part of provisional application Ser. No. 60 / 126,952, filed on Mar. 25, 1999, which are hereby fully incorporated by reference for all they teach and disclose.FIELD OF INVENTION [0002] The present inventions pertain generally to the field of overvoltage and overcurrent protection systems and more specifically to devices and methods for protecting rechargeable elements, such as rechargeable batteries, from overvoltage or overcurrent conditions. BACKGROUND [0003] Electrical circuits that protect rechargeable elements, such as rechargeable battery packs, are well known. However, such rechargeable elements, and in particular rechargeable lithium battery cells, can be dangerous if the operating voltage exceeds a safe limit. [0004] For example, FIG. 1 shows a typical charging curve, i.e., the voltage across the battery vs. time, for a com...

AI Technical Summary

Benefits of technology

[0021] In accordance with another aspect of the inventions disclosed herein, control circuitry for controlling a shunt regulator transistor switch in a protection circuit for use with a charger and a chargeable element includes first and second voltage detection circuits. The first voltage detection circuit is relatively low leakage and is configured to activate the second voltage detection circuit if the voltage across the chargeable element approaches a threshold ON voltage of the transistor switch. The second voltage detection circuit is relatively precise and is configured to activate the transistor switch if the voltage across the chargeable element reaches the threshold ON voltage.

Problems solved by technology

However, such rechargeable elements, and in particular rechargeable lithium battery cells, can be dangerous if the operating voltage exceeds a safe limit.

When charging is in this area, the battery begins to operate in a dangerous mode, with the temperature rising above 70° C., and the pressure inside the battery rising to a range between 3 bars to 10 bars.

Even at this slightly increased voltage level, the battery might even explode.

At this stage, it is too late to save the battery, which is subjected to internal degradation and may explode or combust.

Notably, battery cells in a “fully-charged” state are more dangerous and susceptible to explosion than those in the discharged state.

As will be appreciated by those skilled in the art, the smart protection circuit 21 is relatively complex and expensive to implement with respect to the overall expense of a conventional battery pack.

However, when the composition is exposed to a high temperature due, for example, to ohmic heating from a high current condition, the resistivity of the composition increases, or “switches,” often by several orders of magnitude.

However, the combined in-series resistance of the overcurrent elements 32 and 34 is undesirable across the battery path.

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