Lithium technology batteries provide an energy dense solution for portable power. The advantages lithium batteries provide come with added risk to final applications. To mitigate this risk, battery protection measures must be taken to reduce the possibility of battery failure. One such protection measure, 0 Volt Charging Inhibition, is implemented in many battery protection circuits. The protection is used to prevent the continual charging of an internally shorted battery.
Using a few techniques and features widely available on chargers, voltage based charging inhibition features can be approximated with only a host processor available. Using battery undervoltage detection as a basic battery voltage monitor, and some additional software logic, 2-V Charge Inhibition can be implemented in charging systems to add another layer of protection to the battery charging process.
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Lithium Ion and Lithium Polymer batteries represent the highest energy density among widely used battery technologies today. This type of battery, however, requires the additional protection features to make sure that their use and operation keep them within safe operating ranges. The risk of thermal runaway poses significant liability for any application that uses lithium-based batteries. As a result, battery chargers and battery protection IC’s have developed more robust protection features for given battery-powered application.
One such feature that is increasingly found in battery protection ICs is a 0-V charge inhibiting feature. A battery that has depleted to 0 Volts can be indicative of a damaged battery with an internal short. In most applications, a device enters a very low power consumption state as the battery enters a deep discharge; in some cases, the battery is disconnected from the charging or discharging circuit. If battery voltage continues to fall despite the greatly reduced consumption, this can be due to an internal short. If the battery voltage drops below a certain voltage, the 0-V charge inhibit feature stops charging. For the BQ297x family of battery protection circuit, this voltage threshold can be as high as 0.75 V. Once this voltage threshold is crossed the battery is deemed unrecoverable, charging is inhibited completely.
A low battery voltage, however, is not necessarily indicate of an internally shorted cell. Normal battery self-discharge can cause battery voltage to drop over extended periods of time, but these cells can still be usable. If the battery has self-discharged, but voltage is still above the 0-V charging threshold (1.7 V for the BQ297x), then the battery cell can still be recoverable. This is not necessarily the case for all batteries, battery suppliers needs to be consulted to identify the lowest possible recoverable battery voltage. Recovering a deeply discharged battery requires charging at a low current due to the battery
This feature can be similarly approximated in a charging design with a host controller to provide logic. This can add a layer of protection to systems that can not have a protection circuit integrated into the battery pack. It is important to observe the implementation of this feature and how the design can affect the charger's interactions with a battery protection circuit.