Li-ion has become the chemistry of choice for many industries due to its high energy density, remarkable cycle life and high charge and discharge rates. Li-ion cells, partly due to the characteristics that make them so successful, are however more dangerous than the traditional NiMH/NiCd and Lead based chemistries if not treated correctly.
The correct treatment of a Li-ion battery must first be realised by careful system design and analysis of the end product where the battery will be used, it is critical that a Li-ion host system cannot put a battery into a potentially dangerous operating condition. In addition to system design, clear operating instructions and procedures should be in place to ensure battery packs cannot be mistreated or used beyond their designed operating conditions. Once the above conditions have been met, it should, in theory, be safe to use a Li-ion cell without issue. A Li-ion protection circuit comes into play when the above systems fail, its sole purpose is to maintain cell safety when things inevitably go wrong.
This article covers the critical features that should be present within a Li-ion protection circuit, these are the key features that are required by all reputable cell manufactures. If these key features are not present on a Li-ion PCM (Protection Circuit Module) or a PCM is omitted from a battery design entirely, the cell manufacture will withhold supply.
Over charging Li-ion cells beyond their specified maximum full charge voltage can lead to a process known as lithium plating. Lithium plating is process where lithium ions build up on the anode as metallic lithium. The plating can occur as dendrites, rigid whisker like structures, which can lead to significant reductions in cell performance and immediately raises safety concerns over the cell. Dendrite growth can cause internal cell short circuits due to the potential piercing of the cell separator between the Anode and Cathode electrodes, this in turn can create an internal cell temperature increase which has the possibility of leading to thermal runaway. Li-ion protection circuits must be capable of precisely monitoring all cell voltages within the battery pack and have the ability to shut off charge before any voltage reaches a dangerous level.
Operating a Li-ion cell while in a low voltage state is extremely stressful to the cell, increases in internal cell impedance at low voltages means even low rate charge or discharge operations have the possibility of causing severe cycle life deterioration and permanent capacity loss. More strenuous charge or discharge operation while in a low voltage state, especially in non-optimal temperature conditions, can cause permanent cell damage and consequential safety concerns. Even if treated with extreme caution, deeply discharged cells can suffer from the formation of copper shunts, a resistive short within the cell. Copper shunts lead to increased self-discharge of a cell and can cause excessive heating or thermal runaway if recharged. Li-ion protection circuits must be capable of precisely monitoring all cell voltages within the battery pack and have the ability to shut off discharge current before any voltage goes below the specified safe cell operating voltage. To prevent further decline in cell voltage, a good Li-ion protection circuit should have an extremely low quiescent and shut-down current consumption. It’s important to differentiate between transient voltage drops caused by current spikes and permanent low voltage conditions caused by over discharge, transient drops to low voltage should be ignored by a good Li-ion protection circuit, transient voltage drops are not dangerous to the cell.
Over Current and Short Circuit
Discharging or charging Li-ion cells beyond their maximum specified current ratings will cause overheating within the cell structure. Overheating Li-ion cells will at a minimum cause cycle life deterioration and permanent capacity loss, at worst it will lead to thermal runaway. Li-ion protection circuits must be capable of precisely monitoring both the discharge and charge current flowing through the battery pack and have the ability to shut down the battery before cell damage is sustained. A good Li-ion protection circuit should have shut down reaction times proportionate to the magnitude of over current; for example a short circuit current must be handled more quickly than a comparatively low rate over current condition.