Safety requirements
Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs,[
citation needed] and can be dangerous if mistreated. They may suffer thermal runaway and cell rupture if overheated or overcharged.
[38] In extreme cases, these effects may be described as "explosive." Furthermore, overdischarge can irreversibly damage a battery. To reduce these risks, batteries generally contain a small circuit that shuts down when the battery moves outside the safe range of 34.2 V.
[22][31] When stored for long periods, however, the small current drawn by the protection circuitry itself may drain the battery; normal chargers are then ineffective. More sophisticated battery analyzers can recharge deeply discharged cells by slow-charging them to first reactivate the safety circuit and allow the battery to accept charge. Overdischarge can short-circuit the cell, in which case recharging can be unsafe.
[39]
Other safety features are required:
[22]
- shut-down separator (for overtemperature)
- tear-away tab (for internal pressure)
- vent (pressure relief)
- thermal interrupt (overcurrent/overcharging)
These devices occupy useful space inside the cells, reduce their reliability[
citation needed]; ,and irreversibly disable the cell when activated. They are required because the anode produces heat during use, while the cathode may produce oxygen. These devices and improved electrode designs reduce/eliminate the risk of fire or explosion.
These safety features increase costs compared to nickel metal hydride batteries, which require only a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve