what the safety electronics on lithium ion batteries do

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So what do the safety circuits do?

Like, I get the chemistry behind the lithium ion batteries. Lithium cobalt oxide, or lithium iron phosphate, and then intercalating between the graphite layers and move back and forth to charge. And lithium is pretty light and reactive, so that causes the thing to hold a powerful charge. But what purpose and role does the electronics play in this? What do they do? Other than the complex chemistry involved, what prevented lithium ion batteries from propagating earlier?

In: 7

The electronics protect the cell from excessive discharge or overcharge.

If deeply discharged, lithium cells suffer permanent degradation and are at risk of overheating and fire if recharged.

If fully charged, the energy from the charger goes into heating the battery which can cause thermal runaway.

The electronics monitors battery voltage and if it gets too low, or too high, disconnects the cell from the circuit.

It greatly depends on which specific battery management system you get, in essence they stop all current flowing based on selected parameters, the most important ones are:

1. Over temperature protection
2. Under temperature protection
3. Over current protection
4. Short circuit protection
5. Over voltage protection
6. Under voltage protection
7. Per cell over voltage protection
8. Per cell under voltage protection
9. Protection against large inbalance between cells

When responding to any of these conditions, you generally limit the potential to cause damage, but it’s not foolproof, and mechanical damage to the pack will still bypass these protections (like in an accident)

More intelligent BMSes communicate with the charger to further limit charging at the very low and very high band of state of charge, as well, but this is done for longevity reasons.

Furthermore, some BMSes also include active balancing, which uses power from the fullest cell to charge the emptiest cell – this isn’t strictly necessary if the pack is frequently top-balanced.

All of these protections do nothing in day-to-day running of the battery, they only exist for when the battery goes out of spec.