Editing Talk:Battery firmware lockout (section)

=== Interpreting the specifications ===
There are many values (voltage, current, temperature) on the datasheet of the battery cells and the packs that are made of these cells. For discussing firmware lockout, we mostly focus on two values: discharge cut-off voltage and safety cutout voltage.

==== Discharge cut-off voltage ====
This voltage is a '''loaded voltage''' i.e measured during discharging. It is when the current should be turned off during discharging. Since batteries have internal resistance, this voltage is not the "0% charge" voltage. Assume discharging to 2.5V, the voltage will climb to 3.0V or even 3.3V after removing the current.

The discharge cut-off voltage is highly related to cycle life. The battery's rated cycle life is measured by charging and discharging repeatedly at predetermined voltage, current and temperature.

Discharging to a higher voltage doesn't use up the full capacity of the battery but the actual life will increase. Discharging to a lower voltage gives very little gain at the cost of significant wear. Thus, the discharge cut-off voltage specification is determined with a balance of cycle life and discharge capacity.

==== Safety cutout voltage ====
Commonly referred as "Do not charge" voltage, this voltage is where chemical decomposition occurs and it's no longer safe to charge due to elevated risks. 

This is an '''open circuit voltage'''. If you somehow discharge the battery to this voltage but it bounced back to above discharge cut-off voltage after switching off the device, it's still safe to use, but don't do this intentionally, the gain is not worth the cost. If it bounces back to between these two voltages, see below.

The voltage between the safety cutout voltage and discharge cut-off voltage is the "pre-charge" range, where a very low charging current is applied to safely bring the battery pack back to normal.

A common "self destructive design" is to program the BMS "Permanent Fail" low voltage to way higher than the battery's safety cutout voltage. If the voltage is set to as high as the discharge cut-off voltage, the battery may brick itself if a large instantaneous load is applied(common on power tools) while the battery is at low charge.

==== Zero voltage ====
If the battery pack is discharged even momentarily after one cell has reached zero voltage, internal short circuit will occur within that cell due to reverse polarity. The cell will be destroyed immediately and permanently, without any chance of recovery.