One of the main reasons that battery voltage dropping under load is because the current passing through the battery causes resistance. This resistance creates heat, which in turn reduces the battery’s ability to deliver power. Additionally, as a battery discharges, its internal resistance increases, which also contributes to a voltage drop.
Drivers get plenty of heads-up when the battery runs low, and if it’s low enough, the car will reduce propulsion power. Some EVs will even have the navigation system display the closest public chargers and guide you there. Range anxiety can be a powerful motivator for keeping a fair amount of power on reserve, just in case.
Fully charging or discharging a lithium-ion battery pack accelerates deterioration of its cells. EV makers use battery management system software to prevent this happening, sacrificing around 10 percent or so of the pack’s theoretical capacity to make it long-lasting.
Running completely out of power is known as 'deep discharging' and can lead to the battery deteriorating, reducing its performance and ability to hold charge. If you have less than 10-20% charge left, it’s always best to recharge if possible rather than letting it dip further. How far can an electric car go?
When a good battery is put through a load test equal to its rated CCA (cold cranking amps) its voltage will drop to around 9.6 to 10.5 volts depending on the ambient temperature. It will then shoot back up to ~12.6 volts once the load is removed. A battery with one or more dead cells loses around 2.1 volts with each cell that has died.
It can read a full voltage of 12.6 even though it has a bad cell. However, when a battery with a bad cell is put under load, it will immediately fall well below its real voltage of 10.5 volts. Once the load is removed, it will only bounce back up to its maximum 10.5 volts. So when is 10 volts enough?