Batteries are seldom fully discharged, and manufacturers often use the 80 percent depth-of-discharge (DoD) formula to rate a battery. This means that only 80 percent of the available energy is delivered and 20 percent remains in reserve.
If everything looks good but you’re still having doubts, err on the side of caution and get it tested. If your car battery is at 50 percent, it means that it still has half of its charge left. This isn’t necessarily a bad thing, but it’s something to keep an eye on.
I know both from online websites and from experience that discharging a li-ion battery below 50% greatly decreases its life and capacity. But why is it so? There are two main effects that cause damage to a Li-ion battery due to "deep cycling": During the charge/discharge cycle, lithium ions are intercalated within the electrodes.
So, if your car battery is only at 50% charge, you should still be able to start your car without any issues. Of course, it’s always best to keep your car battery at 100% charge whenever possible. That way you’ll never have to worry about being stranded with a dead battery.
A battery in a satellite has a typical DoD of 30–40 percent before the batteries are recharged during the satellite day. A new EV battery may only charge to 80 percent and discharge to 30 percent. This bandwidth gradually widens as the battery fades to provide identical driving distances. Avoiding full charges and discharges reduces battery stress.
For example, if you decide to constantly fully charge a battery cell (100 %) and discharge it till 20 % you can expect 1.000 cycles until reaching the EOL. However, if you charge it till 80 % and discharge it fully (till 0 %), you can expect to triple the cycles (3.000) before reaching the EOL.