Hi, I have a couple of 18650 batteries and I was wondering what the cut off voltage of these batteries is. I read it's 2.75 V. Also, when I put the batteries together to get a 8.4 V power supply, voltage measurement changes when I add a load.
Say under load multimeter reading of power supply is 6 V and without load reading is 8.4 V. Which voltage value should I use when coming close to cut off voltage.
Thank you
Hi cmore082,
usually the little LiPo chargers use a TP4056 charger (that basically takes care for charging) and a DW01 LiPo battery protection IC. The DW01 will disconnect your battery in a "overdischarge detection case" at a 2.4V.
Some people say, that this is already to low and you should disconnect at a higher voltage (2.9V) but the common sense seems to be: if the current UNDER LOAD drops below 2.4V the battery is empty and you should disconnect - it will recover immediately to a higher, non critical voltage.
However: you have several things in your post, that seem strange to me.
2 x 18650 LiPos will only deliver a voltage of 8.4 if they are absolutely freshly charged. An average voltage I'd expect from two of those type would be ~ 2 x 3,8V = 7,6V, because voltage drops fast after charging.
Second: The LiPos tend to be dangerous. You should try to protect/monitor each cell on it's own. If you ignore that hint the shutdown voltage will most likely ~ 4,8V under load for your two-cell pack. I'd reccomend using protected cells or buy a cheap protector chip for your cells and "upgrade" them to protected ones.
Exactly what batteries are they ? 18650 battery technology has improved a lot recently and they are not Lipos, but one of several different Li-ion types. But if they're dropping to 6V under load when freshly charged then that suggests the load is too much for them. A load that high may well damage the batteries.
For cutoff 2.8V/cell under load is reasonable. You want them to recover to at least 3.4-3.5V after the load has been removed so the loaded cutoff value should be chosen so that works out.
Steve