Store Li-ion batteries at 40% charge?

Question

I've often seen the recommendation to store li-ion batteries at 40% charge for maximum life. Wikipedia states "However, if they are not used for a long time, they should be brought to a charge level of around 40% - 60%. "

I doubt that voltage and "charge level" are linearly related - I imagine that voltage stays fairly constant as the power is used up only to fall off rapidly at the end, in which case, how are we supposed to know what the charge level of a battery is? I'm thinking particularly of a li-ion power-tool battery with no "fuel meter" on the charger or tool.

Thanks

Martin

rubenstein.martin · Accepted Answer

For future reference this is what I've done:

Fully charged the battery and put a small 12v bulb across it. Terminal voltage was 12v. I let this discharge monitoring every 10 mins. Contrary to what I expected, voltage did fall fairly linearly with time until after about 2.5 hours at around 8v it started to fall quite quickly. I let it drop to around 4.5v and then took the bulb away. I left it to stabilise and then measured terminal voltage -

8.25v.

I therefore took 12v as the fully charged state and 8.25v as the discharged state, I therefore took the 40% charge as 9.75v and put the battery back on the charger for a while, discharging with the bulb as necessary until, with the bulb removed and the battery stabilised, I got 9.75v or thereabouts. And then I stuck it in the fridge till needed.

Martin

Store Li-ion batteries at 40% charge?

Join the Discussion

Didn't find your answer?