Store Li-ion batteries at 40% charge?

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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

 

[[email protected]]

I've often seen the recommendation to storeli-ionbatteries at40%
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
around40% - 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 ali-ionpower-tool battery with no "fuel
meter" on the charger or tool.

Thanks

Martin

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