18v li-ion drill battery storage

I have an 18v drill that takes li-ion batteries and last winter I left it in my garage( some days down to -30c) at full charge and wondered if I did any damage? What's the ldeal way to store them this year? I know 40% charge is best but how do I find out what my batteries at?

-30C is definitely on the low side for lithium ion, but I don't think it would damage them when just sat resting. If it's getting down to those sorts of temperatures in the garage, I may be tempted to keep the batteries in your house at the coldest times. If you try to use them when they are that cold, you will probably find that the capacity is significantly reduced, hence it may be a good idea to store them somewhere a little warmer!

If they aren't going to be used for an extended period, then keeping them at the ~40% storage charge may not be a bad idea. I assume that the 18V pack voltage is reached by 5 series cells. This thread may help you estimate the corresponding open circuit voltage for 40% capacity:

http://www.candlepowerforums.com/vb...-Ion-State-of-Charge-and-Voltage-Measurements

So it would be 5 x 3.6v in series? But fully charged 5 x 4.2v (21v)? 40% would be 5 x 3.8v (19v)?

I suspect that the batteries are LiFePO4 chemistry, so fully charged is about 3.6V (so that link isn't quite appropriate, sorry!). I think that 40% is about 3.35V - 3.4V per cell. The best way to check is probably to measure the pack when it's charged and see what the voltage is, that should help confirm the chemistry and number of cells.

Aren't the 3.6v cells fully charged at 4.2 like the 3.7 li-ions?

I leave mine in the box it came with,i sell these for a living and only once in nine years i have been asked for a new battery and that was for a 1/2" drive cordless impact wrench powered by 19.2v which we sell for £90 per battery.

jimbo231 said:

Aren't the 3.6v cells fully charged at 4.2 like the 3.7 li-ions?

Click to expand...

Not for LiFePO4. 3.6V is about the max for this chemistry, and they quickly drop down to the 3-3.3V region.

Interestingly enough, -30C where the OP is converts to -22F.
That's very very close to the lowest temp it would get to inside my Dad's garage (in Alaska) during -50F weather.