[RE-wrenches] discharging Rolls batteries

[Bruce Geddes]

Re: [RE-wrenches] discharging Rolls batteries
  Capacity is reduced at low temperatures.  Now does this mean that there really is less energy in the battery, so that if I take out 50% of this reduced capacity I will only end up with a 50% state of charge once it has warmed, or is this just a performance hit that reduces the voltage and makes the battery capacity appear to be less?  Will the battery actually have less amphours to deliver, or will it just be delivering at a lower voltage?  My point is to ask whether it is legitimate to push the battery to a lower voltage in cold weather on the assumption that it is not really running that low, but just sluggish with cold?
Hi Hugh, When discussing battery characteristics with a chemist at my supplier years ago I was told the capacity at 0 deg c is around 50% of that at 25 deg c because the ion transfer rate within the cell is slowed by the low temperature.  This "loss" of capacity is recovered once the battery warms again.  The same reason why a fully charged cranking battery in a car can fail to turn the motor in cold weather.  Ion rate is so low it simply can't deliver enough current to the starter motor.

So, the answer to the second part of your question is yes.

A good thread with lots of useful background into how batteries really operate.

Bruce Geddes
PowerOn (at the other end of the world)
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