[RE-wrenches] discharging Rolls batteries

[Hugh]

Thanks, Bruce,

I have lived with batteries for thirty years so I do know some of the 
basic things but for most of that time I used secondhand batteries. 
The last couple of years using expensive new ones I find that they 
often perform less well.  I would like to tap into the enormous 
experience of this forum to calibrate some of my assumptions.

I got a nice speedy and informative reply to my first post but when I 
asked too many questions in the second one it produced a lot less 
information.  Maybe I need to go straight to Rolls tech support for 
discharge curves at lower temperatures and lower currents.

I do have one more question for any wrench who programs 'genstart' 
voltages on Outbacks and the like.  At what battery volts do you 
start the generator for a system with a large battery and small 
loads, in the winter?

I would habitually use about 23.5 or 47 volts as a discharge limit 
voltage (time delayed).  However I am finding that from full charge 
at a temperature around 5 degrees C, I am getting under 25% of the 
battery capacity using this rule.  I this what I have to accept?  Is 
the capacity that much reduced by temperature?  Will I reduce the 
life expectancy and invalidate the warranty if I discharge it to 
23/46 volts instead?

I wonder why there is no temperature compensation on low battery 
voltage settings like there is on charging set points.  Do I just 
have to use the generator much more?

There I go asking too many questions again.  I do have a few more 
that I will save for now.  Thanks for any more comments.

best wishes

Hugh
>
>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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-- 
Hugh Piggott

Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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