[RE-wrenches] discharging Rolls batteries

[James Surrette]

Hugh, 

Sorry if you found my response limited but have been traveling since early this am and don't have all information at hand, at the moment. 

Regarding temperature effects on capacity, earlier responses are spot on as the lower capacity is totally as a result of slower reaction times as a result of lower temperatures.   

Approximate Temp effects; 

40C = 104% 
25C = 100% 
5 to 10C = 90% 
-5C = 80% 
-22C = 60%
 
If you need more immediate responses, always feel free forward your questions to support at surrette.com, support at rollsbattery.com (both go to Serge) or phil at Barden-Uk.com (Technical Manager for Rolls Europe). 

Hope this helps, 

Jamie 

>>> Hugh <hugh at scoraigwind.co.uk> 1/14/2010 12:49 PM >>>

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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