[RE-wrenches] Magnum System - Discharged Battery Bank

[Larry]

Here's a drawing to illustrate. When the 70AH is drained, the string 
voltage will drop because that cell will go to 0 volts. Continued 
discharge will reverse the cell voltage. When charging, the 70AH will 
get full first and be overcharged.

On 12/19/14 9:44 AM, Larry wrote:
> Corey,
>
> The reason a cell or a battery in series will reverse polarity is due 
> to that cell having a lower capacity than other cells. For example, 
> you have a string of 100AH cells. One cell only has a capacity 70AH. 
> If you discharge the string more than 70Ah, in order for that cell to 
> continue passing current from the string, the voltage will reverse and 
> begin climbing in a negative direction.
>
> If it were me, I would pull that cell (cells) from your pack and test 
> individually. Fully charge and do a controlled discharge rate. If it 
> comes up less capacity than the others it should not be returned to 
> the bank. For one reason, the entire string is limited to the lowest 
> capacity battery. That battery will always be first to reach full 
> charge and first to totally discharge, reversing polarity again. 
> Another reason is while charging, it will be severely overcharged 
> while you try to reach 100% SoC on the other batteries. If the battery 
> is AGM or GEL, it can go into thermal runaway and may cause a fire.
>
> At the very least, charge the bank to 100% SoC and measure the 
> temperature of the cell(s) that had reversed. Measure before 
> transition to Float charge. If they are hotter than the others, you 
> know you still have a problem.
>
>
> Larry Crutcher
> Starlight Solar Power Systems
>
> On 12/19/14 7:48 AM, Corey Shalanski wrote:
>> Update on my investigation:
>> I visited the customer's house yesterday to set up a pulse charger. 
>> Much to my surprise the three batteries that originally registered 
>> negative voltage are now reading positive. Does this make sense - are 
>> batteries able to shift between negative and positive voltages at 
>> such a low charge level?
>>
>> I do not see any evidence of distorted cases.
>>
>>
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