[RE-wrenches] 24 volt Battery Bank comparison

[Hugh]

I am glad to see this issue with series strings being brought into 
the discussion.

At 11:53 -0600 3/6/11, Ray Walters wrote:
>I've had single strings with equal current through each battery, but 
>some batteries would be at 15.5 volts, while others next to it would 
>be at 13. The charge controller shuts off when the sum of the 
>voltages hits the bulk charge V, but meanwhile some cells are 
>chronically overcharged or undercharged. (parallel strings no where 
>to be seen)

and......

At 15:10 -0300 3/6/11, James Surrette wrote:
>If you hook up 4 x 12V monoblocks for a small 48VDC system and begin 
>charging at 58.8V, total voltage will be 58.8V very quickly but 
>individual battery voltage will not be 14.7V.  Battery voltage will 
>have a large range from 14 to almost 17V.

If you have 4 x 12V monoblocks, they will thrive much better 
connected in parallel than they will in series. The parallel strings 
discussion reappears regularly on this list, and always produces a 
chorus of disapproval, but I think the downsides are over-stated.

Most battery charging regimes are based on bringing the battery up to 
a specific voltage and holding the voltage there for a period of 
time.  This can he achieved with perfect equality for strings of 
batteries in parallel.  (Yes technically you may need to use bus-bars 
to get within millivolts of each other but to be honest who really 
knows the optimum absorption voltage that exactly, and does it 
matter?)   So long as the battery as a whole gets up to and stays at 
the prescribed voltage for long enough, then all the parallel 
batteries will get charged according to the manufacturer's 
instructions.

With batteries in series I am finding that 48V systems are much 
harder to keep properly balanced than in the old days of 12V systems, 
since the current in one string is the same in every cell, whether it 
needs it or not.   Yes you could put diversion load regulators on 
individual 12V batteries in the string, but given the way they are 
priced (by amps) you would end up spending four times as much on 
charge controllers.

To me the issues of unbalanced charging are much worse for batteries 
in series than they are for batteries in parallel.  Now I will go and 
hide from the wrath of the list :-)

have fun,
-- 
Hugh Piggott

Scoraig
http://www.scoraigwind.co.uk
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