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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