Wednesday, June 16, 2010

More on Balancing Batteries

Googling around the last couple of days I discovered that Elecsol (well known maker of leisure batteries) has released a new range of sealed VRLA AGM batteries primarily aimed at solar storage.

They make some impressive claims like that these batteries can deliver 1,100 100% discharges and 1,400 80% discharges.  They offer a 7 year "unlimited" warranty on them.

You can read the blurb on them here:  http://www.elecsolbatteries.com/literature/

The tech book / brochure did have an interesting addition to the traditional star pattern series-parallel wiring scheme.  It had not occurred to me to put balancer cables between the mid-points of the series strings to allow internal equalisation of the bank.  I'd had a problem with one of the six new batteries in my bank being low (out of balance with the other one in series) but I cured it by taking that pair out of circuit and slow charging the weak one to bring it up and then put them back in circuit.  The whole bank appears to be behaving ok now but I periodically measure the volt differences on each block. 

I might put a star equaliser network in (connecting the mid points of all three pairs to a common point so they will equalise er... equally).

The red/black lines are the power lines I have on my bank and the green ones would be the equalisation network that would allow all the weak batteries in pairs to charge up more without over charging the stronger ones.  In my "bad" pair they showed 28.2V across the pair but one was 13.8 and the other was 14.4.  With the equaliser network added, the weak "bottom side" battery might continue to charge by "finding" another weak "top side" battery to pass current through (13.8 + 13.8 is only 27.6 so that pair would continue to charge without over charging the others sitting at 14.1V).  It wouldn't help in every scenario though - if all the bottom side batteries were weak and all the top side batteries were strong then the equaliser network wouldn't help. 

Maybe it's just as well to rotate batteries in strings (like tyres on a car)?

Meanwhile, it's been a great couple of days harvesting.  In those partial cloud surges that I mentioned before, I've seen surges up to 1.98kW from my 1.8kW array (over 109% of rated power). The Sharp ND170 340Wp string stole the show though, putting out 409W (120% of rated power).  They might have managed more but the charge controller capped the output at 15 Amps!  The other charge controller was also close to capping its output to the 60 Amp limit, as it surged to over 57 Amps - the pair pumping an eye-watering 72 Amps into the battery bank.

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