Hi boB, We know that current will flow through the lowest resistive path. In a perfect world, the resistance from end to end of a battery string will be identical. Real world conditions show that one or more cells have a slightly higher internal resistance. Add to that the imperfect terminal crimp, improper torque at battery terminal, and perhaps terminal corrosion and we now have paths of differing resistance. The parallel strings have an unequal distribution of current and ergo undercharged cells. These cells retain a small amount of amorphous lead sulphate that, without correction, will begin to crystalize. We all know what will happen over time.
Granted, this can be prevented by the proper knowledge and attention but I see far too many failures from neglect and/or lack of understanding. My experience comes from 10 years of working with hundreds of small 12 volt systems with 2 to 6 parallel strings of batteries. In our shop we often see 2 to 3 year old battery banks that are damaged and I believe this is a contributing factor. It is often too late by the time the problem is discovered. So, my opinion is to reduce this possibility by decreasing the parallel paths whenever possible. More strings of a given size battery means less stress per string. But, if you decrease strings you are normally increasing cell size to maintain the capacity so this should not be an issue. Battery balancing is a great idea. I would like to see an active system for shunting current on a string to keep an equal current across all strings. I know this is done in the EV business. Larry Crutcher Starlight Solar Power Systems 11871 S Fortuna Road, #210 Yuma, AZ 85367 [email protected] (928) 342-9103 On May 31, 2011, at 11:16 PM, boB Gudgel wrote: > > > OK, I've been hearing for years now that one battery string is best and maybe > two but no more should be used. > > What is the reason for this fear ??? What is the experience with multiple > strings in parallel ?? > > If it is because of current sharing of the strings, that shouldn't be an > insurmountable problem if the wiring is done right. > > If it has to do with the possibility of fire, which Darryl mentioned, seems > that series string fuses could mitigate the possibility of that issue, if > because of shorted cells, just like PV combiners. > > Or maybe it has to do with the trouble of getting into the middle of a pile > of batteries ? Use battery boxes ? > > The strings are in parallel and so they all get the same voltage, > theoretically anyway if the connections are good. > > I would think that the more strings, the LESS stress on individual batteries, > not more. The current in each battery should be less than it would be I > would think. > > So what is the real experience ?? > > Is a battery balancer the answer ?? > > Thanks, > boB
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