Solar Battery Plant Question

[Al Skierkiewicz]

Ed,
Your power output on PV arrays is limited by size and sunlight. In Georgia during the summer, you should be able to get plenty of sunlight provided you can keep the craft in the sun and not under trees. In order to get an array capable of 500 watts, you may have to increase the size of your craft. A quick check found 125 watt units are 25 x 60 inches but the cost is in the $300-$400 range. Current output would be around max charge current for one battery, 6 amps@16 volts in full sunlight. I am guessing you would mount the array flat to provide less wind load so you have to fudge the current and voltage output a little. Without some form of switching, it is difficult to prevent the array from providing some load current which takes away from the charge current. Even at 6 amps, our batteries will take 2-4 hours to fully charge, each.
As to parallel operation, there are varying opinions on this subject. Many people will tell you it is OK to do it, some will tell you never do it. The Battery Institute of America gave me this advice...If the batteries are of the same type, age, temperature, and charge/discharge cycle life then it can be assumed that the only difference lies in manufacturing tolerances. If any of these factors are different, then the individual battery chemistry is sufficiently different that terminal voltage will vary from battery to battery. Millivolt differences will cause current flow from the higher voltage to the lower voltage battery until the difference is equalized. However, the charge state, internal temperature and internal resistance of the higher voltage battery will have changed to the point that it's terminal voltage has now changed. The result is that both batteries will self discharge over some period of time as one feeds current to the other. It is expected that used batteries with our type of load history will have greater than a few millivolts terminal voltage differences due to changes internal to the battery, and the experts expect self discharge to occur in perhaps a day with just two batteries. (Diodes prevent this circulating current.) AGM batteries are not very tolerant of deep discharge and may be damaged by this kind of use. Different battery types (chemistry and construction) differ in terminal voltage variance so blanket statements cannot be applied to all battery types used in parallel. One application that used batteries in series/parallel operation was electric submarines. Remember that these were large, wet cell batteries where the specific gravity of the cell electrolyte was checked several times a day.