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Old 09-05-2006
dave.verry dave.verry is offline
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Hellosailor,

“A connection failure can be remedied with a wrench. A CELL FAILURE is the only thing that would take down the series battery bank.”

Yes but while you found the wrench, and then located the problem your system will be without power. No lights, no autopilot, no GPS, no radio. Not a chance I would want to take. A cell failure in a series bank would reduce the bank voltage, not necessarily take the bank offline. Cell failures, when they occur, tend to fail short. The only open condition that I have observed during a cell failure is when the battery explodes.

“The odds of that same cell failure taking down your parallel bank are double or higher.”

Cell failures are cell failures. The rates of failure should be the same no matter which configuration you use. A cell failure in a parrelled bank will take that battery out, not the others. The equilibrium voltage would be reduced by one cell, about 2V and that one battery would stop discharging into the load. Now before you start, the remaining battery bank will not discharge into the battery with the bad cell, the cell voltages of the remaining cells of the bad battery will be higher then the discharge or load voltage of the remaining batteries.

Cell failures are rare compared with connection failures. Cell failures also happen gradually with the exception of mechanical damage or catastrophic explosions during overcharging. Connection failures, especially in a harsh environment like a marine environment are much more commen. Thermal expansion and loosening, corrosion, vibration all can cause failure of connections. One connection in series with your battery string that fails removes the string from the circuit. While one connection can bring down a paralleled bank, its more likely to remove part of the bank from the circuit rather then the entire string.

“Regarding the loop between the two batteries "The current will be very small and eventually stop. " This is the first and only time I've ever heard that. Have you measured this? With wet or AGM cells? What were the specifics?”

No, this is not tested, but is correct in theory. The theory is sound for all lead acid batteries. If you want I will test this during the weekend. I just happen to have two 92 AH batteries in my garage, one to recycle and one to install in the boat. The test will be simple and I have the equipment.
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