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  #51  
Old 09-15-2006
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Dave,

Thanks very much for your work on this. Very helpful information.

Would you care to go any further, e.g., in drawing any conclusions about paralleling batteries?

Bill
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Conclusion

Sure, it’s what I stated up front.

Paralleling batteries is a practical, safe and reliable way to increase battery system capacity on a boat. Paralleling is naturally redundant and a bad battery will not take your battery bank down, nor will it ruin the remaining batteries in the group. A bad battery will reduce the capacity of the total system only by its own loss of reserve capacity. Paralleled batteries will NOT self discharge into each other and the total capacity of the system is the sum of the individual capacities of the individual batteries.

A single battery bank will not increase the total reserve time over two individual banks, assuming they are both of equal total capacity, but it will simplify charging and help eliminate over-discharging one bank by human error, i.e. forgetting to switch banks.

Parrelled banks will have a higher reliability then a single large bank as not only the batteries are parrelled but so are some of the connections (that’s not to say that a single connection failure cannot bring down both parrelled and series battery banks, but there are a fewer number of connections in a parrelled bank that can bring down the entire system.)

In short, paralleling batteries works, and has some advantages over series individual large cells.
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Originally Posted by dave.verry
Business trip. Very hard to take points when your not there.
I figured it was something like that, but good to have verification. If you were available, I don't doubt you would have taken the readings per the schedule.
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Very interesting results, Dave. I'm not sure I've absorbed them all and had time to think them over, but something bothers me in the data. You say "The final voltage of the batteries at the end of the test were battery #1 = 12.629V and battery #2 = 12.681V. Battery #1 is the one at 90% capacity and battery #2 the one at 55% capacity."

A 35% difference in capacity, measured as how?? And the battery that you show as lower capacity has MORE voltage??? Isn't that, ah, physically impossible? Doesn't the voltage available decrease as capacity decreases?

I'm not sure we're seeing everything here, this 'bump' indicates something is out of sorts.

But more important--enjoy that wedding. A ten day long wedding party, wow, that's got to be fun.
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Test Battery Capacity.

Each battery was in a single 48V string that was in service in my lab for the last 5 years. They were replaced and I load tested each battery in the string with an 11.5A load over several hours and removed approximately 75% of the capacity of each battery and extrapolated the remaining capacity. The one battery that was bad had the test terminated first since its cell voltage reached the termination point (10.88V) much sooner then the others about 3hours 20 minutes compared to about 5 to 6 hours for the others. They were all scheduled to be recycled so I "saved" the two best and recycled the others. The two going to the recycler were the ones tested.
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Also, open cell voltage is a crude approximation of the charge condition of the battery, and tells you nothing at all about the overall capacity. The only sure way of testing capacity is with a load test, which you do each time you have a load on the batteries and are off the charger. More reason you need a good battery monitor that will calculate amp-hours out vs. amp-hours in.
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Good points, but don't you have a wedding to get to?

I suppose the "numbers be damned let's see what happens in the real world" way to double-check this would be to charge both cells, let them stand down, let them stand solo for a week, then load test them to see how many AH could be pulled out of each, singly. Maybe "practical amp hours" based on voltage drop or keeping a headlight lit.

Then recharge both, let both stand down again, leave them connected in parallel for a week, the repeat the load test, in solo, to see where the amps and volts had really gone. In theory...both tests should come out the same, if they're not consuming each other's power.

Maybe in the ten days till you get back, I can find out more. (You know us Missourians, "SHOW ME".)
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Dave and Hello Sailor...

I think the proper test would be Watt-Hours or Kilowatt-Hours, not Amp-Hours.

These are a measure of total ENERGY....amp-hours is not!

It's quite possible to kill a "225 AH battery" by pulling out only 110 AH with, e.g., a 100-amp load. Equally so, it's possible to pull 400 AH from the same battery with a load of only 1 amp.

Bill
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Bill-
You are of course correct, we're looking at watt-hours but if we ASSume a 12v load being used and standardize the useful voltage...then amps is "good enough for gummint work". Give me a 55-watt 12-volt headlight as a test load, and I can measure in amps or hours or watts, and still get a useful number for comparisons. (The only question being, at what point you call the light too dim.)
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