Multiple Batteries in Parallel

Boon Docker wrote:
Here is a quote from "The Battery Experts"

The 100 AH battery will become fully charged long before the larger one. The combined voltage will rise, but by the time the controller turns off the charging sources, the 100 AH battery will be overcharged. Meanwhile, the 200 AH battery will not get fully charged. When the bank is being discharged, the 100 AH battery will go flat and its voltage will fall well before the 200 AH battery. The inverter will eventually cut out but not before the 100 AH battery is excessively drained.

So is the above not true?

I ran a test to see how it went with one of my 100AH 27 batts and my 250AH 8D. (all AGMs) The wiring was not properly balanced in my zoo of a set up and with one 100 missing, but just to get a general idea.

I ran the fully charged 350AH bank down with a 91 amp draw until the Trimetric AH counter said it was 103AH down.

So IMO the 100's share of that would be 100/350 x 103 = 29.43AH which would be 70% SOC approx.

I could not measure the SOC of each battery at that point, being AGMs and so no hydrometer.

I then recharged the 100 by itself and it appeared to take 35AH including some for heat despite the Tri's fixed fudge factor.

Using that 35 anyway, the 100 was down to 65% and the 250 was down 68 AH out of the 103, or at 73% of 250.

65/73 vs 70/70 may not mean much with the margin of error in this test. I believe from previous tests with better wiring balance it would have been more even, but whatever.

The 100 did not go flat! :)