Balmar smart gauge and 10 hour AH testing

lawrosa wrote:
3 hours.

SG 1.250
12.41volts measured @ batts

Your voltage is a tad high wrt the SG compared with the test I did the other day on my "new" 27DCs, which came out to 100% of rated and took the whole ten hours.

Believe the SG, not the voltage so much. Let's see what the voltages do, but don't go by them much for now.

In my case (YMMV) at 5 hours, voltage was 12.1 and SG was 1.250
At 8 hours, voltage was 12.0, SG 1.225, at 10 hours voltage 11.9, SG 1.200. Voltage bounce back from 11.9 was to 12.07 and next morning it was at 12.17 so my 50% notion was pretty close.

Your figures will not line up the same way as mine, but it gives you an idea. When doing 6s, the voltages were lower wrt SG, as to be expected where 6s are known to have more voltage drop under load than 12s. Something is odd about your high voltage readings wrt SG, but worry about that later, doesn't matter for now.

OK so watch your SG, which is already at 1.250 at 3 hours where mine was at 5 hours. I suspect you will get to 1.200 in less than 10 hours. Might be in 6 or 7, we'll see. Don't be surprised. That would be expected with 5 year old batteries of course.

Yes, as battery voltage goes down, the DC lights dim and draw fewer amps, so you do need to add some bulbs to keep amps up during the test. (It is the opposite if you were using an inverter with a 120v lamp as the load. As battery voltage drops, the inverter pulls more amps trying to keep power constant, so you have to reduce the size of the lamp)

On a discharge like this test, the AH count should be accurate, unlike on a recharge where there is heat loss so you have to use a CEF percentage (which gets worse above 80% SOC) Another thing is that SG is fairly linear on a discharge so this test works out well, but on a recharge there is "SG lag" which screws up any attempt to line up voltage/SOC and SG during a recharge.