A few years ago I tested a range of AGM batteries for a safety application. The tests applied a constant current load to a fully charged battery until the battery reached the manufacturer's specified voltage for 100% of discharge. Tests were carried out in a temperature controlled room and measured with equipment calibrated to NATA requirements.
Battery capacities are normally quoted for "Capacity / 20" (C/20) which means that a 100Ah battery is only specified as 100Ah if it is discharged at 5Ah. In the case of my batteries I was interested in their performance at C/10 and my test curves reflect that. As it happens a C/10 discharge rate is probably more realistic for camping use anyway.
The curve below is for a 55Ah battery but will be representative of most (all?) capacities of AGM battery. Keep in mind that as the battery ages and its capacity naturally reduces its discharge time will shorten and the curve will become steeper.
To apply: Measure the terminal voltage of your battery with *no load* applied to it. Let's say your measurement was 12V14 - apply that figure to the graph and you'll note it corresponds to the 5 hour discharge point so at 12V14 the battery is about 50% discharged. At 12V0 the battery is about 60% discharged .
Keep in mind this is not a fully accurate way to determine level of discharge but it's not bad in most cases and is a hell of a lot better than guesswork.
Mike, I wonder if you could explain how that curve could relate to a pair of Gel 100ah batteries. According to what I have read, we should not be discharging the batteries below 50% capacity, and never further than 30%. What would those voltages be.
I don't think the shape of the curve will vary much between the lead-acid chemistries so you can use this as is for your gel battery - I think :)
One thing I should have mentioned is that this data was acquired at 20C. The curve won't vary significantly between about 10C and 30C but beyond those temperatures there will be some difference in which case play safe and recharge early. It's not critical and isn't instant death to the battery if it's discharged to, say, 30% from time-to-time but it's best not to do that often.
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"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
This chart is for an as new Full River 105Ah battery and gives the effect of different loads on just how long before that 12v figure is crossed and the discharge should stop if you want a decent cycle life. You can discharged them to the manufacturers fully discharged limit if you want, but the cycle life shortens severely as can be seen in the second chart.
Interesting to look at the difference between a constant 5.25 amp load (a 60w light bulb) and a 9.5 amp load (114w so not quite 2 x 60w light globes) Roughly 11 hrs for the 60w globe but only a bit over 4hrs for not quite twice as much load. Step it up to a 200w load and the 12v line is cross in less than 1 hr.
The test conditions states a max current of 0.4CA and that equates to the advertised capacity divided by 4, I.E. 25 amps for a 100Ah battery.
