call it 100ah and don't stress out over the details. all these manufactrs use different voltages to get their amp hour ratings, there is no consistency or mandated industry standard to use.one might say 105 ah at 12.0 V the next guy says 100ah but at 12.5V,with no set standard its all just marketing hype anyway.
Actually the Ah rating test is very industry standardized and not rated at 12.5 or 12.0 volts at all. This 20 hour industry standardized testing has been around since the 1920's..
When batteries are designed they have a general idea of what it should produce when compared to the 20 hour Ah discharge test. The manufacturers then run multiple tests on the battery, and periodically sample batches, to determine the industry standardized 20 hour Ah rate. This is why you see some GC2 golf cart batteries that can produce 232 Ah's and others only 215 Ah's in the same case or some group 27's producing only 70 Ah's and others producing 110 Ah's in the same physical case size.
The industry keeps each other honest too. US Battery & Trojan for example are frequently testing competitors batteries against the ratings and this is why you sometimes see Ah ratings change or CCA/MCA ratings change.
The 20 hour rate is how many amp hours the battery can supply before dropping to 10.5 volts under a given load. Each battery has it's own amp load it can support for 20 hours before hitting 10.5 volts. It is not rated at a specific voltage.
They start with a 100% charged battery then discharge it over a 20 hour period until it hits 10.5 volts. It often take multiple tests on a newly designed or recently changed battery to arrive at the amp draw that will go 20 hours before hitting 10.5 volts.
For us it is easy as the manufactures have already figured this out. So a 200 Ah battery can support a 10 amp load for 20 hours before dropping to 10.5 volts. This test is easily repeatable at home if you have a DVM and a known amp load that matches your batters support load.
I have tested new batteries and old using this simple standardized test and new ones are almost always spot on the rating and often very slightly over if the temps are not at spec (77-80F). The toughest part is coming up with a known load that equals the 20 hour rating load if you don't already know it.
To perform the test you simply divide your batteries 20 hour Ah rating by 20 to arrive at what the battery can support for 20 hours before dropping to 10.5 volts.
In Practice: Battery "A" has an amp hour rating of 150, dividing by 20 = 7.5. This battery can support a 7.5 amp load for 20 hours before dropping to 10.5 volts/dead and will produce 150 Ah's while doing so. A battery with a 60 Ah rating will carry a 3 amp load for 20 hours before dropping to 10.5 volts, a 200 Ah battery can support 10 amps for 20 hours before dropping to 10.5 volts. Ah's/20= amp load
The other common test is the reserve capacity test. In this test 25 amps is applied and the minutes counted before it hits 10.5 volts.
A group 27 battery without a 20 hour rating may only be in the 75-80Ah range. Generally if a battery is not meant for deep cycling they don't give it a 20 hour rate.
