Unfortunately the SmartGauge doesn't tell you remaining battery capacity. It is only a state of charge meter. E.g. if your 100Ah battery is only a 90Ah due to aging, a SmartGauge reading of 50% SOC does not mean you have 50Ah remaining. This is the same problem an Amp Hour Meter has in showing remaining capacity, unless its kept calibrated.
In the realities of battery health / DOD to SOC etc.. remaining capacity is the least of our worries. Like anything we learn how long we can run before we hit XX SOC. With the Smart Gauge you will learn how long you can run to 50% and develop a feel for it. The boats fuel gauge does not tell us how many hours we have left either..
Unlike an Ah counter, which is almost never programed well enough to give you an accurate snap shop of remaining capacity, or trying to use a loaded voltage and self compute, the Smart Gauge is really actually pretty smart..
If you move away from thinking of how much you have
remaining and start focusing on "
Do I need to charge soon?" the Smart Gauge becomes a really great tool......
For Don I ran a few tests over the last few days as I had an open spot on my test bench. I threw a 100Ah rated Lifeline battery into my temp stable water bath and let it come up to 77F.
The battery had a known tested capacity of 71.28 Ah's when tested to 10.5V at 77F at a 5A load.
Lifeline states in their technical manual that a Lifeline battery at 77F, under the 20 hour load, will hit 50% SOC at a
loaded voltage of 12.15V. I set my tester for 5A and a 12.15V cut off.
50% of the
actual capacity of that battery would be 35.64 Ah's
The 5A load ran to 12.15V at a battery temp of 77F and delivered just 24.14 Ah's in 4:50 minutes.
At 1 hour the rebound resting voltage was 12.235V indicating approx 50% SOC
At 28 hours the rebound voltage was 12.411V indicating closer to 72% SOC (Note: even at 77F it had not totally stabilized yet)
Based on Ah's removed & the known capacity the battery was really closer to 66% SOC.
The resting voltage got us a pretty close but remember we had 28 hours of resting battery voltage at 66% SOC in order to get there. Sitting at that low of an SOC can lead to hardening of lead sulfate. I would much prefer to discharge to 10.5V then
immediately recharge and keeping the chemistry moving not sitting idle at a lowish SOC..
If you did not already know the actual capacity it would be kind of like throwing darts to find your new 50% SOC range.. Also if you try and go for a resting voltage with battery temps colder than 75F attaining the resting voltage takes considerably longer achieve. I have an Odyssey battery that came off the test bench two weeks ago in my 45F garage still sitting at 13.19V...
Unfortunately using resting voltage to find the new 50% point can be more damaging than a single test to 10.5V as the battery won't sit there for 24 hours each time you draw a few more Ah's out trying to find the new 50% point..
With a Smart Gauge you can really avoid capacity tests altogether and let it tell you when you've hit 50% SOC. You then replace the batteries when you feel they are no longer performing as they should or you can choose to test them..
As Lifeline says in their technical manual:
"These voltage levels are approximate and give an indication of the state of charge of a battery at rest. As the battery ages these voltage measurements will be lower."
Finding the actual capacity is crazy hard, but it is time consuming, a bit tricky to do it correctly and hold current and temp steady..
