State of charge

For whatever it's worth, I have found that if I disconnect the battery from everything for about 15 minutes, I get a fairly accurate measurement of resting charge. It's not as accurate as letting it sit for 24 hr., but I usually do this "quick rest" method when we are camping. So I can't disconnect it for 24 hr.

I developed the "15 minute test" by disconnecting the battery and taking readings with my voltmeter every three minutes. For the first 15 min. after disconnecting, the voltage kept going up. But the curve flattened out after 15 min. and stayed pretty constant for quite a while.

If anyone has different information, I would be very interested in hearing about it!

I am using a pair of Trojan T105s to power a ARB 50 quart frig, I leave for three days with them fully charged. They don’t seem to be being drawn down much, they started fully charged at12.86 volts and only went down to 12.68 volts.

S Davis wrote:
How accurate is using resting voltage as a indicator of state of charge?

Close enough for RV use. I would speculate you are within 5% to 10%.

Are you wondering when to start the generator? Or how long to run it?

It is possible to compile a chart for loaded vs at rest. But it may be more trouble than it is worth. True "at rest" is 24 hours with no charging and no draws.

S Davis wrote:
How accurate is using resting voltage as a indicator of state of charge?

Using resting voltage per SOC is ok while camping since 12.1 is 50% (Trojan) and you will be seeing that as a loaded voltage, you are on the safe side. (no solar making voltage higher)

Not so good with Lithiums (see other discussion on that with 3tons)

for Wets and AGMs fairly useful when camping although they are not resting as such. I use the "morning voltage" which means before any solar can raise the voltage and when not much is on for an amps draw. Between furnace cycles mainly and other stuff not on except the low draw things.

I cross-check voltage vs AH as seen on the Trimetric and have a good idea when they don't "match" so something is on I forgot to turn off, eg., and I have an idea for what counts as being time to recharge today and not wait till tomorrow using the morning voltage.

c.

If AT REST, pretty accurate.

If being discharged/charged, NOT accurate indicator.