How accurate is using resting voltage as a indicator of state of charge?

If AT REST, pretty accurate.If being discharged/charged, NOT accurate indicator.

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.

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?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?

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.

State of charge | Good Sam Community

16 Replies

MEXICOWANDERER
Explorer
Jun 07, 2020
That is a primary charging voltage. -- time limited. When proven good, the battery reaches full charge then loses it's surface charge the voltage algorithm can be used.
S_Davis
Explorer
Jun 07, 2020
pianotuna wrote:
S Davis wrote:
MEXICOWANDERER wrote:
Resize this......

A flooded battery can irreversibly sulfate it's way to death but show resting voltage as being "normal" full charge. Read nothing more or nothing less in your replies thank you :)

What causes this to happen?

Not being fully charged, often from too low a charging voltage.

Oh ok, my charger is charging at 14.75 Volts so that should not be an issue.
pianotuna
Nomad III
Jun 07, 2020
S Davis wrote:
MEXICOWANDERER wrote:
Resize this......

A flooded battery can irreversibly sulfate it's way to death but show resting voltage as being "normal" full charge. Read nothing more or nothing less in your replies thank you :)

What causes this to happen?

Not being fully charged, often from too low a charging voltage.
S_Davis
Explorer
Jun 07, 2020
MEXICOWANDERER wrote:
Resize this......

A flooded battery can irreversibly sulfate it's way to death but show resting voltage as being "normal" full charge. Read nothing more or nothing less in your replies thank you :)

What causes this to happen?
MEXICOWANDERER
Explorer
Jun 07, 2020
Resize this......

A flooded battery can irreversibly sulfate it's way to death but show resting voltage as being "normal" full charge. Read nothing more or nothing less in your replies thank you :)
S_Davis
Explorer
Jun 07, 2020
So this ARB 50 quart frig is pretty efficient, it ran for 96 hours in 70-80 degree temperatures set to 34 degrees and pulled the two Trojans T105s down to 12.68 volts. So less than 10%. I let them rest for about 24 hours before checking the voltage.
RJsfishin
Explorer
Jun 01, 2020
I use a loaded test, tv and 1 lite on, 11.9v time io recharge.
time2roll
Nomad
Jun 01, 2020
S Davis wrote:
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 at 12.86 volts and only went down to 12.68 volts.
No harm at this measurement.
pianotuna
Nomad III
May 31, 2020
Better to turn on a 1 amp load for 10 minutes.
BFL13
Explorer II
May 31, 2020
The 15 min disconnect trick sounds about right to me. If you note how much it goes up in the first 15 min, you can just count that in right off without disconnecting.