Wildly fluctuating battery readings??

[huck55]

I put my fully charged batteries in Airstream, turned on my furnace and lights and battery level readings were at 50% and 12.1 within 5 minutes.

I turned the furnace and the lights off and battery returned to 100% and 13.1 volts almost immediately.
Any thoughts??

[OTRA15]

You have to wait an hour or so, with the batteries resting after removing shore power, and after removing most 12-volt loads, to get an accurate voltage reading. Your owner's manual may have further details.

[ITSNO60]

Originally Posted by huck55

I put my fully charged batteries in Airstream, turned on my furnace and lights and battery level readings were at 50% and 12.1 within 5 minutes.

I turned the furnace and the lights off and battery returned to 100% and 13.1 volts almost immediately.
Any thoughts??

If you apply a load to the batteries while they are not under a charge the voltage will drop. Like Peter says, you cannot get an accurate state of charge with a volt meter until the battery has "rested" both after charging and after any kind of discharging. (The volt meter is not a real useful indicator while charging or discharging)

[OTRA15]

Originally Posted by huck55

. . .
Any thoughts??

Wondering if the "wild fluctuations" have simmered down?

[uncle_bob]

Hi

First off, voltage readings are only a *guess* at the state of a lead acid battery. As others have noted, there is a wait time. They also are highly temperature dependent. If there's snow flakes blowing around 13.1 may be a "half full / stop using" battery.

Next up, the voltage readings in a typical trailer are taken well away from the battery. Put on a large load and you will get a voltage drop. ( If the same path is used for the load and sensor). The battery is still fine, it's just the load throwing off the reading.

Indeed, there *could* be a very real problem. There's no way to really know with a single reading. Grab a digital multimeter and measure at the battery posts. That will take the whole "load on the wiring harness" out of the equation. An even better answer would be to install a shunt based battery monitor.

There's a wide range of things that *could* be going on. You could have a loose clamp on one or the other battery post. The batteries might not be charged (stock charger will not fully charge them at freezing ....). The batteries might be shot. There might be a loose wire someplace else in the wiring harness. Everything could be fine. The only way to sort it out is to dive a bit deeper.

Best bet - get a Victron BMV-712 ....

https://shop.pkys.com/victron-energy...y-monitor.html

Bob

[GammaDog]

Do you have a State of Charge meter of some sort (Victron or whatever) or are you saying “12.1 volts which I interpret as 50% State of Charge based on tables I’ve seen”?

If the later then the above posts describe the phenomenon. If the former... tell us more.

[jeffmc306]

Huck55, do you have Interstate flooded acid batteries (removable caps) or AGM sealed batteries?

If Interstate, a best way to tell the state of charge is the specific gravity using a simple float tester from an auto parts store. Each cell should be close to the others. If you find one that’s lower, the battery may need to be replaced. Interstate does have a good warranty and may still replace it. They replaced both of our 2018 Flying Cloud batteries under warranty.

Hope that helps.