AGM charge.

[Jeff Bittner]

New AS and AGM's. Driving all day and only 12.5 volts when arrived. Ran generator all night to use furnace as temp was 40. Again, only 12.5 volts this AM. With generator on reads 13.3. Is this ok?

[lsbrodsky]

You said 12.5 this AM with generator running all night, then you said 13.3 with gen running. Confusing...which is it? If your generator was running all night did you let the batteries rest at no load before measuring voltage? That is the only way to check battery state of charge. With the charging source running you should see 13.3-13.6 on the battery but that only tells you that it is charging, no how much charge it held. If your batteries were 12.5 at rest(after 30 minutes of no load/no charge) then they are pretty close to fully charged, for which I have always used 12.6. Same thing after a day of driving...was the 12.5 at rest?
larry

[BayouBiker]

To build on Larry's good comments, a couple of other factors may be in play. Since the temperature was 40 F Larry's full charge number of 12.6 is good guidance. The batteries will be 12.8 fully charged, resting for more than 30 minutes and no load at 25 degrees C. They lose about 0.05 per degree C so about 0.10-0.12 so they would be somewhere around 12.6-12.7 depending on condition if fully charged. A tow vehicle will not fully charge them so 12.5 could be about right depending on state at the start of the ride.

Now the generator should have brought them to full charge. So if your 12.5 was resting with no load that is just a bit low indicating an equalization charge will do you some good. If as Larry suggests there was a bit of load say 2 amps, 12.5 is a good number.

Hope this and Larry's comments help.

[uncle_bob]

Hi

A *lot* depends on the plate material in your batteries. You can see the "half full stop using" voltage up around 12.6 to 12.8V (vs 12.0 to 12.2) when it's really cold out. What counts is the actual battery temperature. That may be a bit different than what the thermometer reads.

The problem this all creates is that your stock / un-temperature compensated converter/charger will not get the sub-freezing batteries fully charged. It simply does not put out enough voltage to do so.

Next up is the unfortunate fact that battery capacity drops when they get cold. You might loose 1/3 of your capacity on those cold batteries. Put that in on top of the charge issues and you have a real challenge.

Bob

[Bartek]

Here’s a great Airstream video on power systems, charging, and energy management, including voltage/percentage tables.
https://www.airstream.com/blog/ask-a...power-systems/