Probably a silly question, but I changed the battery set-up for our camper from one group 24 to two group 27's in parallel. Something unexpected happened and I'm trying to understand it. The batteries were fully charged when we arrived at the campsite using a stand alone high quality charger, after 1/2 day, with only the fridge running, (LP heated ammonia with a small fan cycling frequently) the generic 4 led battery level went down to 3 lights. (probably < 12.7V)

Since we were only staying 2 nights, I wanted to watch the drain rate to estimate battery capacity remaining. We continued to use the water pump, lights and fridge, and even showered to use power. We never went below 3 lights.

When we got home (half hour away) I figured the truck would put some charge back in, but not much. I disconnected the battery from the camper and hooked it back up to the charger, and it showed 100% charge?? I know that the charge time for the final 10% of the battery is just as long as for the initial 70% in part because of the diminished charge rate. I expected a much higher drain since it dropped to 12.7V so fast.

I have drawn up a chart trying to show what I think is happening, is there a quick draw down (say...5AH available) to 12.7V and then 130AH available to 12.2V and then another 25AH before it dies?

Is this correct or am I missing something? I should have around 160AH available. Thanks, Mike

 

A trickle charging battery may read 13.7V, but the standing voltage of a good 12V battery is right at 12.7V, which is where your battery was when you got to camp. It may take a day or less depending on the conditions for the battery to get to 12.7V, and even under a very small load it will get there much quicker. 12V Lead-Acid Batteries tend to begin sulfation around 12.4V so If you want a long battery life I'd try to charge it then and not wait till it gets much lower. If possible, never got bellow 12.0V.

checkout:Battery Uiversity for detailed information.

 

The discharge profile and total Amp hour capacity will change.

The first few times a battery is charged and discharged, it will continue to 'form' the plates and develope additional discharge capacity. The specific gravity is also slightly higher than the recommended manufacturer levels. This is all normal because the battery has not yet been 'broken in'. Just keep a watch on it for the next few uses, and you should see it begin to act more like you expect.

 

If you let the camper sit for any length of time you should disconnect the batteries. Batteries connected in parallel will discharge themselves. The higher voltage battery will try to charge the lesser one, and there is usually a difference, albeit small.

 

I didn't know that, thanks, all of you! I guess I'll be getting some disconnects.