Battery voltage drop overnight

We are trying to figure out if we should purchase new AGMs for our boat (IP350 #4). Our current system (one Gp24 and 4 Gp27) give us about 460 Ah but they are six years old. While on the hook we start our night at around 12.65 and by morning we sometimes might be as low as 11.99. All we have on is our fridge and anchor light.

My questions: Is this normal or would new batteries hold their charge much better than what we are seeing? Is it realistic to think I could spend two nights at anchor and not have worry about running the engine to recharge given my current setup?

Thanks for any input.

Hi. My guess is you'd use about 50 amp hours in the overnight. 12.65 is about 75% charge, 11.99 is fully discharged. So if 50 amps takes you from 75% to zero, I'd say your batteries are done. How do you charge them? AGM's require a specific charging regimen to stay healthy. With that said, 6 years isn't too bad.
Check this site for very good battery/charging info
marinehowto.com/

TheCuscus, thanks for replying so quickly. So we have a one solar panel (135 watt) while on the hook and we also have a wind generator. But we don't use that unless we have substantial wind. I believe the batteries need to be replaced but really want the advice from other IP owners. And like you said, six years is not too bad. If/when we replace them, we are going to go to Gp31's. That will give us around 525 Ah vs the 460 we have now.

Thanks again man.

I agree, that you likely need new batteries, But I would add a bit of caution. If it is warm and your box has not been reinsulated you may lose 65 amp hrs over night at anchor - that is what our 350 did at 80 degrees before I reinsulated it. Also, if you have a conventional anchor light rather than an LED you will use about 1.6 amps * 12 hours? = 19.2 amp hrs Plus the panel at .6 amps another 7 amp hrs, and a water tank gauge light at .7 amps another 8.4 amp hrs. So all together perhaps around 100 amps hrs +or-. This is about a 25% of your bank. 12.06 volts is 50% discharged.

Delicia, thanks for the detailed information. So with new batteries holding a charge better than what we are getting now, what would be a reasonable number to see on our charger when we wake up in the morning? Something North of 12.3 ish?

Thanks again for such quick replies. We are ready to make a purchase ($$$) and get ready to have some fun.

FYI, AGM battery voltage as state of charge (SOC)
12.80+ 100%
12.60 75%
12.30 50%
12.00 25%
11.80 0

The leading advantages of AGM are a charge that is up to five times faster than the flooded version, and the ability to deep cycle. AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life.