39 month old group 27 marine crank/deep cycle. East Penn Mfg AGM. Battery will sit for a few weeks between uses and hold 12.6 V. That is enough to start 60HP Zuke. I charged it last night to 13.6 V and overnight it dropped to 12.65 V. I think this is marginal for a boat and needs replacing. This battery is up under the bow casting platform of my 18' Jon boat and takes quite a pounding in rough conditions. Your thoughts? Wow! Batteries have doubled in cost since I bought this one in 6/21.

12.6v is often considered a "good" battery below that may need replaced.On the otherhand 12.8 on a static battery is considered overcharged.

If it starts the motor my thought is it is "OK".

Load test is what you seek and autoparts stores or a battery store can do that for you.

Yep they are high,try buying 2 for F250 diesel truck

the questions are do you have tow boat insurance? can you walk back? are you using an AGM battery charger?

That's for FLA.  Sealed AGM batteries have a slightly higher resting voltage of 12.7 to 12.8 volts when fully charged, and higher charging amperages using proper AGM chargers.  

Personally, if this boat has a single battery, I'd consider replacing it based on age.  If you have dual batteries with a switch, then I'd continue to run it until it exhibited lower overnight voltages or sluggish cranking behavior.  I always converted my boats to dual battery setups, because the idea of getting stuck out in the middle of a lake or far from home on a river, are not appealing to me.  Is this used for trolling motor or electronics as well?  Or cranking only?

"If you have dual batteries with a switch, then I'd continue to run it until it exhibited lower overnight voltages or sluggish cranking behavior".

This is what I'll do. And start shopping for a AGM charger. The ancient century 10 AMP charger I've been using for 30 years easy puts out 15.5 V. A little too hot for an AGM.

Thanks!

Just replaced a 2016 battery in a boat.
Had the key on for about five minutes and it killed the battery. No real draw on it.

I keep the batteries till they pretty much die.


Do have a small lithium jump started that has a pretty big capacity.

Run it until it doesn't start anymore but don't ever forget to take the jump pack from the truck to the boat.  My main starting battery for my 4.3gl is a 2016 I believe and is still going strong.

Battery is still holding 12.7 after sitting without use for 2 weeks. Guess It's OK.

Bought the ARFCOM recommended Noco genius 10 charger. I like it.