Question re Optima yellow top batteries

Roamad

I’ve acquired two used Optima yellow top batteries, model D31T. I don’t know how old they are, but they both currently show a charge of about 8.6 volts. I’m not familiar with these sort of AGM batteries and their durability. Is it possible to recharge these and have them function normally? Or should I take them in for recycling? I’ll have to buy a proper battery charger to recharge them and I don’t want to do that unless they can be successfully revived and put back into service.

C-Wolfe · Posted: Sun Aug 17, 2025 2:53 pm Post subject:

Some auto parts store would recharge and test your batteries for free, might be a good place to start. If they test good, then you can get the proper charger for them.

thataway · Posted: Sun Aug 17, 2025 3:28 pm Post subject:

If both are 8.6 volts, they are probably shot. You can put any type of an automotive battery charger on them for 24 hours and see if they hold a charge. They should read 12.7 volts +/_ 0.1 volt. If they hold a charge, then take to any auto parts store and have them load tested.

Most marine battery chargers will be fine for the AGM Yellow top. For most users a 5 amp per channel would be adequate. For the price of $112, the NOCO Genius GEN5X2: 2-Bank, 10A Onboard Battery Charger - 5A Per Bank is a good buy. If you want the "best" then a Blue Seas, Pro Mariner, Victron or Mastervolt would be considerably more expensive, but would be a faster charger with more bells and whistles. My last C Dory 27 came with a NOCO and was a good charger for the regular or AGM batteries. For a set of LIFePO4, I had an 80 amp charger on the Victron Multiplus 2000 80 50 inverter charger boost.

colbysmith · Posted: Sun Aug 17, 2025 8:45 pm Post subject:

I'd just stick them on a regular car battery charger, and see if the voltage is brought up and how long they hold the charge. If they work, they work. You may be able to find a date stamp on them somewhere. Also, I would check the casing to insure there is no damage anywhere.

Roamad · Posted: Mon Aug 18, 2025 11:27 am Post subject:

Some good tips here. Thank you!

Flashyfish · Posted: Mon Aug 18, 2025 4:23 pm Post subject:

One thing to keep in mind with AGM batteries. If they drop below 10.5 volts, a standard charger may not bring them back up. You have to trick the charger. A battery down in the 10 volt or less range may cause the charger to think the battery is damaged, so it will not turn on the charging operation.

To get around this, you hook up a good 12 volt battery in parallel with the Optima and attach to the charger. This raises the Optima battery voltage up to a level to start the recharge operation. If the Optima starts getting hot, shut everything off and disconnect. The Optima is probably shot internally.
If all is fine, you let this work for an hour and then disconnect the good battery and continue charging on the Optima.
There are several Youtube videos showing the process. I have done this with my AGM battery after running it dead by mistake.