The problem is going to be charging via the engine alternator. You will fry the alternator doing this. It's a little more complicated than just "drop in" replacement.

If you looking to add capacity, you could add a lithium house battery, but you'll need to use a 12V to 12V charger to connect to your existing LA battery system, and likely a dedicated lithium charger once connected to shore power.

Go ahead and replace the existing batteries, they need replacing regardless.

 

Lithium batteries are not what you should add or change out too for running your engines from. As @dtfeld Dave mentioned you need a completely different charging platform to charge those batteries. Great Idea for house batteries and @ttmott and @dtfeld both have threads around here for doing just that. I would stick with what you have in the way of lead acid batteries, just get new ones. JMO.

 

There's a good chance you can charge them back up. Check the fluid level. It's worth a shot, anyways.

 

Agreed, top off the water and let them charge. Have them load tested if you feel it after charging up

 

If they were deep cycle batteries and they aren't real old, you might be able to charge them back up. Try it with a good battery charger, out of the boat.

You may have shortened their life, but you may not have killed them.

If you need new ones, buy Trojan Deep cycles. You DON'T want lithiums. Do some R&D and you'll find out why.

 

FYI, some "smart chargers" may read a low battery as "too low to charge". In that case, give it a go with an old fashioned, heavier duty charger first. Then finish with the smart charger.