batteries in parallel

[curt]

I have two regular group 24 batteries in parallel for my ec21 airconditioner...they are a separate circuit...one battery is dead...
Is there any reason I can't replace that with a deep cycle battery? When the other(newer) battery goes dead, I also would then replace that with a deep cycle. The boat came from the factory with regular batteries, not deep cycle.

 

[LADYBUG TOO]

The "12 Volt Bible for Boats" available from West Marine and others would give you the definitive answer. That being said it is my understanding that both should be replaced at the same time. The R-29 has four batteries in parallel and when one of mine failed I moved the thruster battery into its place. It was the same date of purchase as the surviving three group 27 batteries. I haven't had any problem with that swap but not sure that would be the case with new swapped in with older battery. This thread might prove interesting to all of us if someone has the "Bible" and can give a reference.

Pat & Carolyn, Ladybug, Too

 

[Pearl]

The AC battery bank on my 21ec were deep cyle when I purchased her used. I would recommend replacing both of the AC batteries at the same time because, the older battery will lower the overall capacity of the battery bank. If you do replace just the dead battery make sure it has the same capacity rating of the older battery. Having two different capacity ratings could cause charging and shorten the battery life.

 

[RProffer]

If one battery has gone, you can expect the other one to go soon. Both batteries should be replaced at the same time. Replacing the type 24 batteries with golf cart batteries will increase your output amperage significantly. I used the Interstate GC2-XHD-UT battery https://www.interstatebatteries.com...03,Nr~AND(P_unique_id:Trojan+T105+GC2-XHD-UT). They are slightly larger than the type 24 battery, which means the battery mount will need to be modified. Remove the batteries then remove the battery mount. Cut off the outboard piece of the mount. If you try to put the GC2 batteries in the mount you will see what I am talking about. With the rest of the mount and the hold down straps in place, the batteries will be secure. The 6 volt batteries need to be connected in series - NOT parallel. You will be glad you replaced the type 24 batteries with the GC2 batteries.

 

[knotflying]

I am surprised that one is dead and did not draw the other one down. Usually in a parallel setup if one battery is bad they equalize and the bad one will bring the other down with it. With that said, you should charge up the good battery and let it rest a bit and then do a load test as well as a hydrometer test. If it is good it is good. If the bad battery brought it down too often, then it may be bad also and replacement of both is highly recommended.

 

[HRowland]

As Rick said 6V golf cart batteries are the way to go, more AH and many more cycles. Running the AC in your tug will put many discharge/charge cycles on the batteries. Golf cart batteries are true deep cycle batteries and will last significantly longer than the "pseudo" deep cycle group 24, 27, 31 etc. The typical marine deep cycle 24s, 27s, and 31s are actually dual purpose labeled as deep cycle but they have thinner plates and significantly less cycle life then true thick plate deep cycle batteries. While they will survive more cycles than a pure starting battery they are nothing like a true deep cycle batter. Just about all of the serious cruisers use series/parallel banks of 6V batteries.

When it comes time to replace the batteries in my R27 I may move the start battery and put 4 6V golf carts in the current location connected series/parallel. I have already combined my house/thruster banks and changing to the golf cart batteries would provide a 450AH bank with real cycle capability.

Howard

 

[Newtugnut]

To summarize what others basically have said. Batteries should be equally "good" or equally "bad". To see what's really going on a dc clip-on ammeter will tell you the distribution of current to each battery connected in parallel. An inexpensive one (<$100) can be purchased at Sears for that purpose.