Group 29 and 34 batteries in series???

[Dooley87]

My grandfather is going to replace one of his battery's in his boat, he has two batteries (group 29's) and used to have them seperate, trolling motor etc... on one, depth finder etc... on another, recently he hooked them up in a series, + to + and - to - so it's still a 12 volt system. We have noticed longer run times on the trolling motor this way, he needs to replace one battery and found a great deal on a group 34, can he hook a group 34 and a group 29 battery together or will they work against each other and drain the other battery?

[kfk]

Positive to positive and negative to negative is actually hooking them in parallel and I think the two different size batteries should work fine.

[Dooley87]

Oh yes parallel

[Esox_Magnum]

It will work but the system will only be as strong as the weakest battery....

[Dooley87]

So what's your opinion on the best thing to do if you only purchase one battery, run the 34 on the trolling motor and everything else on the 29 or get another 29 instead to run in parallel? Looking for the longest run time possible on his 55lb thrust trolling motor.

[kfk]

You'll definitely get your longest trolling motor running time by running two batteries in parallel, but I wouldn't recommend running anything else from your trolling motor batteries. In your case I'd probably run the 34 on the trolling motor and eveything else on the 29.

[Northlander]

Make sure to have 2 of the same size batteries.

[delcecchi]

It will work but the system will only be as strong as the weakest battery....

Sorry, if they are in parallel then you get capacity equal to sum of the two.

[Dooley87]

Sorry, if they are in parallel then you get capacity equal to sum of the two.

This makes sense

[Whoaru99]

Yes, parallel you get more run time than if using the two batteries separately. The only problem I see is if the older battery is somewhat weak. If so, then I probably wouldn't use them in parallel to preclude a chance the old battery being somewhat of a parasitic load itself.

Fwiw, I always replace my two trolling batteries as a pair.

[Dooley87]

Fwiw, I always replace my two trolling batteries as a pair.

This is what he decided to do, he is going to buy two new batteries for next spring.