Daisychain car and boat battery on a battery tender?

So thinking next year of leaving the TB in MN for summer use, thinking of using my battery maintainer for the TB and throwing a set of jumper cables from the TB to the Boat battery so both will be maintained. No problem in my book, but looking for someone that might know better.

A few feet of 12ga wire with alligator clips from one battery to the next is all that’s needed. You can connect several batteries to keep them maintained. I have 3 currently wired up and I’ll add 2 over the winter.

Agree with @Rod Knox as long as both batteries are in similar condition. I.e., they fully charged voltages are quite similar. If one is older and closer to death, the tender may cook the better battery trying to keep the older one at the float voltage.


If you put a bunch of batteries in parallel, the most charged (highest voltage) one will attempt to charge the lower voltage one(s), with current limited only by the internal resistance of the batteries, which can be a high current.

Then when you put a charger on that parallel combo, it’s current will be directed to one of them, and which one depends on the internal voltage and resistance.

I’d never try this unless the two batteries are identical and at an identical state of charge.

Battery tenders don’t seem to be that expensive so why not just have one for each battery ?

It seems I’ve been very lucky all these years I guess. I’ve had as many as five 12 volt batteries and two 6 volt batteries wired up to a battery charger over a few winters. There’s always one 12 volt on the system, usually 3, often 5.

But connecting a discharged battery will drop the voltage across the entire array until they all reach a near equal level.

The greatest problem may be over filling a battery which results in hydrogen being trapped in the case. An accumulation of hydrogen can result in a battery exploding.

The boat is a new deep cycle, car is a 3 year old 875 cca, i guess 2 tenders makes sense.

Unless both batteries are deep cycle, I wouldn’t recommend daisy-chaining the batteries

Does your charger even work with deep cycle batteries?

Mine has several settings . . . deep cycle, agm, gel, deep cycle, standard, large battery, small battery, etc.

I’m curious about the difference in maintaining the charge on the various types of batteries. My charger cycles down at about 13 volts where the amperage will fall below 1/2 amp with 1 battery and 2 amps when 4 or more are connected.

My tender has never had a problem with the boat battery, I put it on in the spring until lights are good, put it on in the fall, until lights are good. Like I say 15 year old battery gave up the ghost, just enough to start the 90hp outboard, then went for new.