How to hook up a battery tender on dual batteries?

[L&L]

We replaced our batteries this summer and want to maintain them properly. Have bought a battery tender that is the correct size for the two batteries but cannot get a clear answer on how to hook them up when there is a battery switch involved. Do I have to have the switch on "Both" in order to have the tender charging both batteries. I am thinking that if the switch is "Off" then the only battery being charged is the one to which the positive lead from the tender has been hooked up.

 

[Bt Doctur]

Simply running the engine once a week should be enough to keep good batteries charged. For each battery you should have a dedicated charge lead.
You never want to leave a battery switch on both because if 1 battery goes bad it will take down the other one. With no battery power you have no bilge pump either.

 

[Lazy Daze]

To use a battery charger (or tender) that is only designed to charge one battery to charge two batteries, you first have to individually charge each battery fully, then you hook the charger to both in parallel. Go directly to the batteries, not through the switch.

Make sure you have a GOOD, quality "smart" charger - not just a trickle charger. Many of the cheapies can/will still cook a battery. One with a desulphation mode would be best.

 

[SloBurn]

I would only charge one at a time, unless you have a diode between them, do not hook up in parallel, if one goes bad, it'll kill the other one. Just switch charger leads every time you use boat. That's how I do it.

 

[L&L]

Thanks for all those replies. Most helpful. Tender is a good one but I believe that they sold me the wrong one for my application.

 

[paulswagelock]

You can get a dual bank charger for around $100. Don't mess around, just get the correct charger for the application.