Home Forums Gear Hunting Trucks and Four-Wheel Drives Battery Tender Question

Will one tender (12V; 800mA) service two batteries in a 2-battery diesel?

Seems like it should be fine. Always concerned about unintended consequences.

I’ve read mixed info online and wonder if anyone has connected like this.

Short answer - maybe. Depends on a few details - take a look at: Battery tender FAQs

HawkI seems to be the battery expert here. Maybe PM him, I'm sure he could answer.

Normal current draw on most vehicles is in the 40-70mA range to keep radio and control unit memories alive. If your vehicle has no abnormal draw, an 800mA tender should be fine to keep both batteries charged. I use one occasionally on my truck (Cummins) if I know it's going to be dormant for a few weeks.

The charger will treat both batteries as one, while each needs its own amount of charge.
They make a dual battery charger. 🍻

Thanks. That pretty much my situation. Don’t drive this truck very often in the winter.

Just replaced the 10 1/2 yo batteries and want to get my money’s worth out of this pair! 😉

Really?

So does that mean that there needs to be 2 alternators, or 2 voltage regulators?

Awaiting a new pair of Odysseys here for my Duramax. Only got 8.5 years out of mine. Old Sears Platinum (rebranded Odysseys at the time).

I got gypped, apparently.

Ironbender, yes they do work. A slow, 2.5 amp trickle may not be much, but it heats the batteries INSIDE, which increases cold crank amps.

Heres one memorable example where a trickle charger saved my a s s, from the stupidity of diesels in below zero weather:

I had just finished 14 consecutive 12 hr days on the pipeline and my shift was over. It was 57 below zero ambient in the morning. My coolant looked like a green slushy before start up. I thought that a piece of cardboard in front of the radiator was sufficient enough for driving in that type of cold.

Somewhere between salcha and north pole, i lost lights and even dash lights went dim. What happened was i didnt put a real bib on the grill, so the intercooler was getting blasted by 50 below. It tripped an air temp sensor, so the cummins air intake grid heater was permanentley on. The grid heater pulled more amperage than the alternator could replace and drained the dual batteries.

Emegency pit stop in north pole, i plugged in the truck and overnighted at a rental cabin. I had a napa trickle charger, oil pan heater and block heater.

I made a temporary bib out of some insulated carharts and attached it to the grill with self tapping sheet metal screws. Duck tape wouldnt stick.

Truck fired right up. Off to willow i proceeded, to go run sled dogs! Batteries had plenty of charge to crank over that big cummins 6bt

I agree, as I've done it on many occasions!

A tender will slowly charge until fully charged, and then be careful that you don't over charge your battery as that is also no good for your Batteries.

BTDT, mainah. Used to live down Canaday rd for a couple years. Had to drive from there to UAF without a heater for a week without a working heater when the check ball in the external tank heater (remember those?) got loose and blocked the heater core. It popped out on a back flush.

Anyway, I know they work. I gave three in use now - tractor, gooseneck battery, and a boat battery in the garage.

I was only concerned there might be a reason I was overlooking with dual batteries.


Thanks guys!

I coulda mailed you a pair, Rick. It’s only 50# each of hazmat! 😉

Why you gotta be a smart azz?
Same damn thing happens in your pick up unless it has an isolator on it.
If one of your 2 batteries is bad you’re alternator is working overtime.
One bad battery will ruin the other battery over time. Hence a charger that will control each battery separately.
🍻

No, he asked a simple question, and there is a simple answer. He also has new batteries that he simply wants to maintain. A battery tender can be connected without having to disconnect the batteries to keep them at optimal charge, unlike 2 separate chargers which would require they be disconnected. Just keeping things simple. I'm well aware that batteries age at different rates, and a bad battery is tough on the alternator and the other battery, but I'm not being a smartazz.

Lead a horse to water....
THE BATTERY TENDER WEBSITE ANSWERS THE QUESTION, AND TELLS YOU UNDER WHAT CONDITIONS IT WILL WORK AND WHEN IT IS NOT RECOMMENDED!

Thanks for the link.

Or you can find a charger here that figures it all out as it goes...

https://no.co/brands/noco-genius

I use one of their chargers and and a lithium boost starter.