I have a question or two, I have a F-350 SD 6.0 currently no major mods. I purchased a battery tender for the truck last year and connected it according to their recommendations. The trickle light never came on all winter in Ohio. I thought I might have some problems with the charging system had it all tested at Ford, nothing. Reconnected the tender after taking both batteries out and charged completely.

Three months after keeping the tender plugged in it smoked a battery, replaced that one this an optima red top, shortly after that smoked the other one. Purchased another red top disconnected the tender no problems.

Any ideas, before you say it yes the tender is working correctly sent it back to the mfg to have it checked out. Was told by Tender group the plus would allow for 12/24 volt rigs it would sense the difference. Personally I think they are full of BS but who knows. I do know this it will also smoke you FIMC not cheap, bought a new one cost a grand.

So any one have one and if so does Ford isolate the second battery from the charging system until needed to crank the vehicle?

 

Hello sxkoonce and welcome to FTE.

Your truck is not a 12/24 volt truck, so the company was blowing smoke up your tailpipe.

For does not isolate the second battery. They are hooked up in parallel to provide 12 volts at high amperage. To the Battery Tender it should appear as one big battery.

 

A Battery Tender Jr. would work just fine, might try that out. Never needed one any bigger for any of our dual or even quad battery setups.

 

Did you test the batteries using a digital VOM with and without the tender hooked up? That will tell you if the tender is outputting enough DCV to maintain the battery. You should see 13.2-13.5 ish VDC with one or both batteries hooked up. If the original batteries were in fact 10 years old that's over-time for any batteries life expectancy especially in a diesel. If you do store this truck to keep the rear fender wells from rotting out due to the sponge like materiel pressed between the inner & outer wheel wells which Ford is STILL doing...I digress... then isolate both batteries and buy a tender for each. I store muscle cars. I use high quality (read not china) battery disconnects on the neg terminals while in hibernation with a tender hooked to the posts.