Jumping 12V from 36V pack

[Old MXer]

I always thought I knew what I was doing but the years have proven me wrong more than once.

A few times I've jumped the boss's Mazda with the 36v cart ( because she left an interior light on) & ( because they sit next to each other in the garage) rather than to bring the truck within range.

I've always put one jumper cable on a batterys + post & put the other jumper cable on the next consecutive battery in the series - post. Multi meter shows 12V between those two points. I didn't disconnect those batteries from the rest of the pack. Never had a problem (except for one senior moment when I reversed the polarity. Scary. The good Lord gave me a pass on that one).

I got in a friendly disagreement with a pro mechanic about not isolating the two batteries involved. He says that the other batteries are still adding voltage if not disconnected. I don't think so, but I'm wondering which way is the correct way?

[SouthCo]

You are correct.


Sent from my iPhone using Tapatalk

[2K6TBSS]

Would it matter if you were pulling off the first 2 batteries in the pack vs. the last 2 batteries in the pack?

I don't think it would, but what do I know...

[DB Cooper]

Many people pull 12V for accessories off of a single/first battery and obviously don’t isolate the battery. Those smaller electronics would certainly be more sensitive than the cars 12V battery.

[meimk]

Originally Posted by 2K6TBSS

Would it matter if you were pulling off the first 2 batteries in the pack vs. the last 2 batteries in the pack?

I don't think it would, but what do I know...

Any 2 consecutive 6 volt batteries will give you that 12 volts.

[Old MXer]

Thanks for the backup. I'll have another conversation with the mechanic.

Hey 2K6TBSS. I've done it both ways, & with the middle pair, just in case the jumping causes some additional wear & tear...

Explanation for the reverse polarity episode mentioned earlier:

I don't know if the battery positions are numbered, but I'll call the one closest to the f/r switch, #1. Battery layout = '98 TXT.

So, I was using #1 positive & #2 negative. Car started. Car died. I had already disconnected the negative jumper so I decided to switch to #5 & #6. ( The wear & tear thing.) Moved the positive over to the same PHYSICAL spot on #6. You can probably see where this is going..& did the same with the negative to #5. Proceeded to demonstrate lightning when I made the final connection to the car battery.

I've heard that sending plus down the negative side of modern cars can result in financial ruin. I totally lucked out.

Stuck a trickle charger on overnight & all is well.

[mfinnegan]

your "pro" needs more training LOL

[scottyb]

Originally Posted by mfinnegan

your "pro" needs more training LOL

Whip out your multimeter and show him there is only 12v between your jump cables. Console him with the possibility of pulling additional amperage from adjacent battery but not voltage.
P.S. For better results use 18v when jump starting a 12v car

[Andy4639]

Find yourself a new mechanic to while your at it.

[Old MXer]

This guy isn't a mechanic I've done business with, just met him & got to talking.

He said that years ago his dad taught him about the series hookup/voltage thing & the neccesity of opening the series & isolating whatever voltage you intended to borrow. Sooo, because of that there's no sense in arguing the point. I'd probably get a little defensive if some stranger doubted the hard earned wisdom of my dad. Best to forget it.

( Maybe he was saying volts but meaning amps....)