Can I pull 12V from a 24V system?

[antonmnster]

Another long winter is about to start, and I'm going to be spending serious hours in my work's Pisten Bully groomer out on the trails.

As most heavy equipment does, it uses a 24V system, running two 12V batteries in series. I have some 12V accessories that I'd like to use to keep me company, maybe even a new head unit that has better performance than the Euro-trucker Blaupunkt that's in there now.

Would it be possible to pull 12V from in between the two batteries? Would there be 12V on the series jumper with a chassis ground?

I'd go check with a volt meter, but I'm not at work...

 

[Loobed]

Yes you can. You can either just tap into one battery directly, or you can just use some resistors. I can't remember how to set up (calculate the correct ohms) the resistors. The simplest way is to get 2 resistors that are equal in ohms, connect them in series, then connect the positive and negative between 1 resistor. I am sure if you did a search on the internet you could find something. Also check ebay for a converter: You might be able to get one for $5.

 

[Colt45ws]

Yes, you can tap off of one battery and get 12V. Thing to remember is to split the 12v load between the two batteries. If you run a wire from between the 2 batteries, use it as a ground, and use positive from the truck harness, you will get 12V. If you use it as a positive, and ground to the body/frame, you will get 12V.

 

[eljefino]

I would be concerned about the battery you tap not charging fully. (This would be remedied by at least a little time with the radio turned off.) Also if you tap between +12 and +24 for, say, a stereo, make sure the stereo chassis isn't grounded.

 

[antonmnster]

I'm thinking of just running a tap for just a charger/xmitter for my phone. I'd be drawing less than a watt. Shouldn't upset the battery balance!

There is an accessory plug in the cab, but I've never checked its voltage. I've always assumed it to be 24V, but I suppose it's possible the manufacturer added some resistors or a convertor.

Thanks for the info, fellas!

 

[eljefino]

Once you discharge that battery a little its internal resistance will drop and it will stay "happily" at ~85% charge instead of the 90-95% charge you usually get at float. So you could stick a manual 6 amp charger on the victim battery once a month for a few hours.

 

[meep]

an amp or two should be fine, as said above.

More than 5 amps I'd get nervous-- the "victim battery" will sit a little low and the other battery will run little high on V, lose fluid more rapidly.

The resistor idea only works for small, FIXED loads. And you'd need some hefty resistors = much heat and waste current if you wanted a so-so stable V to run a 5 amp head unit. Cheaper to find 24-12 converter somewhere... they have to be out there...

M

 

[Cujet]

It's not likely you will experience any sort of problem.

We do this kind of thing with 48V golf carts. We use 3 of the batteries for a total of 24V for aircraft starting.

No issues, ever. Very long battery life.

However, I don't use the chassis ground. I use 2 dedicated wires.

 

[onion]

OEM's do this sort of thing all the time with heavy equipment and buses. Just use the jumper between the two batteries as a 12=volt source. For what you're doing, it won't hurt a thing.

Now if you were to draw a high load (say, 20+ amps?) for hours at a time, then it MIGHT become an issue. The solution I've seen (from both bus and heavy equipment manufacturers) is to split up the 12V loads between the two batteries so that they're roughly equal. That method has been used for decades. John Deere did that with their diesel tractors in the 1960's, and lots of tour buses still supply multiple 12-V circuits off each battery.

 

[meep]

^^^^ I like this post.^^^^ even if it's different from mine. reading it, this would put my mind at ease.

 

[PandaBear]

In theory, as long as you do not draw too much from one of the 12V only and never swap their roles once in a while to balance out the charge / wear, you should be fine.

If you are going to run it for a LONG time without some sort of swapping, or individually charge each of the 12V battery to keep their level matched, you may run into problem in the long run.

I'd not use resisters to divide the 24V down to 12V as they are not very good at regulating non constant loads. What about a separate 12V battery (even a very small one that isn't for automotive use) that you bring along and charge off the wall?

 

[eljefino]

Originally Posted By: PandaBear
What about a separate 12V battery (even a very small one that isn't for automotive use) that you bring along and charge off the wall?


That would wear down kind of quickly in the cold, and deep cycling would be worse than the light discharge of a single 12v battery.

OP should run some tests, standing voltage at each battery when idle and when running, then again with his gizmos hooked up. Hundreths of a volt will count.

 

[antonmnster]

In a couple of weeks when the unit comes out of storage and I'm on-site, I can test it out.

As an aside, I have to say that I'm really impressed at the depth of knowledge and helpfulness around here. The quality of discussions is so much higher than it is on, say, BMW boards.

Thanks!