Odd 12 - 24 volt question

glennster · May 18, 2014

Got. Call from a guy wanting to jump start a 24 v system.
Thinks he can series jump from two cars. Told him i didnt think
that was the best idea. Use two stand alone 12 v batteries
would be safer. I dunno, what do you think? Some how i would
be worried a voltage spike would not be good for a late model
car electrical system.

Tvertiz · May 18, 2014

I have done it that way and never had a problem. Hooked one vehicle to each battery. I suppose if time was not a issue you could unhook the battery"s from each other charge them up good then start the system, that would guarantee safety

NCWayne · May 18, 2014

I"ve done it like that on several occasions, not with late model cars mind you, but I haven"t had anything blow up, or short out yet.

4play · May 19, 2014

I've used my Dodge diesel truck to jump start my M35A2 duece. Just need 2 sets of jumper cables and go battery to battery and match polarity per battery and post. Its just like you can run 12v accessories on a 24v series system you just need to get power and ground ran from the same battery.

If you know that only was battery is bad/dead, you could probably get away with hooking up one set of jumper cables to battery posts of bad battery just like you would a regular 12v system. And this could work the other way around to jump start a 12 volt vehicle using a 24v system.

MarkB_MI · May 19, 2014

It will work just fine. I'm not sure what to make of the "voltage spike" to which you refer; where is it supposed to originate?

buickanddeere · May 19, 2014

Don't know how you jumped a pair of 12V batteries in series with a pair of 12V batteries in parallel.

John B. · May 19, 2014

Jumping with two cars and jumper cables in series would subject both cars to 24 volts. Not that's not a good idea if you value your car.

John B. · May 19, 2014

After reading the others' comments I think they have the right idea but mis-understood your questions or I did.

I take your question as you were asking. Is it okay to hook two cars in series with jumper cables to create 24 volts? The answer is NO!

As others have commented. IF you take two cars and hook one car to one battery and jump that is fine.

glennster · May 19, 2014

that was my concern. he was talking about using one cable to jump the pos and neg between the two cars and the jump from the pos on one car and the neg on the other to get 24v. i didnt like that.

John T · May 19, 2014

Assuming it has two individual 12 volt batteries that are in series to make up the 24 volts, you can jump/parallel each 12 volt battery to another 12 volt battery such as say in a car or truck etc. Once jumper cables are properly attached + to + and - to - then you simply have one 12 volt battery (on tractor) in paralell/jump with another 12 volt battery (in truck) and you just do that for BOTH batteries.

I see it as the same that you could charge a 12 volt battery in the tractor using a battery charger, or you could have one 12 volt charger on one battery and another 12 volt charger on the other 12 volt battery, either at the same time or one battery at a time.

Just don't let the jumper vehicles and tractors frame come into contact with each other.

Again, jump + to + and - to - from the truck 12 volt battery to a tractor 12 volt battery and I don't see why you cant jump BOTH of the batteries at the same time using two different vehicles AS LONG AS FRAMES DONT TOUCH.

HOWEVER not being there and not knowing how the tractor is configured I cant say for sure this is right or that it will work SO NO WARRANTY.

Alllllll Im saying is you can jump a 12 volt battery with another vehicle or jump two 12 volt batteries using two vehicles.

John T

glennster · May 19, 2014

that was the question, he wanted to series wire the two cars and then jump a 24 v system.

teddy52food · May 19, 2014

No need to unhook them to charge each one separate.

Bob · May 19, 2014

NO reason it can't be done. Just jeep the cars on the machine being jumped a few few apart so no frame/body parts touch.

I have done it NUMEROUS times with 730 or 830 DEERE diesels or 4020's.

When one's out in the field and needs a quick jump, who wants to take the time to disconnect or remove the batteries?

Mike M · May 19, 2014

Pat Goss from Motorweek cautions against ANY jump starting on these newer cars. Says you can just never tell if down the road a ways if something fails if it was related to a voltage spike from the jump.

glennster · May 19, 2014

the guy has a piece of construction equipment off road. the batteries are not easily accessable. there is a jump port externally on the unit, it is 24 volt. he needs to hook up 24volts to the jump port. he cannot easily get to the batteries to make any connections.

teddy52food · May 19, 2014

In that case you will have to find out if it is + or - ground. Hook up a spare battery between that connection and the battery in the vehicle going in series. Complete the circuit from the vehicle battery to ground.

Owen Aaland · May 19, 2014

(quoted from post at 13:03:52 05/19/14) the guy has a piece of construction equipment off road. the batteries are not easily accessable. there is a jump port externally on the unit, it is 24 volt. he needs to hook up 24volts to the jump port. he cannot easily get to the batteries to make any connections.

It should work but unless you have individual cables you will need three sets of paired jumper cables. From the positive on the machine go to the positive on the first vehicle. From the negative on that vehicle go to the positive on the second vehicle. from the negative of the second vehicle go to the negative of the machine. The vehicles and the machine must remain isolated from each other.

mazemeister · May 19, 2014

assuming that the electrical system on the helper car is otherwise sufficiently durable to jump a 12v system, the only trouble i see with his proposal is if the tires were wet or otherwise conductive, since the cars almost certainly have the negative battery terminal connected to the chassis (ground). so if there were a conductive path from the chassis to soil on both cars you could potentially have them half connected in parallel, then when you connect them in series also, big problem.

in reality, i think it is likely to work fine.

d9gdon · May 19, 2014

(quoted from post at 16:19:22 05/19/14)

It should work but unless you have individual cables you will need three sets of paired jumper cables. From the positive on the machine go to the positive on the first vehicle. From the negative on that vehicle go to the positive on the second vehicle. from the negative of the second vehicle go to the negative of the machine. The vehicles and the machine must remain isolated from each other.

In my mind, this will work except that you have to remove Vehicle #1's existing battery cable because this battery post will be putting out 24V when the last connection is made. It will fry the ECM and alternator(and probably other things) if you don't remove the cable first.

I own heavy equipment and do it all the time with my pickup and two sets of jumper cables along with a spare 12V battery. I throw the spare battery up on the grill guard, hook first jumper cable negative to pickup battery neg post and other end to piece of equipment being jumped(either to frame or negative post of batteries which ultimately is grounded to the frame) then hook first jumper cable positive to pickup battery pos post and other end to spare battery neg post. With second set of jumper cables, just use the positive from the spare battery positive post and the other end to the piece of equipment being jumped(either to the pos starter terminal or the 24V pos connection at the batteries. You don't have to hook the negative side of the second set of jumper cables to anything.

Owen Aaland · May 20, 2014

(quoted from post at 23:24:50 05/19/14)

(quoted from post at 16:19:22 05/19/14)

It should work but unless you have individual cables you will need three sets of paired jumper cables. From the positive on the machine go to the positive on the first vehicle. From the negative on that vehicle go to the positive on the second vehicle. from the negative of the second vehicle go to the negative of the machine. The vehicles and the machine must remain isolated from each other.

Click to expand...

In my mind, this will work except that you have to remove Vehicle #1's existing battery cable because this battery post will be putting out 24V when the last connection is made. It will fry the ECM and alternator(and probably other things) if you don't remove the cable first.

I own heavy equipment and do it all the time with my pickup and two sets of jumper cables along with a spare 12V battery. I throw the spare battery up on the grill guard, hook first jumper cable negative to pickup battery neg post and other end to piece of equipment being jumped(either to frame or negative post of batteries which ultimately is grounded to the frame) then hook first jumper cable positive to pickup battery pos post and other end to spare battery neg post. With second set of jumper cables, just use the positive from the spare battery positive post and the other end to the piece of equipment being jumped(either to the pos starter terminal or the 24V pos connection at the batteries. You don't have to hook the negative side of the second set of jumper cables to anything.

There should be no reason to disconnect any cables. In the system I described there are in effect three separate electrical systems. Each jumping vehicle has its own separate 12 volt system. The 24 volt system is independent of the two 12 volt systems sharing only the batteries, nothing more. As long as the two 12 volt batteries are of the same or close in capacity, the voltage drop under cranking load of the 24 volt system should not cause much difference in voltage between the two 12 batteries. If there is a large difference in capacities the cranking voltage drop may cause the weaker battery voltage to drop below the minimum voltage required to keep that vehicle running.

d9gdon · May 20, 2014

(quoted from post at 16:19:22 05/19/14)

From the positive on the machine...

I reread your post and I think this is where I misunderstood your scenario. On that piece of equipment there are two batteries hooked together(from pos to neg) with a short cable and either one of those can be treated as a 12V pos terminal as far as jump starting is concerned. You must be hooking to one of these in your system, is that right?

The positive that has a cable going to the starter(which also goes to the remote receptacle) is 24V. This is the one I thought you were hooking to in my earlier post, and won't work as described.

Owen Aaland · May 20, 2014

Treat the two batteries connected in series on the machine as one 24 volt battery with one positive post connected to the starter and the negative connected to the frame as ground. Connect the positive from the machine battery to the first jump vehicle battery positive. The connection between the vehicles is from the negative from the first vehicle to the negative of the second vehicle. This puts the batteries from both vehicles is series just like the batteries on the machine are connected. The connection back to the machine is from the negative post of the second vehicle to the negative of the on the machine.

The positive cable from the machine to vehicle one and the return circuit is the negative from vehicle two back to the machine have a circuit voltage of 24 volts. Even though the circuit has 24 volts running through the two vehicle batteries they each have only 12 volts, the same as the batteries on the machine.

Odd 12 - 24 volt question

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