How to jump start 24v with 2 12v batteries

Jump Start a 24 Volt truck from your 12 Volt car

Welcome to this post. Today we look at jump starting a 24 Volt system using a 12 Volt ordinary vehicle. There are a few things to keep in mind and a few mistakes that can cost you big time, so, read carefully and make double sure you have the connections correct!

Disclaimer: Everyone trying this method according to this post, do so at their own risk.

Which 24 Volt systems can be jumped from a 12 Volt system?

I will only speak about first hand experience in this post. As most know by now, I live in a motorhome (Bus). The motorhome has a 24 Volt starting battery configuration. It consists of 2 x 1000 CCA 12 Volt batteries connected in Series to give 24 Volts.

The battery system is connected as in this diagram which is the most common setup for 24 Volt systems in trucks and buses:

The Jumping Procedure

If this is the same way your 24 Volt system is connected, the starting procedure is quite simple.

Hook the + (positive/red) jumper cable to the Red (+) terminal of the 12V car battery and to the [Positive to Starter and Ignition] terminal on the 24 Volt system. (See diagram).

Make sure that it does not touch anything else. Now, hook the – (Negative/Earth/Black) jumper cable to the Earth terminal (-/Negative) of the 12V car battery.

Start the car.

Now,  hook the – (Negative/Earth/Black) jumper cable to the – (negative) terminal marked “B” in the diagram.

Note: Double check that the jumper cables are hooked to the same battery of the 24V system!!

You may hear the revs on the car drop as the alternator starts to charge the truck/bus batteries.

What is happening in fact is that you are charging the one 12 Volt bus battery which in turn is sharing the charge with the other bus battery at 12 Volt. Thus it is perfectly safe. Run the car at higher revs for about one minute before attempting to start the bus/truck. Once the bus starts, quickly remove the jumper cables so that the bus alternator can take over the charging of the 24 Volt system.

The same result will be achieved by connecting the red jumper cable to the + / positive terminal marked “A” and the black jumper cable to the  Earth terminal or Chassis of the bus.

Do you have a classic “JUMPSTART STORY”? Use the comments at the end of the post and tell us what happened!

A great gadget in these flat battery scenarios is this 12 volt battery jumper pack! I’ve had mine since 2014. It holds a charge for months and months.It is tucked away behind the seat in my van. I charge it approx once every 6 months, not that it needs it but I like to keep it full to the brim.

It can easily start 5 and more cars in a row before it needs recharging, it weighs very little and starts up to 2.5L diesels and v6 petrol engines. It does much more than just Jump Start your motorhome, it is a complete Power Bank that can charge your iPhone and anything else that uses a USB charger.

If you have a battery issue and need to replace it, check this post on which battery is best for your application. I also explain the latest Lead Crystal battery technology.

Did you find this information helpful? Please can you be so kind to leave us a comment as a courtesy for sharing it free?

Safe travels and hope to see you on the next post.

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A battery that delivers the correct voltage is essential for almost all vehicles. Deliver too much voltage and components either move or spin too fast, or they overheat and burn out. Deliver insufficient voltage and moving components slow down, or don't work at all. Jump-starting a 24-volt system from a 12-volt system won’t work if only one 12-volt battery is available, but if the 12-volt truck has two batteries, it becomes possible to produce a 24-volt supply.

Step 1

Position the 12-volt truck as close to the 24-volt truck as possible. Orientate the vehicles so their batteries are as close to each other as possible. For example, if both vehicles have side mounded batteries, park the trucks side by side, and if the batteries are housed in the engine compartment, park them nose to nose.

Step 2

Access the batteries in the 12-volt truck. They will be linked in parallel to deliver 12 volts, but 24 volts with the combined current capacity of both batteries. Put on strong gloves. Starting with the battery terminal connected to "ground," use a wrench to disconnect the cables linking the two batteries to the truck. When completed, both batteries are isolated from the truck but they remain connected to each other.

Step 3

Disconnect the battery straps that link the two batteries together. When completed, you have two separate 12-volt batteries, neither of them electrically connected to the truck or to each other.

Step 4

Using short insulated heavy-duty jump leads, connect the positive terminal of the first battery to the negative terminal of the second one. Two of the four battery terminals remain unused, positive on one battery and negative on the other.

Step 5

Connect a jump lead from the positive terminal to the positive terminal on the 24-volt truck battery. Connect a second jump lead between the negative terminal and the engine block or other ground connection in the 24-volt truck.

Step 6

Put the 24-volt truck in neutral and start it following the normal procedure. Once started, disconnect the jump leads from the battery.

References

Tips

• Connect a "memory keeper" device to the 12-volt auxiliary power socket / cigarette lighter in the 12-volt truck before disconnecting the batteries. This protects devices such as radios and GPS units that are designed to stop working when disconnected from a power supply.
• Always disconnect the "ground" side of the battery first. This reduces the risk of electrocution.

Things You'll Need

• Jump leads, 2 sets
• Strong gloves
• Wrench

Warnings

• Lead-acid batteries produce hydrogen gas. Keep all sources of ignition well away from them. Hydrogen plus a spark or flame may result in an explosion. )
• The linked 12-volt batteries cannot be recharged while in use, so take care not to drain them in your efforts to start the other truck.

Writer Bio

David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.

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