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12v Battery in parallel with 24v Battery

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aljamri

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Last week, my uncle was having a flat battery in his 40-passenger bus. I tried to help him to have a champers from his Jeep’s battery but found that the bus is using 2-12v battery (Diesel engine). I tried to explain the situation to him but he told me to connect the cables to one of the batteries. The bus did not started, but I was wondering wither what we did is correct or not ? and tried to make some math to convince him with my idea.
 
The 24v battery could easily have boiled the 12v battery if it had enough charge left in. If the battery in the Jeep was not disconnected he could have fried the vehicle electronics.

Having said that, when I was working as a mechanic, the garage owners quite often jump started "stubborn" cars with a 24v booster .......
 
Paralleling a 12V battery with just one of the two series-connected 12V batteries in the bus might have started the bus if only one of the bus batteries was completely discharged or open. and the other still had sufficient charge (mismatched batteries). You might have got lucky and jumpered your good battery across the bad bus battery on the first try, but if the first try wasn't sucessful, then move the cables to the second battery.

There are many failure modes of lead-acid batteries that might kill one battery, but leave the other still capable of providing cranking current. For example, a dead cell, a shorted cell, an open plate jumper. When this happens to one battery in a series 24V system, both batteries should be replaced with new, because it is important that both batteries have the same capacity. The remaining good battery can be used up in a 12V car.
 
I had one issue where I disconnected the bad battery (clamps on loosely) and started it while being jumped. Once the car started, I tightened the bad battery clamp. Not recommended practice.

ALWAYS remember to make the negative connection to the engine for two reasons: 1) Potential sparks are now away from the battery and 2) the series resistance of the negative wire and the negative post contact resistance are eliminated.
 
If you disconnect a battery while the engine is running or being started you run the risk of toasting the alty.

The car auctions I used to visit frequently started 12v vehicles with 24v, even ones with a ecu.
 
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