Jump Start (vehicle) - Voltage Problem

Voltage Problem

Formerly, especially in cold climates, some jump starts were done with two series-connected batteries to provide 24 volts to a 12 volt starting motor. However connecting 24 volts directly to a 12 volt battery is considered dangerous.

Heavy vehicles such as large trucks, excavation equipment, or vehicles with diesel engines may use 24-volt electrical systems. They usually have a 24V supply using two 12V automotive batteries in series: it is therefore possible to jump-start a vehicle with a 12V electrical system using only one of the two batteries.

Vintage cars may have 6-volt electrical systems, or may connect the positive terminal of the battery to the chassis. The methods intended for boosting 12-volt, negative-ground vehicles cannot be used in such cases.

Passenger vehicles with 42-volt electrical systems may not be possible to "boost" from other vehicles; professional assistance would be required to prevent severe damage to the vehicle and possible personal injury (see tow truck). Hybrid vehicles may have a very small 12 volt battery system unsuitable for sourcing the large amount of current required to boost a conventional vehicle. However, as the 12-volt system of a hybrid vehicle is only required to start up the control system of the vehicle, a very small portable battery may successfully boost a hybrid that has accidentally discharged its 12-volt system; the main propulsion battery is unlikely to also have been discharged.