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Reverse polarity 12V equipment

Using 12V equipment when connected to 12V tapping in a 24V battery system




This Tech Tip applies to any 12V equipment connected to 12V tapping in a 24V battery system with an isolator switch in the earth line of the batteries. The 12V equipment includes:

  • Radios, CDs, Etc.
  • Two way radios, transceivers, UHF Etc.
  • Electric brake controllers.
  • Computers.
  • Inverters.
  • ABS.
  • ECU, engine management systems.
  • 12V fridges.
  • Auxiliary motor starters (Eg fridge motor) Etc.

NOTE: This tip applies whether a Charge Equaliser is used or not.




If any 24V equipment is on while the isolator switch is open (off), the "24V" battery will apply 12V reverse polarity to the 12V equipment. If enough current can be drawn through the 24V equipment, (Eg via 24V starter or lights, Etc) permanent damage can be caused to the 12V equipment.



The NEGATIVE of the "24V" battery connects to the 12V equipment POSITIVE and the POSITIVE of the same battery connects (via the 24V load) to the chassis, therefore the 12V equipment is reversed.

Even if there is not enough current to damage the 12V equipment, the "24V" battery will discharge.




The simple solution is to ensure that all 12V equipment is disconnected before isolator switch is opened.

This is easiest done by having all 12V equipment fed by an ignition controlled relay (or an SBI12 if the maximum 12V current is too high for a relay.)