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Damage from a Reversed Battery (4QD Series)

If you reverse the battery to a 4QD series controller – it is likely to destroy all the MOSFETs. The current that flows with a reversed battery is dependant on the battery capacity and on the battery wiring. Modern MOSFETs are incredibly robust, so it’s possible, if the short is not prolonged that some MOSFETs may survive.

What else fails depends on the length of time for which the reversed battery stays connected and whether an attempt is made to see if the controller is working after it has been blown. Connecting power to a controller thus damaged can blow driver transistors.


A separate page describes dismantling. Once the base plate has been removed, the MOSFET gate leads must be desoldered and all screws and stud nuts removed. The busbar assembly, complete with MOSFETs can now be separated from the driver board.

Checks to be made

If you are trying to repair a controller that has had its battery reversed, you should check

  • All MOSFETs. There is a page on our sister site [] about MOSFET testing
  • Any gate resistor that connected to a MOSFET that has failed. Usually damage is visible, but they can fail high without visible damage.
  • Drive circuitry. If gate resistors are blown, the drive circuitry to that bridge leg may also have been damaged. There is a separate page on testing.
  • Main capacitors. If reversed battery is sustained and MOSFETs are blown, then the main capacitors can overheat, bulge and eventually explode. Any damage to these is visible

Protecting the controller

A power relay or contactor can be fitted as described in the instruction manual.

A fuse or circuit breaker fitted in the battery can give good protection. With fuses, one should always fit the lowest value fuse that doesn’t cause nuisance blowing. Remember that the rating on a fuse is the carrying current of the fuse, not the current at which it will blow. The current at which a fuse will blow is not well defined or specified. Remember also that controller current ratings are motor current, not battery current – the two are not the same. This makes it difficult to give a sensible value for this fuse, but somewhere about 1/3 of the motor currrent rating is usualy adequate in most appliaations. More discussion of fuse ratings are here.