Hi Myles,
This sounds similar to what has happened with many brush motor controllers where the distance from the batteries to controller is long and the load high.
The controller is limited in space internally and so there's only so much bulk capacitance that can be placed at the power input to the controller.
While it is enough capacitance for many instances, the combination of long battery cable lengths, high loads cause either the output silicon devices to fail or the capacitors themselves due to overvoltage.
A straightforward solution is to add external bulk capacitance across the switched power cables near the motor/controller.
In doing so, both the PWM switching transient stresses on the controller as well as the stress induced by the sudden unloading of the long battery cable will be reduced.
It also sounds as if his main power switch is remote from the motors. If so, he should instead be driving a remote contactor/solenoid located near the motors to switch power On/Off.
Also, in doing that, he'll want to consider adding the very common precharge resistor across the main contactor to precharge the controller's capacitors and electronics-
--bulk capacitors can get very stressed by sudden inrushes of current when turned on.
Some of the above may not apply to Torqeedo, but it is very common and applicable to the more traditional EV-scale brush motor controllers.
Search for "curtis precharge resistor", eg. check out the diagram at the bottom of this page: http://evhelp.
com/Wiring_ .Diagrams. htm
In case this helps-
-Myles Twete, Portland, Or.
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