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Hey thanks for your input

So basically your answer is base on expected damage due to a sudden failure of the DC to DC converter -

I don't get the bit about the arcing - When I shut the car off by removing the key the DC to DC cuts all power to the relays anyway ?
I use a lawn tractor battery with 450 cca for the aux.
I keep it charged with a 70 amp alt.
It only charges while the car is moving but I have not had any problems with the setup in the 15+ years I have been running it.

I think what kenny means is if you lose aux. power with the electric motor under load it can cause electrical arching on the contacter and pitting on the points.

I have had the contact on my car open while under load in the past.
Not because of aux. battery but the switch on my emergency brake.

When I first built my car I took off with the emergency brake on.
Didn't go but a tenth of a mile before I noticed it but it was enough that I decided to put an over ride on the on off switch by running the ignition through the e-brake buzzer switch.
Now the car will not go if the e-brake is on.

Great idea and worked fine for many years.
The emergency brake handle is right under my right elbow so I got in the habit of resting my hand on the handle while driving.
The extra weight on the switch eventually caused it to lose contact and shut the power to motor off while driving.
I took several shut downs before I finally figured out what was happening and fixed it.

I checked the contact points and they look to be in good shape.
This is not a recommendation to intentionally open the contacts under load.
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