No! Your traction pack should NOT be grounded to the car body. Especially not if you're connecting the batteries in series (Which would cause a dead short between at least 3 of the batteries). Instead, the traction batteries should be connected +/- to each other, and isolated from the chassis. Something like this:
The accessory battery can connect to the bodywork as usual, but the traction pack should most definitely not.
Code:
/===================[Motor]=====[Controller]====================\
| |
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\=== - [12v] + ==== - [12v] + ==== - [12v] + ==== - [12v] + ====/