Should I connect the driver directly to the battery terminal?
Yes, run the driver supply straight from the battery terminal; the fuse goes closest to the positive post and the negative is not left to the chassis.
Yes, the supply line of a motor driver should preferably be run straight from the battery terminal. The reasons come under two headings: voltage drop and noise isolation. If you feed the driver through the vehicle fuse box, a long distribution bar or a line shared with other loads:
- The supply voltage collapses during the starting surge; starting current is typically 5-8 times the rated value.
- Other devices sharing the same line are affected by that collapse.
- The noise produced by the motor current is carried to sensitive devices over the common line.
The correct order of work
- Run the positive and negative lines straight from the battery terminals.
- Place the fuse at the point closest to the positive terminal; what is being protected is the whole cable in between. Keeping the fuse near the battery terminal is not a preference but the basic rule of cable protection.
- Choose the cross-section for the peak current (KS250 45 A / 5 s, PT500 40 A / 15 s) and verify the voltage drop by calculation.
- Do not rely on the chassis and run a thinner negative line; run it separately and thick.
- Tighten the battery terminals and the connectors to their torque and check them periodically.
At 12V the margin for error is very narrow
The voltage drop rule looks as if it were independent of voltage, but its consequence is not: at 12V a 3% drop is only 0.36 V. That budget is spent not by the cable alone but also by oxidation at the battery post, an unturned terminal screw and the joints in between. So even when the cross-section calculation is right, the quality of the connections decides the outcome. Once the installation is finished, measure the end-to-end voltage drop under load; measurement, not calculation, tells you the truth.
The battery's second benefit: swallowing the energy back
As a motor slows down it behaves like a generator and sends energy back into the line. The battery absorbs that energy. Switch-mode power supplies, because they cannot sink current, let the line voltage climb rapidly in the same situation and the driver sees an overvoltage. Why the supply voltage rises during deceleration becomes decisive above all with high-inertia loads.
Deciding by supply point
| Supply point | Suitability | Reason |
|---|---|---|
| Straight off the battery terminal, fuse near the post | Preferred | Lowest resistance, best noise isolation |
| Short, thick main distribution bar | Acceptable | If the shared impedance is small the effect stays limited |
| Vehicle fuse box, shared line | Not recommended | The voltage collapses at starting, other devices are affected |
| Long, thin intermediate line | Not suitable | Voltage drop and heating at the connections |
Where going straight to the battery is not enough, or does not fit
- The driver is in a panel and the battery is far away. Enlarging the cross-section over a long run is not always economical; here, bringing the driver closer to the battery and extending the motor cable is the better arrangement.
- The system is fed from the mains. With no battery there is nothing to swallow the energy coming back; the braking behaviour has to be assessed separately.
- Consumption while parked is not wanted. A direct battery connection is live with the ignition off as well; if the standby current is a problem, put an isolating switch in the line.
- Welding is to be done on the vehicle. A direct battery connection leaves the electronics exposed to the welding current; disconnect the supply before welding.
Once the installation is complete, do one single check: measure the voltage at the driver terminal while the motor starts under the heaviest load. If the value collapses noticeably, the problem is not in the driver but in the supply line. Tighten the battery posts and the intermediate terminals once more after the first week; on vehicles that vibrate, the first loosening usually happens in that period.
The full topic: DC Motor Driver Selection Guide