The new controller is in and the motor spins, with one wire left over: the light lead from the old headlight. Plugging it into the controller's light output is tempting, and sometimes it works. Other times it has damaged the light circuit. Outputs vary by brand and model, so treat the light output as a signal, not a power outlet, until you know what yours puts out.
What a controller's light output can be
From manuals and owner reports, outputs fall into a few groups:
- Regulated to a low voltage. Some outputs are held near 6 volts, which suits a small lamp built for it. A forum thread quotes a Bafang mid-drive manual that way.
- Battery voltage, straight through. One widely circulated user manual for the S866 display, one of the No.2 protocol units, says its lights power output carries "voltage equal to battery voltage". One owner of a KT-style controller found the light wires coming straight off the incoming battery wires, with no switch or circuit board.
- Pulsed. A forum poster read about 24 volts on a Bafang BBS02B's rear light output with a multimeter, then found a 144 Hz signal at 50 percent duty cycle on an oscilloscope.
A "12 V" printed on a lamp is the lamp's own rating, not a promise about the controller's output. Some manuals list lamp wattages (our guide to one Bafang factory controller shows one), but we could not find a published current limit for the KT-style outputs discussed here, and forum figures contradict each other, so we do not quote one.
Reports of damaged light circuits
These are individual reports, not lab tests.
- A kit seller's page says its controller works with the stock 200-lumen headlight on one family of bikes but not the 500-lumen version. Our Rad controller guide treats bikes with the newer lighting system as incompatible, so do not count on a relay to get around it.
- One owner connected 12 V turn signals to 48 V, lost a front light and fried the daughter board on the KT controller. The owner later powered the lights from a separate DC-DC converter.
- Another added a 12 V LED strip to a KT controller whose original lamp was 6 V, and the light circuit stopped working. A third ran an LED headlight and taillight at the same time and reported two light circuits damaged.
The usual fix: signal from the controller, power from elsewhere
A common remedy is to stop asking the output to carry the lamp:
- A relay. The output energises only the relay's control side, and the relay's other side switches a supply that can carry the lamp. That supply must match the lamp's rated voltage.
- A DC-DC converter. A step-down module gives the lamp its own supply, for example 12 V from a 48 V battery, and the controller's light output stays unused. Its input range must cover a full pack (about 54 V on a 48 V battery).
Check the relay's coil and switching ratings against the port and the lamp. If you cannot tell what the port outputs, treat it as battery voltage with no protection, as the KT-style owner above found.
Check the port before you connect a lamp
- Check the manual, label or seller's page. If the light output is not stated, assume unprotected battery voltage.
- With the battery off, disconnect the lamp. Then switch the display on and turn the headlight function on. With the light off, many outputs read zero. Treat the bare wires as live: keep stripped ends and meter probes apart, and switch the battery off again before changing any wiring.
- Measure DC volts across the two light wires. Do not trust wire colour: that S866 manual notes its colours could be customised. A reading that follows battery voltage means battery voltage; a steady 6 V or 12 V that ignores the battery suggests regulation. Our multimeter guide covers the basics.
- Distrust a steady-looking reading: the BBS02B example shows a basic meter can read pulsing as steady voltage, and no reading shows what load the port can carry. Unless the lamp came with this controller or its manual names it, use the relay or the converter.
Leave the light for last
Test the controller with the old bike's headlight disconnected. Get the motor, display, brake cut-off and key switch working first (the bench-test guide and the key switch guide cover that), then treat the light as a separate job. If you cannot identify the light lead, insulate its end and leave it unconnected.
Where this comes from
The S866 voltage line and the wire-colour note come from a fan-compiled copy of that manual, not a manufacturer release. The rest comes from owner posts on the Endless Sphere and Electric Bike Review forums, one kit seller's page and user guides: real reports, not specifications, passed on as reported rather than as our own test results. If your light behaves differently, the four photos we need shows what to send.