Decathlon Tilt/Hoptown 500E: overvolting stock motor

A friend has given me the titular bike with a dead battery. It runs on 24V, which might be fine for flat areas, but our city is anything but and the assist fades pretty quickly when going uphill.

Given that the battery is already gone, I thought of swapping out the controller for a 36V 500W one (like $35 on aliexpress) and wiring an external 36V battery in a bag.

My only worry is whether the motor can take it. I know from experience with e-scooters that hub motors are usually fairly overengineered and can take 50% overvoltage if you don't go nuts with the watts, and 500W-nominal should count as "not nuts" while enabling the bike to take uphills a lot better than it does now.

The motor says "DEC8R24V250W" followed by a long number which I take it to be a date/serial. I tried googling and only found a couple hits about the same bike, but no technical info on the motor itself. this thread mentions a "Hulong" company, while this one in a hungarian forum suggests it's made by Bafang.

Fallingwater said:

A friend has given me the titular bike with a dead battery. It runs on 24V, which might be fine for flat areas, but our city is anything but and the assist fades pretty quickly when going uphill.

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Depending on the battery type and capabilities, even when new, much less now, it could simply be that the voltage sag under load is so great that the motor can't get enough power to keep assisting under those conditions.

If you have any way to test it with a new 24v battery, I would recommend trying that first just to save complication and risk of changeover for a higher voltage/etc system.

My only worry is whether the motor can take it. I know from experience with e-scooters that hub motors are usually fairly overengineered and can take 50% overvoltage if you don't go nuts with the watts, and 500W-nominal should count as "not nuts" while enabling the bike to take uphills a lot better than it does now.

Click to expand...

FWIW, if it's a 250w motor (implied by the p/n you found on the motor), then 500w is 200% of the original wattage. That's usually more important than how much overvoltage it is, for how well a motor can handle it.

Whether it can take that, or the 36v vs 24v (150% of the original voltage) depends partly on the type of motor, and partly on available cooling, and partly on usage (how long it gets pushed that hard).

In general, brushless motors can handle any voltage up to the failure point of the winding insulation, which is usually at least a couple hundred volts, and possibly a lot higher.

But brushed motors may begin to be damaged from arc-burn on the commutator, especially at higher currents, at voltages much higher than "rated". At the least, the brushes and commutator segments will wear faster.

If it's a geared hubmotor, doubling the power input especially under high loads and low speeds, is likely to generate a whole lot more heat inside, which will be trapped by the double insulation of a geared hub (two separate airgaps to pass to get heat out of the motor core and out to the air), and cause the motor core to heat up a lot faster and hotter than at it's original power level. Whether that's a problem or not you'd have to test to find out.

If it's a direct drive hubmotor, or a well-exposed chaindrive motor, then heat retention is less of an issue, though it may still spike quite high for longer periods of high loading.

If it's a chaindrive motor (vs hub) then you can change the gear ratio between motor output and system input to bring the motor RPM-wheel-RPM ratio at full throttle back down to what it would've been for 24v, and that will help it operate closer to it's original design intent. This keeps the motor cooler under higher loads and lets the system use battery power more efficiently, too.

If it goes thru the pedal chaindrive, and you can shift gears post-motor-input-point to let the motor stay closer to it's more efficient RPM range, and that will also do the above.