Observed Voltage While Accelerating

My bike is powered by a brushed WE BD36 hubmotor with a 48V battery pack. There is a voltmeter (el cheapo multimeter) with probes on the plus and minus poles of the battery pack.

At a stopped position, no throttle, fresh pack, the voltage reads about 51V.

When I accelerate I observe the multimeter register voltages up to about 150V, which slowly drop as the bike reaches top speed to about 43V. The voltage reading seems to stay stable as long as the same speed is held. I would expect to see a voltage drop as soon as the hubmotor is included in the circuit, but the multimeter reads a big increase in DC voltage, well above the battery pack voltage.

What is the cause for the meter to register the high voltage? Apparently a change in current affects the reading.

Is something in the circuitry of the multimeter that causes it to read high voltages?
Is there AC noise riding on the DC current?
EMF from the hubmotor?
Does the controller create "noise"?
Perhaps PWM frequencies are affecting the voltmeter?

Anybody have an idea what could be causing the odd readings?

If you have the meter wires bundled together with other wires you may get that effect

Dunno?I've had an experience kinda similar thou...

I've got a cheapo Wally World digital multimeter($10.00),and when checking the voltages individually of my batteries while charging (36v sla pack/36v2amp charger) with this meter it would read voltages in excess of 100v. The first time this had happened I must of jumped 3 feet into the air while disconnecting the charger,batteries weren't hot and no bulging .... Rehooked up the charger and tried again with another multimeter and all was fine.For some reason the cheap Wally World meter will read crazy voltages when the individual battery voltages are checked.If I measure the voltage across the whole pack it reads fine.I was never able to figure out why this happens and had forgotten about it until reading this post.I use a different multimeter now and a Watts Up.

Eric

sometimes the PWM of chargers or controllers can fool multimeters.

Sounds like a false reading from your multimeter.

On my digital V-Ohm meter, and probably most others, the voltage is determined by comparing it to a reference voltage provided by the 9V battery. When the 9V starts to lose voltage everything starts reading high.

I don't see how a Wally Meter could get a 3x voltage, unless it just went crazy, but you can't get a legitimate voltage this high from the battery itself.

I made a voltmeter from a Radio Shack panel meter. It came with a resistor for 15V full scale, but I need 24V. Instead of changing the resistor, I biased the meter with a 9V alkaline battery. That way its range is 9-24V not 0-15V or the 0-24V it would have been with a larger resistor.

The needle is faster than a digital V meter since it doesn't take time to digitize and process the input voltage.

Don't know. But I know that inductors have the property where voltage changes with changes in current. Specifically, V = L*di/dt or the voltage is equal to the inductance times the change in current. Since the motor itself has coiled loops of wire, I'd imagine you're having some inductance in the circuit somewhere(Basically, coiled loops = inductor)... I bet if you slowly change the current, you'll have less of a voltage spike but if you quickly change current, you'll have more of a voltage spike(Don't know if you have the much control over your controller, though.).

PWM might be to blame but I don't totally understand its role in controllers and motors so I can't really comment. Also, digital sampling might be quirky but I don't totally understand your meter's digital sampling method.

I bet the PWM is messing up the accuracy. When you reach top speed and the controller is not PWMing any more (100% duty cycle), the pulses go away and your readings return to the real world. You could possible make a filter from a resistor and capacitor to filter the input to the meter in order to get more accurate readings.

Yeah, I had the same problem with one model of a tiny pocket DMM from Canadian Tire. Charging and at rest it was fine. Start the motor and the readings were jumping all over the place, seemingly randomly, and probably from actual battery voltage to 150v or so like yourself.

I suspect it's a cheap DMM issue, combined with some noise from the controller. Perhaps some small filter caps across the DMM leads would fix it but I never bothered; just used one of my many DMMs without this problem. Small caps are usually better for pulse filtering than electrolytic.

I just went for a ride on battery power only, with the hybrid generator not connected in the system, and did not have the voltage swings. So it appears to be related to the generator system. Coils in dc motor and PWM is a likely candidate.