EE needed: E-Bike controller problem

[Testarossa]

Installed a no-brand Chinese electric bicycle conversion: 48v, 1000w. And it works.

BUT only with an ammeter in the circuit between the LiFePo battery and the controller.

53.5v at the controller power lead.   .001a with switch off, .078a with switch on. 1 amp and up with the throttle twisted and the motor spinning the rear wheel, no load.

Without the ammeter in the circuit, nothing.

Does the ammeter provide enough voltage drop to spoof an overvoltage protection circuit in the controller?

[elvisboy77]

An inline ammeter provides very little impedance, if that is the kind you are talking about, ie not some sort of power resistor measuring voltage drop.  So it does not make a lot of sense.  It could be that it is enough impedance for the controller to "see" that the battery is there, if the battery is too low impedance by itself.  Most electronics (battery chargers etc) require a certain amount of impedance and voltage to work.

I am sure there are more erudite explanations out there.

[Testarossa]

It's a simple digital multimeter. The shunt is something ridiculous like .0001 ohm.

How do you measure the impedence of a battery?  Doesn't an ohmeter work by sending a small voltage through the circuit to measure the drop? How would that work when the battery itself is pushing 53 volts?

If it is a matter of low impedence, I guess I could just wire in a small resistor. Does that make sense? What should the resistor impedence be?




upload photo on web

[Wayne Orwig]

1000 watt
48 volts.
That means close to 21 amps.

That meter, and the wires on that meter, will NOT handle 21 amps. I am shocked that it did not pop something.

And no, you can not use an ohmmeter to measure the battery impedance.

[Testarossa]

Yeah, it's billed as a 1000 watt motor but I don't believe it. 750 is the max legal in Europe and that's probably the limit for the controller. The circuit length is only about three feet so the wiring should be able to handle about 15 amps. I put a 20-amp fuse in the battery lead.  As I said, I'm pulling less that 1.5 amp with the wheel spinning free. When I get the system running I'd like to hard-wire a volt-ammeter to see what's really going on.

Apparently you can buy 1000 watt controllers for off-road use.

[Wayne Orwig]

You will need a shunt at the battery, feeding a remote meter. Should be easy to find such a thing on ebay.

What might be happening. The electronics might sense too much inrush current on normal startup for some reason, and automatically shutting down. I assume it is a brushless motor of some type. So it may be protecting the drivers. Putting the way undersized ammeter in line my be enough for the electronics to not see an overcurrent condition.

Maybe. Maybe not.


Can you try it at a much lower voltage?

[Testarossa]

I have a 36v battery handy. Will try that. Thanks!

[Aaron D.]

750 watts is the limit for the US. Europe is mostly 250, there's a class with 350, but European bikes are severely speed limited.

Go to electricbike.com which is dedicated to the DIY crowd and is run by the mad scientist of e-bikes at Luna Cycles. Parts, kits and advice.

My wife is riding a Luna e-bike and for the first time in our marriage can ride at MY pace, together. Fantastic!

[Testarossa]

Here's the label on the controller, which seems to imply that it can handle 26a.



pic upload edit

[Aaron D.]

Should be fun. The Bafang BBSHD will handle 30 and 50 amps without issue, so hopefully this one will do the job!

[Testarossa]

More info: The 36-volt battery (from Gail's ebike) pushed enough current to light up the battery-level warning LED, but not enough to spin the motor. So to drain the 48v battery down to some level that might not trigger the overvoltage circuit, I hooked up a 40-watt AC bulb. It's slow to drain -- after an hour only down to 53.3 from 53.6.

[Kiwi_Roy]

Make your own crude meter shunt from a length of standard copper wire. Just take a length of wire and bare the insulation at a couple of spots away from the main current connection.
#12 the spots are 7.6 inches apart Example 10" of #12 with a tap 1" from the end and another at 8.6"
#14 the spots are 4.5 inches apart
This will give you 1 mV per Amp
Solder a couple of small wires to the bare spots to run to your multimeter, wire size or length is not important as there is only a tiny current flow.
It's very important not to terminate the main current at the tap points an inch or so away is good.
You may be able to tap into the bike wiring also but you would need to know how many mV per foot per Amp.

[Aaron D.]

I'm thinking the controller won't allow the motor to run under 38 volts, a lot of folks push 52V through 48 volt motors and the low cutoff is around 42

[Testarossa]

I drained the the battery to 52v and the bike now runs nicely. Trick will be to monitor the recharge cycle so as not to exceed 52v in future. I guess I'll just have to hook up the voltmeter part way through the charging charging cycle and keep an eye on it.

Thanks for all the advice!

[Wayne Orwig]

For something that runs from Lithium batteries, it likely will have undervoltage protection. I am surprised that it shuts off with over voltage like that though. That is more a charger issue.