Broke Back Rack Tributary Thread

I removed the old, broken rack this morning and went for an unpowered ride. I noticed while riding about 10mph that my voltmeter, though I thought disconnected from the batteries, read 10 Volts.

Then I realized this was the hubmotor's regen I slowed to zero, and sure enough the voltmeter needle moved to zero too.

I don't understand though why my ammeter, also wired to a shunt between the controller and batteries, only read zero while I was pedaling or not.
(I'd like to think my pedaling was generating at least 10 amps at 10 volts!)

And I also don't understand why the controller's freewheel diodes didn't block the regen current the voltmeter sensed.

The controller is always turned on as I use a switch on the battery pack (now removed) to switch system power.

My poor, back rack -n- battery pack emasculated bike (I still have 6s15p lithiums on the midframe, but the ~24 volts is below the controller's cut-off):

Whenever the wheel turns, the motor is generating voltage. The freewheel diodes in the controller circulate the voltage back toward the batteries. Other than the tiny amount of current used by your voltmeter, you won't see any significant current unless you go fast enough to generate more voltage than the battery pack. At that point you will regen into the pack.
Try going down a really steep hill and see what happens. Once you hit regen speed, you will not be able to go any faster, even pedaling (well, maybe slightly faster). You wouldn't want to do this very long with lithium batteries unless you had a BMS on board.
What kind of controller are you using? You should be able to lower the cutout voltage if you want to.

Thanks Fechter....I have the popular 72v 35a crystalyte controller.

I thought this controller blocked regen at any speed with diodes on the motor output.

I'd consider breaking open the controller to remove the LVC, and add a wire to the current-sensing shunt, but I installed it under the midframe battery box, and it'd be a huge pain to remove.

I need to lower the weight on the back so the next rack doesn't crack. So I'm going to take 2s of the 12s on the back and move them up front. That'll give me 8s on the front, and when I get a new rack, 10s on the back. Probably I'll need one more subpack to get the voltage under load above the controller's 29v LVC. To this end I'll buy another 15 18650s....this way too, if I ever have to pull the back battery pack, I can still use the bike at ~36v (9s x 4v).

The arrow points to the controller on this early project pic below...

You wouldn't want to do this very long with lithium batteries unless you had a BMS on board.

Click to expand...

The batteries still attached to the bike are disconnected from the controller (because the electricity is routed in series through the back battery pack now removed from the bike). Does that have something to do with its regen behavior?

freewheel diodes are mounted in parallel. I dont see how they would effect regen getting back to the batteries. The internal body diodes in the mosfets would allow regen voltage to get back to the batteries.
Xyster if the batteries are open circuit then you might as well not have batteries attached. Being that as far as your motor is concerned you have an open circuit there would be no current flowing. What does bother me though is that your only making 10 volts at 10 mph, i would think that at 10 mph you should be making more than this, at least 15 or so. You can assume that your losing 0.7 volts ( actually less being that theres no current flow) in the body diodes,no matter here. If you have 1 volt per mph then that means no load wheel speed would be 72 mph with your 72 volt set up.
Joe

I should be back in black with a new back rack next week, hence there's very good reason not to unmount batteries which would be a pain-and-half to mount again.

From my motor's graph, is it reasonable to surmise it's no-load speed is 53mph?



I'll recheck my regen voltage numbers next time I ride.

Good to hear that you worked out the rack.
53 mph and 72 volts works out to 13.5 volts at 10 mph. Being that your going to drop about a half a volt in the controller and then theres probably some error in the bike computer and also that you have an anolog voltmeter thats hard to read very acurately it could actually be a-ok.
Joe