Lowering controller amperage

Sorry if this question has been asked before, likely has.

Bought a POS Chinese bike from an online sleazy vendor (yes I should have spent more time checking him out first) and are now trying to get it going. I've basically run the problem down to a faulty controller. It's up inside the top of the integrated battery down-tube, looks to be held in place with silicone. I'm assuming it'll get destroyed just getting it out.

The supposed 750W rear hub is actually only a 250W, so I plan to likely replace it at some point with a true 500W or 750W motor. Getting to the point ... so I want the new controller (something from KT to match the current KT LCD3 display) to be suitable for the future larger motors, say a 25 amp model controller. As it seems this would be overkill on the current 250W motor can I simply dial back the amperage of the new controller using the C5 "Controller Maximum Current Adjustment Mode" to make it more suitable for the existing 250W motor?

The documentation for the LCD3 seems to indicate I can use the C5 function to dial "maximum current vale (A)" which I assume is amps, by up to 5 amps. In effect making a 25 amp controller operate like a 20, or a 22 like a 17 amp controller.

Is this correct? Any and all comments and thoughts greatly appreciated.

Thanks.

It probably is correct...but only taking 5A out of 25 isn't doing much to help a 250w motor at presumably a 48v battery (you don't say which one you have, but they're more common for "750w" systems than 36v would be). 48v x 25A = 1200w, so about 5x higher than the motor is meant for. 48v x 20A = 960w, so only about four times higher.

Even at 36v, thats 36v x 25A = 900w, about four times higher, vs 36v x 20A = 720, about three times higher.

So...it's better than at full current, but....

It's generally possible to run a motor up to at least twice it's "rated" power for a few seconds at a time, and sometimes longer, depending on the motor and the conditions it's done under. It's all about how much waste heat is generated inside the motor, and how fast it can get rid of that heat so it doesn't build up inside and damage things.

It's a fair bet that a 250w motor is a geared hubmotor, and the major limitation in those is the plastic gears, as far as getting to hot goes--they get softer the warmer they are, and eventually the teeth can be damaged or stripped off. If it's a sensored motor/controller setup (vs sensorless), then the hall sensors can be damaged by the heat and the system won't run correctly or at all as it doens't know how the motor is spinning to correclty send power to it. Kept too hot for too long, the windings themselves will fail, and this can take out the controller too if it shorts phases together.

Even if it doesn't get too hot, too much torque applied ot the gears or the freewheeling clutch inside it can damage or break them, so using a soft-start controller, or "feathering" throttle slowly from off to engaged can help when a controller is just too powerful for it.



An alternative in your case is to check inside the new controller for what kind of and how many battery-current-measuring shunts it has. THey may look like small black squares with silver ends, or they may look like fat silver wires. If there is more than one of them in parallel (common) then removing one (or more) will lower the current limit proportionally. If there are two, taking one out cuts it in half. If three, remove one, by 1/3. Etc.

Hi AW,
Thanks for the detailed response. Yes the battery is a 48V integrated fork tube battery. The hub is a geared hub, looks like a typical Bafang but doesn't have a branded sticker on it. It's serial number includes 250W48V so there is no doubt what it is.

You've convinced me, I can get a 15amp controller for only A$50 which is what it costs me to buy food for dinner. Put it all together and when I discover, which I expect pretty quickly, it won't get me up the hill I plan to use the bike on, throw the whole system out a stick in a BBSHD mid drive. I'll just stick a second battery decent size 48V with a smart connection in a rear rack type setup.

Another learning, go walk the hill you want the bike for PRIOR to purchasing something. This thing is just going to be a 'fishing trolley' to get to a remote spot (about 7 kilometers each way), it is steeper in sections than I'd anticipated, and will require some slow speed uphill grind over badly eroded sections.

Regards,

Freddy1 said:

Another learning, go walk the hill you want the bike for PRIOR to purchasing something. This thing is just going to be a 'fishing trolley' to get to a remote spot (about 7 kilometers each way), it is steeper in sections than I'd anticipated, and will require some slow speed uphill grind over badly eroded sections.

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I suspect the middrive that you can downshift the rear wheel gears on to help the motor make more torque at the wheel for those low speed sections is going to perform significantly better for this purpose than a similar-power hubmotor drive, unless the hub is wound and the system setup for that slow a speed for that torque to begin with.

It may help to use the http://ebikes.ca/tools/simulator.html to setup a test configuration or few for your expected worst-case conditions, to see what it will "really" take to do what you want, before diving into any new parts.

C5 set to 03 cuts the current by half. That's cheaper than buying a 15A controller, isn't it?

What were you getting on the LCD3's wattmeter before?

docw009 said:

What were you getting on the LCD3's wattmeter before?

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The bike has never actually run, other than a couple initial test spins with the rear wheel raised. Then it completely died. I'm still waiting for one of those test box things to arrive to confirm what the multi meter tests show, that one of the 3 phase wires from the present controller is an open circuit.

Then I guess you gotta buy that 15A controller.

I wish they had a tester where we could hook up the 3 phase and 5 Halls from the controller, and it would simulate the motor. That little box actually tries to do that, but you only get one cycle. If the phases are incorrect, you get a different pattern.

https://www.youtube.com/watch?v=hCJIo6H9VB0