Relationship between amps and motor power?

[guerney]

Does anyone know what the relationship is between amps drawn by my BBS01B controller, and motor power? Presently, my controller is set to 15A - this adequate for all the tasks I've asked of my 20" wheeled folding bike so far, including hauling heavy bike trailers up steep hills, for much of the summer. However, I am unclear as to how much motor power I can expect, if I set my controller to 20A. Below is a graph of my new prospective PAS settings - I didn't want a completely straight line to 20A. I wanted a straight-ish line to 15A, because I know my battery and controller cope fine with 15A. The progression from 15A to 20A is cautious, in case something goes up in flames or explodes. Levels 5 to 9 would only be used in exceptional circumstances, as would level "0", which at 1.2A, as far as I can discern, is the minimum required to overcome motor resistance, plus a bit. At the moment, I almost never use less than 15A, unless running low on battery (19.2ah with LG MH1 cells, 22A BMS).

Edit: it's a 36V battery.

 

[Bonzo Banana]

250W shared by 15A is less than 17 volts so is this a reading after the controller has dropped voltage in order to maximise current/torque? Is this a 36V battery?

The more you push amperage the harder the individual cells work, this shortens their life and could lead to some cell failures over the long term. That's why the smaller capacity battery packs in entry level ebikes typically don't last as long as each cell has to work much harder with regard discharge rates. When I see cheaper hub motor based ebikes for sale locally as faulty it almost always reads as if the battery pack has failed rather than mechanical issues. My point is maybe your battery has far greater capacity and much better cells so not an issue but I would be conservative within the specification of the cells, not take them close to their maximum discharge rate. If you calculate what each cells is expected to discharge based on the controller settings and then compare that to the specification of the cells you should see how close you are.

 

[guerney]

Is this a 36V battery?

Apologies - it's 36V, I should have mentioned. I initially assumed the motor power was 250W @15A, because 15A was set on the controller, when bought.

I suppose what I'm asking is: What's the actual motor power per amp, after losses?

Amps	20	A	@36v
percentage	PAS level	amps	watts
6	0	1.2	43.2
23	1	4.6	165.6
40	2	8	288
55	3	11	396
75	4	15	540
82	5	16.4	604.8
85	6	17	612
90	7	18	648
95	8	19	684
100	9	20	720

 

[matthewslack]

If you use the gears to keep the motor rpm up, then motor output mechanical power will be around 80% of electrical input power.

Your table and graph make sense as calc of electrical input: volts x amps x whatever the controller has been told to do.

 

[guerney]

If you use the gears to keep the motor rpm up, then motor output mechanical power will be around 80% of electrical input power.

Your table and graph make sense as calc of electrical input: volts x amps x whatever the controller has been told to do.

Thank you @matthewslack - that's exactly what I needed to know.

 

[guerney]

The more you push amperage the harder the individual cells work, this shortens their life and could lead to some cell failures over the long term. That's why the smaller capacity battery packs in entry level ebikes typically don't last as long as each cell has to work much harder with regard discharge rates. When I see cheaper hub motor based ebikes for sale locally as faulty it almost always reads as if the battery pack has failed rather than mechanical issues. My point is maybe your battery has far greater capacity and much better cells so not an issue but I would be conservative within the specification of the cells, not take them close to their maximum discharge rate. If you calculate what each cells is expected to discharge based on the controller settings and then compare that to the specification of the cells you should see how close you are.

Thank you @Bonzo Banana - I intend to be careful, especially because this battery is now two years old. I won't be levels which draw more than 15A, unless it can't be avoided - quite when that might be, I can't imagine at present. My bike has coped with everything thus far... but I'm towing heavy bike trailers, sometimes over very rough ground. Extra power in reserve if needed, might be useful. Some time ago, @Nealh rated my battery pack at about 25A (continuous), and the BMS is limited to 22A - I suppose the manufacturer was being careful.

 

[sjpt]

Look at https://ebikes.ca/tools/simulator.html?motor=MBBS02&mid=true&gear=1
Doesn't seem to have many recent motors, no BBS01, but should give lots of info if you interpret it correctly.

 

[WheezyRider]

20A for a 19.2 Ah pack is not a problem, especially as the cells are rated at 10A each and there are 6 in parallel in each bank. The BMS sounds a bit weak for such a pack with only a 22A rating. My Yose power 15.6 Ah pack had a 20A rated BMS and it failed in less than a year. I replaced it with a 50A unit and noticed less voltage sag etc. My second Yose power pack has just gone the same way and I'll be upgrading the BMS in that too.

Here is the MH1 being torture tested...

Test of LG 18650 MH1 3200mAh (Cyan)

 

[guerney]

20A for a 19.2 Ah pack is not a problem, especially as the cells are rated at 10A each and there are 6 in parallel in each bank. The BMS sounds a bit weak for such a pack with only a 22A rating. My Yose power 15.6 Ah pack had a 20A rated BMS and it failed in less than a year. I replaced it with a 50A unit and noticed less voltage sag etc. My second Yose power pack has just gone the same way and I'll be upgrading the BMS in that too.

Here is the MH1 being torture tested...

Test of LG 18650 MH1 3200mAh (Cyan)

Thank you @WheezyRider - that's interesting information. Before I bought my BBS01B kit with battery from the seller on Amazon (I wasn't aware of this forum then), I asked a lot of questions about the quality of the batteries he sold - the seller said that mostly the issues are caused by BMS failure, then he offered to sell me a spare BMS. If I have problems, I'll certainly be enquiring about what BMS to replace it with.