question about battery discharge rate and PWM

[rocwandrer]

I commute by bike year round. I'm starting to have cold weather linked knee pain while hill climbing that is really slowing me down a lot.

I'm planning to run a 250w motor geared below my average speed (for hills and unplowed snow) with the smallest batteries I can get away with. I've tracked my commute, and I think I need/want only about 11 minutes of assist time between charges, and don't need full power for all of that time.

This brings up a bunch of questions. The first one is simple, but it seems really hard to articulate what I am asking. Bear with me:

If I run a 50% duty cycle on the controller at 7 amps for 20 minutes, will the battery behave like it saw a 7 amp draw for 10 minutes, or like it saw a 3.5 amp draw for 20 minutes, or something in between? Does the same apply for all battery types?

For batteries that give less total amp-hours at higher amp rates, what will be the effect of duty cycle? A Li-Ion battery I already own has a 2.1 amp-hour rated capacity and a max discharge of something less than 3c. I know it can't take the 100% duty cycle 7 amp load, but it can take a 3.5 amp load. Can it take a 50% duty cycle 7 amp load? I know many batteries say they can take momentary discharge at higher rates, but it is bad for them. Does this scheme mean I will be wearing out my batteries much faster than I would at 100% duty cycle and the same average power?

 

[bobc]

lead acid will behave like 7A for 10 minutes.
Lithium will behave like any combination of 7x10 amp minutes ;^) (the lipoly cells in my experience have no puekert effect & give back as many electrons as you stuff into them - period!)
I think your later questions beg the question, why is the limit there? if it's thermal then you have probably an rms current limit allowing short term overcurrents. But if the cell is not designed for high discharge rates (at 3C it doesn't sound like it) you may find the series resistance reduces the output voltage to useless levels under these circumstances, regardless of heating or other damage.
Might be worth thinking about a more suitable battery, they're not crazy expensive.

 

[Drunkskunk]

Welcome to the forum.

A 250W motor running at 50% duty cycle is only putting out 125w power minus what ever it's efficancy loss is under the load and speed at the time. At best that would be 95w output, but it could be 0W: not be enough power to lift the extra weight of the motor up a hill.

Basically, a 250W motor is good to power an average sized person on flat ground at 12-15mph. its too small for any real use on hills, and useless at less than 100% power.

I assume you want a geared motor since you're going to be peddling often, but the minimum size I'd recommend for hills is a BMC or MAC 500W motor and a 36v 20-25A controller.

I assume you want a small battery to cut down on weight. Unfortunately, smaller that 36V10Ah for most batteries won't be able to handle putting out the amps under load without damage. If you have some experience with Lipo, it is possible to run 36V 5Ah of a good quality Lipo battery. They weigh around 13-1400 grams, but are extremely dangerous if mishandled. A much safer for noobs, non-cobolt based Lithium battery in the 36V 10Ah size range can weigh 5-6 pounds, and there are version the size of a large water bottle.

It may be oversized for the distance you want to go, but not for the load it may be under.

 

[rocwandrer]

That answers a lot for me, thanks! I've also dug into the motor simulators and now I think I understand how the throttle works. The part I was missing was that I thought of the throttle as a torque control, not how it really works. I thought i was going to be able to feel peek efficiency in the throttle travel, which, based on torque, power, and efficiency curves, now seems highly unlikely.

I am still thinking that what I would like to be able to do is gear for a speed just above what I can maintain on the steepest uphill on my commute when my knees aren't bothering me. I find that when my knees bother me, gearing down for the hill is almost worse, because it prolongs the problem. For this reason I want to use the electric assist instead of gearing down without assist to maintain climbing speed as needed. If i pedal faster than the motor can keep up, great!

I was/am thinking like this. If I can comfortably put out 175 watts for as long as I like even when my knees are bothering me, and it takes 300 watts to climb a hill at a reasonable speed, then I was thinking I'd run say 50% assist power with really high efficiency by dropping throttle. Clearly this isn't going to work out like i thought because it is so hard to know where the peek efficiency is, but i still feel like if I gear for what i can do when my knees aren't bothering me, and run 100% throttle, most of the time, I'm going to be using much less than peak current.

I'm wanting to run Ryobi Li Ion battery packs. I have 4 nicads and 2 Li Ion packs, and 2 chargers at home, plus a gang charger at work. If I could get away with using the 2 1.2 amp-hour (I think) 18v packs I already own in parallel and get 10 minutes of assist, mostly at well less than a 5 amp hour discharge rate, that would get me started to see if this solution is workable before i spend money on dedicated batteries. The Ryobi batteries already have PCB's built in, and I already have dedicated chargers, etc. I was originally thinking that since I use those batteries for other things anyway, I'd get 4 of the 2.2 amp-hour for a more permanent solution, but that looks way more pricey than something like a pair of these: http://www.all-battery.com/185volt-3000mah15cli-polylipo5cellbatterypack.aspx

Thoughts?
Richard

 

[Drunkskunk]

you're on the right track, but those Ryobi batteries aren't going to be able to handle the output needed with just 2. If you have 8 of them together, maybe.

The Lipo idea is better. But that battery isn't what I would recomend. For half the cost and better quality, you could get a 20C 5S 5000mA Tenergy from Hobbyking like this one. http://www.hobbyking.com/hobbyking/..._5000mAh_5S_20C_Lipo_Pack_USA_Warehouse_.html
thats about the smallest you can run. the 3000mA just won't cut it.

Two 3000mA batterys, combined to make a 36 volt pack will give you about 100 watt hours. a 20 amp controller will draw 720 watts an hour, so the 3000 pack will last about 9 minutes and be dead, and draining lipo dead is always bad. the 5000mA pack can be drained dead in about 15 minutes, so it can handle the 11 minutes you need it.

 

[amberwolf]

FWIW, you can *make* a torque control rather than voltage control; the simplest way is to use the Cycle Analyst to intercept your throttle signal, modulate it based on battery power draw and a few user-settable options, and feed it's output to yoru controller's throttle input.

You could also build an analog electronics unit to do it, or program one up with an MCU, but either of those is a task I wouldn't want to have. There are some ongoing projects here on ES that would end up doing this, though, once they are complete.

There may even be controllers that already do torque control, but likely they are the expensive ones, not intended for ebikes (and capable of MUCH more power than what you need to control).

 

[rocwandrer]

That last post brings up another question. What happens when the motor is demanding, say 25 amps and the controller is rated for only 20 amps? Does it clip and deliver less voltage? duty cycle? fry itself?

 

[gartmu]

That last post brings up another question. What happens when the motor is demanding, say 25 amps and the controller is rated for only 20 amps? Does it clip and deliver less voltage? duty cycle? fry itself?

If the controller is rated for 20A it never gives more than 20A if it is working properly. To do that the controller has a current sense circuit and changes the duty cycle to stay in the 20A limit.

 

[rocwandrer]

That last post brings up another question. What happens when the motor is demanding, say 25 amps and the controller is rated for only 20 amps? Does it clip and deliver less voltage? duty cycle? fry itself?

If the controller is rated for 20A it never gives more than 20A if it is working properly. To do that the controller has a current sense circuit and changes the duty cycle to stay in the 20A limit.

Thanks! exactly the info I was looking for! Does that mean the motor never sees more than 20 amps, but the battery could have an instantaneous draw greater than 20 amps?