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motor/controller/battery compatability

Posted: Jun 30 2020 4:41am
by crofty
Hello

I'm new to the forum, I have had an electric bike 5 years, it is a UK 250w hub motor Oxygen and is ok on the hills where i live but now in my seventies I am finding the hills more and more difficult and my speed on the hills often drops to around 6 mph which is not too good for the motor. So i thought I would build a new bike. I have chosen the Bafang BPM 48v 500w code 13 motor laced into a 700c rim with a KT 36/48SVPRM-YCF01 controller rated at 12a and max 25a. My battery is a 52v 17.5Ah. The build is well under way but a friend who is more up on electric stuff than I suggested this setup might pull too much current from the battery and shut it down. Any advice appreciated.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 6:20am
by amberwolf
crofty wrote:
Jun 30 2020 4:41am
So i thought I would build a new bike. I have chosen the Bafang BPM 48v 500w code 13 motor laced into a 700c rim with a KT 36/48SVPRM-YCF01 controller rated at 12a and max 25a. My battery is a 52v 17.5Ah. The build is well under way but a friend who is more up on electric stuff than I suggested this setup might pull too much current from the battery and shut it down.
The battery should have a number of specifications from the manufacturer.
Nominal voltage
chemistry (brand/model of cell inside)
number of cells in series (like 14s)
number of cels in parallel (like 4p)
Nominal capacity (Ah, sometimes listed in Wh which is Ah x Nominal Voltage)
Continous current (A) capability
Peak current (A) capability, and for how long.

Those last two numbers determine whether it can meet the demands of a specific controller; they need to be at least as high as the controller's similar numbers.

Those numbers are properties of both the actual cells used, and their wiring configuration, and of the specific BMS protecting the cells, so they can be determined by knowing those things instead, if necessary.

If you have none of that info, a guess can be made that most batteries are capable of 2C continous current output. The "C" means capacity, in Ah, but is used as an Amps (A) multiplier. So if you have a 17Ah pack, it's "C" is 17.5, so if it's 2C capable, it can do 35A continuous (in which case it would last about half an hour if you did that from full to empty without stopping or reducing the load).

Even if yours is not very good and only 1C capable, that's still 17.5A continous, so it would work fine for the continous controller rated current, but would be stressed by the peak current.

Since the controller itself would *also* be stressed by that peak current (which is why it's a peak/max and not a continuous rating), then that's ok, becuase you don't want to run any of the parts beyond their continous ratings for more than a few seconds at a time, typically.

The motor also wouldn't handle continous use at that max 25A either--it's only meant for 500w, and though it can take more than that for short periods, if it was really loaded to pull 25A it's taking about 1300w, or more than twice it's design. Geared hubs don't shed heat very well, so all the extra heat it's making inside will build up and damage it (over time if not on that first ride), if it's used in a way that keeps it above it's designed use.

So it would actually be *good* for the battery to shut off when heavily loaded (above the 12A or so continous rating of the controller), to help you prevent damage to the other system components.

You can go to http://ebikes.ca/simulator to see how various things in your system affect and are affected by other things, including how it's used, headwinds, hills, wheel size, etc. Takes a bit of playing with to get the idea, but the page has info below the charts to show you how to use it all. If a particular part you need is not listed in hte menus, you can pick something close to it, or use "custom" to input the specs for the part if you know them.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 7:53am
by crofty
Thank you amberwolf for a clear and thorough reply.
I understand now about the battery which is a Chinese generic Hailong with claimed quality cells. I will try and get hold of the specs for the battery.
Before buying the parts i noted that the companies I looked at in China recommended 20A controller for 36v 500w BPM and 25A for 48v 500w BPM motors. If i have any doubts I can always buy a 20A controller to reduce the risk and give me better safety margin.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 8:22am
by donn
Your controller can be programmed, and I would guess that one of the things you can change is the maximum amperage draw. That's how you match the controller to the battery.

I gave it a quick look online to verify this, but the only source I could find was a stinking video, so you're on your own, but the ideally you got the programming software and instructions along with the controller (ideally = not much chance of that.)

Re: motor/controller/battery compatability

Posted: Jun 30 2020 12:52pm
by crofty
No i didn.t get any programming software lol, I wouldn't know how to go about it anyway .
Nice to know i can get it done though should I decide to, thanks for the idea.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 5:19pm
by motomech
I noticed this the other day;
https://www.evcomponents.com/annual-spe ... oller.html
I don't know anything about the Co., but it looks like a pretty good deal.
i have any doubts I can always buy a 20A controller to reduce the risk and give me better safety margin.
Even though you hinted at THE PROBLEM when you related your slow climbing speed on hills, you apparently don't understand how system power levels effect the ebike's ability tackle hills.
The idea is to do what you can to hit the bottom and maintain the climb w/ as much speed as possible and to use an "under-powered" controller is the exact opposite of what you want to do.
It all boils down to the 1st. law of hub motors; That is, when the climbing speed falls to less than half of the top speed, the motor will start to produce more heat than forward locomotion. So a 30 Amp controller will carry you further before the speed drops to this "no-go" speed than a 20 Amp controller will.
The key element is to know when the bike will soon be at too slow a speed (or worse yet, will soon "lug" to a stop) and jump off and push first.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 7:06pm
by Sunder
motomech wrote:
Jun 30 2020 5:19pm
Even though you hinted at THE PROBLEM when you related your slow climbing speed on hills, you apparently don't understand how system power levels effect the ebike's ability tackle hills.
I think you got the wrong problem though. The problem with his motor slowing down on hills was his old bike. His new problem is:
crofty wrote:
Jun 30 2020 4:41am
a friend who is more up on electric stuff than I suggested this setup might pull too much current from the battery and shut it down. Any advice appreciated.
Getting a smaller controller will definitely lower the risk of the bike pulling too much current from the battery and shutting it down.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 9:00pm
by donn
Sunder wrote:
Jun 30 2020 7:06pm
Getting a smaller controller will definitely lower the risk of the bike pulling too much current from the battery and shutting it down.
Or he could 1) find out what his battery will actually support, and 2) make sure his existing controller is configured accordingly, for the zero risk/ no cost solution. Unless it doesn't have that configurable parameter, which would sure surprise me.

Re: motor/controller/battery compatability

Posted: Jun 30 2020 11:48pm
by Sunder
donn wrote:
Jun 30 2020 9:00pm
Sunder wrote:
Jun 30 2020 7:06pm
Getting a smaller controller will definitely lower the risk of the bike pulling too much current from the battery and shutting it down.
Or he could 1) find out what his battery will actually support, and 2) make sure his existing controller is configured accordingly, for the zero risk/ no cost solution. Unless it doesn't have that configurable parameter, which would sure surprise me.
What would surprise me, is if a 17.5Ah battery couldn't handle 25A. That's less than 1.5C. They could have very badly paired an undersized BMS to it, but that undersized? Who knows.

Re: motor/controller/battery compatability

Posted: Jul 01 2020 6:23am
by crofty
Thanks for the replies. Motomech i am aware of heat generation when the motor is running slowly, but what do you do, I can't ride any faster up the hills. I do get off regularly and even feel the motor from time to time. Thanks for the link, thats a great deal, unfortunately shipping cost to the UK was over a $100! .
donn & Sunder, like you I would be surprised if my battery was so bad that it couldn't support the 25A, it was sold by one of the Chinese companies that boast Samsung cells.
Thanks you for all the advice, I now understand alot more about batteries. I have ordered a 20A controller and a LCD3 which shows how many watts I'm getting so I will have more idea how much I'm asking of the system.