crofty said:
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.
