The current limit on the BMS is for the cells of the battery. (assuming it has a current limit that shuts it off when exceeded, to protect the cells).
(but it does have to be capable of high enough current so the current pulled thru the BMS doesn't burn it up).
The BMS is there to protect the battery.
You must choose a BMS with a limit that is less than what the cells can easily handle (not their maximum specs, unless you prefer to run them really hard and age them quickly, and replace the pack often).
So you have to know what the cells themselves are capable of for average current, and the number of those cells in parallel, so you can multiply the two to get the current you want the BMS to protect them at.
Then you have to know how the pack itself is built, and whether or not it's interconnects and wiring can handle that current. If not, then you have to lower the limit to what that can handle.
Hopefully you can trust the seller to know this, but they don't all. Most of them probably don't, and don't care, because if you use the pack hard and wear it out early then they get to sell you a new one.
So...since we don't know which specific cell model the pack is built from let's make an assumption that the current each cell can easily handle (not it's max) is 5A. (it could be less)
We don't know how many paralleled cells there are, so we'll calculate that out by dividing pack capacity by cell capacity you've given. 24Ah / 3.5Ah = 6.87. That's almost 7 parallel cells, but not quite, so I don't know if they've derated the pack capacity or rounded up. We'll assume they've rounded up and so call it 6 cells in parallel.
So 6 cells x 5A per cell = 30A as the easily-handled current the pack can deal with.
The max current is probably around twice that, but max current pushes the cells hard, and it heats them up (which ages them faster), and it causes more voltage sag, so you don't get as much power (or capacity) out of them using them at that rate. If it's only a few seconds at a time, it's not a big deal, but if you're pulling that current for long periods, it heats up the pack (how much depends on the cells themselves, the pack construction, airflow, etc etc).
Keep in mind the numbers above are assumptions, and may have nothing to do with your cells--you will have to find out what cells they are and what their specs are to get the real numbers.
But if they are correct, then as long as you're using the pack at 30A or less most of the time, with only bursts of higher current, then the pack would be ok for your controller, and you would use a 60A (or less) BMS.
However, if you will be pushing the pack hard a lot, pulling the full (or nearly so) current the controller is capable of, you would probably want to double the size of the pack so it's not being pushed so hard.
In any case, you must size the BMS to protect the pack, not to provide what the controller needs.
You have to size the whole pack to provide what the controller needs, first, though.
, but travelling along at 18-20 mph most the time in city, i am looking for a decent range- thus the 24ah. I wanted to go higher 30-35 AH, but was getting out of my budget. 95% of my riding is relatively flat, with some bridges and overpasses. If needs be- or someone sees a 1500w motor more practical, please speak your mind, i am completely open about this.
