E-driver_ said:
What BMS do I need for a 52volt battery that will have 91 18650-cells?
I want to have a pretty powerful controller. Haven't decided which controller and kit for it yet.
But to be able to pick rom high powered ones I should go higher on the BMS-side also right?
50 or 60 amps perhaps?
You need to know what the complete system requirements are before you can start picking parts. "powerful controller" doesn't actually mean anything, for instance. (to me, a powerful controller would be at least several *hundred* amps, battery-side...to others with lesser needs, a 20A controller would be powerful).
What matters is whether the system has enough power to do the job you need it to do. So you first have to define what you want the bike to do for you, under the specific conditions you ride in, with your riding style, on the terrain you will ride on. Without knowing those things then anything you get may or may not be a waste of money; may or may not be capable of doing what you want.
You can use the http://ebikes.ca/tools/simulator.html to experiment and see what kind of power requirements your usage/situation/etc will need, by using the various pre-setup parts on there, or customizing it if you need more power than those can supply.
Other notes:
THe BMS is there to protect the cells. So it's current limit needs to be based on what the cells are designed to handle--you don't want a BMS that will allow more current to be drawn than the cells can safely output.
However...The BMS (and cells) must also be able to output *more* current than the controller will ever demand of them, so they are not stressed, even as they age. So generally it is safer to specify a battery pack that is capable of at least 25-50% more than you will ever need from it, both in Ah capacity and in A(mps) capability, so that even as it ages, or in adverse conditions like cold weather, it can still do the job you need it to without stressing (damaging) it.
Since many BMS may be made cheaply with FETs that dont' really do the job they say they will, a programmable BMS that is built to handle much more current than you will ever need, but can be set to only allow a small portion of that (whatever the cells can actually handle, minus a safety margin), would be better than a BMS rated for exactly the current you need. (it also means you have room for upgrading the system a little bit without replacing the BMS, just adding more parallel cells for greater current handling as needed).