Can I add a separate BMS controller? I am asking if it is possible.
If your battery has no BMS, then yes, you could, but you'll have to disassemble the battery to:
--find out if it already has one
--access the cells to connect the BMS wiring to
and if there is no BMS in it already, then the case probably won't have space inside it for one, so you would need to make a new case that can hold both cells and BMS, or add the BMS outside the case with a hole in the case for all the wires to go thru, and then make something to attach to the case to protect that wiring and the BMS.
If you can do all that, then you'd need to find a BMS that:
--has a 7A output limit (assuming that's what the cells can handle, which we don't know and have no information available to verify with)
--has a balancing function
--is for a 10s (36v) li-ion pack (not lifepo4, etc)
and any other functions / etc you want it to have.
If you want it to use the same connector for charge and discharge, then you need a common-port BMS.
If you want separate connectors that are independent, you need a separate-port BMS.
(you can wire a separate but not independent charger connector if you want, on the common-port type)
And yet, you do not answer my question how to charge the battery with the DC charger.
I answered it as best as I possibly could, because we know nothing about what charger you have, or it's connectors, etc. See the quote below of my original post:
The controller doesn't charge the battery; the charger that comes with the battery is used for that.
If it didn't come with one, then I'd recommend asking the seller for one that is specifically compatible with the battery you bought from them.
If your charger was not made for that battery specifically, then you'd have to verify that it is compatible with that battery based on the charger label and the battery webpage info (that I quoted previously from the page you linked to).
I don't know where to plug it to a controller or wherever.
If you mean the battery, that depends on your specific controller. Each one will have some cables for the battery supply, but they can be different plugs, appearances, markings, etc.
If you mean the charger, it doesn't get plugged into the controller, as I already said; it gets plugged into the battery.
If it came with the battery the plugs should match; if they don't you need to talk to the seller about that, since it probably means the charger wasn't meant for that battery and it may have other things that aren't compatible either.
Or you need to buy a charger that can do what the battery says it needs, which is 42v final charge output for lithium-ion, and I would recommend at 2A or less (since we don't know what the cells actually are capable of, and the battery and it's webpage have conflicting info that I wouldn't trust any of).
If you get one that has a matching connector to that already on your battery, then you can directly connnect them when charging. (when not charging you can then connect the battery to the scooter (controller, etc). If you get one with a different connector, you'll have to either change it's plug or the one on the battery, or make or buy an adapter wiring harness to go between them.
In all cases, you must verify (with a multimeter / voltmeter) the polarity of the charger output vs the polarity of the battery input, since you can damage or destroy either or both of them if that's wrong (or even start a fire).
Now I am veeery confused. But I understood that the battery doesn't match the controller. What I have to choose better? Maybe the battery? Because many of the controllers are about the same
I don't know how to explain it any better than I already have.
We can't know what battery or controller you need to use. We don't even know if your motor has hall sensors, or not, if it's a hubmotor, a brushed motor, brushless, etc. (it's probably brushless 3phase BLDC, but there's no guarantee of that). These are things you'll have to find out for us to recommend a controller.
Normally the best thing to do when starting a project is to figure out what you want that project to do for you, under what conditions. Then you can use that info to figure out how much power is needed. Then you can use that to find a motor and controller that will do that job for you. Then you can pick out a battery to run those for the range you need.
Since you're not doing it that way, and you already have some parts you want to use, then you'll have to find out what those parts *are* before you can find out what you need to run them.
In previous posts, I've already outlined things you can do to find out what you have, and your wiring, etc., so that we can then help you find things that will do what you need.