Today I received the BMS (Battery Management System). Had to get this all the way from America since we don't have anything like this here. Or well, only 3-4x more expensive. Pretty quick delivery, bit less than 2 weeks. Also didn't have to pay much tax for import, way less than expected.
The BMS is there to take care of the batteries so you can't under- or overcharge (very important for lithium batteries). The batteries also get balanced while charging so that you can get the maximum capacity out of them for a bit higher range.

Here you can see the whole system. All the small printboards will come on each of the batteries and are connected in series. I ordered a few extra just in case I'd destroy one making a wrong connection.

On bottom left is the head-end board as they call it. The whole loop of battery boards will be connected to this and this one basically manages everything.
In the middle is a sensor that can measure the current, voltage, temperature and some more things. It sends all this information to the display you can see on top right. On this display you can also see the capacity of the batteries.

Head-end board coincidentally fits in the small box that I got with the charger. This was actually meant for the little relais but I can place this in there as well now.

Last week I still got some more parts, will post them here as well since it's all part of the project.

Black cable glands in trade for the wrong gray ones. Rubber insulator caps for the terminals of the controller and motor. Some more cable shoes with different sizes (will still have to order more of the big M6 ones since the batteries also need M6 instead of M10). Orange heat shrink only for color coding on the black cables I received for the charger and DC-DC converter since this is also high voltage.
Then there's still a small charger that can charge one 3.2V lithium battery at a time. I will need this to charge all of the batteries for the first time to get them balanced. The batteries will all be connected in parallel for this. After that the BMS will keep them balanced. Came all the way from Czech Republic, just can't get these things here. This won't be fitted to the quad bike but it does make sure the batteries are immediately ready for use.
Also 2 steel blocks. I will have these routed together with the motor mount so they'll get a bit smaller, sloped and lighter. After that I can get them welded on the rear swing arm and the motor mount will be attached to these blocks.

The plate for the motor mount is also ordered already. It was pretty hard to find the correct size. I again couldn't find it here so I ordered it in Germany.