Foildrive Max battery caps

If you decide to build your own FD max format battery these are the end pieces to help you do that.

DISCLAIMER: Building your own lithium-ion battery pack carries severe, inherent safety hazards that can result in catastrophic failure, permanent property damage, or critical personal injury. Mishandling cells during assembly—such as accidental short circuits, improper spot welding, or mechanical punctures—can instantly trigger uncontrollable thermal runaway that produces self-sustaining chemical fires and toxic gas emissions. Standard fire extinguishers are typically ineffective against lithium battery fires, making accidental blazes exceptionally difficult to suppress in residential or non-industrial workspaces. You assume full personal liability and safety responsibility for any assembly, testing, or usage, and you should never attempt construction without advanced electrical expertise, certified safety gear, and dedicated fire-suppression protocols.

Building a battery is a moderate to high complexity task. Do not attempt building a lithium pack unless you have the skills and experience. This is better left to pros who build and service electric bike batteries, etc.

The list of materials is as follows:

1. You need to pick good 21700 cells - you need 33 of them. Currently the best cells for the task are Reliance RS50 or RS60, Tenpower 50XG or 60XG or Linkdata 60P or 65P. There are a few source online in the USA to get the cells.

2. Battery end connectors (female): https://www.aliexpress.us/item/3256807002231878.html

3. Bluetooth BMS necessary for charging and balancing the battery. This BMS is not used for discharging, e.g. the power rating is irrelevant, the space is tight so pick the 20A or 30A versions https://www.aliexpress.us/item/3256806823280934.html When installing the BMS, you may need to trim the plastic around the UART connector (unused) so that the slanted end cap can fit correctly.

4. Copper welding strips - there are some good currents flowing so 0.3mm copper is preferred. FD max electronics limit current to 60-70Amps but still... Plain 0.3mm copper is super hard to spot-weld so get the premade wellgo copper/nickel strips from here: https://www.aliexpress.us/item/3256806126848308.html You need the 21700 2P Malposition model.

5. You need a spot welder, AwithZ has some great models, for the wellgo strips above you can get away with a lower end model. If you are trying to weld plain 0.3mm copper with stainless steel sandwich on top, you need a lot more energy and a higher end model like P60F etc.

6. Get some plain 0.3mm copper roll from amazon, you will need it to make the bus bar to take the negative from the back of the pack to the front connector.

7. Get a roll of good kapton tape - use it as an insulator and also as a protective sheet when spot welding the cells. Make sure you don't have any metal tools, blades, etc around. If a metal piece shorts two copper plates of welded cells you will get sparks and likely a fire.

8. Get some 10 gauge bare solid copper wire - you will need it to make the connection from the XT90i connector pins to the positive and negative bus bars. Use plenty of solder when soldering these to prevent them from coming apart during peak currents.

9. Get some 14 gauge THHN insulate wire - you will need to run this from one of the small pins of the XT90i connector to the C- on the BMS for charging. These batteries charge using the main positive terminal of the XT90i connector and one/both of the small pins for negative. The big negative pin is wired directly to the battery and is used for discharging only.

10. Make sure the cells are packed tight, if you allow glue or anything else to get in between them you will end up with a pack that is too long and wont fit/allow the cone to close against the seal.

11. Get some G10/FR4 sheets from amazon - cut them to size and use them to cover the side copper plates of the battery before you lay the copper negative bus bar on top and the ribbon cable for the balancing wires.

12. Get to permatex ultra black sealant and use that to tack the cells together, stick the G10 board to the sides, coat exposed connections, etc. The stuff dries non-rigid and acts as an insulator.

13. Potting the battery - you need to figure this out yourself - there are guides on the internet. Again make sure that your battery doesnt get to big, if it doesnt fit this will all be a wasted effort.

14. Get some 120mm shrink wrap from Aliexpress or amazon - this is what you wrap the battery once you are done.

15. Programming the BMS. This is a standard BT BMS that sends various stats. You can use the "Overkill Solar" app for iPhone or Android - it's free. After install, you will need to adjust the BMS to match the battery architecture. Using the app, go to settings and change battery series from 14 (default) to 11 (this is crucial). After that you can adjust total battery capacity (that will be 3x cell capacity), the over volt and under volt thresholds, etc. Also turn the balancer ON and set it to 15mv or 20mv. You can read up on the other parameters and adjust over time. Again, this BMS will not stop over-discharging the battery but it will protect it in the charging cycle as well as balance it.