Mate X battery 52V 15Ah – tesla 21700 cells – DIY

This quick post aims to detail the build of a 52v 15000mAh battery fitting in the aluminium enclosure of the Mate X.

This battery pack features 42 x 21700 cells (5000mAh) x 3 (parallel) x 14 (series).

One choice I made was to trash the BMS: remove a SPoF (Single Point of Failure), my previous original battery pack was killed by the stupid BMS (fully drained cells) and you get pretty angry when you discover 65×18650 cells dead because of a 20$ board failure.
I simply split the 14 cells series in two blocks of 7s which will be easy to charge with a classic external RC lipo/liion battery charger. Simple and less parts will be exposed to the bike vibrations.

Total cost for components under 200 Euros ($240)

  • 42 x BAK N21700CG 5000mAh: 3.90 Euros
  • new aluminium enclosure (if you are not reusing the original Mate X): ~30Euros
  • 2 x balancing 7s cable: 5 Euros

Material and tools if you don’t already have:

  • Charger Li-Ion >=7S: ~50 Eur
  • Spot welding machine: ~12 Eur
  • Silicon cable and nickel sheet plate: 10 Eur
  • 3d printer… and ABS filament.

BAK N21700CG cells rocks. 5000mAh is the top of the line and it’s a well-known high quality brand which aims to clone Samsung cells. Every cell I received where all precisely charged with a maximum difference of 5mV between them, impressive.

The 21700 cell holder I designed is a perfect fit. I used ABS for the printing material.

     

The cell holder features 11 slots allowing 4 stacks: 11 | 10 | 10 | 11 cells like this:

The assembly (very ugly picture 😉 sorry)

Operational: >50 Volts

Balancing cables: 2 x 7s

Packing and isolation: epoxy plates and isolation foam which allows to compress make the packs tight and secured:

One half:

Charging, some time later… of course you can charge both half with two charger and upgrade the PSU to allow fast charging (I am limiting at 2Amps because I have a poor 65w psu in front of my 1000w ISDT T8)