How to Upgrade Cheap RC car battery to Lithium Battery

This is the first experiment on how I am upgrading my son RC car, using 18650 Li-ion battery cells from an old power bank.
I build a 2s2p battery pack with BMS, this pack voltage is 7.4v and 4000mAh using 4 2000mAh cells, max voltage: 8.4v.
Before the upgrade, this RC car using 4*1.2v 700mAh ni-cd cell is very weak. My son only plays for around 5 minutes. And after, he can play above one hour.

If you want to adjust the voltage to fit old battery pack, you can use the LM2596 buck module: https://amzn.to/2MHgWdW or Aliexpress: http://s.click.aliexpress.com/e/NZwedubI

Parts:
2s 20A BMS: http://s.click.aliexpress.com/e/dlDaSoPq
8.4v Li-ion charger: http://s.click.aliexpress.com/e/K1a7oaxe
18650 cell: http://s.click.aliexpress.com/e/rsBljzba
Nickel strip: https://amzn.to/35bfk39 or http://s.click.aliexpress.com/e/dUJXwr08

*Spot welder in this video: https://youtu.be/Kh2zizuSgF0

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