40AH Li-ion Battery Project

Today I started a new build project that I think others may want to consider. A 40AH Lithium Battery kit for portable operations. Why a kit and not buy a ready-made battery? Considering the costs of new batteries I found kits and parts to build a portable battery for less than have the price of a ready-made battery. Yes, less than half the price.

So, what’s in the kit?
1 12V 3S7P Rechargeable Battery Box Pack with 40A BMS+Charger+Holder Pack+DC Cable
21 3.7V 6800mah 18650 Flat Head Top Li-ion Red Rechargeable Batteries

An optional item you may want to consider is:
DIY Mini Spot Welder Machine Welding Power with Pen for 18650 Battery USA

When considering this kit, it is important to consider what you want to spend on the batteries. In my case, I will be using 3000MaH batteries. There are lesser amperage batteries available which are fine but will yield a shorter runtime.

Assembling this kit will require welding the straps to the batteries, this is where the spot welder will make your life easy. Spot welding will prevent you from damaging the batteries with heat from soldering irons or guns.

I will post an update as I build and test, enjoy and let me know if you start a building project of your own. I would love to share your builds with the club.


Jim Lathan, W4JLP

[email protected]

A note from KU5N, Charles

I have been building Li-Ion batteries from recovered and new 18650s since 2019. Initially with caps from https://vruzend.com/ that allow you to press-fit and connect. Later, I got a DIY Arduino-controlled spot welder from https://malectrics.eu/product/diy-arduino-battery-spot-welder-kit-v3-2-2-full_bundle_car_battery/ and still use it when the need arises. Most of my battery arrays are 4S, running the voltage as high as 16.8v, and I use a regulator to bring the voltage down. Bringing it down rather than up from 3S was recommended because the efficiency is better. All of my arrays have a BMC built with them.

I use the Li-Ion arrays primarily when camping. The density for weight trade is better for Li-Ion than LiFePo4, even though dollars per cycle over time for LiFePo4 is less expensive. For camping, I use new higher capacity batteries. For others, I have probably 20-30 recovered 18650s that I have used a charger to charge, discharge, recharge to measure then group by mAh capacity. 

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