Electric bicycles use batteries made from lithium ion cells. One of the most common types is a cylindrical cell called an 18650 cell, named so because it is 18 mm in diameter and 65 mm long. I'll show you how you can create your own DIY electric bicycle battery from these cells for much less than the cost of a retail ebike battery.

It's actually quite easy, and because the prices for these cells get even better when you buy more of them, I often buy an extra large pile of cells and just make extra batteries to sell locally. That way the batteries I make for myself end up being free!

To make your own electric bicycle lithium battery you'll need the following, and more details about each are included below:

Lithium 18650 cells

All lithium-ion cells are 3.7V, and you'll need to wire them in series to get the correct total voltage for your ebike battery, and in parallel to increase the capacity. There are a bunch of different cells on the market, each with their own advantages and disadvantages. I used Panasonic 18650pf cells in this battery, which are 2.9AH each and can deliver a maximum of 10A continuously. If you want a little more capacity, you can go with Sanyo 18650GA cells that are 3.5AH each and also provide 10A continuously. If you don't need as much power though, the most economical cell is the Samsung 26F cell, which is 2.6AH and can provide about 5A continuous. The Samsung cell is better for ebikes that don't need as much of a high power battery. Most of my ebikes use Samsung 26F cells because I like to build packs with larger capacities and use them on medium power ebikes. Just remember that because they can only provide 5A continuous per cell, you might need to use more of them in parallel. For example, in a 30A continuous pack, you'd need at least 6 cells in parallel.

I get my cells from Aliexpress, where payment to a vendor is held in escrow until you receive your goods and confirm that the goods match the description. I prefer it better than ebay, because this way I know my money is safe and I can get it back if I have an issue with a seller. But I only use reputable sellers of battery cells, like those linked above, so I haven't had an issue with cells yet.

Battery Management System (BMS)

You'll need a BMS to monitor your cells during charging and discharging. Basically it protects the cells from getting drained too far or getting overcharged. When choosing a BMS you need to match two main factors: voltage and current rate (more important for discharge than charge current). If you are building a 36V battery, you'll need a 36V BMS (or usually called 10s, meaning 10 cells in series) to match your battery. A 48V battery uses a 13s BMS and a 52V battery uses a 14s BMS. Just make sure you choose a BMS configured for the same amount of cells as the battery you are building. Also remember to check the discharge current. If you want your battery to be able to handle 20A continuously, choose a BMS that is rated at least 20A, and higher is better to give you a safety buffer.

Spot Welder

You really need to use a spot welder to make a lithium battery out of 18650 cells. It is technically possible to solder the cells together, but it creates a lot of heat on the end of the cells that can damage them and prevent them giving their full capacity. I have a few different inexpensive spot welders. Even with the price of the spot welder, your DIY lithium battery is likely to end up costing less than a retail ebike battery. Plus you can make a few more batteries and sell them! I got all of my spot welders from Aliexpress for the same reason as my cells - because I know I'll get a good product or my money back! I like to use a fairly simple spot welder without hand probes which I got on sale for about $150, but I've also had good experiences with the 709 series welders and others that have extra hand probes, though they cost a bit more.

A friend of mine wanted to build his own battery but didn't want to invest in a spot welder. He ended up buying one, building his battery and then selling the spot welder for more than he paid for it on ebay, since they are pretty rare in the US. Whatever works for you!

Nickel strip

You'll use nickel strip to join your cells together. Make sure you get 100% nickel strip and not nickel plated steel, which is cheaper but has much higher resistance. Be careful, some vendors try to sell nickel plated steel strip as real 100% nickel strip since it is nearly impossible to tell. To ensure you received genuine 100% nickel strip you can either use the spark test or the salt water test as described here.

Heat shrink tube

You'll need some large diameter heat shrink tube to seal your battery. I picked up some 10 meter rolls of many different sizes of heat shrink tubing from 110mm all the way up to 300mm. You can get it by the 1 meter length though if you aren't building as many batteries as I am.

The rest of the parts and tools you need are smaller and I've covered them in the following steps.