DIY-battery

Intent: This page is created as a guideline whether if assembling/designing your own battery pack is a viable option compared to buying a pre-made battery. Prerequisite software/knowledge (before building a pack)

Ability to draw 2D schematics/diagrams. - you need to do this if you are trying to make a pack with a certain dimension constraint. use of CAD is encouraged.

You know how to use the simulator - You need to know how much current your battery will be drawing.

Ohm's law. Since we are not going into AC power, complex number/angle is not needed.

Ability to read battery cell datasheet. - As the wiki page becomes older, more cell info will be added to the page; but for cells that are not on this wiki/internet in a simplified format, reading datasheet will be necessary. LG M50LT datasheet, from batteryspace, Simplified datasheet of M50LT, courtesy of Batemo.de

You will most likely see terminologies that you've never encountered before, this will be explained along the article.

Safety

Batteries become explosives if they are uncontrollably discharged. Therefore, at all times, you must respect the cell and equipment that you are using and know how to operate them. This would mean:

• Do not solder directly onto cells, unless you know what you're doing. you are thermally stressing the cell. In practical term, this would be putting soldering iron onto cell junction for extended period of times. The industry uses spot welders or screw-on terminals to reduce thermal stress on cells.

• Do not leave individual cells outside packaging unkept. Store them in a container, if possible.

• Do not use used cells, if possible. Used cells are not recommended because they will have different internal resistance (IR) and it will be harder for BMS to balance packs with different IR and increase chance of cell failure.

• Do not make a battery pack without a BMS. Always use BMS from reputable sellers with appropriate discharge rating. For example, 30A battery should have at most, 30A BMS, preferably slightly lower for safety margin)

• Do not place/use conductive objects when making battery, if possible. This is generally unavoidable when using spot welders. In this case, cover the areas that you are not working on to reduce likelihood of shorts.

• In case of actual fire, do not try to put out the fire, Li-ion fires are uncontrollable and specialized fire extinguishers are needed. Even this is not enough for large li-ion fire

Equipment requirement

Soldering iron

depending on design, you may not even have to solder anything, but you most likely will for balance wires. Balance wires are used by the Battery Management System (Hereinafter BMS) to read a pack's voltage for charge/discharge/storage. Pinecil will work for balance wires, but a higher power soldering iron will be necessary if you are soldering bus wire/plates.

Battery case

The shape/volume of the case determines how much cell you can fit in the case. You can 3D Print a battery case, and this will be explained further. In general, injection-molded/hardcase plastic battery case (if designed/manufactured well) will be almost always superior to a 3D printed/handmade case. Therefore, the latter cases are for more experimental purpose that market does not provide i.e. very large in dimension, custom mounting/hole for heatsinking, etc.

Spot welder

Spot welder of your choice, and something that won't fail after using it for an hour. They are either battery-powered, or supercap-powered. NASA's guideline on spot welding. Most li-po based spot welders on ecommerce site doesn't seem to last long. Expect anything under $100 be a "lottery", more or less.

Multimeter

used for troubleshooting. Red probe goes on V+, Negative probe goes on V-. Do not mix these up. Cheap, reliable multimeter will do. we're not measuring high voltage here. Something like fluke 101 works.

2D/3D CAD software

To your preference - there are hobbyist/entry level software that usually has limited/featureset comapred to hobbyist-level and professional (career)-level software, but for this purpose, as long as you can do 2D sketch and do 3D model using sketch, it will work. Example: Tinkercad Fusion 360