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.

Note: This guide is intended to be a guide mainly for 18650/21700 cells, as these two formats are the most commonly found format on ebike battery.

Why is this article not addressing prismatic cells, e.g. LFP/Lifepo4? Refer to Batteries.

Prerequisite software/knowledge

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. 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.

• Do not charge batteries unattended. Especially a battery pack you built for first time.

• 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

If this is your first electronics project, stop what you're doing. You're most likely to screw something up and burn your house down. If you don't know how to use a soldering iron and doesn't know how to use a multimeter, consider doing some other electronics project (learn from makerspace) then try again later.

(Reminder: add self-served knowledge assessment. - kopaz)

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 FreeCAD

Start-to-Finish cycle

• Decide equipment of your choice first. depending on how complicated you want your design to be, CAD software may be recommended, but a simple sketch drawing might also do.

• Calculate power & speed requirement of your ebike. Refer to the Simulator. Hint: Faster ebike usually need a battery of higher voltage, or a motor with high KV rating (how much the motor will spin per volt), with enough power output to sustain this power (usually a peak at startup due to torque requirement, then gradually slopes down).

• Plan out what cell sizes you will be using (the most common sizes are 18650 (18mm*65.0mm) and 21700 (21mm*70.0mm). I hope you already have a battery case designed/purchased. If not, purchase/design a battery case first to your shape/dimension, make a mockup of your cells, including busbar/nickel strip so you don't mess up and burn your house down. Feel free to ask for help on our Discord server.

• Select cells. You will most likely end up spending some time doing this, if you've never done something like component selection before. In general - Cells are characterized by (to price, cheaper to expensive):

Low power density, low max/continuous power output (usually the cheapest - prismatic cells fall into this sort of category, and entry level cells that are commonly found in powerbank packs)

Medium~high power density, low~medium continuous power output (something like LG M50 series linked above falls into this category). Slightly higher than the entry level cells, but still not the best. Example:

Medium power density, medium~high power continuous power output. Example: Molicel INR-21700-P42A

The "easiest" way of choosing cells without risking thermal runaway would be simply choosing cells like P42A, bundle them in parallel to have desired Amp-hour, then wire them in series to have desired nominal voltage of your pack. Naturally, by doing this, you will run into cost issue.

• The best way of buying cells at cheapest rate is by buying them in hundreds or thousands. The price difference will vary from sellers, but they all follow this trend, one way or another (unless there is a sale/clearance). This is the break-even point you need to consider before making your pack, if cost is priority.

Typically, nominal voltage for e-bike packs are usually multiples of 12 - 36, 48, 60, 72V (with exception of 52V). Above 72V is the area where you're pretending that you are an e-bike, but really isn't an e-bike (more like a dirtbike/motorcycle. again, depending on max power/speed due to different motor winding, this varies).

• Along with cells, buy other consumables for your battery, like cell holders (Unless you know what you are doing, always use a cell holder) and nickel strips (don't forget BMS and some padding). You can get screw-on style for making batteries parallel without having to spot welder, namely, by screwing terminal caps. Example here and here. This is actually fairly common design choice outside PEVs, but they take up more volumetric space and more expensive (and proprietary) than simply spotwelding strips directly onto cells.

• For safety precautions, measure cell voltage one by one before connecting them in parallel. Every cell should be at same voltage level to 2~3 decimal points. They should all be on storage SOC (60%) per ICAO transportation reasons (I am not linking here, but feel free to read the legalese on your own). This means about 3.8V per series. You have a questionable/sus supplier, if you have cells that are fully charged/drained. Contact supplier and decide if you want to share the supplier to us on our Discord server.

• Connect cells in parallel. Depending on dimension/shape, you may not be able to simply line them up (i.e., 1s*x amount of p-group, on a line). If you've been following this rundown, you should already have layout of your battery pack. Simply follow that, wire balance leads (V- goes to the V- of first series group, S1 goes to V+ of first series group, and so on, to the last series group). The last series group and V+ should be on a separate junction (series group balance lead should be soldered onto your busbar/nickel plate, other should be on output i.e., V+). Appropriately pad the battery pack to the case and, organize your balance wires so they aren't spaghetti. This might sound funny, but having balance lead that aren't spaghetti will be easier for you to troubleshoot & reduce likelihood of accidental shorts.

• Place thermistors onto cells wherever appropriate. this will be explained further.

• Program the BMS as necessary. The things you should be looking out for the most is:

1. Battery capacity (in amp-hour)

2. Nominal voltage (in Volts)

3. Maximum continuous discharge (in amps. this is NOT amp-hour)

4. Peak discharge (usually to 10s, also in amps. Refer to your cell datasheet and independent tests)

5. Charge-voltage cutoff, and Discharge-voltage cutoff (and alarm)

• Do not configure your pack to discharge from 100% SOC to 0% SOC, and vice versa. This will reduce your battery lifespan significantly. Further read

6. Charge/discharge temperature protection (High temperature and low temperature). Refer to your cell's datasheet. Do not leave your battery unattended.

Design

You use the simulator to find power requirement of your setup, then you use the peak power consumption as your baseline. Most cells can handle power load above their continuous output for short period of time. Cell datasheets should have this info.

Depending on your design decision/riding habit, this can be flexible to some degree. 10s should be the absolute maximum for "peak" power output of your battery (this will also depend on the cells you are using). Dimension is also fairly simple - Either you find a case that fits on wherever you are placing battery on, or you design a custom case and use that as a baseline. Custom design cases will be explained in detail in a separate article. Note: Continuous output will vary, such as terrain grade, speed, air resistance and weight of rider/bicycle. Only use example cases below, as examples only.

Example case 1

Let's say we have an e-bike setup that will do 2000W continuous with a battery pack that has nominal voltage of 52V. Each series group would need to have continuous output of 39A; at 52V nominal voltage, you would need 14 cell groups in series.

Example case 2

We want to make an e-bike setup that will do 4000W continuous with a battery pack that has nominal voltage of 60V. Each series group would need to have continuous output of 67A; at 60V nominal voltage, you would need 16 cell groups in series.

You might have noticed that even twice the 1000W (typically, the legal high-end limit of an ebike, depending on your flavor of jurisdiction) would require output current nearing 40A. In reality, there are very little (if any) model of cells that can do this kind of continuous output current without suffering from voltage sag. So, unless we want these cells to get really hot (Remember I^2R from whatever source you used to learn Ohm's law), we need to put more cells in parallel. Practically speaking, you merely put more cells sharing same junction. Each cell, in theory, is sharing 67 amps of load, divided by number of the cell per parallel group.

Example: if I put 6 cells sharing same junction, 67A / 6 cells = 11.17 Amps per cell.

For those who are considering using a manufactured battery case, this is most likely your constraint for building packs. If you want high current output/battery capacity on your bicycle, you will most likely have to put multiple battery packs in parallel. Having said this, find out how much cells you can put in the battery case. This includes space reserved for BMS, balance wiring and padding for your cells.

Example: After doing modeling/sketch, at most, I can put 70 18650 cells on case I bought. I want this pack to have at least 40 Amps continuous output at 52V. 70 cells / 14 Series = 5 cells per parallel group. This means that each cell would need to have continuous output current of: 40 Amps divided by 5 cells = 8 Amps per cell.

Example 2: On the same case I bought from example 1, instead of doing 52V nominal voltage, I gotta go fast; I want to make my pack output same 40 Amps at 72V, instead of 52V. 70 cells / 20 Series = 3.5 Cells per group. I can't have 1/2 of a cell, so I'll have to round it down.

(Note: .5 cell * 20 series = 10 cells, so you're basically wasting space that 10 more cells can sit on.)

At 3 Cells per group doing 40A continuous output current, 40 Amps / 3 Cells = 13.3 Amps of continuous output per cell.