It is very important to only buy high quality lithium ion batteries. Improperly build battery packs can lead to catastrophic fires resulting in loss of property and life.
Lithium-ion batteries can combust when improperly charged or discharged and can't be extinguished by water. as such there are only a few retailers we are willing to endorse. Its highly recommended to go with these resellers or at least do serious research on who you plan to buy packs from. Do not buy battery packs off random ebay/amazon/aliexpress pages, their quality often leaves much to be desired.
DO NOT bypass BMS to "fix" a battery! BMS is usually the last-line of defense for safety and overriding will have serious consequences, like overcharging cells risking fire & injury. If batteries don't charge, it doesn't charge for a reason. forcing charge into it will have serious consequences.
Common battery pack formulas
To find out nominal voltage of your pack, do: Number of series * 3.6V (3.2V for LiFePO4). e.g, 15S4P would have, 3.6V*15S = 54V Nominal voltage.
For capacity for the pack, First, find the datasheet for the cells used in the pack, then find nominal energy capacity of the cell (in Wh, watt hours). Then, do series*parallel of the pack (e.g, 15s4p=60 cells) * nominal energy of the cell (let's say, 18wh per cell). = 1080Wh.
To find maximum continuous discharge capacity of the battery pack, look at the datasheet again, and look for "standard discharge" or something along the lines of standard discharge.
They are almost always a number followed by a C. After you find the C-rating (that is what it's called), use the above formula to find capacity of the pack, then multiply the number by the c-count. that is the absolute maximum your battery pack can handle for discharging.
Divide the number by nominal voltage for that number in amps. E.G. 15S*4P*18wh=1080Wh. I found the cells to have 2C rating. 1080Wh*2 = 2160W.
I already know 15S = 54V Nominal, 2160W / 54V = 40A. So, in this case, the minimum (with no safety margin for BMS) BMS I should get would be 40A, capable of taking 15S Battery.
*Note: This number is absolute maximum continuous rating. Therefore, it is advised that you use this number (for programming BMS) as PEAK DISCHARGE, and for continuous usage, use 70~80% of what the actual datasheet, unless datasheet says otherwise (i.e. explicit mentions of peak discharge capability, usually described as ~amps/watts, 10s.
If you are unable to find proper documentation/datasheet for the cells you have for the battery pack, we suggest not using it.
Common cell formats
The vast majority of E-bike batteries are based on 18650/21700 Li-ion cell type batteries. This is due to:
- Better customizability due to shape of shells. This results in more variety of configurations without losing volumetric space; bicycles often have complex non-rectangular spaces that batteries are attached to.
- Pouch cell (prismatic) type batteries (small format, we are talking about size of phone batteries here) do not have structural integrity unlike cylindrical cells have. this results in worse reliability for these cells against impacts.
- Pouch cells often have inferior c-rating (continuous current output) rating than cylindrical cells.
- Larger prismatic cells are harder (if not outright impossible) to be used on ebikes, due to their size being a constraint. See point 1.
- Lastly, LFP/Lifepo4 cells have inferior power density compared to li-ion. This becomes a less of an issue for larger electric vehicles (i.e. motorcycles, cars, boats), whereas on a bicycle weight becomes an issue because most bicycles are not designed to have tens of kilograms of extra weight apart from rider's weight. This would mean that you would need to upgrade the frame/tires/motors to ones that can support more weight, and at that point you're essentially turning an electric bicycle to a motorcycle. Additionally, typical LFP/pouch cell battery packs (often prismatic/rectangular) are never meant to be mounted/tied to a bicycle.
Where to buy:
We currently recommend the following dealers.
EM3ev - High quality hand-made batteries shipped from China. Comes equipped with a bluetooth compatible BMS so you can monitor your batteries health and performance with a cellphone app. Highly recommended.
Luna Cycle - High quality batteries shipped from USA
Affordable E-bikes.ca - Sells/ships batteries from Canada.
Where NOT to buy
1. All batteries available on large e-commerce websites (i.e. Amazon, Ebay, Aliexpress, Alibaba, etc).
These websites are filled with low-quality battery manufacturers & drop sellers that wants to maximize cost. a typical ebike battery starts around $400USD, give or take. They often take 10~20% on listing/maintenance fee (excluding taxes), so you can kinda see what kind of raw cost we're dealing with here.
2. Unit Pack Power
They used to be recommended, but after several fires & reports from community & outside community, they are no longer recommended.
These guys seem to use pouch cells, and unless you know what you're doing, you should not be using pouch cells on e-bikes.
Remember, a good ethical manufacturer will give you at minimum, what cells they are using & reasonable margin for continuous current/peak current of the cells.
Reputable Cell Providers
Here, we are listing battery cell suppliers known by members to sell legit cells. Cells are purchased only when someone wishes to make a DIY battery pack.
DIY battery packs
If you cannot find battery packs that satisfy your needs, or you want to make/design on your own, DIY-battery can help you get started.
Finding a quality case that meets your needs can be challenging; DIY-case can help you with designing one.
Use cells to their rated-C, do not overheat them. Some heat is inevitable/preferred even, because batteries' IR generally increase as cell temperature drops. There's a whole control method to proper temperature balance of cells.
Generally, you should strive for 20~80% SOC for charge-discharge cycle, but if you want to further lengthen cell lifespan, you can charge-discharge on a narrower SOC range, but this has diminishing returns (compared to 0%-100% to 20~80%). Further read/Reference
Disclaimer: The above quiz is not meant to be a certification and should only be used for educational purposes only. The test mostly covers about cylindrical cell formats, as they are most common type of cells used in E-bike batteries.
Questions/Concerns/Comments of that quiz should be forwarded to Kopaz at E-bike discord server.