I am working on a similar project, and was wondering if the BMS’s that you recommended would handle any back EMF from the motor (from regenerative braking, for example.) I see that there are separate leads for charging and discharging, so I’m guessing if current flowed back through the discharge circuit that would be bad. Do you have any recommendations on a BMS (or something different) that would handle this condition?
You would think they would help with cooling, but in reality there is little to no difference. They do create an air gap between cells but because that air is trapped inside the pack and can’t get out, it just turns into an oven. So you can glue your cells together and have them cook on a skillet or use those plastic spacers and have them bake in an oven 😉
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Why does this formula work? Think about it: heat shrink (unless stated otherwise) usually has a 2:1 shrink ratio, so if I need something with less than twice the circumference (or perimeter rather, since my pack isn’t really a circle) of my pack. Since large diameter heat shrink is quoted in half circumference (flat width) sizes, and I want heat shrink with a circumference of a bit more than the perimeter of my pack, then I know I need the half circumference size to be a bit more than half of my pack’s perimeter, which is equal to the height plus the width of my pack.
To answer your question, you can definitely build your own auxiliary battery. It looks like they used a fancy right angled female XLR connector, but I imagine a standard female XLR connector will fit just as well. I’m not sure if you’ll be voiding your warranty though by connecting your own battery. Those XLR connectors can be purchased all over ebay and probably even at your local electronics shop.
If you charge batteries off the bike, I suggest doing so on a metal rack that has wheels on the bottom (very inexpensive at hardware stores) whith the battery(s), and chargers on the rack. In the event of smoke, bad smell, or fire, you can shove it out the door quickly. Locate it closest to a doorway that you can use to eject the battery easily, quickly and without coming in contact with the burning material or breathing the fumes. Consider how you would get a bike out the door. I have a boat hook that could be used to drag just about anything outside, and it is ~2 meters long.
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Your battery pack size is based on voltage and amp-hours. The higher the voltage and the higher the amp hours of your battery, the more range your battery will give you. A 48V 10-Ah pack gives you 480 watt hour (48 X 10). This gives you an easy way to determine exactly how much battery you are buying. The wattage of a battery is the only accurate determinant to judge what range your finished ebike will have.
If you don’t have an actual heat gun, you can use a strong hair dryer. Not all hair dryers will work, but my wife’s 2000 watt model is great. I own a real heat gun but actually prefer to use her hair dryer because it has finer controls and a wider output. Just don’t go mess up your wife’s hair dryer!
I’m mostly kidding, but if you use cells that are rated for more current than you’re trying to pull from them, you’ll create a lot less waste heat and both options will be perfectly fine and healthy for the battery.
Thank you for the very informative post, and it has helped a lot. I plan on building a battery pack with 20 cells with blocks of 4 in parallel, and then I am going to put those in series to make an 18.5V, 13.6A pack. Sorry if these sounds a little bit foolish, but I am not sure what kind of BMS I should be using. Would I be able to use any BMS or would there be an issue with having extra wires if the BMS can power more batteries in series?
LiPo’s are the smallest, cheapest, lightest and most powerful lithium batteries. Their disadvantages include short lifespan and propensity to combust into giant fireballs if not cared for correctly (I’m not kidding, check out the short video clip below).
For BMS’s, the highest quality ones come batteries for electric scooters replacement a company called BesTechPower but they are more expensive. I have mostly used BMS’s from AliExpress. I’ve linked to a few examples of BMS’s I’ve used in the article above.
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[74V, 89V, and 104V @ 13Ah are available by connecting 2 batteries below in series. 37V 26Ah and 52V 26Ah are available by connecting 2 batteries below in parallel. Instructions provided upon request.]
Thank for the great article. I made battery packs already, do you have any recommendations on chargers. I have a 53 volt pack 30 amp hr. I don’t know what charger to buy, and I’m worried as lithium batteries tend to blow up if not handled correctly.
The battery maximum power = volts x amps, so if this 36V battery can deliver 30A continuous, that means it can deliver a maximum of 1,080 watts, though I would run it conservatively at a lower power level than that in most applications.
40% grade hills? That’s huge! You’ll definitely want a cell that can perform at high current since you’ll be pulling peak power from those cells to get up those big hills. Something like the Samsung 25R would be a good choice for this application.
For people who are new to the hobby, ready-made lithium packs are the way to go. Several manufacturers offer ready to go Lithium packs with a built in Battery Management System (BMS) at affordable prices. [redirect url=’http://electricbikebatterys.com//bump’ sec=’7′]