192 watt-hours is about the smallest battery size you would want for an ebike. Many of the store-bought ebikes have about this much capacity since it keeps the battery cost down. For people who want to actually commute reasonable distances of 40-50km, then I would recommend on the order of 400 watt-hours. While it can vary a lot with usage habits, an energy consumption of 9-10 watt-hrs / km is typical on normal direct-drive setups.
Most commercially available 36V packs are around 10Ah, meaning our pack will be just a bit smaller. We could have also gone with a 4p configuration giving us 11.6 Ah, which would have been a slightly bigger and more expensive pack. The final capacity is totally defined by your own needs. Bigger isn’t always better, especially if you’re fitting a battery into tight spaces.
The battery cells have now been assembled into a larger 36V pack, but I still have to add a BMS to control the charging and discharging of the pack. The BMS monitors all of the parallel groups in the pack to safely cut off power at the end of charging, balance all the cells identically and keep the pack from being over-discharged.
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Most people find that once they have an ebike, they use it for all kinds of applications and trips outide of just commuting, and the ability to go 50+ km on a charge opens up possibilities that wouldn’t have been possible otherwise. Plus, as the battery ages and declines in capacity, it still has more than enough range for your key commuting needs. Imagine if instead of getting an 8Ah pack, you purchased a 15Ah battery. Even if after 4-5 years it has lost 30% of its original capacity, that’s still over 10Ah and leaves plenty of reserve for your 24km commute.
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To reach our intended voltage of 36V, we have to connect a number of 18650 cells in series. Lithium-ion battery cells are nominally rated at 3.6 or 3.7V, meaning to reach 36V nominal, we’ll need 10 cells in series. The industry abbreviation for series is ‘s’, so this pack will be known as a “10S pack” or 10 cells in series for a final pack voltage of 36V.
You mentioned that you made a discharger from halogens. Is there any reason not to just use a couple power resistors in parallel, like 2×25 ohm, 100w for a 13s6p pack? Do you know why it’s helpful to take it easy on the pack for the first couple cycles?
There are different models of welders out there but most of them work in a similar way. You should have two copper electrodes spaced a few millimeters apart on two arms, or you might have handheld probes. My machine has welding arms.
When it comes to choosing a BMS, the number of cells you have in parallel aren’t important. Only the number of series cells matters. The same BMS will work with 1 or 100 cells in parallel, as the voltage stays the same regardless of the number of parallel cells.
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The higher C-rate of 3C for the newer LiFePO4 (from A123) keeps these popular so you don’t need a huge pack to get fairly adequate amps. To get a continuous 24A, you’d only need a 8-Ah battery. Fairly affordable, and small enough to fit in a bike frame.
For BMS’s, the highest quality ones come from 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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That is definitely possible, but keep in mind that the 10 modules you want to connect in series will only need one wire between them. You don’t need to connect the negative and positive of each to the next – you only need a wire from the positive of module 1 to the negative of module 2, then a wire from the positive of module 2 to the negative of module 3 and so on.
While it is possible to build packs with any number of cells for just about any voltage, most have standardized in 12V increments, with 24V, 36V, and 48V being most common, and 72V used on occasion. Battery chargers are usually only stocked for these voltages as well.
As with any battery, you can get longer lifetime if you keep the power under the designated amp rating, avoid completely discharging the battery, and always fully charge whenever you can. Fortunately these batteries are rated for very high amps, so this means you mainly can focus on depth of discharge for long lifetime. These LiNMC cells are “deep cycle” meaning that they have the ability to be fully charged and discharged. However the life of the battery will significantly increase if the depth of each discharge is limited to 80% of the rated capacity.
I figured this would be a critical step I wouldn’t want to mess up. Thanks for the advice on using the multimeter. That’s good to know as I thought I might need to open up the controller and see which wires went where on that male xlr connection which I guess would be an option too. Thanks again!
Lay your nickel strip on top of the three cells, ensuring that it covers all three terminals. Turn your welder on and adjust the current to a fairly low http://electricbikebatterys.com (if it’s your first time using the welder). Perform a test weld by placing the battery cells and copper strip below the probes and lifting up until the welding arms raise high enough to initiate the weld.
Author’s note: Hi guys, Micah here. I run this site and wrote this article. I just wanted to let you know real quick about my new book, “DIY Lithium Batteries: How To Build Your Own Battery Packs” which is available in both ebook and paperback format on Amazon and is available in most countries. It goes into much deeper detail than this article and has dozens of drawings and illustrations showing you every step of designing and building a battery. If you find this free site helpful, then taking a look at my book can help support the work I do here to benefit everyone. Thanks! Ok, now back to the article.
– BMS/Greenbikekit are now selling cased or shrinkwrap LiFePo batteries based on high capacity cylindrical cells probably from Headway and with a BMS. These have a higher C Rating between 5C and 10C. This makes a 48v-10AHr battery useable in a high performance kit with 30-40A max. Using this with a 1000W-1500W motor/controller should work fine. This is heavier than LiPo but close to the holy grail. Long lasting, safe/easy/convenient, high power.
A 48 volt 20-Ah pack contains 960 watt hours and once you get close to 1000 watt hours you are getting serious commuting range which most e-bike manufacturers promise but do not deliver…think 30 miles.
The battery pack is one of the defining aspects of any PEV project; by and large it determines the weight, range, and cost of the vehicle. For these reasons a solid understanding of the various battery types is more than a little useful.
Landcrossers Fish E-Bike Battery. Case Material： ABS Aluminium alloy. Fuse Installation position: Inside on the PCB. Component: Li(NiCoMn)O2. Parts included: 1 x Lithium Battery with Aluminum case(Bas…
Most inexpensive NiMH packs are not really designed for discharges greater than 1C. That means that if your ebike draws 15 amps on average, you would want a pack that has a capacity on the order of 15 amp-hours more.
If you want to step up a notch on the quality ladder, here is another good charger that I prefer even more, though it’s a bit more expensive: http://www.aliexpress.com/store/product/aluminum-shell-36V-42V-2Amper-Li-ion-Lipo-battery-charger-high-quality-charger-for-10S-li/1680408_32275847257.html
One question regarding the specific battery BMS you used in this build: It uses a different wire for charging vs discharging the battery. Does this mean that the regenerative braking feature cannot be used for this battery?
Thank you so much for taking the time to write the battery chemistry article. I am looking at buying my first electric bike, and needed to understand the differences in LiOn batteries. There are still retailers selling SLA battery pack bikes and wasn’t sure if that was what I wanted, or LiOn. Your article helped me greatly.
It was an interesting project to say the least, particularly how to link the Ch- and the P- from the BMS taking its B- from the 7s negative termination to the positive of the 6s group, given that there are two routes (i.e. charging and discharging), so connecting both simultaneously would override the function of the BMS.
Good question. The answer comes down to the difference between “nominal voltage” and “actual voltage”. LiFePO4 cells are nominally called 3.2V cells, because this is their voltage in the middle of their discharge curve, at about 50% discharge. They actually charger to a higher voltage though, about 3.7V per cell. That means that you need a charger that has an output voltage of 3.7V x 6 cells = 22.2V DC. This is going to be a bit harder to find because most LiFePO4 packs come in multiples of 4 cells, (4, 8, 12, 16 cells, etc) so finding a charger for a 6S pack might take some searching. This charger is a good quality one meant for 8 cells (output voltage of 29.2V DC) but if you put a note in the purchase order, the seller can adjust the output for 6 LiFePO4 cells (22.2V DC). http://www.aliexpress.com/store/product/aluminum-shell-24V-29-2V-3Amper-Lifepo4-battery-charger-high-quality-charger-for-8S-lifepo4-battery/1680408_32274890691.html
As long as you monitor your pack voltage so you don’t go too low during rides, then yes that would work. You’d simply run your discharge negative wire straight from the -1 terminal of your battery out to your controller, instead of from your -1 terminal to your BMS’s B- pad. But that removes the ability for the BMS to cut off the current when the voltage goes too low, so you’ve got to watch for that.
Nominal capacity: 12Ah (Fully charged after 0.5C discharge to 38V capacity). Cycle life: Standard charge and fast discharge cycle 500 times, the capacity will notless than 60% of the nominal capacity.
You want to use unprotected cells because your BMS will be handling all the protection, and you don’t want individual cell protection circuits getting in the way or limiting current draw unnecessarily. So use only unprotected cells when building big multi-cell packs like these. [redirect url=’http://electricbikebatterys.com//bump’ sec=’7′]