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Project E-bike part 2: Battery packs

Last time we looked at how to handle the act of spot welding. As we progress through this quest for an electric bike, we must first acquire some batteries. Now how hard can that be? Well… it’s not that hard, but like everything I seem to find an interest in, it requires some good old fashioned wine-fueled research.

In the first part of this e-bike adventure I mentioned how battery packs are made. You take a single battery called a cell and connect it in series/parallel to get the voltage and the capacity you need. A lithium ion cell is 3.7V nominal and typically 4.2V when fully charged. Nominal means, as far as I can fathom, the average voltage of the cell within it’s safe working range. Most of the cells I’ve looked at have a minimum recommended voltage of 2.4V. An e-bike battery in Europe is typically 36V which is kind of misleading, but then again, calling it a 42V battery would be equally so.

They are 36V because the packs consist of 10 cells in series (37V nominal but we call it 36V to confuse people) and then capacity is added by adding more parallel groups. Your run-of-the-mill e-bike battery is 10s4p (sometimes as low as 10s3p) meaning 10 cells in series paralleled 4 times giving you a capacity of cell capacity x parallel groups.

There are a bunch of different cells commercially available ranging from 1500mAh to a whopping 4000mAh per cell. A rule of thumb is that lower capacity gives you higher peak current, all else being equal. An e-bike motor is fairly low current compared to what these cells can deliver so anything near 10A per cell will work just fine. Given some cell and a price for that cell we need to calculate the price and the capacity of the final battery pack. This is not hard but gets tedious when you have 50 different cells to look at in two or three possible configurations.

To make these calculations faster an less error-prone I wrote a simple web-based battery pack calculator called That 18650 calculator which persists the input values in the URL so you can bookmark them for easy comparison.

Which cells do I buy?

This is the age-old question. There are a couple of features you need to consider: Capacity, discharge rate, price, quality.

Capacity

You can build a battery pack with let’s say 10Ah capacity using a couple of different cell combinations. You can use 5x 2000mAh, 4x 2500mAh or 3x 3300mAh. A good rule of thumb is that more cells equal more kapow and more endurance. So by all objective measures 5p packs can handle more abuse than a pack with 3p, assuming the quality of the cells is the roughly equal. The thing is cells are not cheap and even the more shitty ones are not that cheap. The very high capacity cells are way more expensive than their weaker siblings. A 3500mAh can literally be twice the price of a 3000mAh cell of the same brand, so there seems to be a price/performance advantage there.

I am going for a 15Ah pack in a housing that can hold 50 cells. That puts the capacity requirements around 3000mAh.

Discharge rate

This is something you need to consider if you intend to build a pack that can drive something with a very high peak current. An e-bike actually has quite modest current requirements in the range of 15-20A. So if you buy the largest capacity cells they can maybe discharge with 6-7A max so you would need at least 3p of them to get in the 15-20A range. While technically within spec the cells will most likely heat up reducing the lifetime of your pack.

A 50 cell pack will be 10s5p so we are looking for cells that can handle 5-10A each, which will bring us comfortably above the 15A minimum.

Price

This varies almost daily but most normal cells can be had for somewhere between 3 and 6 euros. I need 50 of them so it’s better to aim for 3.

Quality

The quality of the cell can be hard to determine without proper testing. There are a lot of fake 18650s out there and many of them have very good knockoff labels/print making it hard for a consumer to spot the difference. So when buying cells find a reputable reseller in your area. I went with NKON. They have a wonderful selection, fair prices on both cells and shipping and they are located in EU. They were recommended on a couple of forums and I have been very pleased with their products and customer service.

Now for the brands. Anything made by Panasonic, Sony, LG is probably good enough for an e-bike. Sanyo is also quite popular because they are a teeny bit cheaper. I didn’t bother looking at other brands, but there may be some hidden gems here and there.

The verdict

Evaluating all of the above I settled for the Panasonic NCR18650BD 3100mAh 10A. The price at the time of my purchase was 3.7 euro per cell. In september 2020 NKON had them for 2.76 and they are now up to 4.30, so the prices fluctuate a lot. The rating fluctuates between 3100mAh and 3200mAh, not sure why that is, it may be binning.

50 of those puppies rated at 3100mAh in a 10s5p configuration cost me 185 euros. With a total capacity of 15.5Ah it’s a very nice price. I ordered them and they showed up in a timely fashion.

Now I have everything needed to build a pack, or do I?

Thanks for reading if you’ve made it this far.

/J

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