(Linked from Conversion to eBike)
Making it yourself, or not
Choosing a voltage means going for 48V or 36V. I chose 48V which seems to be the trend (2017).
Why make your own battery pack? The cheap packs have lousy cells, the ones with proper cells are very expensive, even on aliexpress.
Buying battery cells on the net is dangerous, chances are that you get imitation. Too good to be true? Don’t buy! Do your research, search the net for suppliers name and review.
Choosing a battery
Go for 18650 form factor. Lots of battery cells equal lots of weight. So you want batteries with high capacity, more than 3000 mAh. After searching for a proper size I ended up with a good deal: 110 cells for USD 344 (http://queenbattery.com.cn). Form factor 18650, brand LG, type F1L, capacity per battery cell ~3350mAh. For specs see: LGC_INR18650_F1L_v1.pdf (additional LG_INR18650F1L_TI.pdf). However, for two batteries, 2 * 78 cells were needed so I needed a second batch, including ~ 5% extra cells in case of malfunction. Update September 2018, for battery number 3 I ordered 100 * NCR18650B, with a considerable lower tag: USD 260. For specs see: NCR18650B.pdf.
Understanding a bit of battery technology won’t hurt. You may want to read https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries before creating packs and constructions. Fully charged batteries and higher temperatures are bad for example.
- If you chain 13 cells of 3.7V, you get 48.1V
- So if you put 6 chains parallel you have still 48V, but more capacity.
- Each 6 cell block is connected, or 4 cells as in the example below.
- This configuration is abbreviated as 13sp6, below is an example of 13s4p.
Stupid enough to buy a high powered motor, it needs a proper supply. 1500 W, ~50 V. That means 1500 / 50 = 30 A (Amps). Each cell offers 5 A, we need 6 lanes to get 30 A. This determines the size of the battery block: 13s6p, 78 cells, < 4 kg. Big, but acceptable. Each cell weights ~50 grams.
Choosing less lanes means that the motor cannot work at full capacity and cells are abused under full load. That is why you always need to check battery specifications and motor specifications.
I bought a really nice set of batteries. But multinationals think they can form cartels for price-fixing , don’t pay taxes, lobby for their own profit and believe they are obliqued to terrorize competition. This is a text from a Chinese supplier:
Due to LG/sanyo/panasonic/ sony/samsung/sony is International brand ,We’ll use blue shrink wrap to cover it to avoid custom issue
Like this sample photo .Please find enclose
After get cells ,please remove blue shrinkwrap, that’s all
Yes, it gets that bad! Politicians, read this, do something against it. Competition, even from abroad, is nearly always a good thing!
Battery Management System (BMS)
The BMS is very important and I had really bad luck, ending up with a totally destroyed battery pack, because of a failing BMS.
So BMS has the honour to have its own page here: https://vanderworp.org/en/battery-management-system-bms/
A solution that is easy to service means: No spot welding of nickel strips on batteries. By the way, batteries with pre welded strips (no need for spotwelding!) have similar prices as without. They can be used in several battery containers.
The promising solution: http://vruzend.com/ This system contains end caps for individual cells with nuts. Easy, not expensive. However it is a brand new product (2017) and I expect it to have some start problems. I love this system but it looks like it is not available right now. This means I have to think of a different solution…
Long story short, I ended buying a cheap spot-welder, knowing I started a new adventure. A picture of the battery and more on welding can be found here: https://vanderworp.org/en/sunkko-787a-hack/.