Making battery packs requires your full care and attention.
You are working with a lot of stored energy and pretty much all the tools you use are metal and can cause a short circuit. In a short circuit, something is going to get hot, and probably melt.
This is a 3 article series I wrote quite a long time ago. I now recommend only 18650 packs for bikes. Also my old website got hacked and I lost part 1 of this article. I’ve summarized it here, but I do apologize that’s not as complete as before. Without any more delays, let’s get started.
Tools needed for this step
Double Sided Tape
The first step is to visually inspect all the cells. Look for any pouches that are damaged; have holes, dented or otherwise compromised. Don’t use damaged cells. Nothing good can come of that. Replacing a cell is MUCH more dangerous than building the pack in the first place. You must use higher forces with tools and there is a significantly higher chance of a short circuit.
This is an example of a physically damaged cell. The voltage is fine, but the cell has been punctured and air is getting into the pouch which will degrade it. Don’t use cells with physical damage. It’s dangerous.
Next, use the multi-meter to check the voltage of each cell. They should be relatively the same, though it’s not critical. With parallel packs it’s critical that all parallel cells have the same voltage. Otherwise one pack will discharge into the other pack and could cause a problem. With series cells, it just makes the balancing board work a bit harder while charging for the first few times. If you do have cells that are not the same voltage and you are sure they aren’t damaged, just charge the pack at a very slow rate for the first time and the BMS should be able to correct the issue. Remember slow is key. The BMS cannot discharge cells at a high current so it’s crucial that this be done slowly. Many BMSs have this listed in the specs. Mines states that it cannot balance at a charge current above 7A. It balances much better at 2A.
The next step is to crimp the cells together. To keep them in place after and during assembly, use some double sided tape. Don’t use too much or you will never be able to get them apart in the event of a problem. Start by placing a few strips of double sided tape on each cell.
On my cells, this is the tab that can be soldered to. You can see it’s actually been crimped to the tab that goes into the battery. You simply cannot solder to some types of metal, leaving crimping, bolting or other mechanical bonding as the only methods to make electrical connections.
Each cell has 2 leads. One is positive, the other is negative. We are connecting the cells in series. If you don’t know what I mean, STOP now. You need to go read about basic DC circuits. On many packs, only 1 lead can be soldered too. The other is made of a metal that can’t be soldered. We are going to crimp the cells together, not solder them. This is a folding process that will give you a flexible and very solid joint. It’s a very good method of joining cells together. You are using metal tools to do the crimping so be very careful. Don’t be fooled by the benign nature of individual cells, a short can be very dangerous. Tabs can melt, tools can become red hot or worse! Believe me, I know and it happens in the blink of an eye.
Back to the process. Align the packs to the positive and negative tabs are aligned. Now fold the tabs over keeping the tab than can be soldered on the outside. We will need to solder to it later. I think the picture shows this fairly well.
Keep crimping cells in series. Here you can see a pack partially assembled.
Once the pack is all assembled, make sure the voltage across the whole pack is correct.