Now that we have a crimped pack, we need to attach a battery protection board and insulate the pack from short circuits.

Tools needed for this step

  • Insulating Tape
  • Insulating Cardboard (Business cards are perfect)
  • Foam double sided tape (2mm or 3mm is fine)
  • Soldering Equipment (Iron, Solder, Flux)
  • Battery Protection Board & Wiring harness
  • Heat Shrink Tubing (Optional)

A battery protection board limits the current output of the battery, prevents over discharging & over charging as well as balancing cells during charging. In RC packs, most of this is handled by the charger, but since this is for an electric bike, it makes things safer to have a protection board, not to mention you can use a less intelligent charger.

The first thing to do is figure out where you want to locate the battery protection board in the finished pack. I specifically picked out this board because of it’s shape. I wanted it to sit on the side of the pack. I could have put it on top (wouldn’t fit in this particular case) or in front or behind the cells. I decided to use the side this time. In practice, the top is the most convenient place to access the board, and if my case had the height, I would probably have put it there. In most cases you don’t need to access the board, but they often have lights showing which cells have reached full charge and those can be useful diagnosing bad cells. Also sometimes you get stuck and the board shuts off the battery due to low pack voltage. What if you are almost home and don’t want to pedal? You can open up the pack and bypass the board and get yourself home. I am not recommending that, but I’ve done it and it will get you home at the expense of battery life and a possible battery fire.

I temporarily double sided taped the board onto the pack. It will need to be insulated before being permanently installed, but I needed to know how long to make the wires.

Since the contacts are fairly big and I didn’t want to spend a long time waiting for things to heat up slowly (which can actually add more heat to the packs) I used a big soldering iron to tin all the contacts.

You can see all the contacts are now tinned. I then switched to my regular iron to, carefully, solder all the wire in place. It’s best to tin the wires before trying to solder them to the battery contacts. If both sides are tinned, things should go very smoothly.

Here you can see all the wires soldered in place. There is one wire missing that will be soldered to the positive battery terminal, but I can’t solder it until the positive battery lead is soldered in place.

Here you can see the connectors and where they sit on the pack. I’ve also soldered the terminal wires to the protection board and the battery pack. Now it’s time to insulate.

I have taken to using my business cards to insulate my batteries. I did make one pack with other people cards, but it’s a real pain if you figure out later that you need one of those phone numbers! (Update: They do make special paper for insulating batteries. Typically it’s green but I’m not sure the English name for it. I’ve never had any issue with business cards, but the green paper holds up better when wet. For god sake don’t get your pack wet!) Cut the cards so they completely cover each terminal, but don’t force them in between the cells as this will only server to mushroom out the top of the pack making it harder to fit in the case later. I then tape all the cards in place. I used Kapton(polyimide)  tape to hold things together. I think it’s a good tape for batteries. It’s strong, insulates well and is high temperature resistant.

Keep going with the tape until you are sure it’s both insulated and won’t move. This is where the double sided tape we used assembling the cells really helps. You can also use PVC heat shrink tubing to hold things together. I have it, but decided not to use it here as I don’t think it’s needed and I didn’t want to put the protection board inside the same heat shrink as the pack, just in case I need to access it in the future. In my experience, everything eventually needs repair, and in China, eventually isn’t always that far away.

I used PVC heat shrink to insulate the protection board. Since there are through hole components on the PCB, I used two layers of heat shrink just be safe.

Here is the insulated pack and I have securely double sided taped the protection board to the side of the pack. You can see I added bullet connectors on the protection board so that I can bypass, or replace it, without having to disassemble the entire pack. I like to think I learn from my previous mistakes ?

Step 3 is to install the battery in a protective box and install it on the electric bike.