After about 2 years, 300+ charge cycles, and ~1200km of riding, my belts broke and my battery started to go downhill. Fast. If you have similar issues I hope that this thread will help you if you’re a DIY-er like me.
I embarked on this project as a studying MechE, interested in learning more about BMS and electronics. This was my first battery build, and I have to thank my electrical engineering friend Mazen for helping me purchase the right parts. I was able to build this in my EV FSAE teams’ shop, so I also already had access to all the tools required.
I suggest you READ the whole writeup before you purchase your cells; there are a few tips to guide you throughout.
For insurance purposes, I have to state that I am by no means an expert at this, and if you follow this guide, you do so at your own risk. You assume full responsibility.
With that out of the way, let me begin with some of the symptoms:
- Reduced range
- Board shutting off
- Random buzzing on the remote
- Lack of power under 50% battery
- Weird smells around the battery area
- A crack in the battery casing (in my case, unrelated to the failure of pack)
I experienced all of the above, but any of these signs will be a sign that your battery is beginning to go downhill. Let’s begin.
Tools required:
- 4mm Allen key (5/32 works too)
- K-Weld or similar battery spot welder
- Glue for the crack + temp sensor
- Electronic grade silicone for NVH
- Kapton High Temp Tape
- Soldering Iron + Solder
- Nickel strip (1-2m)
- Side cutters
- Xacto knife or razor blade
- Multimeter
- 21700 batteries
- Time (4-5 hours)
- Cost ($150-200 [cells only])
Battery selection is up to you, but I personally chose to spend a bit more and got what seems to be the best cells on the market, P42As from Molicel. These cells are rated to discharge at 45A and have a 5% nominal increase in battery capacity over the standard cells. I ordered mine from 18650batterystore.com.
For those interested in the standard cells, in my pack I had Lishen LR2170As, which can be found at the link below.
Lishen is by no means a bad brand, but it was clear that the Molis were more up to the task when comparing the spec sheets.
Regardless of what you choose, make sure to choose a cell with a discharge current over 30 amps, preferably as high as possible, due to temperature constraints and no cooling. Another reason to choose the 45A P42As, as they are currently the best on the market in terms of discharge rate.
The cells are arranged in a 12s2p mounting system (346Wh pack), so I ordered 30 in case I had issues. I still have 6. You might be able to get away with only 24, but in the case of a bad cell or cells, it’s good to have a few extra.
Okay, so now that that’s out of the way, here comes the build montage:
Step 1: Remove the battery pack by undoing the 4mm screws.
Be careful of the connector here!
Step 2: Remove the ESC similarly to above (6 4mm Allen screws). You should have a result like this. We’re removing the ESC to figure out how much current we can draw.
Step 3: Undo the Phillips head screws on the ESC to check its current rating.
Check the white sticker, mine said 15Ax2, meaning 30A output capability. Again, make sure your battery output is higher than this per cell! Avoiding excess heat is the key for long battery life.
Package the ESC back up after this.
Step 4: Order all your parts. When everything arrives, open up the battery by removing the Phillips head screws. Remember where everything goes! I suggest taking a picture. Then, remove the lid, again being careful for the cables that connect to it.
Step 5: Remove all the cables being careful not to rip anything off. Also, pay attention to the Kapton tape on the top. Remove that. Also, take a picture here to remember how the cables were routed. Putting the lid back on is a pain due to the wires.
Step 6: Remove the battery pack by prying up from the side with the cable connection. This is hard because the bottom is attached with adhesive. DO NOT use a screwdriver to pry it, you will risk a cell puncture. If you pull on it and work your hand around, you will be able to pop it off. Then remove the black plastic with scissors or a knife, careful not to puncture the cells or scrape the PCB.
The heat sensor is attached there w/ more Kapton tape.
Step 7: Remove the polycarbonate protective sheet on the bottom. Don’t short out these terminals. Even a sweaty forearm is enough. Carefully cut and pry off the nickel sheet with some side cutters. Also, maybe Google your batteries if you find it interesting. The writing on the side will tell you what it is.
Step 8: After cutting off the nickel strip on the bottom, then you have to remove the PCB. I found it easiest to desolder the connection points, and then slice the silicone with a blade and then slowly pull the PCB up, careful not to bend it too much or break it. If you can’t get it out, then move on to the next step and remove it after you have the batteries out.
Then remove the nickel strip in the same way as before.
Step 9: Start to wiggle the cells out. You have to break the silicone here too. I also cut the silicone here with a thin blade.
Step 10: Remove the protective covers and attach them to the new cells, then place the new cells in the plastic retainers.
Step 11: Reassemble in reverse order. Use the K Weld to weld up the nickel pieces in the same configuration as originally. I used a setting of 23J, but it can be tuned to your comfort level as long as its making a good connection. Make sure to use enough pressure during welding.
Step 12: Place the board back on top with some electronic-grade silicon. Use the multimeter to figure out which way is positive and which is negative. If the value is positive, then your red probe is where you want to put the red wire of the board, and black goes on black. If negative, vice versa.
Put silicone everywhere that had silicone originally. For the temperature sensor, use some quick-cure epoxy (I know this is exothermic, but as long as you don’t use a massive glob, it should be okay) or some sort of glue that doesn’t exothermically react. The last thing you want to do is overheat a cell at this point.
Cut the tabs ONLY AFTER you solder them; notice I left them long through this point so that I could easily solder away from the cells. Again, avoiding the heat.
Step 13: Resolder all your joints. I’m missing a picture of this, but mimick the same style that the original had. The solder will attach well to the nickel due to its chemical properties. I put heat-shrink around the longer wires as was there originally, but I was very careful in shrinking them. I used a heat gun, but again, only do this if you feel comfortable. It is easy to overheat a cell at this point and damage it. If you’re not comfortable, don’t do it.
Step 14: Now wrap the Kapton tape all the way around the pack.
Step 15: Reassemble everything in reverse order. Start with the pack into the casing, then close up the casing; be careful to avoid crimping the wires (it’s really easy!) Screw the pack and ESC and all pack on and reconnect them. Then glue the pack when it’s on the board if you have the crack like I do.
I hope this write-up helps someone feel more comfortable bout their battery replacement, and as always, leave some questions if you have them. I am more than happy to answer them. I know I have more resources than some, and I love to share, so help me help you. Thanks for reading!
