Batteries are the greatest enemy of electric vehicles. There’s a strong agreement here that li-ion batteries are safer than lipos. And while yes, a single li-ion cell is safer vs a single lipo cell, li-ions have a weaker point. And this is the need to assemble 40-78 cells. With lipos, it’s easy to solder new wires to the tab. But with li-ions, you need to assemble it with either spot welding or soldering. Spot welding is the easiest solution, but it is less effective with copper, the ideal material for keeping heat down while discharging batteries, vs nickel. This leaves two other options, soldering copper wire to the cells or solder copper stringing to the cells. I copper wire in the past, and while it worked great, it increased the 65mm width of the cells to 80mm. So I decided the new idea, solder copper stripping. Calculated the amount of copper needed in each sheet to get the same copper content as a 10 AWG wire, cut them out, cut all of the corners and rounded any sharp edges. I had the copper strips bend up over top of the each p so that when connected, there would be a flexible joint (another reason I couldn’t spot weld the tradition way li-ion packs are put together). I put multiple layers of good quality 3M electric tape between the + and the - of every cell (since everything except the cap is a -). I thought I had eliminated all of the possible risks. But I had certainly missed something:
Just some context of what you are looking at. This is my prototype Rocket Deck with 4x hummie v4 hub motors. This was a 12s4p (48 cell) 30q li-ion battery pack assembled as described above. It included a speedometer, working brake lights and head lights, as well as turn signals that are controlled via the 2nd channel (the wheel) on the mini trigger remote. It had 4x 4.12 chaka vescs, and my really big spark switch (as I like to call it, haha).
Also, in this photo, your looking at the front, the same angle as the videos below. The esc’s are in the back right corner. There’s 9 p’s of 4 cells each on the left side and 3 p’s of 4 cells each on the right side. This fire started at the 9th p of the battery pack.
2 factors that I hadn’t considered.
- What if vibrations cause the copper to pierce the electrical tape and the cell’s thin layer of heat shrink.
- What if the cell balancing wires (which the for the 9p pack, comes out of the top, the side closet in the photo, wraps around and goes back to the middle of right side where it is used) catches a copper strip, pierces the insulation and shorts.
I will likely never know which one is was. But In my mind, it most defiantly was one of those 2 cases.
As to the story of what happened, I was on a joy ride with a friend who was biking. I took this ride so I could put more miles on these wheels, since it has been the biggest issue with all hub motors, including hummies in the past. The wheels (and board) had been ridden over 200 miles at this point. About a mile in while in the bike lane and slowing down for a stop sign (during regen), I heard a loud hissing sound. In the past, with v1 hummie hubs, I heard a similar sound, which was I lost my wheel and I’m ridding on a metal hub now (totally ridable btw, happened at over 25 mph multiple times with out any trouble). So I jumped off almost immediately, since I was only going maybe 10 mph at the time. I realized quickly that I had all four wheels on and that the cells where in fact venting like crazy. So I pulled my phone out and started shooting some video and taking photos while I waited about 30 minutes for the fire department to arrived and finally put it out. In the mean time, one cell after another goes into thermal runaway, catches fire, and spreads to the next cell. At one point, a cell or two exploded, shooting one off like a rocket.
In the end now, about 3k was lost, motors seem to still be ok, though still not tested.
What I’ve learned in the process:
- Li-ion batteries are still very dangerous.
- If lipos blow up, it’s likely you mishandled the cells or there was a manufacturing error when they assembled the internals of the cell (assembly error far less likely than li-ion).
- Li-ion batteries need to be assembled very carefully and monitored more carefully than lipos (as theres 3-8 times the number of points of possible failure)
- Li-ions are much better at dealing with abuses than lipos, but again, more points of failure = more likely to fail (generally speaking from an engineering standpoint)
So which is actually safer? The Answer: They are both very dangerous.
I have built 2 boards with li-ion packs and 6 more with lipos, and my 2nd li-ion pack is the first one that caught fire. Granted, the failure was on my part, and a properly assembled li-ion pack is safer than a lipo any day of the week. But this is a DIY forum, where many new builders ARE building their own li-ion packs from the cells bought in these group buys. So if factoring in the likely hood of an assembly error or oversight, is li-ion actually safer than lipo for DIYers who are assembling the packs themselves? Well… I’m running lipos again, haha.
The moral of the story: Make sure you know very well what you are doing if assembling a li-ion pack and if not, make sure to buy one from someone who does (preferably with cell level fuses, which might have save me in this case). If not, lipos are easier if you have good soldering skills, but make sure you use a bms!
Lipos are more likely to have manufacturing faults that show up later and cause a fire. Li-ions are more likely to have an assembly error, which could lead to fire.
So if Li-ion’s are properly assembled, there’s an extremely low chance of fire (assuming you use the cells properly). With lipos, there’s an unknown chance of manufacturing faults that cause cells to blow up, but assembly error causing fire is much lower.
Again, which is safer? Li-ion in theory. In the DIY world though, where we try experimental shit, neither is safe. This could have been my house, my university, my work, ect…
Without further a due, the after math:
Short Edited Video:
Full Uncut Video:
Be safe everyone!