Long story short, as of a few weeks ago one more friend, who knows well how to do everything right in the Care and Feeding of RC LiPo, had a catastrophic fire resulting in the complete loss of his bike. What went wrong? Unknown, and that’s the killer. He was simply charging like he always does. That’s pretty much it for me and my affair with RC LiPo, since I know these guys and they know their shit… and still had fires.
So that leads me to the A123 26650 cells. I got my hands on a few hundred of them and after weeding out the bad ones got enough to build a small, 23AH 80-somethingV pack. These things can dump a crapload of amps. They’re rated at 70A per cell continuous which figures out to 700A when you parallel them in strings of 10. Which is sick shit. Here’s their spec sheet, which is downloadable from A123, but if you look for it on their site they make you do a form submission: A123 Systems ANR26650 Data Sheet
This pack is going to weigh about 48lbs, and what you see above is half of it.
Time for a little battery pack review. I was trying to figure out the size of the conductors I needed on the strings, and had to go back to The Basics. The stuff you see here is 1/4″ tinned copper braid, which is the equivalent of around 13AWG and can handle maybe 20A, just for reference, and mostly the reason I used it was to keep the heat transfer down when I soldered the connections. (I’m using a bigass 125W iron from around 1923, from my gran’pappy’s tool chest.) I didn’t want to go bigger, but I’m afraid this stuff is too small for all those amps.
So. Paralleling cells gives you the same voltage, but adds the amperage as well as the amp-hours. These are 2.3AH cells, so each string stays at 3.3V or so, but yields 23AH. It also adds the amps, so if the cells can discharge safely at 70A, then ten of them gives you 700A. Which is way more than this conductor can handle. Just to complete the explanation, when you series cells you then add the voltage, but the amps/AH stay the same. 24 of these parallel strings gives me 80V or so. Thus, 23AH, 80V, for a total of just short of 2KWH. (Remember – KWH = AH x V?)
But let’s back this up a bit. My controller maxes out at 400A, and honestly I’ve never seen it pull higher than 300A, even with the RC LiPo in it. So, though each string may get pulled as much as 300A at 3.3V, each cell is going to be pulled at 30A. Still, I’m worried about those conductors.
My solution at the moment will be to add wire braid – this time 3/8″ along each row before shrinking them together. This will simply be a contact fit, not a soldering job, and we’ll see how that goes. Worst case will be to tear the pack down and put in copper bus bars to keep heating down.
In the meantime, it’s charging time, and discharging, and charging again – keeping track of the capacity of each string to weed out the bad boys.