Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
- Messages
- 21,189
I saw a short circuit test of a CALB 180ah cell on YouTube. He pulled 2500 amps on a dead short with a wrench. That's about 13c. Round up to 20c for a safety margin. That's also an older model CALB cell, so newer cells could potentially push out way more than that, especially ones like headway and other lithium chemistries.
A 280 amp hour cell at 20c is 5600 amps. A 2p version of that (which a lot of people have, and more) is almost 12k amps, which is higher than most MRBF or ANL fuses can handle. If you have a large bank, you could potentially short with tens of thousands of amps.
It's a class T for me! Unless I had a cell bank that was under 100ah, then I would consider another, still high interrupt rated fuse type. Even so, I have a class T on a 25ah LiFePO4 power bank.
Fuses are the absolute last line of defense against catastrophic failure, don't skimp on them.
Nice enough demo, but I wouldn't use the raw numbers reported to select a fuse.
3.3V cell
2.2V under load
2220A
Not quite maximum power transfer, with 4880W delivered; max would occur at 1.65V (and same power dissipated inside cell)
He dropped a large steel wrench across a single cell (effectively).
Steel ranges from 13 to 70 micro-ohm centimeters http://www.rfcafe.com/references/electrical/resistivity.htm
I'll use 30
Consider a wrench 20 cm long, 1.5 cm wide, 0.5 cm thick.
30 micro-ohm cm x 20 cm / 1.5 cm / 0.5 cm = 800 micro ohms = 0.0008 ohms.
Compare to a cell with spec 0.00025 ohms, typical measurement 0.00017 ohms.
Wrench plus typical cell is 5.7x higher resistance than cell alone. (hmm, doesn't agree with my estimate of close to max power transfer)
Consider same wrench shorting out a 16s pack. Then, resistance would be just 1.3x the cells alone.
I don't have cell resistance figures for his cell, but I would base fuse AIC rating on cell resistance figures. Using What I have seen quoted:
3.4V/0.00017 ohms = 20,000A. So class T would be good for a single string of LiFePO4 cells.