liveforphysics
100 TW
So, I've been doing cell testing on the cells sent to me by Cell_man. They are 15Ah and 20Ah cells rated for 30C discharge.
There has been 2 threads about them:
http://endless-sphere.com/forums/viewtopic.php?f=14&t=14719
http://www.endless-sphere.com/forums/viewtopic.php?f=9&t=14832
I badly underestimated the amount of work it would be to test these cells.
I first made 4awg leads, and used 1/16"x1/4" buss bars for terminals. The problem is, when you're trying to pull ~500amps at just a couple of volts, keeping the voltage drop happening at the loadbank (where you want the heat to go) is tough! Pulling 500amps for a minute continous also takes professional crimped connections on 4awg, and turns them into a smoking mess that melts into the carpet. lol So, I thought, hmm... lemme double up on all the cableing... NOPE! Still a fail! lol
For anyone looking to do cell testing at these current levels, I will save you the time and expense of doing it wrong twice so you can just start out doing it right. Use 000, or 0000awg cable. Use cable ends that say they are rated for 2,000amps continous, the little "500amp" ends have too much V-drop, and get hot, and V-drop is a big problem when trying to pull absurd currents at tiny voltages.
For bus bars, use something absurdly oversized. You will be glad you did, and this was the only way I finally got my conductors and connections to stay cool.
When doing a FET stage for 450-600amps at ~1.8-2.0v, you're basically SOL. I designed for 800amps, this worked for about 4 seconds before I got fireworks. I designed the second one for ~2,000amps, and it just died today. The FETs do NOT have a good Rds at 2v, and they do not behave like they do at higher voltages, even just 5v they perform fine, drop them to 2v, and all bets are off. From looking at the scope, they seem to switch slowly, which I'm thinking adds the boatload of heat to the silicon, which in turn jumps up the Rds, and they quickly fail. Also, I can't stiffen this with caps and things, because that would spoil the clean cell voltage ripple I need to read on the scope to read Ri on the cells. Because of no caps and crazy currents, could the FETs be doing inductive current loops or other wacky things that are killing them?
I'm determined to get some 100hz, 1khz, and 10khz Ri measurements from these cells at full 30C discharge, so the new plan is to go with 20 of this fet:
http://www.irf.com/product-info/datasheets/data/irfp4004pbf.pdf
I'm going to mount them directly with solder paste onto a 1.5"x1/4" x 20" long copper buss bar using the solder paste/hot-plate method, then snip the center tabs off each package, tie the gates together with a 10ohm resistor in series with each gate, then lay a 1"x 1/8" copper buss bar across all the source legs, and drill the ends of the bus bars and bolt the 0000awg terminals onto them. I've got 7amp FET driver chips to drive the gates.
Does anyone with good experience at dealing with high currents at extremely low voltages have any special insight to help? Does my new FET bank idea seem like it would be tough enough? On paper it should good to switch about 3,800amps, but I'm not sure if it will handle 500-600amps at 2v, because the last bank I built should have been good to 2,000amps, and it made a sound like firecrackers and blew up testing at 1khz and ~500amps, yet the sink never really got warm. It was like the FETs just exploded inside rather than overheating.
I'm struggling with this, and I'm long past-due on wanting to get this all done. Any advise from you pro-cell testing guys would be great!
Thanks!
PS: Here is a video of this poor 15Ah cell taking some abuse. I've kept one 15Ah cell freshly cycled, and not abused, and this other poor guy has been my little test dummy. He has been dead shorted a few times by mistake, he vaporized a wrench for me, he has been stepped on and folded a bit (oops!), and has taken 500amp continous discharge from 3.7v to 2.0v about 10 times. Amazingly, his capacity has actually increased just a whisker, and he still performs great, though I think I've managed to increase his Ri a little (I got him too hot a few times). This idea was to determine of these things were delicate cells or not, because sh*t happens in EV applications, and it's important to know your pack isn't going to be killed if a controller shorts inside or something. I think we can rest assured these cells, and the tabs on the cells are very much up to the task handling the full 30C. I've hit this 15Ah cell with 700amps (47C) and the tabs stayed fine, voltage dropped quite a bit though. Here is a quick little video to see the sort of abuse I'm putting the cells through, and they take it like champs. This is my whipping-boy abused/damaged/overheated cell, and he still performs. Note that voltage gauge is a 0-200vdc gauge that has crap accuracy at these voltage ranges. The current gauge is a 0-800amp shunt type, and I calibrated it to match my fluke, I wouldn't trust it right down to the amp, but I would definately trust it to be +-10amps. I use a scope when I do Ri measurements of course, but it doesn't show up on video worth a damn unless you're focused on on it, so the meters are just to be visible in the video and give an idea of the current these things can pump.
[youtube]i4CefGEnryc[/youtube]
There has been 2 threads about them:
http://endless-sphere.com/forums/viewtopic.php?f=14&t=14719
http://www.endless-sphere.com/forums/viewtopic.php?f=9&t=14832
I badly underestimated the amount of work it would be to test these cells.
I first made 4awg leads, and used 1/16"x1/4" buss bars for terminals. The problem is, when you're trying to pull ~500amps at just a couple of volts, keeping the voltage drop happening at the loadbank (where you want the heat to go) is tough! Pulling 500amps for a minute continous also takes professional crimped connections on 4awg, and turns them into a smoking mess that melts into the carpet. lol So, I thought, hmm... lemme double up on all the cableing... NOPE! Still a fail! lol
For anyone looking to do cell testing at these current levels, I will save you the time and expense of doing it wrong twice so you can just start out doing it right. Use 000, or 0000awg cable. Use cable ends that say they are rated for 2,000amps continous, the little "500amp" ends have too much V-drop, and get hot, and V-drop is a big problem when trying to pull absurd currents at tiny voltages.
For bus bars, use something absurdly oversized. You will be glad you did, and this was the only way I finally got my conductors and connections to stay cool.
When doing a FET stage for 450-600amps at ~1.8-2.0v, you're basically SOL. I designed for 800amps, this worked for about 4 seconds before I got fireworks. I designed the second one for ~2,000amps, and it just died today. The FETs do NOT have a good Rds at 2v, and they do not behave like they do at higher voltages, even just 5v they perform fine, drop them to 2v, and all bets are off. From looking at the scope, they seem to switch slowly, which I'm thinking adds the boatload of heat to the silicon, which in turn jumps up the Rds, and they quickly fail. Also, I can't stiffen this with caps and things, because that would spoil the clean cell voltage ripple I need to read on the scope to read Ri on the cells. Because of no caps and crazy currents, could the FETs be doing inductive current loops or other wacky things that are killing them?
I'm determined to get some 100hz, 1khz, and 10khz Ri measurements from these cells at full 30C discharge, so the new plan is to go with 20 of this fet:
http://www.irf.com/product-info/datasheets/data/irfp4004pbf.pdf
I'm going to mount them directly with solder paste onto a 1.5"x1/4" x 20" long copper buss bar using the solder paste/hot-plate method, then snip the center tabs off each package, tie the gates together with a 10ohm resistor in series with each gate, then lay a 1"x 1/8" copper buss bar across all the source legs, and drill the ends of the bus bars and bolt the 0000awg terminals onto them. I've got 7amp FET driver chips to drive the gates.
Does anyone with good experience at dealing with high currents at extremely low voltages have any special insight to help? Does my new FET bank idea seem like it would be tough enough? On paper it should good to switch about 3,800amps, but I'm not sure if it will handle 500-600amps at 2v, because the last bank I built should have been good to 2,000amps, and it made a sound like firecrackers and blew up testing at 1khz and ~500amps, yet the sink never really got warm. It was like the FETs just exploded inside rather than overheating.
I'm struggling with this, and I'm long past-due on wanting to get this all done. Any advise from you pro-cell testing guys would be great!
Thanks!
PS: Here is a video of this poor 15Ah cell taking some abuse. I've kept one 15Ah cell freshly cycled, and not abused, and this other poor guy has been my little test dummy. He has been dead shorted a few times by mistake, he vaporized a wrench for me, he has been stepped on and folded a bit (oops!), and has taken 500amp continous discharge from 3.7v to 2.0v about 10 times. Amazingly, his capacity has actually increased just a whisker, and he still performs great, though I think I've managed to increase his Ri a little (I got him too hot a few times). This idea was to determine of these things were delicate cells or not, because sh*t happens in EV applications, and it's important to know your pack isn't going to be killed if a controller shorts inside or something. I think we can rest assured these cells, and the tabs on the cells are very much up to the task handling the full 30C. I've hit this 15Ah cell with 700amps (47C) and the tabs stayed fine, voltage dropped quite a bit though. Here is a quick little video to see the sort of abuse I'm putting the cells through, and they take it like champs. This is my whipping-boy abused/damaged/overheated cell, and he still performs. Note that voltage gauge is a 0-200vdc gauge that has crap accuracy at these voltage ranges. The current gauge is a 0-800amp shunt type, and I calibrated it to match my fluke, I wouldn't trust it right down to the amp, but I would definately trust it to be +-10amps. I use a scope when I do Ri measurements of course, but it doesn't show up on video worth a damn unless you're focused on on it, so the meters are just to be visible in the video and give an idea of the current these things can pump.
[youtube]i4CefGEnryc[/youtube]