I'm going to throw this post I made last night for the cell testing thread into this thread as well, because I think it's very relevant to this pack design thread and the cooling needs. As my tests showed here (and surprised me!) these cells don't need heatsinking, but rather just need heat-spreader plates sandwiched between the cells.
Here goes the post:
So, this is the reason why my FET setups kept exploding until I went to the 100ohm gate drive resistors with the 7.1v zenner clamp diodes. The inductive kickback is brutal! This is the reason why caps are so very important on things with FETs, and it's a damn shame I can't use them on a cell testing setup (because it would influence the cell performance readings).
Ok, I snuck my test-rig into work to have access to the $14,000usd fluke thermal imager, and took some very slick pics.
I layed out some engineering blueprints to protect the table, then setup the whole mess right on the 20ft long confrence table. lol
These are thermal tests from the minty fresh undamaged cells. I tested the 15Ah cell until I ran it to LVC, then swapped to the 20Ah cell, and ran it almost to LVC, but I had an unexpected problem with my test setup that you will see later in the pics. lol
The colors auto re-calibrate to fit the temp range, so blue might be 500deg if there are parts in the picture at 1000deg, or white/red could be -20deg if there are parts in the picture at -100deg, so you must use the temp tags to read the temps, or look at how the temp scale is setup for each indivdual picture.
This is 2 minutes at 150amps on the 15Ah cell. (5Ah, or 33% of cell capacity used)
97deg peak temp. Ambient temp was 77deg F, so 150amps for 2minutes just brings the cell up to human body temp, and all localized at the cathode. Pretty damn cool running cell.
I loaded the loadbank up to 300-315amps, and let it sit another 1 minute (Another 5Ah, 66% of cell capacity used).
107.7deg F temp peaks. Feels about Luke-warm to the touch in that cathode area, about body temp for the rest of the cell. Still very cool running for a cell getting hit with 20C discharge continously...
I was going to step up the discharge to 450amps for the last 5Ah left in the cell, but I was dicking around tweaking the manual focus and trying to range the camera that it all ready had another 30seconds at 300amps, so I just let it finish at 300amps to LVC. (Another 5Ah, ~90-100% of the cell used.)
At this point, I was pretty proud of how my setup was working, and I wanted to take a pic of the FETs.
Shoot! Barely warm! Little bastards were doing a hell of a job, and I owe that to BigMoose's advise about the higher resistance gate drive resistors and the zeners to protect gate bounce. Thanks Moose!
So, feeling pretty confident with my FET setup, and wanting to make the most of my time with the IR imager, I decided to hit the 20Ah cell with a worst case scenerio sort of situation. I did 600amps, 80% duty cycle (480amps PWM average), 10khz switching frequency.
So, it starts out humming right along, this was roughly 40seconds in (5.3Ah, 26% of the cell used), I didn't get a clean time marker when I started exactly. Oops! I decided to just run the cell into LVC and snap pics along the way. lol This is pretty much an absolute worst case scenerio jump in your EV and just stay wide open on max current limit from a full charge all the way to empty. lol If a cell can handle this, it can handle pretty much anything an EV is going to throw at it.
The FET bank was really making a strong humming sound, so I decided to snap another pic.
I could see the loadbank start to relax, and I could see waves of heat riseing off of it. lol I wanted to check on the FETs again.
WHOA! 371deg F FETs!!! Not good! lol I decided to just let it run it's course and see what happens though.
This was about 95% discharged, keeping up the brutal amp rate the whole time. Why didn't I run to 100% discharge? You will see why below.
The solder on my FET bank MELTED!!!! WHOA!!! Amazingly, I tested the FETs afterwards, and they still seem to work perfect! Notice that crazy delta-T between the copper and the FETs! Can you say thermal run-away? lol