Yes, that's about right. This is the only aluminium profile I could get with suffiecient area to complete this task on the weekend. It was only temporary and has been disassembled after I discharded the battery again. Essentially I wanted to:
- charge it with a 48V inverter close to full (I did, I stopped when one of the cells reached 3.51.)
- then I set up bus bars for parallell and I topped it up all the way (to 3.59 to be exact - which still took ~4h with 30A)
- then I left it for 4h
- finally I set it up as you can see on the picture and I discharged it down to 3.2V by running a 2kW heater for 7h+ (final SOC about 10% judging by the amount of energy used - it was showing 3.14V under load, but recovered to 3.2V which was my goal to avoid any swelling when I remove the compression fixture)
- then I removed the busbars and I disassembled the battery.
This is only 3mm mild steel plate and ~204mm (8in) between bolts. It doesn't take "a crazy amount of pressure" to bow the middle out by about a mm, maybe 1.5mm tops.
In fact I was curious how much force does it take. I have an app on my phone called beam design (for when I DIY various metal things) and according to it this amount of bow is pretty much 300kg. So it seems I've hit the mark without even knowing :-D
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But seriously, I was concerned about all of that force beeing concentrated on the edges too. No doubt the insulation sheet protected it as after I removed the fixture I can't see any damage to the casing. Not even an imprint in plastic. For the permanent version I'll weld supports onto this plate so there is no flex and I've ordered disc washers for the bolts. The fact this plate flexed is probably good as it avoided applying too much force.
After some thinking I decided the battery doesn't really need insulation as it will be in a basement where I never had the temperature go below zero (it's around 5C all year), but I'll add heaters just in case.
It definitely would be my preference, but all I could get without ordering in advance was 20,30,40 mm in 2mm thickness (under 1/8in thickness, about 3/4'',1in,1.5in) then there was this 50x3mm (2in x 1/8) bar that would leave me ~6mm clearance (1/4) so not much if a washer is dropped. But I was extra careful and it is dismantled by now.
I can do sketchy for a short time and not regret it... I'd not leave it like this.
Edit: I have an extra bit of info someone might find interesting:
Initially (after the battery was charged, but before bus bars were removed) the plate flex was almost inperceptible. I mean I looked at it and I wondered, is this plate flexing? It looks like it does a bit, but I'm not sure. I wish I used feeler gauge to measure, but I didn't. It certainly was a tiny flex. Then I removed the busbars to do the parallell arangement and only then the plate flexed by 1mm (maybe up to 1.5mm as said before, but most likely between 1 and 1.5). This means that just the busbars with screws torqued with a torque wrench to 6Nm were holding the batteries.
This makes me think, if you put your busbars on when batteries are discharged, torque them, then charge, you may actuallybe applying "compression" to all but the end cells without even knowing.
Also, the batteries did shrunk again, after I discharged them back to (recovered) 3.2V. I believe it is not ideal to store them too long at such low SOC (10%) but I wanted to ensure they go back to the same size and it is hopefully only for a week or two at most.