Pressing on,
I'm building a 48 volt battery for my RV. I plan to compress my Keheng 280s using springs, even before the first top balance. It's not hard to imagine they are similar enough to the EVE 280s that they also would benefit from ~12 psi. If anyone has more data on them, I'd love to have it.
I have 16 cells, which will be connected in series for a 48v battery. Due to the length, I'll have two rows of eight cells, each with it's own separate compression box. To connect the two boxes, one of the bus bars will be longer than the others. I don't think that will be a problem but if anyone sees an issue with that please point it out.
I'll be using four 5/16" threaded rods for each box, covered with flexible tubing. On each end of each stack I'll use a 1/2" HDPE plate and two pieces of aluminum angle stock or aluminum unistrut to ensure there is no bending or flexing.
In between each cell, to prevent abrasion from vibration, as well as prevent heat transfer in case a cell starts to overheat, I'll have a sheet of thermal barrier (
link) and possibly a sheet of 3/16" closed cell rubber (
link), which is neoprene and SBR (styrene butadiene rubber), so if a cell is overheating, I will have more time to detect it before it infects the neighbor cells.
They will sit on a smooth sheet of HDPE so they can slide smoothly as they expand and contract. The bus bars will be braided aluminum. If anyone knows the best place to get them, I'd appreciate it. I'll put a sleeve of heat shrink on them to reduce the amount of exposed metal.
The cell dimensions are 6.75*8", so 54 sq in, at 12 PSI is 648lbs, so 162 lbs/rod in a four rod setup. If I remember correctly someone noted their 8 cell stack had an expansion/contraction range of about .25 inches. If set at 50 SOC that's only 1/8" (0.125") in either direction.
Leesprings has a spring that does 162lbs/in (
link), and it's 2 inches long, which seems short, but given the very small amount of travel, it looks like it would work. If it's compressed to one inch while the stack is at 50% SOC, the 1/8" variation would only change by 20.25 lbs (162/8). The resulting PSI range would then be 10.5 at zero SOC and 13.5 at 100%.
I have already ordered an overkill BMS which unfortunately doesn't do per-cell temperature monitoring. The cells will be under my couch in my RV, which will be vented to the living area, using standard HVAC vents, so they benefit from my air conditioner or heater as weather dictates. If I'm comfortable, they're comfortable, so no worries about them being too hot or cold. Any surface that is not a vent will be covered in Hardie board (
link) in case of fire.
Given that's pretty much my only storage option, I'd also like to have a pressure sensor on each box, and per-cell temperature monitoring. Has anyone successfully implemented those? Though unlikely, I'd like to do everything I can to detect a misbehaving cell before it becomes a serious problem.
Lastly, I'd like to keep the SOC between 10-90% to maximize life. With that in mind, when doing the parallel top balance, to what voltage should I charge the cells?
I know I have a lot of stuff going on here that may seem unecessary, but I want to take every reasonable precaution to minimize the risks and maximize life of two critically important, expensive items: The RV (fulltimer) and the cells.
Thoughts and advice from the community would be greatly appreciated before I start buying the parts and putting them together.
Thanks all!
