hey
@JeepDaddy thanks for making this post. I was torn between building a plywood box, buying something bigger, or re-using a plastic battery box from one of the group 31 lead acids I'm replacing in my van. Your post gave me confidence I could make a 4s 280ah battery fit in the box (and space) I already have, and I'm really glad I did. I don't mean to hijack your thread here but figure I'll add details of my variant in case it helps someone down the line.
My primary requirements were:
- low relative motion, I'm driving an old sprinter down plenty of nasty roads and there's nothing smooth about it
- low stress on cell terminals with the inevitable relative motion that does occur
- some compression but not the high (in my opinion) 12 psi that others are aiming for
- distribute pressure across entire cell face to avoid pressure points where bare cells would make contact if pressed directly against each other
The build:
- 1/2" plywood end plates, slightly thinned out at the bottom to fit inside the tapered box. I would have used thicker if space allowed
- 1/8" foam from McMaster between each cell and end plate ("Flame-Retardant Oil-Resistant Blended Buna-N Foam").
- compression with 1/4" bungee cord. I bought 50ft of this. I hung a few weights from single length of bungee and found that at 50% elongation the tension is ~ 5lbs. So, each wrap all the way around the assembly at 50% stretch = 10lbs force. 50' bungee at 50% elongation could get me 22 wraps = 225 lbs. 225lb/56 = ~4psi. I ended up doing 14 wraps and it feels super solid. Pressure ~ 2.5 psi (with just under 150lbs total force. I can pick it up and shake it around by the loops of line of line on each end plate and nothing moves at all.
- after compressing I made 2AWG cell interconnects instead of bus bars. It was easier to do diagonal than U-shaped loops so I did that with a slight arc
- the 120A JBD BMS doesn't span the entire length of the cells so I put a plate across the top, solidly mounted to one end plate and bungeed down to the other to account for expansion/contraction. I had to raise the lid by 3/4" by adding spacers inside the lid where it sits on the lip of the box
- discharge on/off switch mounted to the lid. The BMS app is great but it's nice being able to switch the discharge off and know it's off right at the battery when making connections
- battery main studs on the sides of the box