diy solar

diy solar

Quick top balancing "Best Practice" question

Rednecktek

Solar Wizard
Joined
Sep 8, 2021
Messages
5,444
Location
On a boat usually.
Hey all, my project with my nephew is about to start and I just realized I wasn't sure about the compression aspect. It's going to be a 4s pack of the Eve 304Ah cells. When I made all of my previous packs for my brother's pop-up camper, the solar dolly and the portable diesel heater project I did the initial charge with a battery charger, then disassembled the pack and spun the cells around for the top balance, then spun the cells back around for final assembly where I actually strapped the cells together.

Looking back, I never really worried about compression, I just strapped them together real tight with tape and called it a day. Now however, if I were to leave them loose while charging/balancing, they're going to swell up, right? Once I strap them together, they're going to shrink a bit and loosen up, right? My plan for this pack is to get some used mouse pads for padding between the cells to keep them from rubbing around on each other, but that's mostly for the environment it's going to be living it.

Basically, would it be better to strap the cells together BEFORE the initial charge and then make a harness for the top balancing so everything is all nice and tight before they swell up? Or am I worrying too much about nothing significant? Calendar aging is going to be a concern LLOONNGG before cycle count is even looked at.

Thanks y'all!
 
Cells should not swell if you don't overcharge them.

Swelling is electrolyte decomposition, usually due to overcharging, although it can also happen due to extremely low discharge voltage (like near zero cell volts), or extremely high charging currents.

The cell laminate wrap in metal can cells is open at the top of laminations where metal foil terminal connections are brought out that allows any electrolyte decomposition gases to escape the top of wrap into the sealed metal can. It is just the metal can that is bloating due to gas pressure build up. The real damage is not the gas bloating, it is other electrolyte decomposition components that is hydrocarbon tars that coat the electrode surfaces increasing cell impedance.

Any benefit of compression is to help prevent delamination of electrode material from the metal foil current collector. You can damage cell with too much compression pressure which is why I am not a proponent for DIY'ers to do it. It can crush electrode granules which can leave them electrically isolated or crush the thin porous plastic separator that can cause cell to short out.

Electrolyte normally keeps separator nice and slick with oily electrolyte. If you decompose enough electrolyte and dry out an area, the separator can stick to electrode which is more likely to pull on electrode enough to separate it from metal foil.

Delamination of graphite from copper 2.jpg
 
I individually charged and discharged my 9 280Ah Eve cells unrestrained and stored them fully charged for 5 months. I never noted any swelling. There is published full charged expanded dims in the datasheet, but I didn't measure it. At no time did my cells expand in a way that I couldn't gently press them together and remove any gap.

IMHO, if cells are high quality and healthy and operated within spec, there is no risk of "bloat" or "swelling."

However, given the datasheet lists a compressed requirement for cycle life, I would certainly compress the final configuration while in operation.
 
I'll be the odd one out, I always use compression when charging. Here is a picture of the balancing and test bench. I use 2 Overkill Solar 12V BMS's, 2 power supplies and even another 12V battery charger. I arrange the cells for series connection in the fixture. The fixture is just 4 rods from the cabinet the cells were installed in this thread, plus 2 of the end plates. Rods were set for proper compression with 3mm Poron at 25% compression, nut inside each plate limits compression to the correct length. Cells are charged in series with the Overkill BMS high voltage cutoff at 3.65V on the first cell. I do burn some cells down with a light bulb if the delta between cells is high once cells hit 3.4V to speed things along. I then cheat a little, I remove the BMS and busbars then install a set of 10 gauge positive and negative wires with ring terminals to allow putting the cells into parallel using diagonals, you can see the short wires alongside the cells. Then use the power supply to finish top balance of those 4 cells to 3.65V. BMS is then installed again with busbars and I capacity test those four cells. I work my way down the line, top balancing testing and recharge. When cells are tested and recharged, I then parallel the 16 cells together using diagonal positive and negative wires to 3.65V. It doesn't take long. Only then do I break the pack down and move them to the cabinet. I recently did 64 cells this way. I installed all my cells then attached the busbars so all would balance out before turning my system on. Top balancing.jpg
 
Back
Top