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Judge my battery box [images], and 4s2p advantages?

Oranjoose

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Mar 27, 2021
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I just assembled a battery box with some inspiration from the forum. This is intended to go in a vehicle application. Here's a picture of it outside of its shell:

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The cells are separated by felt purchased from an arts and crafts store. The cells are being compressed by 1 inch pine wood pulled together by 1/4" threaded rods. The foam between the rods and cells were packaging from a gas generator, and it's the exact same kind of foam that the cells were packaged in.

There's a recent thread on the forum about fire safety and it's spooked me. I hope the pine wood isn't a serious fire hazard, especially being fairly far from the terminals. I suppose I can cover the edges of the wood with kapton tape?

Here are the cells placed inside the container:

1617516341675.png

The container is a rugged plastic tote that I bought from Menards for $7.99. The cells fit great inside of it. The cells are sitting on a cut-to-fit slab of the same foam (unpictured at bottom of container) that's between the threaded rods and the cells.

I have them wired in parallel to top-balance them while I wait for the BMS to arrive. The cells didn't come with enough bus bars, so I made those two 2AWG wires just for the top-balance stage. I used an infrared temperature reader to verify that all battery terminals were reading the same healthy temps. That silly little charger is what I had on-hand, but it's a max of 2 amps, so even though I verified by clamp meter that it was working perfectly fine, I shut it off and am waiting for a 10 amp charger.

If you kind folks would, I invite critique on this build so far. Any obvious problems or recommendations? I was thinking about mounting the BMS directly to one of the wood panels. Bad idea? Fire hazard? This is supposed to go in a vehicle, so I feel like everything needs to be packed tight, but also allow for some shock absorption, right? I was planning to put the whole box on foam and rubber.

As a separate question, if I later decided I wanted more capacity, I was hoping to build another box and wiring them in parallel. Will's video and some on this forum suggest 2p4s with a single BMS for a 12v 8-cell configuration. However, aren't there some advantages of the 4s2p arrangement?

1. More modular to have two battery boxes, which could be especially handy in the vehicle application I'm putting them in.
2. I feel that mixing these will-be-older cells with newer cells in the future would be better if the sets of cells are in their own separate batteries, instead of trying to put them all in one battery.
3. Having two separate BMSs should allow for bigger loads, right? Unless I bought a higher-rated BMS if I did 2p4s, but I'll already have the one BMS, if I was going to buy another BMS, might as well get the same one for a second battery.
4. Redundancy of BMS and battery seems prudent in case something goes wrong. I'd still hopefully have one functional battery while I repaired the other one, God-willing.
5. Some argue that it's less work to make the 2p4s configuration, but if I'm starting with a 4 cell battery, then I'd argue that it would actually be more work, since I'd have to find a new container, get longer rods, etc. With 4s2p, I can just duplicate my existing battery assembly.

Final question: if I do 4s2p, wiring two of these boxes in parallel, then the wires connecting the boxes will have to be considerably longer than the busbars wiring each set of sells in series. Is that going to be a problem? For 2p4s, all the "wires" (i.e. bus bars) in the whole arrangement are all the same, extremely short length. Maybe if the wires are beefy 2AWG wires like the ones I made to replace the bus bars in that photo for the top-balance, I'm hoping that will work okay.

Any input is most appreciated!
 
I like the look of the box, it's the same design I am currently thinking of. I will be building dual 4P's for redundancy, mounting the BMS's in the end of the packs.
 
I do not think felt is substantial enough to provide protection from case wrap shorting. I would find a source for fiberglass insulating board sheets. Thin plastic cutting board sheets would also work, but fiberglass has better properties.
 
Also, most reports I have read about these aluminum cells shows not for use in mobile environments. I would want the entire setup secured to a steel box, and the steel box secured to the vehicle at a minimum.
Many posts are showing flaws with these cells. Venting electrolyte, fires, etc.
 
Also, most reports I have read about these aluminum cells shows not for use in mobile environments. I would want the entire setup secured to a steel box, and the steel box secured to the vehicle at a minimum.
Many posts are showing flaws with these cells. Venting electrolyte, fires, etc.
You don‘t need a steel box, if then aluminum….but better use nylon boards as walls as it’s isolating and put a simple fire blanket made from Kevlar (not the cheap aluminum foil ones) inside the box before putting cells in.
isolating the cells use Silikon backing mats, they are 0.4mm thick and withstand 250 degrees Celsius and don‘t burn If hotter.
nylon board box, one layer fire blanket as Kevlar is conductive, one layer Silikon backing mats as isolation and then cells in.

you see fire in big banks with a huge number of cells in series and parallel where drifting and mechanical expansion is a real problem. The BMS sees one battery but if 10 are parallel one can get overcharged very easily.
a simple battery like this with 4 cells the bms sees every cell and expansion is not a problem at all.
 
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