Idiot's guide to a self build battery

[MrPablo]

I would suggest that the Black B.M.S. cable is not tye rapped in the bundle with the positive cables from each cell. B.M.S. fault and lots of current could flow. I have also soldered a fuse / holder to each cell cable.
I might be a bit over the Top. ?

The balance leads are 22awg, others have stated they're basically self fusing at 15a. I could add in fuses per wire but I still have a good few balance wire offcuts. It could be an interesting experiment to see exactly when they fail.

[ALAN/ALAN D]

Ref “ others have stated they're basically self fusing at 15a. “

From a fire risk point of view I went down fuse holder / glass fuses. Best to have molten metal inside a glass enclosure.
I bought some 40 amp bolt on fuses with a plastic enclosure around the fuse section. Done a test and the plastic caught fire / burnt away. They are now all in Landfill.

[marshman]

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[ALAN/ALAN D]

Training course needed. LIFEP04 battery.
Compression of cells to increase battery life. ?


People seem to Compress all 16 cells side to side.
But not all 16 cells end to end.
Even if you compress side to side and end to end.
They can expand top to bottom.

If they are compressed the cells are still joined together at the terminals with solid copper bar links.
Which will cause failure where the studs go inside the battery.
Should they have flexible links between cells. ?

Why are they not made in a more solid case so that they do not need compression.
If they are made in solid case. They will still expand at the top / terminals. ?

Is there any information from the battery manufacturer about how to best look after them. ?

I Dooo want them toooo last.

Checked smart meter readings over last three days.
No energy imported.

[marshman]

EVE LF280K Product Specification sheet

It is the first one I have seen with specific notes on "compression". I have attached the relevant pages below - sorry about the "watermark" but couldn't attach a PDF.

It only mentions compression of the "sides".

The sides are by far the largest in area and without compression the large sides do bulge. The top/bottom and "other" smaller "sides" are much smaller in area so less total "force" to make them bulge and I also suspect (don't know for sure) that the construction of the cells means that any expansion pressure is to the sides.

Some research found that a torque of around 8 in lbs (approx 0.8Nm) gives a compressive force of 165lbs - x 4 (four bolts) total of 660lbs over the side of the cell gives around 12lbs/sq inch = as per the EVE spec.

Having said all this, there are loads of packs out there running uncompressed with rigid busbars. So probably comes down to your own gut feel, and on how much time and money you have to throw at the project.

[Stinsy]

Training course needed. LIFEP04 battery.
Compression of cells to increase battery life. ?


People seem to Compress all 16 cells side to side.
But not all 16 cells end to end.
Even if you compress side to side and end to end.
They can expand top to bottom.

If they are compressed the cells are still joined together at the terminals with solid copper bar links.
Which will cause failure where the studs go inside the battery.
Should they have flexible links between cells. ?

Why are they not made in a more solid case so that they do not need compression.
If they are made in solid case. They will still expand at the top / terminals. ?

Is there any information from the battery manufacturer about how to best look after them. ?

I Dooo want them toooo last.

Checked smart meter readings over last three days.
No energy imported.

There is a lot written about compressing the cells and some people get very tetchy about exact forces. The science isn’t settled however there are a few things that we can be pretty confident about.

- An enclosure that resists expansion is desirable. Some initial tension is beneficial to longevity too.
- The batteries will naturally expand on their largest side. Top/bottom and small-side expansion isn’t as problematic.
- The cells have been manufactured with only an ultra-thin lightweight, casing. You are expected to provide a casing suitable to the application.
- Many link bars have one hole elongated to accommodate expansion however this obviously requires the pack to be isolated and the terminals loosened/retorqued periodically.
- The importance of all this depends on how the batteries are used. The applications discussed on here apply very little stress to the cells. We’re talking 0.25C (16s 280Ah 3.6kW) with most installations I’ve seen discussed here having a much lower “C”.

[marshman]

There is a lot written about compressing the cells and some people get very tetchy about exact forces. The science isn’t settled however there are a few things that we can be pretty confident about.

- An enclosure that resists expansion is desirable. Some initial tension is beneficial to longevity too.
- The batteries will naturally expand on their largest side. Top/bottom and small-side expansion isn’t as problematic.
- The cells have been manufactured with only an ultra-thin lightweight, casing. You are expected to provide a casing suitable to the application.
- Many link bars have one hole elongated to accommodate expansion however this obviously requires the pack to be isolated and the terminals loosened/retorqued periodically.
- The importance of all this depends on how the batteries are used. The applications discussed on here apply very little stress to the cells. We’re talking 0.25C (16s 280Ah 3.6kW) with most installations I’ve seen discussed here having a much lower “C”.

Agree with all of that apart from the last line............."The applications discussed on here apply very little stress to the cells. We’re talking 0.25C (16s 280Ah 3.6kW) with most installations I’ve seen discussed here having a much lower “C”.

The expansion is mainly dependent on S.O.C. and not how "hard/fast" you charge/discharge - though I accept temperature must have an effect as well. The EVE spec. sheets specifies a certain compression force at 40% S.O.C., then specifies a maximum "pressure" at 100% S.O.C. Batteries packs used for energy storage are regularly cycled down to 10 to 20% and then charged to 100% - so lots of potential for movement. The expansion occurs every cycle.

[Fintray]

With regard to the busbars, I had the same thought about stress on the terminals. So I went for "flexible" busbars. Not cheap but they are far superior to the ones supplied with the cells. Large cross sectional area of copper and no "hot spots". When I built my first 28kWh pack (2p16s) I noticed a few of the busbars getting warm to the touch at 100A - warm in the centre section - I confirmed this with a thermal camera. The flexible ones stay stone cold - no detectable rise in temperature.

Out of interest, what busbars did you end up using?

[marshman]

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[Fintray]

Cheers Marshman.