?When to compress?

[Lasertrapper]

Very much appreciate feedback....

I'm building qty 4 4s4p packs using 302ah CATL prismatic cells and 4 JK 200a BMS for our Sprinter van, so size is important.

I want to mechanically connect the 4 cells in a stacked "L" shape and incorporate into an aluminum 80/20 structure that partially surrounds the wheel well, and will be at the bottom of full height cabinet.

I'm using a sheet of Poron foam in between the cells to provide compression force & using flexible busbars.

I have the prototype cell enclosure about 70% done (see pictures) and my cells are all sitting at 3.30v, ± .01v

My question is, at what point should I compress the cells?

Should I compress, then top balance? Or, vice versa?

 

[robby]

You don't want to compress the cells.
What you want to do is drain them to 20% SOC and then fixate them in place but with no extra force than what is needed to snug them together. Then when you charge the batteries back up they will try to expand but they cannot expand.

 

[Lasertrapper]

Thanks so much!

The cells are all sitting at 3.30v, is that close enough to 20% SOC?

 

[MisterSandals]

The cells are all sitting at 3.30v, is that close enough to 20% SOC?

Hang on to this, its a good reference,

 

[Lasertrapper]

Perfect, THANKS!! I looked all over for data specific to the CATL 302ah & was unsuccessful.

 

[MisterSandals]

I'm building qty 4 4s4p packs using 302ah CATL prismatic cells and 4 JK 200a BMS

Assuming you mean 4P4S ... and 4 of those? Wow!

Are you planning on matching the 4P groups of cells with each other so that each groups capacity (capacity of all 4 cells) is equal to all other groups capacity?

This would be a LOT of work but would give you the best possible batteries.

And what on earth are you powering with that?
3.2V x 64 x 302Ah = 61.8kWh

How will you charge this? 48V?

 

[Lasertrapper]

I have total of 16 cells, 4 BMS. Planning groups of four 3.5v 302ah cells in series, 14v 300ah per pack, then wiring the four 14v packs in parallel, so total 1200ah at 14v. If I have a bad pack or BMS on the road, I can disconnect it and run on 3 packs, or 900ah.

Wife wants to run the roof air conditioner overnight if we boondock for a night.

Charging via shore power, alternator, and solar.

 

[MisterSandals]

I have total of 16 cells, 4 BMS

Ohhh! I saw "qty 4 4S4P" and read that as a schload of cells!

If I have a bad pack or BMS on the road, I can disconnect it and run on 3 packs, or 900ah.

True but running in parallel it might be hard to notice if one pack goes bad. I dunno. Thats an awesome battery bank.

 

[robby]

Nice bank of batteries, I am curious as to what you are planning to use to charge them?

 

[Lasertrapper]

Nice bank of batteries, I am curious as to what you are planning to use to charge them?

Thanks! Charging via shore power (Victron Multiplus II), alternator (Victron MPPT), & solar TBD. Will probably add Nations second alternator at some point.

 

[MisterSandals]

Charging via shore power (Victron Multiplus II), alternator (Victron MPPT), & solar TBD.

You should get as many charge cycles on these 4S packs as you can before what looks like an involved install. Then, test the batteries as 4P to make sure your wiring is balanced and equal (equal length wires and/or bus bars).

I've mess with cells across batteries for months at a time getting the cells in each battery as equal as possible. I'd swap low capacity of one with high capacity of another taking comprehensive notes.
I am finally doing capacity tests on 19x 280Ah cells which will make arrangement more precise but it takes a long time (into my 3rd week, hampered by clouds in front of my power source...).

Long story short, don't expect to just plop them in your awesome enclosures, bolt them into place and be good to go.

 

[Lasertrapper]

Ohhh! I saw "qty 4 4S4P" and read that as a schload of cells!

True but running in parallel it might be hard to notice if one pack goes bad. I dunno. Thats an awesome battery bank.

Wow, you have my respect!!

I did pay for "matched & batched" cells from Docan but haven't tested anything other than voltage.

I have the EBC-A40L load tester which discharges & charges at 40A, but I'm thinking I'll just top balance the cells 4 at a time in parallel, then connect in series. Thoughts?

The JK BMS I'm using have 2amps of active balancing, and Andy from off-grid garage on youtube likes them.

 

[MisterSandals]

I have the EBC-A40L load tester which discharges & charges at 40A, but I'm thinking I'll just top balance the cells 4 at a time in parallel, then connect in series. Thoughts?

Since you have the tester and the cells right there, I'd test a few cells to see how well matched they are in terms of capacity. Especially if you are not pressed for time and have other things to do in the mean time.