DIY 16s 48v parallel wiring question?

Here is my setup. See pictures.
My main concern is when I put the batteries in parallel at the busbar, is the length of the + cable and - cable. (See pictures, green arrows are areas of concern) my positive cables are much longer than the negative cables. The BMS is my negative cable and goes directly from the battery to the -busbar. 12" blue cable and 12" black cable. The positive cables run up to the breakers and then onto the +busbar, roughly 60". Positive cables are 4/0 from battery to breakers, 2/0 from breaker to busbar.
Negative cables are 2× 4awg coming out of both sides of the BMS.
Every cable that I made is the exact same length for each section.
Is there going to be a problem with the negative wires being shorter than the positive wires?

The picture with the 2 battery cells is just to show location when fully installed, the last picture is how the battery cells will be laid out. There will be 4 total 16s 48v batteries, 2 on each side of the BMS stand. The main positive and main negative will all be facing the BMS stand.

Brettw said:

My main concern is when I put the batteries in parallel at the busbar, is the length of the + cable and - cable.

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Its ok that + and - are different. BUT, all the + wires need to be the same, and all the - wires need to be the same.

What's up with the odd 16S arrangement of cells? That would be awkward for trying to constrain/compress your batteries.

And do yourself a favor and label your cells (i like A, B, C...) because the cell position in a battery are numbers (and cell 1 is the negative end).
Take notes!

Also put a big red mark/tape/something! on the positive side of each cell. It is REALLY easy to make a mistake since black terminal is usually negative.

How are you going to choose which cells go in which battery?

MisterSandals said:

Its ok that + and - are different. BUT, all the + wires need to be the same, and all the - wires need to be the same.

What's up with the odd 16S arrangement of cells? That would be awkward for trying to constrain/compress your batteries.

And do yourself a favor and label your cells (i like A, B, C...) because the cell position in a battery are numbers (and cell 1 is the negative end).
Take notes!

Also put a big red mark/tape/something! on the positive side of each cell. It is REALLY easy to make a mistake since black terminal is usually negative.

How are you going to choose which cells go in which battery?

Click to expand...

I don't plan on compressing the batteries, I've read and watched several different opinions on the compression subject and I decided to go this route. Doing it this way puts zero stress on the terminals so I can use the busbars that came with the batteries. Off grid garage and Will Prowse have done videos on not using compression. I got this layout from Will.

Not sure what you mean by choosing cells for different batteries?

Thanks for the answer and advice on labels. Every cable that I made is the exact same length in each section. And the BMS cables all attach directly to the batteries and then to the -busbar, so those are the same length as well.

Brettw said:

Not sure what you mean by choosing cells for different batteries?

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You have 64 cells. How will you determine which 16 cells go into each of your 4 batteries? Random?

Do you have the ability to keep this test data associated to speck cells? It looks like it could be very handy for your individual notes. And, maybe there is some info that helps you form better and more uniform batteries?
If those are cell capacities, put the 16 weakest together, the next 16 together and finish with the 16 strongest together.
It may seem unintuitive but within a battery, you want your cells to be as equal as possible. You'll see when you start charging.


For compression, my belief is "firm constraint" rather than a tensioned compression.
We all have our beliefs!

MisterSandals said:

You have 64 cells. How will you determine which 16 cells go into each of your 4 batteries? Random?

Do you have the ability to keep this test data associated to speck cells? It looks like it could be very handy for your individual notes. And, maybe there is some info that helps you form better and more uniform batteries?
If those are cell capacities, put the 16 weakest together, the next 16 together and finish with the 16 strongest together.
It may seem unintuitive but within a battery, you want your cells to be as equal as possible. You'll see when you start charging.
View attachment 133088

For compression, my belief is "firm constraint" rather than a tensioned compression.
We all have our beliefs!

Click to expand...

Those specs came from overkill bms. They test each bms before they send them out.
Hopefully my cells are matched, I bought them that way supposedly ?

After they're done top balancing and have had a few days to settle, I will probably put cells of similar voltage together. Not sure if that's a good strategy, but it's all I got. I'm noticing upwards of 10mv between cells that have been top balanced, after they had a day to settle.

Brettw said:

After they're done top balancing and have had a few days to settle, I will probably put cells of similar voltage together. Not sure if that's a good strategy, but it's all I got. I'm noticing upwards of 10mv between cells that have been top balanced, after they had a day to settle.

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Yea, thats not much to go by. I spent a LOT of time trying to correlate cell settling rate and amount to cell capacity without significant results.

I had a LOT of downtime (lots of rain so no golf) and did capacity testing of 19x 280Ah cells. It took a couple months running as close to continuous as solar charging would allow. I chose the best 16 for my 16S but have no 48V equipment so in holding pattern.
FWIW, the 16 best cells from capacity testing were different than the 16 cells i selected when i got them.

Brettw said:

Hopefully my cells are matched, I bought them that way supposedly

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I have 16 x 280 ah cells and saw it’d take at least 10 days to capacity test them, so I opted out of measuring and matching.

Among my limit was a 20 amp discharge tester. 14 hours per battery. Then to charge them on a 15 amp charger.

Before delivery, I did have a picture of the batteries and an excel spread sheet emailed to me with battery capacities including internal resistance, and that data showed them all evenly matched.

I always wonder how they figure the cell capacity. I doubt the drain a 280 ah cell with a 20 amp charger and charge at 15 amps.

chrisski said:

I have 16 x 280 ah cells and saw it’d take at least 10 days to capacity test them, so I opted out of measuring and matching.

Among my limit was a 20 amp discharge tester. 14 hours per battery. Then to charge them on a 15 amp charger.

Before delivery, I did have a picture of the batteries and an excel spread sheet emailed to me with battery capacities including internal resistance, and that data showed them all evenly matched.

I always wonder how they figure the cell capacity. I doubt the drain a 280 ah cell with a 20 amp charger and charge at 15 amps.

Click to expand...

Yeah, at this point I don't plan on doing any kind of substantial capacity testing. I am at almost a month of constant charging to get them sort of top balanced. (Close, but not all the way to 3.65v) I just need to get the system running.