16S BMS for a 16 cell 24v battery?

[Mjc4924]

I built a 300ah 24v lifepo4 battery using 16 150ah cells and used a Daly 8S 250 amp BMS. So each cell lead is wired to two cells in parallel. The battery works ok but the it cuts off at 24.4V. I haven’t looked to see what the individual cell voltage is.
I’m wondering if I should use a16S BMS. Does it matter that the cells are wired to provide 24v instead of 48v?
Or should I use 2 BMS units?
Last question is, can the cut off voltage of the BMS be adjusted somehow?

 

[Steve_S]

16S is NOT for 24V, 8S is what is used for 24V.
Cells in Parallel ONLY work if they are properly Matched & Batched, Bulk Commodity cells do not do well in parallel, they diverge too much.
Many thing can contribute to such issues. Often these ARE contributors, it's common.
Cells unbalanced, never top balanced.
Poor connections with busbars, because of dirt, oil, waxes, or burs & ridges on busbars.
Not properly tightened to spec, loose / poor connections.
1 or more weak cells or runners may be in the pack, these cause serious havoc if paired with a strong cell.

 

[TomC4306]

Deleted

 

[Mjc4924]

Thank you Steve. That answers my questions. So I should essentially have two 24v 8 cell batteries each with it’s own bms then put those in parallel to double my capacity.

 

[rhino]

Check the resources section here on different options for configurations. Click the download link at top right: https://diysolarforum.com/resources/cell-configurations-for-12v-24v-and-48v-lifepo4-batteries.42/

 

[Bob B]

Also .... make sure to top balance the cells before putting them in series .. or you will have the kind of problem you are describing.
Some cells will be fully charged before the others causing the BMS to shut off before the pack voltage gets as high as you want.

 

[Steve_S]

@Mjc4924 most of the info you want & need is in this document which I prepared for Luyuan Tech a few months ago. There is Preparation & some troubleshooting info in it as well.

Luyuan Tech Basic Lifepo4 Guide

This document was posted to the forum by Amy from Shenzhen Luyuan and written by Steve S. It was meant to be posted as a resource. Below is her description of the resource: Dear all friends in the forum, This file in the attachment was...

diysolarforum.com

With 150AH cells, you can discharge at a Max Rate of 1C or 150A for one hour and take a MAX of 75A for Charge input for two hours. NB, At those rates the cells will warm up noticeably.

I would suggest a BMS which can handle 150A out & 75A in, we generally recommend having a "safety margin" to not take a BMS to its operational "edge", commonly suggested to oversize a BMS by 20-25%. A fuse at 105A would be a perfect fit as well.

With 24V based systems, you can use MRBF fuses (All my packs have them, I gave on the Little Fuse T-Fuses (they can break apart)..
REF Link to one of member vendors, good folks!

Amazon.com

Good Luck & Have Fun
Steve

 

[MisterSandals]

So I should essentially have two 24v 8 cell batteries each with it’s own bms then put those in parallel to double my capacity.

You can still make a single battery from 16 cells in a 2P8S configuration and use a single 8S BMS.
I do this with 8 cells to make a 2P4S battery with a 4S BMS.

You'll be monitoring the parallel pairs so making the pairs equal to one another is beneficial. So maybe you have the strongest paired with the weakest, second strongest paired with second weakest... Its the buddy system with pairs of cells working in unison.

And it does not make any difference in regards to capacity how you arrange them.
You will end up with 300ah x 3.2v x 16 = 15360wh in every configuration.
You can vary the voltage which will change the amp hours but the capacity will be constant.

 

[Mjc4924]

Thank you all! From Steve’s article I see that at 24.4v I’ve used most of the batteries capacity anyway. What I really have to do test the cells that are in P and make sure everything is balanced. The I should be able to maximum capacity before cutoff.

 

[Steve_S]

Thank you all! From Steve’s article I see that at 24.4v I’ve used most of the batteries capacity anyway. What I really have to do test the cells that are in P and make sure everything is balanced. The I should be able to maximum capacity before cutoff.

Before anything, top charging & top balancing the cells to make sure they are all at the same "starting point status". You test individual cells with a cell tester machine or a whole pack (with BMS Attached) for a pack capacity test.

See here:

Battery Capacity Testers

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www.mobile-solarpower.com

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CBA IV 4 Computerized Battery Analyzer

Scientifically Analyze Battery Performance. Test any type of battery. Test quickly at real world load conditions, 40 amps max. Results are graphically displayed with easy to use software. USB interface. Built-in safety features for battery protection.

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