Lifepo4 batteries discharging unevenly

Of course, such a bank should not be built by paralleling strings, imbalance issues are inherent in such layouts.

3.2V cells at the target Ah capacity, all in one string is preferred, each cell gets monitored and balanced properly by the one master BMS. In this case 16* 100-150Ah cells in series.

If not possible (why not?) then put the lower-Ah cells in parallel groups at the bottom level, then connect in serial to get to the desired voltage, e.g. 2P16S, 2*60Ah *16 in series.

But then you're not able to monitor at the per-cell level.

Having multiple "sub BMS" connected without central coordination increases the complexity geometrically, unreliable may even be dangerous.

What are the exact batteries used? BMS? Links please.

Who advised you these units could be used this way?

Also a more detailed diagram of the wiring layout. For example are these BMS common-port type, or separate ports for charge vs discharge?

John61ct
Thanks for the reply.

I should have mentioned this system layout worked good for 3 months in my boat then all of a sudden the 2 opposite batteries started to discharge differently.

Each GTK 24v battery (from China Attached picture) is in it's own Stainles Steel Case with it's own BMS I do not know what kind of BMS in in each battery. I do have a diagram sent by the battery company. attached is the diagram for a 48v BMS the 24v version is the same just has B1-B8.
As with most Chinese companies comunication is not good. Each battery pack has its own 24v-10amp charger.
Specks on battery below.

Thanks John

Specification
1 Charge cut-off voltage DC 29.2V
2

Nominal voltage 24V
3 Nominal capacity 60Ah
4 Continuous discharger current 80A
5 Charge current Standard charge: 10 Approx 6 Hour
Rapid charge: 20 Approx 3 Hour
6 Standard Charging method 0.2 CC(constant current) charge to 73V, then CV(constant voltage 58.4V) charge till charge current decline to less than 0.02C
7 Max. charge current 60A
8 Max. discharge current 120A
9 Max.Pulse Current 100A(2S)
10 Discharge cut-off voltage 50V
11 Cell 3.2V Lifepo4
12 Operating temperature Charging: 0°C - 45°C
Discharging: -10°C- 60°C
13 Storage temperature -10°C to + 45°C
14 Battery Weight Approx. 11 kg
15 Battery Dimension 175*225*265 mm
16 Battery outgoing voltage 20V-29.2V

Lots of very poor makers & suppliers in this market niche.

I would never buy just based on price, only from top quality makers through well-recommended and widely trusted suppliers.

Ideally located on and shipped from the same continent as me.

And never a battery where an unknown BMS is embedded in the battery, difficult to isolate, remove and/or replace if it proves bad.

Routine access to individual cells is essential.

Systems: OPE-Li3 (Lithionics/Ocean Planet), Victron, MasterVolt, Redarc (Oz specific?)

Bare cells: ​Winston/Thundersky/Voltronix, CALB, GBS, Sinopoly and A123 (now Lithium Werks / Valence / Super B)

You are isolating each battery to charge right?

If not, are those chargers electrically isolated?

If not, they should probably not be put in series like that.

There are chargers explicitly designed to work stacked in parallel together, but in series is pretty rare.

A single charger at 55-57V would be the usual way to charge such a bank,

**if** the batteries were designed to be parallel/serially connected like that.

I did exspect a bad cell or BMS, just did not think I could get 2 at the exact same time.

As far as cost @ the time these batteries packs cost $750 each and I chose them because of the S Steel cases and built in BMS. Link below.

I am isolating each battery when charging each has it's own AC charger as well as MPPT solar charge control all are isolated and fused in & out.

The BMS on each battery will not let me charge with 48vdc.

I can take apart the batteries and extract the BMS and could change it if I need to replacing seperate cells are a bit harder they are lap top battery cells the link has a picture of how they are welded together.

If I do change the BMS for each battery what do you recoment as a good source?
Thanks John

Here is a link to a NPR story about my solar electric sailboat. https://www.wbur.org/hereandnow/2019/09/23/climate-solar-powered-electric-boat-motor

Link to GTK battery on Alibaba
https://www.aliexpress.com/item/32845144671.html?spm=a2g0s.9042311.0.0.27424c4dxC0caJ

I'm not saying anything failed, I think you need a knowledgeable person to do a proper system level design of your 48V battery bank.

Did you rely on the Ali seller rep to advise you? They very often have no clue.

sakara777 said:

the link has a picture of how they are welded together.

Click to expand...

I see no such pic, why not just post it here, likely in future all these links will break anyway.

By "laptop" cell you mean those little cylindrical ones, 18650 size for example?

At 8S to get to 24V, say rated 2.5A each would be 8P as well, so maybe 64 cells each * 4 total 265 cells needing re-welding.

For 48V you need 16S so if above count is correct, 16P each 3.2V "group", will act as a single big cell.

If re-using the nice box, each gets 4 of these groups, will need another central box for the new 16S BMS

(I don't have specific reco there, but can help with general selection criteria)

plus wiring, four pair connecting to each box.

This the sort of project you want to learn / DIY?

The spot welder might be a couple hundred, for not much more, you might be able to find someone do it for you.

Or test and see if they're OK, and sell the old and buy new?

Note that a BMS expert may come along and specify 4* ones better suited to this 2S2P interconnection of whatever is inside each box.

I'm basically a noob to this stuff, and there's always multiple ways to skin a cat.

Hi
All of the "New Batteries" have a very high failure rate - even the "expensive ones"

You are a LOT better off getting a battery from a crashed EV

The level of quality control by the car makers is simply light years ahead of the sort of things that are sold as
New Batteries"

AND they are cheaper!!

I'm using most of a Chevy Volt pack - 16 kWh cost me $1800US

Here is a thread
https://www.diyelectriccar.com/forums/showthread.php/2012-chevy-volt-battery-93101.html

I know it's long but it's well worth reading

You can also get Nissan Leaf batteries and modules and Tesla modules
But I have no personal experience with them

Exactly.

LFP's power density is OK, but energy density maybe half of what the recent LI chemistries are getting.

But they are **very** thermally unstable (boom bad),

much shorter-lived cycling lifetimes

and neither of the density factors are critical with the OP's use case,

in which I think LFP is the right choice.

All
Thanks for your response I admit I do not come from an EV background but from a off grid solar one. We have used LFP batteries in off grid systems and they have performed well but not this brand.

At this point either I sell these as separate 24v packs and go with a rebuilt EV battery or I salvage the cells and configure them in to 2 separate 48v – 60ah packs with a new BMS and new cases.
If I do reconstruct the cells what companies do you recommend for a good BMS @ 48v.

If I do go this route can I create 2 - 48v 60 ah packs with separate chargers for each yet configure them in parallel to get 120ah. (remember my primary charging system is a MPPT solar charge control) plus a 120vac-48vdc 10amp charger if needed.

Or keep them separate and switch between packs when I need power to run the boat.
Thanks John