Test Lifepo4 batteries under load - voltage drop

[Owlbawl]

Hi, tried to search forum but didn't find clear answer for me.
Had to use Solar inverter with LiFePo4 battery attached to it because of regular power outages in my place.

Would appreciate for your help and advice!

Just bought Must PV18-3024 (3kW) solar inverter on Aliexpress, where I've also got the Lifepo 24V 100Ah for it from "KEPWORTH".





Battery was once tested with 100A load (2400W) for 1 min (had flashing low voltage alert on inverter, but hold).
For now I had two ~50% discharges on 0,1-0,2C for about 3-4 hours outages.
Battery is connected with 25mm2 ( 4AWG ) copper cable.

The thing is kinda make me worry - is the state of the battery I bought.
When inverter switch to battery - both times I saw starting voltage 26,2 and 25,8 at the end of cycles. With 200~400W average load (about 0,1-0,18C).

Made manual load test (can't attach the screen video I've made):
Made control measurement with voltmeter on battery terminal with it disconnected from inverter, was showing 26,7V at start (battery not fully charged).
The inverter itself shows 26,6V
Switched off the grid - load 200W - voltage dropped to 26,2V // 250W - 26,1V
When I connect 1000W load - I see the voltage drop from 26,6 to 25,6.

Is it normal behavior for "new" LiFePo 8s with BMS? Or it could be showing it made of degraded / used cells?

The case is glued, so no chanse to measure the inner resistance / voltage on cells without damage disassembly of it...

 

[sunshine_eggo]

4awg is too thin for 100A.

Triple check all connections for quality and proper torque.

You've already measured the system resistance:

P = I * V

1000W / 25.6V = 39A

V = I * R

R = V / I = (26.6V - 25.6V) / 39A = 25mΩ

This is about 5X higher than a quality lead-acid battery. You have thin wires and possibly a bad connection, or a bad battery.

 

[Owlbawl]

4awg is too thin for 100A.

Took it from the inverter manual and checked with DC calculators I found... The cable is just 1.4m long. What is recommended for 100A 24V or point where check it again please.

But, anyway, the test was made with 42Amp load, which is much less.

Made the new tests just now with ampermetr directly connected to batt. terminals.
27,05 - 26,40 = 0,65V drop with 911W (wattage measured by inwerter, not sure losses are included)

And second run 42Amps
26,99 - 26,25 = 0,74V drop with 999W (wattage measured by inwerter, not sure losses are included)

If calculated correctly - its 20 mOhms for whole 8s battery with BMS... Is ok?

 

[MisterSandals]

both times I saw starting voltage 26,2 and 25,8 at the end of cycles

Your battery at 26.2V is about 60% charged. 25.8V is about 30% charged.

View attachment 128792

The cable is just 1.4m long. What is recommended for 100A 24V or point where check it again please.

 

[sunshine_eggo]

Took it from the inverter manual and checked with DC calculators I found... The cable is just 1.4m long. What is recommended for 100A 24V or point where check it again please.

But, anyway, the test was made with 42Amp load, which is much less.

I ran the number for a 39A load.

Made the new tests just now with ampermetr directly connected to batt. terminals.
27,05 - 26,40 = 0,65V drop with 911W (wattage measured by inwerter, not sure losses are included)

And second run 42Amps
26,99 - 26,25 = 0,74V drop with 999W (wattage measured by inwerter, not sure losses are included)

If calculated correctly - its 20 mOhms for whole 8s battery with BMS... Is ok?

Did you check every single connection?

 

[Owlbawl]

Your battery at 26.2V is about 60% charged. 25.8V is about 30% charged.

View attachment 128792

Cant see the attachment. I have the feeling the inverter lies... would check voltage next time with voltmeter directly..

Thank you, counting on this - my wire is ok then. 4AWG = 25mm2 I used with copper hydraulic cable crimping on the ends.

 

[Owlbawl]

Did you check every single connection?

Yes. I even ran one test withot DC circuit breaker with direct cabling connect.
And measurements were made directly on batt. terminal with voltmeter.
The batt. spec resistance =<20mOhm i found on paper included...

 

[MisterSandals]

Cant see the attachment. I have the feeling the inverter lies... would check voltage next time with voltmeter directly..

Fixed attachment.
Voltage needs to be battery at rest (not being charged or discharged).

I ran the number for a 39A load.

Can you point me to your chart? Link or attachment.
Curious as to why the difference between your chart and the one i have.

 

[Owlbawl]

Thats hot... on full load. Seems it's better to switch to 35mm / 2 AWG

I ran the number for a 39A load.

Yes, it's what inverter shown itself

 

[Owlbawl]

Can you point me to your chart? Link or attachment.
Curious as to why the difference between your chart and the one i have.

they both looks correct, but one showing the temp, second counts the length )
who'd connect them )

 

[sunshine_eggo]

Well, my 48V DIY battery bank with 294 cells and 588 threaded connections, 28 aluminum bus bars, many feet of 2awg cable and undersized cell interconnects has an IR of 9mΩ, and I'm thankful given the ungodly number of connections.

20mΩ seems outrageous to me.

Source (this draws from the national electric code, marine grade cable can handle more, but you can get excessive voltage drop):

American wire gauge - Wikipedia

en.wikipedia.org

 

[Owlbawl]

20mΩ seems outrageous to me.

had to grab what was on market and for adequate cost with current shortage, spent 690$ on it.
the only way to figure out deeply is to damage and disassembly it, what is bed for guarantee if it real

 

[Owlbawl]

So, I found out how to disassemble the battery carefully, it's slide-to-click )
Can't find out the BMS used, but I doubt it has a temperature control. Maybe someone could help?
The cable used for terminals inside is 7AWG ~10mm2 / 200C
The cells seems to be 8s2p 50Ah pouches.

 

[Alkaline]

Well, my 48V DIY battery bank with 294 cells and 588 threaded connections, 28 aluminum bus bars,

Ok we need to see some pics of this beast.

 

[sunshine_eggo]

Ok we need to see some pics of this beast.

NMC Bank, Bus Bars and Batrium Love

It has been years in the making, but I finally have the NMC battery poised for deployment... (okay, only half, but still). Summary: 7X Ford Fusion/C-Max Energi PHEV batteries, 7.6kWh rated per pack. Each battery: 84S 25Ah 3.7V NMC cells. Four subpacks of 21S in series Packs have tested...

diysolarforum.com

 

[Alkaline]

Ok well I wasn't quite expecting that but yes...you have a lot of cells ?