"Like New" LiTime 12V 100AH LeFePo4... drained to 7.9V -- BMS issue! ???

[Scoob-SanDiego]

I have a new MPP LV2424 and a pair of LiTime 12V 100AH batteries that I bought in their "Like New" section (for $203).
Setting up my MPP with 4 solar panels and my two batteries in series for the prescribed 24V battery bank.
All was working OK but my 2 batteries were barely enough to last through the night before the sun would come back.

During the initial setup, I had the MPP AC input connected to the wall outlet using a GFCI extension cord.
One morning I came in and everything was OFF. I figured I had not configured the MPP correctly and the batteries ran low (then the MPP shutoff).
When I measured my battery voltages, I noticed that one battery was at 12.19V and the other was alarmingly at only 7.9V !!!

I promptly disconnected and recharged the batteries using a separate charger.
I reviewed the MPP setup and all seemed in order. So I reconnected everything and after a couple of day the same thing happened again
with the same distressing LOW LOW voltage on that one battery.

Further investigation revealed that the GFCI breaker had popped and when the battery would reach 23V in the middle of the night, the MPP was unable to feed itself (50W) and the load (45W at night) using the line-in and instead kept on sucking on the battery.

The MPP is set (default value) with a Low DC cut-off voltage of 21.0V (the manual doesn't elaborate on what it does when that threshold is reached, cut the load or shutoff the entire MPP (which it should!)

I'm not too sure what exactly happened to deplete that battery that much.
A few things come to mind:
1) It seems WEIRD that one battery should be depleted so much deeper than the other one
2) 12.19V + 7.9V = 20.09V... Presumably the MPP "low DC cut-off voltage" would have been triggered... but if the MPP stays on, it would keep sucking 50W!
3) I would expect the BMS on the LiTime battery to shutoff somewhere between 10 and 11V. Right????

QUESTIONS:
A) Does anyone have a better understanding of what took place?
B) Does anyone know exactly what the behavior of the MPP "low DC cut-off voltage" is?
C) Am I right in believing that I have a deffective BMS on that second LiTime battery?

Thanks in advance for your input!!!
- Christian

 

[sunshine_eggo]

A, B, C)

Plain and simple.

You depleted your batteries.

The battery at 7.90 is not a real voltage. It's in BMS discharge protection mode. It will not discharge. That's likely what cut power. Its actual cells are likely in the 10.0 to 11.6V range, but the BMS is reporting an arbitrary value.

A battery at 12.19V is at most 12% charged and at this level, it's very possible imbalance in the cells means you're very very close to 0%.

So there's no evidence your batteries are in any way defective.

Did you:

Fully charge each battery individually to 14.4V for at least two hours before wiring in series?

When you received the batteries, they were:

1. Probably not charged much above 30%.
2. Not at the same state of charge as each other.
3. Likely not top balanced.

If you failed to charge them individually to 14.4V for two hours prior to wiring in series, then they were not balanced, thus the fullest battery determines when charging stops, and the emptiest battery determines when discharging stops. It's very possible that this imbalance means you have no more than 60-70% of your total capacity available.

Recommend you charge each 12V to full and then wire in series.

 

[ryankshaw]

yep, like @sunshine_eggo says, one of your two batteries is hitting it's low voltage cutoff protection.

when you said:

I promptly disconnected and recharged the batteries using a separate charger.

what charger did you use? was it something that you can verify is charging at 14.4-14.6v? because if it is a charger made for lead acid batteries (like car battery chargers), it is probably set to charge to a different voltage.

I have the exact same Li Time Batteries and just did this myself. I used a bench power supply and set the voltage to 14.6 and charged each battery separately. then put them together

@sunshine_eggo I'll jump in to ask you one question about that, since you know what you are talking about:
If you ever need to rebalance 2 batteries wired in series, can you do so by attaching the alligator clips to the + and - terminals of one battery and charging it till it gets to 14.4 and zero amps, and then attaching them to the other battery and doing the same, without disconnecting them from being in series? or is it mandatory to disconnect the series before top charging them?

 

[sunshine_eggo]

@sunshine_eggo I'll jump in to ask you one question about that, since you know what you are talking about:
If you ever need to rebalance 2 batteries wired in series, can you do so by attaching the alligator clips to the + and - terminals of one battery and charging it till it gets to 14.4 and zero amps, and then attaching them to the other battery and doing the same, without disconnecting them from being in series? or is it mandatory to disconnect the series before top charging them?

Yes. Timing and load on the 24V system can complicate it.

 

[Scoob-SanDiego]

A, B, C)

Plain and simple.

You depleted your batteries.

The battery at 7.90 is not a real voltage. It's in BMS discharge protection mode. It will not discharge. That's likely what cut power. Its actual cells are likely in the 10.0 to 11.6V range, but the BMS is reporting an arbitrary value.

A battery at 12.19V is at most 12% charged and at this level, it's very possible imbalance in the cells means you're very very close to 0%.

So there's no evidence your batteries are in any way defective.

Did you:

Fully charge each battery individually to 14.4V for at least two hours before wiring in series?

When you received the batteries, they were:

1. Probably not charged much above 30%.
2. Not at the same state of charge as each other.
3. Likely not top balanced.

If you failed to charge them individually to 14.4V for two hours prior to wiring in series, then they were not balanced, thus the fullest battery determines when charging stops, and the emptiest battery determines when discharging stops. It's very possible that this imbalance means you have no more than 60-70% of your total capacity available.

Recommend you charge each 12V to full and then wire in series.

So If I get your right, once I think my 12V battery is already charged (say by connecting two batteries in series and charging them with a 24V LifePo4 charger), THEN I would connect each individual battery on a bench power supply and leave it at 14.4 for at least 2 hours??? Can I leave them on the PSU for too long or leaving them at 14.4 will just maintain them?

I remember seeing one of Andy's offgrid garage video explaining the lifePo4 charging curve but it got lost in the million of solar info I have been ingesting these last few weeks LOL. And he doesn't have much of an index to his insanely large collection of videos.... do you have a good reference (URL) to the charge cycle of the lifepo4 battery, prescribed voltage, duration, curve, explanation of the various stages of the curve etc?

Thank you so much for the heads up!
- Christian

 

[sunshine_eggo]

So If I get your right, once I think my 12V battery is already charged (say by connecting two batteries in series and charging them with a 24V LifePo4 charger), THEN I would connect each individual battery on a bench power supply and leave it at 14.4 for at least 2 hours???

This would work. Loads on the 24V system should be minimized. Best to shut down if possible.

Can I leave them on the PSU for too long

Yes, but we're talking weeks/months.

or leaving them at 14.4 will just maintain them?

The purpose is to ensure the battery is held at elevated voltage, so the BMS can balance the cells.

I remember seeing one of Andy's offgrid garage video explaining the lifePo4 charging curve but it got lost in the million of solar info I have been ingesting these last few weeks LOL. And he doesn't have much of an index to his insanely large collection of videos.... do you have a good reference (URL) to the charge cycle of the lifepo4 battery, prescribed voltage, duration, curve, explanation of the various stages of the curve etc?

you have options:

Fast charge: 14.4V, hold for 30 minutes, float at 13.5V
Slow charge: 13.8V, hold for two hours, float at 13.5V

The slow charge lessens the peak voltage and achieves peak voltage at a lower current. This is generally regarded as optimal for maximum cycle life and to provide additional time for cell balancing.

Once balanced, a 12V balancer will help keep the 12V at the same voltage. Something like this:

https://www.amazon.com/dp/B09GLMCNXK

 

[Scoob-SanDiego]

This would work. Loads on the 24V system should be minimized. Best to shut down if possible.

Yeah, no problem... my system operates with 2 x 12V batteries and I just got a second set of 2 x 12V. so I will:
1) Maximum fill my backup pair while they are off the system.
2) Swap with the live pair and then maximum fill my original pair while off line
3) Pull all the batteries off line and maximum fill everything,
4) THEN connect them back in the live system.

BTW:
Is it better to do 2x 12V batteries in parallel then connect those in series OR to make two 24V batteries THEN put those in parallel?
In terms of voltage and power it's theoretically all the same... but I figured it's best to ask

I just ordered some cheap battery volt meters so I can easily keep an eye on the individual battery voltages for a while.
https://www.amazon.com/Battery-Setting-Capacity-Indicator-Monitors/dp/B08W47GYJ2

And if I observe that it drifts, then I will get the following LiTime battery balancer:
https://www.amazon.com/Battery-Equalizer-Balancer-Supports-Trolling/dp/B0CLTZ2XQX

Thank you for all the feedback and info!

PS: Any link to a page with full info / graph on LifePo4 battery charging cycle?

- Christian

 

[sunshine_eggo]

Yeah, no problem... my system operates with 2 x 12V batteries and I just got a second set of 2 x 12V. so I will:
1) Maximum fill my backup pair while they are off the system.
2) Swap with the live pair and then maximum fill my original pair while off line
3) Pull all the batteries off line and maximum fill everything,

#3 isn't really necessary.

Steps
Top balance charge 12V in string 2 (backup), put in 24V service.
Top balance charge 12V in string 1 (current).
Wait until both strings are within about 0.2V of each other and put in parallel with string 2 for 2S2P 25.6V battery bank.
charge to 28.8V and hold for a couple hours.

Cycle normally spending at least 1-2 hours per day above 27.6V.

4) THEN connect them back in the live system.

BTW:
Is it better to do 2x 12V batteries in parallel then connect those in series OR to make two 24V batteries THEN put those in parallel?
In terms of voltage and power it's theoretically all the same... but I figured it's best to ask

Best to be done string by string. You could technically put them all in parallel and hold 14.4V for 8 hours, then build your 2S2P bank.

I just ordered some cheap battery volt meters so I can easily keep an eye on the individual battery voltages for a while.
https://www.amazon.com/Battery-Setting-Capacity-Indicator-Monitors/dp/B08W47GYJ2

And if I observe that it drifts, then I will get the following LiTime battery balancer:
https://www.amazon.com/Battery-Equalizer-Balancer-Supports-Trolling/dp/B0CLTZ2XQX

Looks good.

PS: Any link to a page with full info / graph on LifePo4 battery charging cycle?

I posted this recent thread. One of the final images shows an actual charge profile. As you can see there's really no absorption time unless you're charging at high C rate.

Pseudo self-discharge exercise

Battery: 4S 25Ah Topband Navitas 3C cells. 120A JBD BMS UART to USB adapter from Overkillsolar Bluetooth module uses the BMS UART connector, so it was not installed. VRM: Raspberry Pi 3 B+ 1GB (RPi) 256G MicroSD Venus OS 3.11 start, 3.12 end @Louisvdw serial battery driver There were NO LOADS...

diysolarforum.com

Highly similar in concept to lead acid except almost all the charge is taken on below 13.8V for LFP vs. 20% of the lead acid capacity taken on at 14.4V.