Are my batteries cooked?

[Steve Dally]

Hey all, hoping you can help me out here...
I built up a 560 ah lifepo pack for my van about 4 years ago and it has worked perfectly until just a couple weeks ago. It still charges all the way up to 14.2 volts with good balance etc, but then after a full charge the voltage drops down to 13.4 after just 10 minutes or so and then seems to deplete over the course of two days off grid instead of a week... I can't think of any thing I might have done to abuse them... My BMS should protect them, right? I have them adequately heated, etc. They've gotten quite a bit of use, but nothing near the 100,000 gazillion cycles they're capable of.
I guess my main question are these:
is this the typical sign that the battery is cooked?
What kinds of abuse might I be overlooking that could be avoided in my new build?
Is 4 years of weekly use about the lifespan of lifepo (i don't think so)
Could it be a faulty bms (nothing has seemed fishy about it)
Anyway, thanks in advance,
Steve

 

[Kornbread]

Can you see individual cell voltages?

 

[robby]

Do you have a shunt to see how much power is going in and what is coming out.
You can then compare that to what they batteries are rated for.

 

[justgary]

Do they get really hot in the summer?

 

[Brett V]

That sounds more like a load draining the battery.

 

[sunshine_eggo]

Hey all, hoping you can help me out here...
I built up a 560 ah lifepo pack for my van about 4 years ago and it has worked perfectly until just a couple weeks ago. It still charges all the way up to 14.2 volts with good balance etc, but then after a full charge the voltage drops down to 13.4 after just 10 minutes or so and then seems to deplete over the course of two days off grid instead of a week... I can't think of any thing I might have done to abuse them... My BMS should protect them, right? I have them adequately heated, etc. They've gotten quite a bit of use, but nothing near the 100,000 gazillion cycles they're capable of.

Definitely respond to the 3 questions.

I guess my main question are these:
is this the typical sign that the battery is cooked?

Without knowing more details, it's hard to say. Definitely need cell data.

What kinds of abuse might I be overlooking that could be avoided in my new build?

Heat

Is 4 years of weekly use about the lifespan of lifepo (i don't think so)

If operated in nominal conditions, no.

Could it be a faulty bms (nothing has seemed fishy about it)

Maybe.

 

[SomethingAppropriate]

Perhaps they are not balanced.

Take your bets now.

 

[wpns]

still charges all the way up to 14.2 volts with good balance

How do you know the cells are balanced?

Where are you, and what are your temperatures like?

 

[Steve Dally]

Do they get really hot in the summer?

Not all that hot... I'm in the Seattle Tacoma area so...
But it does heat up in the van on a summer day, 120 Fahrenheit, maybe?

 

[Steve Dally]

Can you see individual cell voltages?

Yes, right now, for example, the pack is 13.3, and the cells are 3.319, 3.320, 3.320, and 3.321, so an imbalance of .002
The battery was charged to full capacity last night and has just been sitting in driveway... 10 hours with just the fridge and heater (diesel espar heater) running and a few charging devices, so between 3 and 6 amps

 

[DougL]

Were your battery cells new when you got them? Also not a good sign that you don't know how hot it gets in your van or how hot the batteries got.
Heat is the primary killer of Li batteries. You should have had a solar panel on the roof directly operating a fan which pulled out hot air inside the van and reduced the heating. I'd also want to validate the voltages on the cells were exactly as the BMS shows since 3.55V top charge is very near the max voltage I've seen spec'ed on LiFePo4 batteries(3.65).

 

[Steve Dally]

Can you see individual cell voltages?

That sounds more like a load draining the battery.

I don't think so... I can see the current load with my Bluetooth app, and I don't see any unexplained load. Right now, for example, I can see that I've been drawing 5 amps or so for 10 hours (heater, fridge, chargers) and it shows that I've used 50 amp hours. That all seems right, but my voltage is down to 13.3, so it seems like my capacity is suddenly way less than 560 amp hours...

 

[SparkyJJO]

Yes, right now, for example, the pack is 13.3, and the cells are 3.319, 3.320, 3.320, and 3.321, so an imbalance of .002
The battery was charged to full capacity last night and has just been sitting in driveway... 10 hours with just the fridge and heater (diesel espar heater) running and a few charging devices, so between 3 and 6 amps

Your in the middle of the SOC curve where you can't stay if they really are balanced. At those voltages you could have a cell at 30% and another at 70%. Charge it up to it looks full, then look at cell voltages.

 

[HRTKD]

Not all that hot... I'm in the Seattle Tacoma area so...
But it does heat up in the van on a summer day, 120 Fahrenheit, maybe?

The top end of the working range of EVE cells is 140°F. That's quite high and you probably have decreased the life of your cells at 120°F. In my camper my cells never get over 100°F and that's an unusual day. I have active air exchange to keep my cells and the rest of my equipment cool.

 

[Steve Dally]

Perhaps they are not balanced.

Take your bets now.

Balanced... hmmm.... Maybe you're right. They are pretty balanced (.001-.002) until I start getting up to near full charge. Once I hit 14.2volts they show a much bigger difference like 3.633 and 3.394 for highest and lowest cell. I only charge them to 14.0 volts for this reason, but that would still give me like 95% of my 560 amp hours, right?

 

[SparkyJJO]

Balanced... hmmm.... Maybe you're right. They are pretty balanced (.001-.002) until I start getting up to near full charge. Once I hit 14.2volts they show a much bigger difference like 3.633 and 3.394 for highest and lowest cell. I only charge them to 14.0 volts for this reason, but that would still give me like 95% of my 560 amp hours, right?

Nope it wouldn't, if they are way out of whack you're not actually charging to 95%.

You need a proper top balance to start with so they're all in line. Then see where that lands you.

 

[Steve Dally]

Were your battery cells new when you got them? Also not a good sign that you don't know how hot it gets in your van or how hot the batteries got.
Heat is the primary killer of Li batteries. You should have had a solar panel on the roof directly operating a fan which pulled out hot air inside the van and reduced the heating. I'd also want to validate the voltages on the cells were exactly as the BMS shows since 3.55V top charge is very near the max voltage I've seen spec'ed on LiFePo4 batteries(3.65).

 

[Steve Dally]

They were new. I got them from Amazon. I did the top balance routine and everything seemed great until just recently. I appreciate your comments about heat... We do vent the van during the summers, but not, specifically, the battery space (enclosed under the passenger seat).

 

[SomethingAppropriate]

e like 3.633 and 3.394

that is a problem
3.4 could mean 80% on that cell.
So you are only charging to 80%

 

[upnorthandpersonal]

They are pretty balanced (.001-.002) until I start getting up to near full charge.

The only way to see imbalance is to take the cells either to the top or bottom. Voltages in the middle of the SoC curve are meaningless.

Once I hit 14.2volts they show a much bigger difference like 3.633 and 3.394 for highest and lowest cell.

Your cells are not balanced.

I only charge them to 14.0 volts for this reason

Which means there is no time for any balancing to happen if you have a passive balancer.

but that would still give me like 95% of my 560 amp hours, right?

No. It will be dictated by the capacity of the lowest cell.

 

[Steve Dally]

Nope it wouldn't, if they are way out of whack you're not actually charging to 95%.

You need a proper top balance to start with so they're all in line. Then see where that lands you.

Should I pull them out and try another top balance, or have I ruined them, do you think?

 

[Steve Dally]

Nope it wouldn't, if they are way out of whack you're not actually charging to 95%.

You need a proper top balance to start with so they're all in line. Then see where that lands you.

Is it too late now, after four years, to top balance and reinstall?

 

[sunshine_eggo]

Yes, right now, for example, the pack is 13.3, and the cells are 3.319, 3.320, 3.320, and 3.321, so an imbalance of .002

The only way to see imbalance is to take the cells either to the top or bottom. Voltages in the middle of the SoC curve are meaningless.

Your cells are not balanced.

Which means there is no time for any balancing to happen if you have a passive balancer.

To elaborate on this, BALANCE is about STATE OF CHARGE, not voltage. Voltage only correlates meaningfully OUTSIDE the normal operating range of 3.1-3.4V. Even a notably imbalanced battery will have a very tight cell voltages.

that is a problem
3.4 could mean 80% on that cell.
So you are only charging to 80%

wut?

Thread 'LFP full charge @ 3.4V/cell'

Feb 11, 2024

Thought I'd try a little something on a cheap 12V I've been testing...

Charge to 20A @ 13.6V (3.4V/cell), 2A tail current.

When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged to about 95% SoC @ 3.4V.

Here's a plot of the charge:



0.2C charge
2.5 hour bulk
5 hour absorption
0.02C tail current
104.6Ah input (tested at 105.5Ah)

Have about a 0.12V drop in the leads @ 30A, so it's pretty negligible at 2A.

With sense leads, it would have spent more time in...

• sunshine_eggo
• Replies: 47
• Forum: General Battery Discussion

• 

summary: A 100Ah cell held at 3.40V until the current drops to 2A will yield 99%+ SoC.

Should I pull them out and try another top balance, or have I ruined them, do you think?

No. If you have a benchtop power supply, simply set it to 3.65V prior to connecting to cell and charge the cells starting with the lowest voltage first, cycling up through the cells and then hitting them again AFTER all 4 cells have been driven to 3.65V (takes just a couple minutes per cell) to ensure all are at 100% and very nearly the same voltage.

You can do this with the battery in-place and without disassembling. It will be helpful to remove loads.

What BMS? Please include your BMS settings.

 

[SparkyJJO]

Nope not too late. Do it.

 

[SomethingAppropriate]

wut?

Yes, given zero absorption time.
I am prepared to be wrong tho.