diy solar

diy solar

Are my battery cells defective?

N

newsolardude

New Member
Joined
May 10, 2021
Messages
7
I bought 4 3.2V 120Ah LiFePo4 cells and just got them today. I measured the voltage and they're all around 2.8-2.9V. Does this mean that I have defective cells? How do I get them up to the 3.2V it should be? Will it be a problem using them if I was planning on using them for a 12V system?
 
Seems like you need to do the normal top balance and capacity checking before you declare them bad.
 
And if you aren't yet equipped to top balance you can connect them in parallel for a day and then connect them in series and charge them. Then you can pull some power out of them and see how they do. However, you do want to top balance eventually before putting the battery pack in service.
 
newsolardude said:
I bought 4 3.2V 120Ah LiFePo4 cells and just got them today. I measured the voltage and they're all around 2.8-2.9V. Does this mean that I have defective cells? How do I get them up to the 3.2V it should be? Will it be a problem using them if I was planning on using them for a 12V system?
Click to expand...
2.8-2.9 volts is pretty low for brand new cells.
Are these cells brand new?
2.5 volts is empty and 3.65 volts is full.
 
newsolardude said:
I measured the voltage and they're all around 2.8-2.9V.
Click to expand...
Are you sure the meter is reading correctly? Unless the meter is able to read to three decimal places and is a (ideally calibrated) quality meter, the reading could be some way off.
Ideally you need to charge each cell separately to 3.60 volts, taking care not to over charge using a suitable charger, and test the capacity to verify the cells are OK.
Before using the cells as a 12v battery read up on the advice given in the resources section regarding balancing the cells. After reading around the subject if you still have questions ask on the forum.

Mike
 
newsolardude said:
I bought 4 3.2V 120Ah LiFePo4 cells and just got them today. I measured the voltage and they're all around 2.8-2.9V. Does this mean that I have defective cells? How do I get them up to the 3.2V it should be? Will it be a problem using them if I was planning on using them for a 12V system?
Click to expand...
I have this too, it is normal. I also had 2.9v on each cells because the battery took 2 months on shipping.
2.9v means there is 30% charge left. So, for a LiFePO4 cells, they are safe.
 
crozen said:
I also had 2.9v on each cells because the battery took 2 months on shipping.
Click to expand...
A healthy LFE cell is not going to go from a 50% SOC to a 30% SOC in just 2 months. Most cells are shipped with a voltage of around 3.2 volts and are received with a voltage of around 3.2 volts. One of the advantages of LFE cells is their very low self discharge rate. I would bet your cells and the OP's cells were shipped with a voltage of 2.9 volts. That doesn't mean there is anything wrong with them as you noted. :)
 
crozen said:
I have this too, it is normal. I also had 2.9v on each cells because the battery took 2 months on shipping.
2.9v means there is 30% charge left. So, for a LiFePO4 cells, they are safe.
Click to expand...
2.9v is more like under 2%. Not even close to 30%.

That is not self discharge unless they have sat on the shelf for years. More like they were discharged and then shipped. Cells fully discharged to 2.5v under a 40A load will bounce back to 2.7v within seconds after the load is stopped.
 
Last edited:
Just John said:
2.9v is more like under 2%. Not even close to 30%.

That is not self discharge unless they have sat on the shelf for years. More like they were discharged and then shipped. Cells fully discharged to 2.5v under a 40A load will bounce back to 2.7v within seconds after the load is stopped.
Click to expand...
2% of capacity on 2.9v ? no way ?
Try it by yourself, put a load from 2.9v to 2.5v and monitor it, you will see how much left.
 
crozen said:
2% of capacity on 2.9v ? no way ?
Try it by yourself, put a load from 2.9v to 2.5v and monitor it, you will see how much left.
Click to expand...
I have done it while monitoring and logging every second. More than 50 times. I have actual hard data, and a lot of it.

As an FYI, I have previously posted an example of it:

diysolarforum.com

Fast/Large capacity cell tester.

I managed to buy an EBC-A40L. I say managed, because apparently I got a bargain price, $173.63 off Aliexpress. Good luck finding it for that now. https://www.aliexpress.com/item/1005001971982159.html?spm=a2g0s.9042311.0.0.7e074c4d7VEoFR They raised the price and raised shipping by quite a bit...
diysolarforum.com diysolarforum.com
 
Last edited:
Hi all,

Thank you all for your help. I bought a 10Amp power supply and will charge them/top balance them later this week. I contacted the seller and they said that anything from 2.7-3.7V is normal. It also did take about 2 or so months for it to ship over to the US so hopefully the low voltage is because of that? Will followup after the top balancing is done.
 
newsolardude said:
Hi all,

Thank you all for your help. I bought a 10Amp power supply and will charge them/top balance them later this week. I contacted the seller and they said that anything from 2.7-3.7V is normal. It also did take about 2 or so months for it to ship over to the US so hopefully the low voltage is because of that? Will followup after the top balancing is done.
Click to expand...
3.65 is absolutely full for lifepo4.
I would not accept cells that came at 3.7 volts.
Are we talking about lifepo4?
 
newsolardude said:
Thank you all for your help. I bought a 10Amp power supply and will charge them/top balance them later this week. I contacted the seller and they said that anything from 2.7-3.7V is normal. It also did take about 2 or so months for it to ship over to the US so hopefully the low voltage is because of that? Will followup after the top balancing is done.
Click to expand...

Yes, the 2.7v is very low. Much lower than recommended storage voltage, it doesn't drop that low from shipping, the cells were discharged and shipped that way. If the vendor is telling you 3.7v is normal, then the vendor doesn't know what they are talking about.

diysolarforum.com

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
diysolarforum.com diysolarforum.com

Top balancing is a good idea. Feel free to ask if you have questions.
 
You must log in or register to reply here.

Similar threads

J
Cell voltages not matching
Replies
9
Views
319
kommando
K
C
Replacing EVE lifePO4 3.2v 310ah, need advice.
Replies
11
Views
478
CharlieBB
C
2
Building a 12V LiFePO4 battery with components from a 48V battery with one bad cell
Replies
3
Views
157
sunshine_eggo
sunshine_eggo
J
Top balancing tail current? How low?
Replies
7
Views
427
Jet
J
J
DIY 16s 48V Cells
Replies
11
Views
656
augiedoggy
A
Back
Top