Is 3.28v resting voltage right for 26650 lifepo4 cells?

[GT877]

I have had an old lifepo4 48v 21ah sitting around in my house for quite a few years now.
I first bought the pack from china in 2011, it worked very well and used it up untill 2013 when the motor started cutting out, taking off the ducttape that the pack was surrounded in, I noticed quite a few cell tabs had corroded or came loose. I realised I had not been running it at its full capacity of 21ah for a while.
I understood then I had not been taking care of this pack very well. I used it in heavy rain quite often with a rack bag that was not 100% waterproof.

Untill recently it had be sitting in my house since 2013 but I could not find anywhere to get it spot welded back together so I went ahead and bought a spot welder. It was either faulty or it just kept tripping the power switches in the house every time I turned it on.
So now I've decided to simply use lots of continuous copper wire scunched above each cell terminal as padding and tightly wrapping it with ducttape. Testing each cell pack as I go along. I found there were quite a few dead or faulty cells and some had also swelled up but were fine. So after casting those cells aside I went with 48v 18ah
Unfortunately back in 2012 I had absent mindedly crossed the BMS charge port with the battery output or blew the charger, I cannot really remember what happened but the battery still worked fine then and I simply charged it cell by cell instead.

When I tested the voltage of each cell pack after wiring and ducttaping it all together, each cell pack read 3.28v and with just 2 packs at 3.25v
I have have just been charging each 18ah cell pack with a 3.6v 10ah lifepo4 cell charger, The cell charger usually stops charging after a while once the cells are fully charged and it will enter the float charge mode but while charging most of these cell packs, the cell charger just keeps charging them for more than 2 hours. There were only a few that entered float charge. If I disconnect the charger the cell voltage will read 3.32v on any of the cell packs and will drop back down to 3.27v - 3.29v
I know these cells have seen better days but I was wondering if anyone knew if that resting voltage is ok? I am going to connect an ammeter to my handlebars to see what it can reach before cutting out, I have a 1000w motor so I know with 48v 18ah its only 864w so the BMS will cut out before I can use full load but the ammeter will show me at what current it will cut out.

The lifepo4 cells are 26650(MP-Fe 2VX09E19)

Sorry for the ramble!

Thanks

 

[amberwolf]

balancing and full charge usually occurs around 3.6-3.65v.

typically 3.2-3.3v could be anywhere in the main part of the charge curve, so there's no good way to know by voltage whether any of those cells are fully charged or not.

You'd have to get the actual specs and charge curve for your specific cells from their manufacturer.

Also, I don't know that the pack repair/assembly method chosen will provide sufficiently reliable connection to the cells, as duct tape stretches. IF connections are not good you may only be charging a few cells out of a parallel set, so some may take a lot longer to charge because more cells are connected, so take more charge.

Can also be that some cells are damaged in the parallel sets taht don't take as long to charge, so they can't hold as much.

You'd ahve to individually charge and capacity test each cell, with a load, to be sure they are all working correctly. There are threads for that sort of thing, too.


You might look around at the various DIY pack build threads for potentially better connection methods. Or at the various spot welder threads to see if there are fixes for your welder problem.

 

[dmwahl]

3.45V is pretty much full, 3.3V is ~50%

3.45V (at low current) is basically full, but 3.3V is not necessarily 50%. LiFePO4 will rest around 3.2-3.3V for most of the capacity so in that range there's no reliable way to determine state of charge just by voltage.

 

[999zip999]

The numbers are on a curve for capacity. 2.8 is close to zero and 3.45 is about 93% full. But it spends must of it's time between 3.2 and 3.4v so 3.0 to 3.5v is very large number for amount of it's capacity so.
Edit 3.5 not 3.9.Sorry.

 

[GT877]

Hey thanks for the replies!
I have reached float charge stage on some of the 18ah cell packs and the voltage reads 3.33v and returns to 3.29v - 3.30v resting voltage
I think these cells can no longer hold what they originally could, perhaps they can now hold around 80% of their original charge?
I recall when I first had the battery the resting voltage was somewhere inbetween 53v and 54v but now the overall resting voltage is 52.5v although I have yet to reach float charge for all the cell packs, some just keep on charging up. Either way they don't seem to gain much more voltage from 3.28v to 3.30v.

Once I get the battery onto my bike I will finally test to see what current draw the motor can take before cutting out, perhaps that will help me understand what capacity the battery is at.

Thanks again!

 

[dmwahl]

3.45V is pretty much full, 3.3V is ~50%

3.45V (at low current) is basically full, but 3.3V is not necessarily 50%. LiFePO4 will rest around 3.2-3.3V for most of the capacity so in that range there's no reliable way to determine state of charge just by voltage.

This is a common misunderstanding. With LiFePO4 you just need to include the hundredth of a volt to determine relative SoC. Of course age, temperature, manufacturer, etc. affect that but once you know your battery, the voltage is a quick estimate of SoC.

Perhaps after you "know" your battery, yes, voltage is a quick rough estimate. But if you don't have a detailed knowledge of how your individual cells perform, then it's not a very accurate measure. For example, the LFE cells in my lawn mower read ~3.25V after I've been using it for anywhere in the range of ~90% to 25% SOC, and after an hour or so of resting with no load, recover to 3.28-3.30V/cell. I've done similar tests with a variety of cell capacities and manufacturers and while the discharge curve is similar for all of them, I haven't been able to reliably correlate voltage (which I measure/record to the mV) and SOC except at the nearly full/empty points. The problem is just as you pointed out, it varies due to age, temp, manufacturer, etc, as well as from cell to cell within a batch.

 

[GT877]

The cell charger reads 3.65v while charging the cell packs. I have finished charging most of them already where the charger automaticly shuts off and testing the voltage of each cell pack straight after the full charge reads 3.33v and drops back to 3.29v within 15 minutes.

I have now got everything connected on my bike and tested it out. Before taking the bike out I just gave the motor a spin in the air to make sure everything was connected ok. The motor had no problems and worked fine. I then tested the overall voltage and cell voltage and I noticed it had immedietly dropped on all cells from 3.29v to 3.26v-3.27v.
Anyway I took the bike out and motor runs well and I can climb hills and reach 25mph no problem.
I can also reach 30 mph but the bms cuts it off after a few seconds, The ammeter read 25A when it cut out at 30mph. I was expecting it not to go over 18A?

Thanks again for the replies!

 

[GT877]

I dont think the battery reads 3.65v during full charge. The voltmeter is connected to the cellpack while the charger is also connected, it most likely just shows the charger voltrate rather than the battery volt level. Once the charger is disconnected there is no quick or gradual voltage drop witnessed as it immedietely shows 3.33v upon disconnection.

For example charging a lifepo4 cell at 3.0v, immedietely upon connecting the cell charger to that cell, my voltmeter shows 3.65v and if I were to then disconnect the charger right away my voltmeter will immedietely show 3.0v again so I can't actually see the volt level of the any cell while it is charging

I definitely agree about something not being quite right with these cells, its my fault for not taking care of them in the past. When I eventually get a new pack I will take every precaution and effort to maintain them.
Unfortunately I cannot monitor the voltage while riding, I will need to get more suitable voltmeters for that however I will trial and test the battery over the next few days seeing what range it can deliver.

 

[999zip999]

Gt where you at ?

 

[GT877]

So far after a bit of testing, the battery proves to hold quite a fair amount of charge. It certainly does not hold as much it would of have when I first bought it but it does seem to hold enough for commuting. So far I've done about 15+ miles with it on the initial charge with no cut outs. Battery has gone from 52.5v to 52.2v. Ammeter usually shows a load of 5 to 10 amps, I don't often go over that unless I am climbing a very steep hill but I always pedal to help.

Thanks again for all the help!