Need some LiFePO4 help

F

FesterJester

New member
Joined
Sep 22, 2018
Messages
5
To start, hello to everyone in the forums.
I have used a few answers from this forum over the years, but never joined until now.
So, I have recently started getting into re-purposing cells from old battery packs.

Recently, I got an K2 Energy LiFePO4 battery from a recycle center.
It was a 12V drop in replacement design.
It was kinda bulged on the sides, but it was holding a decent charge.
After a little research I found it was a bunch of small cells with anintegrated BMS.
I decided to carefully cut open the case and rescue what cells may be good.
(Performed in the middle of my lawn with a bystander ready ;) . All went well!)
Unit was setup in what appeared to be 4 groups of 6 cells.
After getting the spot welded plates off the cells, I now have 24 LFP26650EV cells and a 12V BMS board.

I used a volt meter and found all the cells to be 3.17v - 3.36v.
I am in the process of charging each cell with a Nitecore Intellicharger NEW i2.
I will recheck the voltage a few days after charging each cell to look for voltage drops.

Question is:
How do I properly test each cell to see how "healthy" it is?
I have no problem building my own tester if needed, I have small circuits knowledge and experience.

Thanks in advance,
FJ
 
Obviously look for physical damage eg bulging first, sounds like you've done that.

Next step would be a capacity test eg charge each cell separately to full & hold at full voltage (approx 3.45 to 3.5V per cell, not 3.6 or higher for LiFePo4) for a couple of hours. Look for the current dropping away.
Apply a load like 5A short term & look at the voltage dip. They should not drop more than about 0.2V (might be more if you've charged them higher).
Then discharge them at 5A or so through some resistive loads. Time the discharge to approx 3.0 - 2.9V. Figure out the capacity.

With the voltages on LiFePo4, there's a fairly flat "plateau" when they are in their normal range, you're looking for the edges of this when it starts rising fast at the top it's full, & dropping fast at the bottom edge it's empty. You want your cells on that plateau all the time.
Li-ion the plateau is more of a slope & harder to read.
 
Redpacket said:
Next step would be a capacity test eg charge each cell separately to full & hold at full voltage (approx 3.45 to 3.5V per cell, not 3.6 or higher for LiFePo4) for a couple of hours. Look for the current dropping away.
Click to expand...
If I am understanding my research properly, would this be constant voltage charging?
Basically letting unrestricted current (within the limits of the battery), but limiting voltage to 3.5v and waiting until current drops to a specified value (say a few 100ma).
Would TP4056 or TP5100 chargers be safe to use?
Some sites selling them say Li-Ion and some justsay Lithium. :huh:

Apply a load like 5A short term & look at the voltage dip.
Click to expand...
Define "short term" please. Seconds? Minutes? Probably not hours. :D

Then discharge them at 5A or so through some resistive loads.
Click to expand...
I have some high current, ceramic resistors that will do the job well. :D
 
TP wont work they are higher voltage. Test with your rc charger at LiFe settings

there is no use splitting such pack up. Test them as is in each P config. I would have keept that pack together with that bms if it was working. and just tested each cell inside in P pack first ie 4 tests.


You test them basically between 2.5 and 3.65V. Same as with any other Lithium but with different voltage settings.
 
FesterJester said:
Redpacket said:
Next step would be a capacity test eg charge each cell separately to full & hold at full voltage (approx 3.45 to 3.5V per cell, not 3.6 or higher for LiFePo4) for a couple of hours. Look for the current dropping away.
Click to expand...
FesterJester said:
If I am understanding my research properly, would this be constant voltage charging?
Basically letting unrestricted current (within the limits of the battery), but limiting voltage to 3.5v and waiting until current drops to a specified value (say a few 100ma).
Click to expand...
Click to expand...
"unrestricted" from a big source or connecting a flat cell directly to a full one wouldn't be good, but since a charger or solar, etc will have practical limits... typical chargers might do 500mA or 1A which would be fine.
Yes it's constant voltage with implied constant current due to the source

FesterJester said:
Would TP4056 or TP5100 chargers be safe to use?
Some sites selling them say Li-Ion and some justsay Lithium. :huh:
Click to expand...
Like daromer says, you need a charger capable of LiFePo4 specific voltages, not Li-ion which would fry these batteries.

Apply a load like 5A short term & look at the voltage dip.
Click to expand...
FesterJester said:
Define "short term" please. Seconds? Minutes? Probably not hours. :D
Click to expand...
Yes seconds or a minute or two (as long as you like!) but long enough to show the cell holds a voltage under load vs drops away fast.
A full capacity test shows the rest.

Redpacket said:
Then discharge them at 5A or so through some resistive loads.
Click to expand...
FesterJester said:
I have some high current, ceramic resistors that will do the job well. :D
Click to expand...
Perfect !
 
emuland-metroman said:
i have 700 lifepo4 to test.

if You have anche Opus,. You Can switch opus to lifepo4 opening and setting ti 3.7v
Click to expand...

You should not charge LiFePo4 cells to 3.7V, that's definitely too high. Some manufacturers say 3.65 but even that's too high. For long cell life, 3.45 to 3.5V is the max.
 
I understand that 3.4v to 3.5v per cell is the recommended charge voltage cutoff.
Would 1amp be the recommended current to charge each cell at?
K2 Energy's spec sheet recommends ?1.6 amps.
I am assuming charge current varies from cell mfg to cell mfg as well as cell size.



So, just because I can, I plan to build my own setup for charging and testing batteries.
I don't think I am going to go large scale yet, but I am going to keep large scale in mind while building my setup. ;)

I think I am going to use this circuit for charging. (to be referred to as charger)
https://hackaday.io/project/9072-open-autonomous-domestic-robots/log/41973-battery-charger-circuit
(Unless someone out there knows of a good, somewhat cheap board similar to the TP5100/TP4056)
Mainly because I have most of the parts already.
I also, if I understand it right, the circuit can be adjusted to charge other lithium varieties also. (Li-Ion? Li-Po?)

As a controller/monitor I plan to use an Arduino, maybe a Raspberry Pi. (to be referred to as controller)
(Arduino has several analog inputs already built in that I can use to watch voltage.)
(RPI has none, but analog to digital converters are pretty cheap)

For charge/discharge current measurement, I'm thinking ACS712 5-amp version.
https://www.allegromicro.com/en/Pro...p-Integrated-Conductor-Sensor-ICs/ACS712.aspx

For load testing, I have high current, ceramic resistors. (to be referred to as resistive load)

So, my ideais as follows:
Code: 
Insert cell into holder
Controller detects voltage and monitors it (say 1 minute?)
Controller then switches charger relay on (charges cell) and monitors the current
Controller will switch charger relay off when current drops to a defined level
A defined amount of time will be waited for the cell to rest
Controller reads cell voltage
- If cell voltage is within defined good range (3.3v-3.6v?)
-- Controller switches resistive load on for a short defined time and the back to off (checking for quick voltage drop)
Controller reads cell voltage again
- If cell voltage is still within defined good range (>3.2v?)
-- Controller switched resistive load on and monitors cell voltage
-- Controller switches resistive load off when cell voltage reaches <3.0v
-- Controller records time taken to discharge cell and performs capacity math
-- Controller switched charger relay on and monitors the current
-- Controller switches charger relay off when current drops to defined level
Test complete

Still a work in progress, but that is the basics.
Each step will have some diagnostic lights or feedback of some kind.
I want to design everything to be compatible with Li-Po, Li-Ion, and LiFePO4. (My hopes anyway)
I may input the mfg rated capacity at start and, if the tested capacity is close enough, recharge to full charge or storage charge at the end.
This is a very similar idea to Brett's Arduino 8x Smart Charger / Discharger, but I do believe it's only compatible with Li-Ion.
Ideas, thoughts, criticism, and comments are welcome. :)

Thanks,
FJ
 
The hackaday circuit looks pretty good.
R5 (sets the cell voltage) should be more like 390 ohms (cell max =~3.47). The shown 470 value = cell max ~>3.7V, way too high.
Only thing is there's nothing to protect it if a full Li-ion or overvoltge cell was accidentally connected - I'd add a 1W resistor in the collector (bottom) leg of Q1, eg 0.5 ohms.
Cell reverse protection could be easily added with a FET.
A tail current cutoff circuit could be added too via a comparitor, FET (dual use above reverse one) & few parts
 
Redpacket said:
...
Cell reverse protection could be easily added with a FET.
A tail current cutoff circuit could be added too via a comparitor, FET (dual use above reverse one) & few parts
Click to expand...

Are you referring to a P-Channel FET?
I was looking at a simple polarity correction circuit using 4 - 1N400X diodes.
I am not familiar with FET's, so I am reading up on them.
Quick reading on FET's makes me think they are a better choice though. :D
Any suggestions on what to use as a FET?
 
FesterJester said:
Redpacket said:
...
Cell reverse protection could be easily added with a FET.
A tail current cutoff circuit could be added too via a comparitor, FET (dual use above reverse one) & few parts
Click to expand...

Are you referring to a P-Channel FET?
I was looking at a simple polarity correction circuit using 4 - 1N400X diodes.
I am not familiar with FET's, so I am reading up on them.
Quick reading on FET's makes me think they are a better choice though. :D
Any suggestions on what to use as a FET?
Click to expand...

If you use diodes in the circuit, your voltage measurements will be quite inaccurate & vary with current & temperature even if the cell voltage was magically perfectly unchanged.

Google "reverse polarity protection circuit mosfet". With batteries & chargers like this project, you just need the FET, nothing else.

FETs are awesome devices, easy to drive, switch large currents easily. Use these instead of relays -cheaper, simpler, smaller.

Something like anFQP30N06LN-ch, TO-220 package,approx 60V, high current, "logic level" gate threshold (so works with batteries down to 2.5V), ~$1.73each from Mouser
For P-ch, something like IRF9Z14PBF,TO-220 package,approx 60V, high current, "logic level" gate threshold (so works with batteries down to 2.5V),~$1.39 eachfrom Mouser
Or lower voltage versions like
NDP6020P P-ch
 
Redpacket said:
emuland-metroman said:
i have 700 lifepo4 to test.

if You have anche Opus,. You Can switch opus to lifepo4 opening and setting ti 3.7v
Click to expand...

You should not charge LiFePo4 cells to 3.7V, that's definitely too high. Some manufacturers say 3.65 but even that's too high. For long cell life, 3.45 to 3.5V is the max.
Click to expand...



you are right. but opus need to be set 3.7 to charge 3.65 life
 
You must log in or register to reply here.
Back
Top