This is Andy from Australia. I am new to the forums. You have a great forum going here! Thanks to all contributors.
I have a question. Even though, Ive death with Sla based RV solar/dual battery systems, I am new to lifepo4. I am building a power tank that weighs a few kgs that I can take to places.
I bought a 12ah 12v lifepo4 battery with bms from a local internet vendor. The nominal voltage on the specs is 12.8. After it's fully charged the voltage reading I get is 12.85V. It discharges to 11.80 and disconnects. I am using a victron charger with modified lithium ion profile. I measured the capacity both charging and discharging and I get ~150w which is about 11.80ah based on 12.8v which is pretty close to claimed 12ah.
I read everywhere that voltage of lifepo4 batteries at 100% soc is 14+V and it's 12.8V when it's at 20%.
Is the battery I have faulty? I wanted to ask the experts before returning it as I really like the size of the battery.
Cheers
Andy
12.85 is only 3.21V per cell - that is pretty low. Now, voltage is not a good indicator of state of charge, but typically a fresh, fully charged cell should be around 3.4V per cell at least, or 13.6V for your pack. However, if you charge slow you might not get to that voltage, so it depends on your controller settings and charge current.
If you pull advertised capacity however, you're fine. I assume you pull 150Wh, not 150 Watts.
12.85 is only 3.21V per cell - that is pretty low. Now, voltage is not a good indicator of state of charge, but typically a fresh, fully charged cell should be around 3.4V per cell at least, or 13.6V for your pack. However, if you charge slow you might not get to that voltage, so it depends on your controller settings and charge current.
If you pull advertised capacity however, you're fine. I assume you pull 150Wh, not 150 Watts.
I tried charging at 2A and 5A, both with same results. Yes, my bad, it's 150Wh. When I charged at 5A, i only got 130Wh so charging at less than 0.2C gave me better capacity. I may reach out and ask if this is specific to their batteries.
Nominal voltage is not the fully charged voltage, LiFePo4 cells have a nominal voltage of 3.2V, with 4 in series that equates to 12.8V. The fully charged voltage is 3.65V per cell, or 14.6V.
If in fact they are LiFePo4 the best charging voltage would be around 3.45-3.50 volts per cell, 13.8- 14V for a 12V nominal battery, I would verify the charging settings because they may be for a different chemistry, like LiPo, often referred to as lithium ion.
Nominal voltage is not the fully charged voltage, LiFePo4 cells have a nominal voltage of 3.2V, with 4 in series that equates to 12.8V. The fully charged voltage is 3.65V per cell, or 14.6V.
If in fact they are LiFePo4 the best charging voltage would be around 3.45-3.50 volts per cell, 13.8- 14V for a 12V nominal battery, I would verify the charging settings because they may be for a different chemistry, like LiPo, often referred to as lithium ion.
The charging program is basically constant current with increasing voltage until 14.2v then absorption at 14.2v until fully charged. I also tried 14.4V with same result. Would you suggest me to try 13.8V?
Thank you
The charging program is basically constant current with increasing voltage until 14.2v then absorption at 14.2v until fully charged. I also tried 14.4V with same result. Would you suggest me to try 13.8V?
Thank you
The rational behind not charging to maximum voltage is because there is negligible capacity to be gained, only added stress to the cells, 13.8V is probably sufficient to be in the mid to high 90% SOC, which will theroitically extend the cell cycle life expectancy, but may not provide rated capacity, it's a tradeoff, loose a few Ah for more cycles.
The rational behind not charging to maximum voltage is because there is negligible capacity to be gained, only added stress to the cells, 13.8V is probably sufficient to be in the mid to high 90% SOC, which will theroitically extend the cell cycle life expectancy, but may not provide rated capacity, it's a tradeoff, loose a few Ah for more cycles.
What is the spec of the battery?
When you charge the battery, you should monitor the charging current to see if the current starts going down after reaching the 13.8V point.
What is the spec of the battery?
When you charge the battery, you should monitor the charging current to see if the current starts going down after reaching the 13.8V point.
I tried a multimeter, an energy meter, a voltmeter. They all read same at the terminals.
I used an led strip that draws 1.5ah to drain the battery and the energy meter showed 11.8 wh when the bms switched off at 11.8V. But what would be the reason that the battery's voltage is lower than what is in the market? And would this have a negative effect with devices that it will power?
I tried 14.4 and 14.2 for absorption for 2 hours. I am trying 13.8V today. Float is disabled.
I've attached graphs which I've just realised existed. If you look at the open circuit voltage, the voltage at 100% SOC is 13.3V. considering my garage would be at 15°C during the day and colder during the evening, would the voltages I get be actually correct and there is nothing wrong with the battery apart from having a overall low voltage?
The bulk/absorption is a target value for constant voltage , the charging source needs to either hold that value for a predetermined period, or be able to terminate at a given current draw. Should there be no means to hold the voltage as described, as soon as the battery reaches said target, a transfer to will occur from bulk to absorption then immediately to float, this is undesirable because it's not allowing the current to catch up with the voltage. This is a little difficult to explain, so I'll use my system as an example in the description below, it's a 24V nominal but the theroy is the same just double values.
My battery is 400Ah the bulk/absorption is set for 28V, the absorption is set to terminate after 180 minutes, or if the current drops below 1% of nominal capacity, whichever occurs first. When the bulk reaches the 28V value it transitions to absorption, still at 28V, the current is still high at this point but gradually drops as the battery becomes saturated at the 4A termination preset. The 180 minutes hold is to compensate for cloudy days because it's strictly solar charged, on most days the current will drop to 4A before the time expires.
The settings used account for around 1.5 kWh of additional capacity over what they would be if the voltage was the only factor to determine transition, to absorption then float, the crux of the matter is just because the battery has reached the target vslue, it still needs time to absorb current. This, in theroy, is the same as lead acid, but due to the low resistance of LFP, the process generally happens a lot sooner.
Every system is different due to environmental conditions, so the settings will be system specific if solar is the sole charging source, grid charging is more predictable but the same principles apply, having the correct charging equipment is paramount to achieving your goals.
I've attached graphs which I've just realised existed. If you look at the open circuit voltage, the voltage at 100% SOC is 13.3V. considering my garage would be at 15°C during the day and colder during the evening, would the voltages I get be actually correct and there is nothing wrong with the battery apart from having a overall low voltage?
Open circuit LFP voltage does not change very much over temperature, but the ability for it to move lithium ions to supply current demand is greatly reduced at cooler temps. It cannot support as much load current at low temps without significant terminal voltage drop. It's rate of internal impedance rise increases below +10 degs C.
I've attached graphs which I've just realised existed. If you look at the open circuit voltage, the voltage at 100% SOC is 13.3V. considering my garage would be at 15°C during the day and colder during the evening, would the voltages I get be actually correct and there is nothing wrong with the battery apart from having a overall low voltage?
Voltage is a poor way to determine state of charge, the charge/discharge curve is extremely flat between the knees with LFP, a 1mV difference is an extremely large difference.
I did try 13.8V as the highest absorption voltage last night and the final voltage was indeed increased to 13.6V in the morning and dropped to 13.4V by evening. Big improvement over 12.8V!
When I used voltages like 14.2 or 14.4, it would charge bulk until it reached that voltage and the could add another 0.1ah in the absorption phase which I guess means it was charging until it was almost full. But this time it charged to around 85% and then added another 2ah in the absorption phase.
Does this look normal or can be improved further?
I am discharging it now and can try a different charge voltage.
I am tempted to try 14.0V
Charging to 3.450 V per cell is sufficient, going higher adds very little it the way of capacity doing more harm than good.
Did the current drop to near zero in absorption and how long was that phase ? Do you have the ability to control the length of time absorption is held ?
Charging to 3.450 V per cell is sufficient, going higher adds very little it the way of capacity doing more harm than good.
Did the current drop to near zero in absorption and how long was that phase ? Do you have the ability to control the length of time absorption is held ?
Yes, I can adjust the time for absorption. It actually charged for another 0.1ah after absorption where it does storage.
Yes, the current gradually dropped to 0 during absorption but I don't know how long it took to reach 0 current.
Please the photos. Cycle 2 was yesterday with 14.4V absorption and the OCV after charging was 12.8v. cycle 1 is the most recent one with 13.8V absorption and the OCV is 13.5V. You can see each phase in the attachments.
Cheers
I did try 13.8V as the highest absorption voltage last night and the final voltage was indeed increased to 13.6V in the morning and dropped to 13.4V by evening. Big improvement over 12.8V!
When I used voltages like 14.2 or 14.4, it would charge bulk until it reached that voltage and the could add another 0.1ah in the absorption phase which I guess means it was charging until it was almost full. But this time it charged to around 85% and then added another 2ah in the absorption phase.
Does this look normal or can be improved further?
I am discharging it now and can try a different charge voltage.
I am tempted to try 14.0V
Fully charged Lifepo4 is 3.4V per cell, resting. That is, letting it sit disconnected overnight, then checking voltage. After charging to 3.65V, the voltage should settle to 3.4V overnight. If it doesn't, you probably overcharged it.
With that in mind, if you charge to 3.5V per cell, then you will have a longer slower absorption stage until fully charged. If you charge to 3.65V, you will have a faster, shorter absorption stage. At 3.65V, some people skip the absorption, and stop charging as soon as 3.65V is reached.(I think that is very smart btw to reduce the chance of overcharge)
In either case, the capacity the battery is charged to should be the same, or very very close to the same. You get there faster at 3.65V, but you will get more cycles at 3.5V (or even a bit lower) It is even possible to fully charge a Lifepo4 at 3.4V per cell (13.8V for 4 cells), it just takes a lot longer.
Interesting to catch up and see...
I run several packs in Parallel and not all same capacity packs. Much testing including hard Abusive Thrash Tests (Pushing system to Max Edges for charge & discharge rates).
I run with Midnite Solar Classic-200 SCC & Samlex EVO Inverter Charger.
I run a profile with Absord till EndAmps / TailCurrent reaches 0.05C-Rate of largest capacity pack (280AH = 14A) which then immediately kicks over to Float ! Runner Cells WILL take off at 3.400Vpc at which point cells differentlials increase, with Bulk/Commodity cells that can be up to 1mv per AH of Storage Capacity... so 280mv for a 280AH cell. THESE ARE OBSERVED & REPEATABLE ! BMS's should be set to allow for such deviation otherwise either a Differential Cutoff or HVD Cell cutoff can & will occur (pending on BMS type). THIS IS OK, as FLOAT (Constant Voltage, Variable Current) will continue to trickle amps as packs can take it and ultimately will end up pushing 0A once packs are properly fully saturated at the set voltage point. At this same time, the Hi Volt cells WILL SETTLE (that is normal) while the lower cells take in the trickle.
OF NOTE: If BMS has Passive Balancing a few other things occur as it burns off Hi Volt Cell energy. Active Balancing transfers Hi Volts to Lo Volt cells and this accelerates the balancing & leveling. NOT ALL BALANCERS ARE APPROPRIATE FOR THIS USE - Chemistry !
On Average, my system (30kWh) is in Float Mode by 12:00-13:00, within one hour of reaching that level, the packs tend to be around 25-30mv differential within 2 hours the cells will be at <10mv differential in all packs. And floating at 3.4875Vpc which is the top of the "Working Voltage Range".
BMS Trickery !
A SmartBMS is pretty much a Mandatory thing with DIY Batteries (all actually IMO) so things can be tweaked up properly. Default settings are most often inappropriate and some are downright BAD for LFP.... Hi Volt cutoff at 4.0V and lo volt @ 2.00V and temp control (or lack of). The REALITY is that with LFP you have to use a gentle hand (well all Lithium) and it also means allowing for things because we are NOT dealing with EV Grade Matched/Batched& Binned cells..... Allow for 200mv differential will allow runners to run without cutting off the other cells as they charge up and while that may go against some Common Sensibilities, it is fine once you see & understand out it all plays out.
AGAIN, I will restate, I use Charger BMS' with Passive Balancing (on only during charge mode). I use a QNBBM Active Balancer on every production pack and JBD BMS' & a Heltec Active Balancer on my utility packs.
Click the Blue Buttons Below to view specs/settings.
Setting
Default
My settings
NOTES
Over charge P Voltage
3.65
3.65
Over charge R Voltage
3.55
3.55
Over charge Current
50
86A / 150A
0.5C rate for 174AH / 280AH
Over Discharge P Voltage
3.00
2.65
CUTOFF Trigger
Over Discharge R Voltage
2.00
2.75
Release @ this V.
Over Discharge Current
300
175A / 250A
1.0C Rate for 174AH / 280AH
Low SOC cutoff
20%
0%
forces cutoff @ % (faulty)
High Temp cutoff
50C
70C
Diff of Batt Temp
10
15C
Diff of cell Voltage
30mv
200mv
Unmatched cells drift,
Temp Unit
C
C
Key Beeper
ON
ON
LCD Backlight
10
10min
Cut off Delay Time
10
10S
Current Calibration
-SET-
Temp Alarm
ON
ON
Cell Empty Voltage
2.50
2.50
Cell Full Voltage
4.20?
3.65
Default Setting
Enable
Balance Parameter
-SET- (OFF)*
Passive is ON charge only, start at 3.40V, 30mv diff.
Battery Capacity AH
1
174 | 280
Label Value of cells
Battery Power WH
1000
4554 | 7168
FORMULA (NominalVolts * #ofCells * RatedAH) (3.2*8*280=7168)
Low Temp cut off in Charge
2C
2C
Low Temp cut off in Discharge
-10
-10C
All equipment MUST BE Voltage Corrected & Calibrated (VERY IMPORTANT) see link in my signature on how to do it.
Divide Values X2 for 12V. Multiply X2 for 48V. Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost) Equalize: OFF Float 27.9V (3.4875vpc) MIn Volts: 22.0 (2.750vpc) Max Volts: 28.7 (3.5875vpc) Rebulk Voltage: 27.7 (3.4625vpc) End Amps: 14A (*1)
(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A. NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.
** Coulumbic Efficiency for LFP is 99%
A FINAL POINT !
Using ONE Battery Pack is fairly simple and straight forward. Running with Parallel packs in a bank is also not that hard and works very well when done right. But when using Multiple Packs the game changes and how the batteries interact when charging & discharging. Once you cross into Mutiple Batteries the use of Dumb BMS can become quite harzardous especially if they have craptastic BMS settings. The BMS is the Brains, The Guard Dog & Safety Keeper of YOUR Investment, not the thing to futz around with.
Hope it helps, Good Luck and check out the links in my Signature.l
Steve
