LiFePO4 Voltage Chart?

gnubie

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The wheat is, voltage as a SOC indicator for lithium batteries isn't much use as the voltage only really starts to move towards fully discharged and fully charged. For most of the state of charge there is very little variation and that variation will be completely swamped by the effects of voltage drop in wiring etc by the load pulling a lot of current. People often make the mistake of trying to gauge SOC with a load (or charger) present.
 
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Stevelk

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The wheat is, voltage as a SOC indicator for lithium batteries isn't much use as the voltage only really starts to move towards fully discharged and fully charged. For most of the state of charge there is very little variation and that variation will be completely swamped by the effects of voltage drop in wiring etc by the load pulling a lot of current. People often make the mistake of trying to gauge SOC with a load (or charger) present.
Thanks a lot for the response. Pretty sure I understand everything you said up to maybe the last sentence.

WRT that, I think I understand you to mean that you don't want to check SOC using current if there is already current moving into or out of the battery (ie, current with current currently :). So then the correct way to check SOC would be.. what?
My guess: A shunt and a coulomb meter. But used how and when?

P.S. I'm trying to begin bringing up a 280ah 48 volt system.

thanks again.

--
Steve
 

j_t_l

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Just read this whole thread. As a newbie I have to admit it is mostly confusing. Seems like, in this case, we've defeated the purpose of using a forum to gather accurate and meaningful data.

I wish there was a way to separate the wheat from the chaff as you say. Guess I'll have to just keep reading reading reading, experimenting and measuring. I wish I understood this stuff better.

Thanks everyone I know you are trying to help.:)
Unfortunately, voltage curves for lifepo4 batteries are either really complicated or only semi-useful.

I think the quick answer is "don't do it" -- but no one likes that answer :).

Depending on the use case: do you really care exactly what the SOC is? If you just want a rough hand-wavy guide, then this table works well for a 4s battery: (from http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/)

Voltage readingAssessmentCapacity
13.3V or moreNear fullOver 80%
Above 13.2VPlenty of reserveAt least 70%
Below 13.15VGetting on the low sideLess than 40%
Below 13.0VDefinitely getting lowLess than 25%


To get something more accurate, it depends on temperature, how long the cells have been resting, whether they were last charging or discharging -- and even after you take all that into account you can hardly tell the difference between 40% to 70%.



And even THEN for just the hand-wavy chart, if your meter is off by 0.05v the guestimate is way off.

I looked for the first result I got for a search for "DMM" on amazon, an inexpensive $35 unit.

This has:
  • Digital Display: 5 ¾ , 6000
    • (note: the "5 3/4" is bogus -- I'm going to believe the "6000", which is 3 3/4 digits)
  • DC Voltage: 600mV/6V/60V/600V ±(0.8%+3), 1000V (1%+5)

So if you're trying to measure 13.2v (and you believe their specifications), you can get an error of up to:
13.2v * 0.008 + 00.03v --> 0.1056v + .03v --> 0.14v
So you'd see something between 13.34 and 13.06. This makes even the hand-wavy chart a bad guess.



Or you can shell out for something a bit ridiculous like the $250 121GW (https://www.eevblog.com/product/121gw/):
  • Dual 50,000 count display with bargraph
  • 0.05% + 5 Basic DCV Accuracy
For the same 13.2v:
13.2 * 0.0005 + 0.0005v --> 0.0066v + 00.005v --> 0.012v
So you'd see something between 13.212 and 13.188.


FWIW: if you combine the el-cheapo meter with a decent reference voltage, you can figure out a calibration factor and get pretty good results. Something like this thing: https://www.amazon.com/gp/product/B087Q541YH


Stevelk said:
So then the correct way to check SOC would be.. what?
My guess: A shunt and a coulomb meter. But used how and when?
And as a final note: if you want/need an accurate SOC meter (and can't deal with the handwavy voltage meter), a shunt & coulomb counter is the way to go.
You can play around with cheaper things like two of these. You need two, because they only measure current one way -- you need one for charge, and one for discharge. IMO it's just not worth it if you want something easy to use.

Or you can get more expensive ones from Renogy or Victron.
 

gnubie

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My guess: A shunt and a coulomb meter. But used how and when?
This is the best way to tell your battery's state of charge. The shunt goes between the battery post and everything else, ie the 'output' terminal on the shunt effectively becomes the battery terminal. Stevelk linked to some. Smarter units such as Victron's also monitor the battery voltage to determine when it has been fully charged and can use that as a reset point and self adjust to your battery's capacity to some degree. At the very least the cheap meters will tell you the net state of your battery regarding amp hours in and out, which is far better than looking at resting voltages.
 

dRdoS7

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Hi,

3 (or 4) other options:




The first 2 have a relay output, so can be used for control, and also self-reset to 100%, the last one is the cheapest. The second is a Victron copy.

Sorry about the AU ebay links, just wanted to show other coulomb counters available. There are others, but one I've tried isn't as good as the above.


Although, to be fair, I put it in place of an ammeter that had failed, and is in the cable to the inverter, which means it's not reading the output of the SCC. I'll rectify that when my LiFePO4 bank is installed. It will probably be OK as a SOC gauge then. Appears accurate amps reading wise. Certainly was cheapest. It also has a relay output IIRC.

I should also add that I removed the shunt from the circuit board, and mounted it separately, There were 2 reasons: Firstly it was getting hot, second the protective cover couldn't be replaced once the cables were attached, and couldn't attach the cables with the cover in place (catch22) . Might be OK with small wires.

dRdoS7
 
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dRdoS7

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Hi,

The first 2 have a relay output, so can be used for control, and also self-reset to 100%, the last one is the cheapest. The second is a Victron copy.

Sorry, the first doesn't have a relay contact, I was thinking of the BMV712.

dRdoS7
 

Capt Bill

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Charging LiFePO4 is a two step process … FIRST step uses constant current (CC) to reach about 60% State of Charge (SOC); And then STEP 2 kicks in until the charge voltage reaches 3.65V per cell. Turning from constant current (CC) to constant voltage (CV) means that the charge current is limited by what the battery will accept at that voltage, so the charging current tapers down asymptotically.
n learning mode here on my first DIY LiFePO4 24v 280 Ah LiFePO4 set from Xuba: I want to better understand the specifics for properly charging one 3.2v 280Ah LiFePO4 cell with a Bench Power Supply rated for up to 30v / 10A. I wonder if someone can confirm my recent thoughts (after kind of experimenting with no specifics in a good manual to read). ... If I want to to get constant current, I think the Red Light next to my lower current knob needs to be "ON" (as in lite up, & the Green Light next to the Voltage Knob goes OFF when that Red Light is ON) ??? and That Indicates Constant Current (CC) ??? Is that correct thinking? ... I notice If I turn voltage knob up full, I can get 10.4 amps out of my bench supply. Then if or when I adjust voltage knob downward for less voltage, the green light comes on at about 3.8 volts. turning back up just a little, and the green light goes off, and the red light goes back on. I think the green light indicates CV, and red light CC, but not sure. REQUEST: Please confirm if this is correct thinking on my part, or please give me some educational clues.

Then at about 3.29 v measured with a multi-meter at battery being charged (which is about 60% SOC), I think I want to get my Bench Power Supply to switch to Constant CV ??? I notice by turning the the voltage knob down from 3.8v to 3.65v or 3.6v ; the Green Light next to the Voltage Knob Turns ON / Red Light goes OFF ??? and I think that means this Bench Power Supply is then in Constant Voltage mode ??? Is that correct thinking? ( I have not yet seen any specific confirming explanation in a good read manual).

Also: Please Confirm or correct me: ... and Then to fully charge a new or newish 3.2 * 280 Ah cell to FULL (like for a following up Ah testing procedure) .... Keep on Charging via my Bench Power Supply, with Green Light "On", and Voltage knob adjusted to 3.65 volts ... until I get a 3.6 or 3.65 volt reading at the battery with my mulit-meter. Does this thinking match yours ? If not, ... please fill me in :+)

Also, I do not know if setting the Voltage at 3.65v at will protect the cell once it reaches 3.65v, and/or if that Geen Light will Stay ON (when battery reaches its' full 3.65 cut off voltage), or possibly go OFF when the LifePO4 cell reaches 3.65 volts. I think I need to be monitoring this reaching full voltage junction so I can turn off the power supply close to then ??? ... Then I wonder how long should I wait before testing the cell for Ahs to see how it compares to it's Ah rating? I have thought to let battery sit overnight, then test for Ah. Any clues you from you more experienced folks will be Appreciated? ... Thanks In Advance :+) Bill
 
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BiduleOhm

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I think the green light indicates CV, and red light CC, but not sure. REQUEST: Please confirm if this is correct thinking on my part, or please give me some educational clues.

That's correct from your description ;)

Also, I do not know if setting the Voltage at 3.65v at will protect the cell once it reaches 3.65v, and/or if that Geen Light will Stay ON (when battery reaches its' full 3.65 cut off voltage), or possibly go OFF when the LifePO4 cell reaches 3.65 volts. I think I need to be monitoring this reaching full voltage junction so I can turn off the power supply close to then ??? ...

Yes, it'll protect the cell, but don't let your it float at 3.65 V for days either.

The usual order of operations to set a bench PS are: set the voltage with no load, set the current limit to min, short the output, set the current limit to the wanted value, unshort the output and connect your load.
 

PeterBC

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Observations of voltage behaviour... I'm experimenting with some tiny 6AH LiFePO4 and Electrodacus SBMS0 with Heltec 5A Active Balancer too sometimes.. and a bench 5A supply. For these cells 5A is almost 1C rate.

I did an initial top balance with all in parallel to 3.6V. Now in series as a 24V battery, just letting them drop via SBMS0 load (30ma .. to approx 200hrs est life), but with the Heltec active balancer on some runs (an additional 10ma I think when its mostly balanced).

When I let it all discharge to 2.8V .. then tried to charge at 5A .. within 5 mins one cell was over 3.55V , my target max (and default disconnect for SBMS0). This wouldnt be good in a real set.. Seems to indicate a dodgy cell. This prevents even a minimal 1C charge. If I dropped that to 2A, and later 1A then even less it stayed mostly under the 3.55 limit. But the voltage diff between cells was about 140mv without the Heltec Active balancer. When I put the heltec on, it quickly drove the imbalance down to about 40mv at 1A charging. As amps dropped to 0.1A the differential went to < 10mv after 30mins or so.

When no charging, heltec maintains 7mv differential. But if I remove it the dodgy cell drops about 20mv ... making a 27mv delta, so its the low cell of the pack now. Put the heltech on it jumps up almost immediately to be just 7mv diff. Leave it on for 20hrs, it still drops 20mv when I take off the Heltec

Voltage of the pack under charge, especially 1C or 0.5C rates is VASTLY higher (relatively for LiFePO4) than at rest after 30mins of no charge.
I havent tested under high discharge rates, but I expect similar behavior.. Under 1C or 0.5C discharge rates Volts will be considerably lower than the at rest rates.

This might be a bad internal resitance cell... Not sure how to measure internal resistance with just a simple Fluke 4 digit voltmeter ?
 

Csimmer

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Hi, I am using the SOC relay in a Victron BMV 700 to control a contactor which switches the pv panels to my chargers. This way I can charge my batteries with a high voltage and have it stop at say 80% SOC to increase my battery lifespan. This works very well. The only issue is that I need to be aware of is the BMV drifting as it never get close to 100% SOC to reset. I check my battery voltage and BMV SOC in the mornings after my batteries have not seen any large charging or discharging events and compare it to the voltage/SOC chart I found at this site. My load is never zero so my voltage will be not a true resting voltage because the fridge, freezer and standby loads are about 500w (I have 16s 2p 280ah). From the SOC/V table I have interpolated a voltage for every 1% SOC and made this spreadsheet to keep a check on my BMV. I use a google form to enter the readings via my phone.
Any criticisms or advice.
 

dRdoS7

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Hi,


Hi, I am using the SOC relay in a Victron BMV 700 to control a contactor which switches the pv panels to my chargers. This way I can charge my batteries with a high voltage and have it stop at say 80% SOC to increase my battery lifespan. This works very well. The only issue is that I need to be aware of is the BMV drifting as it never get close to 100% SOC to reset. I check my battery voltage and BMV SOC in the mornings after my batteries have not seen any large charging or discharging events and compare it to the voltage/SOC chart I found at this site. My load is never zero so my voltage will be not a true resting voltage because the fridge, freezer and standby loads are about 500w (I have 16s 2p 280ah). From the SOC/V table I have interpolated a voltage for every 1% SOC and made this spreadsheet to keep a check on my BMV. I use a google form to enter the readings via my phone.
Any criticisms or advice.

Could you set the capacity lower? Maybe alter the settings at which it decides to reset the Ah/SOC to 100%? The BMV measures Ah, so you'll probably be able to tweak the settings to get what you want/need.

The BMV in my caravan used to reset too soon, so I changed the settings, and it's OK now. But I charge them to 100% (14.6V-3.65/cell) because I want them as full as possible, even if it is only a few Ah difference.

My home LFP I charge to 63.5V-3.53V/cell. I am lucky in that I have a inverter with solar connected which takes care of loads, and a separate SCC which looks after charging. The inverter only charges if the voltage of the bank is low(ish), but it has a FW bug that cuts it short and goes to Float too soon, so leaves the SCC to do most of the work.

I figure, they are going to deteriorate no matter what. Either due to high charge voltage, SOC, or ageing.

So why worry. There'll be better, cheaper batteries around by then.

>3500 cycles (according to specs, ~3700 on their graph) is about 10 years, and that is from 2.5V to 3.65V, and back to 2.5V. That is at 1C. Cycling them to only 50% each day is already going to increase the time. Lower charge/discharge current too. Probably past my remaining life time. But that's what my NiFes said too. I beat them!!!

Only my opinion, and you know what they say about them!

When you're charging, have you watched the difference the voltage setting makes to charging? Noted the different Amps going in to the batteries? They may charge just as well, by using a lower voltage setting in the charger, and without using any external controller.

No matter what the voltage setting, it's going to put maximum amps in until the SP is reached.

dRdoS7
 
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Csimmer

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Hi,




Could you set the capacity lower? Maybe alter the settings at which it decides to reset the Ah/SOC to 100%? The BMV measures Ah, so you'll probably be able to tweak the settings to get what you want/need.

The BMV in my caravan used to reset too soon, so I changed the settings, and it's OK now. But I charge them to 100% (14.6V-3.65/cell) because I want them as full as possible, even if it is only a few Ah difference.

My home LFP I charge to 63.5V-3.53V/cell. I am lucky in that I have a inverter with solar connected which takes care of loads, and a separate SCC which looks after charging. The inverter only charges if the voltage of the bank is low(ish), but it has a FW bug that cuts it short and goes to Float too soon, so leaves the SCC to do most of the work.

I figure, they are going to deteriorate no matter what. Either due to high charge voltage, SOC, or ageing.

So why worry. There'll be better, cheaper batteries around by then.

3500 cycles (according to specs) is about 10 years, and that is from 2.5V to 3.65V, and back to 2.5V. That is at 1C. Cycling them to only 50% each day is already going to increase the time. Lower charge/discharge current too. Probably past my remaining life time. But that's what my NiFes said too. I beat them!!!

Only my opinion, and you know what they say about them!

When you're charging, have you watched the difference the voltage setting makes to charging? Noted the different Amps going in to the batteries? They may charge just as well, by using a lower voltage setting in the charger, and without using any external controller.

dRdoS7
I might be wrong but if I set the capacity lower, I think the BMV will still drift as it has no reference.

A SOC reset by voltage is problematic as well because charging and discharging affect the voltage at any given SOC.

Is it a MPP solar inverter because I had the early float issue as well, there is a firmware upgrade from third parties that will fix some MPPs.

You're right about the longevity IF the sellers specifications are true.

I have noticed a significant increase in charging amps at a higher charging voltage.
 

dRdoS7

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Hi,

I might be wrong but if I set the capacity lower, I think the BMV will still drift as it has no reference.

A SOC reset by voltage is problematic as well because charging and discharging affect the voltage at any given SOC.

Is it a MPP solar inverter because I had the early float issue as well, there is a firmware upgrade from third parties that will fix some MPPs.

You're right about the longevity IF the sellers specifications are true.

I have noticed a significant increase in charging amps at a higher charging voltage.

I would have thought if it's counting amps, it should be OK. What efficency are you using? My Classic SCC is set at 98%. Over the last 3 months, the measured eff. is 98.5%. So, if I'm right, there's slightly more going than is it's displaying. The BMS I have also counts, but is always less than the Classic. I figure it's always getting to the max. because the amps tail off to 2A, below which the SCC switches to Float.

Mine is a MPP 5048MG, so no patched FW. I haven't read of any factory FW that fixes it either. AFAIK, MPP haven't even acknowleged that there's a problem. Same response to the Error #8 where the inverter trips if the panels are wet, and the inverter is not running on battery. At least there's a workaround for that. I reported it to MPP, and got an "Our engineers are looking in to it" reply. I've had it for nearly 4 1/2 years.

dRdoS7
 

Csimmer

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Hi,



I would have thought if it's counting amps, it should be OK. What efficency are you using? My Classic SCC is set at 98%. Over the last 3 months, the measured eff. is 98.5%. So, if I'm right, there's slightly more going than is it's displaying. The BMS I have also counts, but is always less than the Classic. I figure it's always getting to the max. because the amps tail off to 2A, below which the SCC switches to Float.

Mine is a MPP 5048MG, so no patched FW. I haven't read of any factory FW that fixes it either. AFAIK, MPP haven't even acknowleged that there's a problem. Same response to the Error #8 where the inverter trips if the panels are wet, and the inverter is not running on battery. At least there's a workaround for that. I reported it to MPP, and got an "Our engineers are looking in to it" reply. I've had it for nearly 4 1/2 years.

dRdoS7
My BMV is set at 98% which seems pretty good so far. The firmware fixes I mentioned are third party and I found them here https://forums.aeva.asn.au/viewtopic.php?f=64&t=4332
 

dRdoS7

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Hi,

My BMV is set at 98% which seems pretty good so far. The firmware fixes I mentioned are third party and I found them here https://forums.aeva.asn.au/viewtopic.php?f=64&t=4332

I am on that forum too.

No Float Bug fix for the MG sadly. I'm pretty sure the Error #8 isn't fixed. There is an external 08 "fix" (isolates the PV if it's >70V), for which I have bought the main part, but have yet to go to an electronics supplier for the other minor parts. Have been running off solar & battery 99.99% for the past 4 months, so it hasn't affected me. If we go away on short trip, I'll manually isolate the PV to to the inverter. I turn eveything off if it's going to be more than a few weeks.

dRdoS7
 

CDell

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I've quickly scanned thru the forum concerning chargers. All seem to say constant current then voltage. Why can't it be just voltage . I have a Xantrex c-60. Can anyone tell me if it will work?
 

jasonhc73

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I've quickly scanned thru the forum concerning chargers. All seem to say constant current then voltage. Why can't it be just voltage . I have a Xantrex c-60. Can anyone tell me if it will work?
When a (any) charger charges, it pushes current until the voltage nears the desired voltage. Then the current lowers until the voltage equalizes.

That is just how chargers work.
 

CDell

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When charging from solar panels the available current is not the same all day. So when they said charge a lifepo4 with a constant current it can't be done. So why have mppt charger if the current is going to vary anyway. I'm a beginner so bear with me
 

smoothJoey

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When a (any) charger charges, it pushes current until the voltage nears the desired voltage. Then the current lowers until the voltage equalizes.

That is just how chargers work.
Current flow from the charger to the battery is determined by the voltage differential between the charge voltage and the battery voltage.
Until the voltage differential based draw is at or below the chargers rated amperage the charger regulates the current flow by adjusting the charge voltage down.
When the charger no longer has to adjust the charge voltage down then you are in cv mode.
 

CDell

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Doesn't a MPPT charger supply a constant current for the first 60% no matter the voltage?
 
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