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Old 06-09-2021, 11:34 PM   #29
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It is always interesting to go back and reread through that white paper.

My take on it is a lithium battery can be charged to 100% SOC as low as 3.42vpc (13.68v) as mentioned in the paper a few sections above where you quoted. The difference with 3.35vpc (13.4v) is this voltage will hold a “fully charged” lithium battery at or near 100% SOC, but I don’t think it will charge a discharged lithium battery to 100% at that voltage. The two are not the same.
I'd say any voltage that will hold a LiFePO4 at 100% SOC will also get it to 100% SOC ........ though it might take hundreds of hours at 13.4V. I don't think a battery that eventually reaches some chemical state where it rests at it's nominal open circuit voltage (usually about 13.34V) can have two different SOCs.

For your theory to hold, a LiFePO4 battery sitting at a given voltage (whether 13.4 or 13.6) could have two or more possible SOC levels. That can happen transiently (i.e., the voltage is settling after being charged) but it's not possible in the steady state.

For instance, you charge a LiFePO4 to 14.4V (call that 100% SOC) and the charger switches to float at 13.6V. The battery voltage will fall slowly to 13.6V and sit at that 13.6V and remain at 100% SOC (or maybe 99.8% since it takes some internal self-discharge to bring the voltage down to 13.6V).

Now take the same LiFePO4 battery at 80% SOC and apply a CC charger set at 13.6V (with no switch to float or a switch to float at 13.6V). The battery will draw a charge current and that current will eventually go to zero with the battery sitting at 13.6V indefinitely.

So, intuitively, the battery chemical state is the same in both cases. In either case removing the 13.6V source will let the battery voltage settle and it will go to 13.34 (typically) which is considered 100% SOC. So, can the battery be at two different charge states from these two different charge processes that leave the battery sitting at 13.6 volts? I don't think so. Similarly, a LiFePO4 sitting at, say, 13.20V, will be at some particular SOC regardless of what process got the battery to that voltage. LiFePO4 SOC vs voltage curves are a single line, not a band of possible SOCs. At least in the papers I've read.

I think there's some redundancy in the above paragraphs. I should not write at this late hour.
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Old 06-10-2021, 10:49 AM   #30
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I'd say any voltage that will hold a LiFePO4 at 100% SOC will also get it to 100% SOC ........ though it might take hundreds of hours at 13.4V. I don't think a battery that eventually reaches some chemical state where it rests at it's nominal open circuit voltage (usually about 13.34V) can have two different SOCs.

For your theory to hold, a LiFePO4 battery sitting at a given voltage (whether 13.4 or 13.6) could have two or more possible SOC levels. That can happen transiently (i.e., the voltage is settling after being charged) but it's not possible in the steady state.

For instance, you charge a LiFePO4 to 14.4V (call that 100% SOC) and the charger switches to float at 13.6V. The battery voltage will fall slowly to 13.6V and sit at that 13.6V and remain at 100% SOC (or maybe 99.8% since it takes some internal self-discharge to bring the voltage down to 13.6V).

Now take the same LiFePO4 battery at 80% SOC and apply a CC charger set at 13.6V (with no switch to float or a switch to float at 13.6V). The battery will draw a charge current and that current will eventually go to zero with the battery sitting at 13.6V indefinitely.

So, intuitively, the battery chemical state is the same in both cases. In either case removing the 13.6V source will let the battery voltage settle and it will go to 13.34 (typically) which is considered 100% SOC. So, can the battery be at two different charge states from these two different charge processes that leave the battery sitting at 13.6 volts? I don't think so. Similarly, a LiFePO4 sitting at, say, 13.20V, will be at some particular SOC regardless of what process got the battery to that voltage. LiFePO4 SOC vs voltage curves are a single line, not a band of possible SOCs. At least in the papers I've read.

I think there's some redundancy in the above paragraphs. I should not write at this late hour.
While I donít have the test equipment Rod has to come to the conclusions that he did in his paper the fact remains that he states a lithium battery can be fully charged at a voltage as low as 13.68v, not lower. He also states that a fully charged lithium battery can maintain near 100% SOC at 13.4v.

I am not saying a lithium nor any battery can have two different SOC there can only be one. I am referring to charge voltages not resting voltages.

While a low value there is still a small amount of resistance to charging (ie not 100% efficient) as well as self discharge to a lithium battery. Would 13.4v be enough to overcome these inefficiencies and maintain a near 100% SOC, I think so, is it enough to fully charge a discharged battery I donít believe so. I can think of no other plausible explanation for Rodís differences in the two paragraphs, can you?
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Old 06-11-2021, 03:55 PM   #31
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While I donít have the test equipment Rod has to come to the conclusions that he did in his paper the fact remains that he states a lithium battery can be fully charged at a voltage as low as 13.68v, not lower. He also states that a fully charged lithium battery can maintain near 100% SOC at 13.4v.

I am not saying a lithium nor any battery can have two different SOC there can only be one. I am referring to charge voltages not resting voltages.

While a low value there is still a small amount of resistance to charging (ie not 100% efficient) as well as self discharge to a lithium battery. Would 13.4v be enough to overcome these inefficiencies and maintain a near 100% SOC, I think so, is it enough to fully charge a discharged battery I donít believe so. I can think of no other plausible explanation for Rodís differences in the two paragraphs, can you?
Good observations. I don't know what Rod had in mind. The very precise number suggests he's not saying something like charging is too slow to be useful below 13.68V.

I'm headed out for a couple of weeks, but will look into this when I get back. I have the equipment to charge and monitor charging at lower voltages. E.g., I can top-off my battery (14.4V), draw it down to 90%, and apply 13.5V for a few days and watch the current and SOC. The SOC monitor will be reset when I top off so should still be quite accurate during this process.

Not very meaningful, but at this minute, a 300W solar array is charging my 300 Ah Lifeblue at about 6 amps with the battery at 41% SOC (open circuit voltage around 13.1V I'd guess) and the solar charger set to charge to 13.2V and float at 13.2V. The question is what "13.2" means. It's probably rounded up or down and not all that accurate to start with; presumably it's the 0 current float voltage. I've never checked it with a voltmeter. I do know the BMS reads lower than the solar controller at times when they should agree (zero current flow). Though the BMS only outputs three significant figures as well so isn't very meaningful. The BMS does tell me the individual cell voltage to four significant figures so I could add those up. Someday.
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Old 06-11-2021, 06:50 PM   #32
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Thanks for the very extensive discussion!



In the meantime I have installed the new LiFePo4 battery, set the Progressive converter to the Lithium settings and got it all to work. Also, I set the Renogy solar controller to its generic Lithium setting (without the float function).


So far I have been camping only with shore power and noticed the converter/charger brought it up to 14.5V without further current displayed on the battery monitor. 5 days later the voltage is still 14.5V. I guess that is because the battery just does not take more charge and the converter just keeps this constant while supplying the 12V equipment. I hope that is O.K. for the battery.



Oh well, next 4 days will be boondocking and I can check what the solar controller is doing.
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Old 06-11-2021, 10:07 PM   #33
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Thanks for the very extensive discussion!



In the meantime I have installed the new LiFePo4 battery, set the Progressive converter to the Lithium settings and got it all to work. Also, I set the Renogy solar controller to its generic Lithium setting (without the float function).


So far I have been camping only with shore power and noticed the converter/charger brought it up to 14.5V without further current displayed on the battery monitor. 5 days later the voltage is still 14.5V. I guess that is because the battery just does not take more charge and the converter just keeps this constant while supplying the 12V equipment. I hope that is O.K. for the battery.



Oh well, next 4 days will be boondocking and I can check what the solar controller is doing.
Actually keeping a lithium battery a 14.5v for 5 days is very harmful to the battery. It should charge to 14.xx volts and then STOP charging. If your type of lithium batteries have balance boards then leaving them at 14.xx volts for a short period of time (think minutes not days) to balance the cells occasionally is fine but what you are doing will ruin them in a few hundred cycles.

Why not set up a CC/CV charge profile (if able) on your converter/charger and solar controller and be done with it. Not a big fan of these chargers with ďlithiumĒ settings because most of them donít match what is required for a particular lithium battery.
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Old 06-11-2021, 10:16 PM   #34
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Good observations. I don't know what Rod had in mind. The very precise number suggests he's not saying something like charging is too slow to be useful below 13.68V.

I'm headed out for a couple of weeks, but will look into this when I get back. I have the equipment to charge and monitor charging at lower voltages. E.g., I can top-off my battery (14.4V), draw it down to 90%, and apply 13.5V for a few days and watch the current and SOC. The SOC monitor will be reset when I top off so should still be quite accurate during this process.

Not very meaningful, but at this minute, a 300W solar array is charging my 300 Ah Lifeblue at about 6 amps with the battery at 41% SOC (open circuit voltage around 13.1V I'd guess) and the solar charger set to charge to 13.2V and float at 13.2V. The question is what "13.2" means. It's probably rounded up or down and not all that accurate to start with; presumably it's the 0 current float voltage. I've never checked it with a voltmeter. I do know the BMS reads lower than the solar controller at times when they should agree (zero current flow). Though the BMS only outputs three significant figures as well so isn't very meaningful. The BMS does tell me the individual cell voltage to four significant figures so I could add those up. Someday.
You might be able to get some general useful information from your tests but without a regulated power supply with dedicated voltage sense leads etc not sure how accurate it will be to one decimal place let alone two. Let us know how things go.
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Old 06-11-2021, 10:52 PM   #35
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You might be able to get some general useful information from your tests but without a regulated power supply with dedicated voltage sense leads etc not sure how accurate it will be to one decimal place let alone two. Let us know how things go.
Should be fine. It's my solar controllers that are a bit vague about what voltage they are providing. For this test I have a WFCO 55 amp converter that won't go into bulk mode at LiFePO4 voltages, so is always in float mode. There's a pot on the control board that lets me adjust the float voltage from 12.X to 14.1 volts. The regulation is fairly tight so it puts out a good bit of current when its open circuit voltage isn't much above the battery voltage. I'll set the open circuit voltage at 13.5V using the Fluke and hook it up to the battery.

The BMS only gives me one decimal place, but also gives me individual cell voltages to the millivolt. I don't know how well calibrated the cell voltage sensors are, but they do show as little as 2mv difference after a charge so are likely pretty good. I can add those four to get the LiFePO4 battery voltage on the battery side of the BMS to three places after the decimal (I can't get to the cells with the Fluke). BMS voltage drops are very small ... my Overkill BMSs have only 10 or 11 mv drop when charging or discharging at about 30 amps. I'd guess the Lifeblue is in that ball park and that will let me get all voltages (cells, BMS terminals, and charger voltage) plenty accurate for this purpose.

The BMS SOC monitor also seems very accurate. I've had occasion to draw a constant current (90 amps for 20 minutes) and compare the Ah with the SOC. The Lifeblue BMS has a built-in Ah test that updates if you get the battery low enough. My last update indicated 322 Ah and I use that as 100% SOC.

Should work fine.
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Old 06-18-2021, 05:32 PM   #36
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Thanks also for the additional info.
That constant charge voltage held at 14.5V made me nervous and I am now just going to turn off the battery disconnect switch when on shore power (except if I need that to recharge the battery).



I am wondering if Progressive Dynamics just added the switch for lithium batteries with a different end voltage to be "compatible" with lithium without exactly knowing what they are doing. But I would hate to replace the converter/charger because it is integral to the power distribution and breaker system.


I am even wondering to build my own disconnect system that turns off shore power charging with a solenoid switch when reaching 14.5V and turns it back on when the battery voltage drops below e.g. 13.2V . If there is sufficient solar panel output it would never turn on.



Or is there something on the market that does that already?
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Old 06-30-2021, 07:48 PM   #37
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Test done. You can charge a LiFePO4 battery with 13.5V

Quote:
Originally Posted by 757driver View Post
While I don’t have the test equipment Rod has to come to the conclusions that he did in his paper the fact remains that he states a lithium battery can be fully charged at a voltage as low as 13.68v, not lower. He also states that a fully charged lithium battery can maintain near 100% SOC at 13.4v.

I am not saying a lithium nor any battery can have two different SOC there can only be one. I am referring to charge voltages not resting voltages.

While a low value there is still a small amount of resistance to charging (ie not 100% efficient) as well as self discharge to a lithium battery. Would 13.4v be enough to overcome these inefficiencies and maintain a near 100% SOC, I think so, is it enough to fully charge a discharged battery I don’t believe so. I can think of no other plausible explanation for Rod’s differences in the two paragraphs, can you?
A couple of weeks ago I promised to see what sub-13.68V charging would do on a LiFePO4 battery. Long story short, you can charge at 13.5V and get to 100%. It just takes a while. Here's the story.

I was planning on topping off the battery to reset the SOC monitor then discharge to 90% but didn't do that. After a recent few weeks out in the wilds I got home with the 300 AH Lifeblue LiFePO4 bms reading 68%. I put the solar on to get it up to 90% sitting in the yard. It was surely close to 90% but the SOC might have drifted a bit and might have been off by a percent (last top-off was a couple of days before getting home).

So I connected up the old WFCO converter with the float voltage set at 13.50V with no load on the WFCO. And connected it to the LiFePO4. Initial current was about 12 amps (the WFCO does not have tight regulation). Within about six hours the current had dropped to about an amp. At about 8 hours the current was down to 0.3 amps. The SOC was reading 97%. The Lifeblue bms does not register charge or discharge current under 0.7 amps, so I'm guessing the SOC monitor stopped advancing when the current dropped below 0.7 amps at around 7 hours.

So, without a reliable SOC reading, I turned the solar (set at 14.4V) back on and watched the voltage for about 7 minutes. The current was 17 amps and voltage rose at an increasing rate from 13.5 to 14.0. I concluded the test when voltage hit 14.0V which indicates about 99% SOC. So it took about 2 Ah (17 amps for 7 minutes) to get the 300 Ah battery from where the WFCO left it up to about 99% or a bit more. Hence the WFCO set at 13.5V had taken the LiFePO4 up from about 90% to over 98% after 8 hours and was still charging (albeit at 0.3 amps).

With the WFCO sitting at 0.3 amps after about 8 hours, and drifting down from there albeit slowly, it seems clear that another 10 or 15 hours (averaging about 0.15 amps) from the WFCO sitting at 13.5V would have gotten the LiFePO4 up to 100%.

So, I don't know what Rod had in mind when mentioning the 13.68V requirement. My Lifeblue LiFePO4 clearly will get to 100% with less than 13.68V. And, presumably will do so at 13.4V; though that might take multiple days.
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Old 06-30-2021, 08:02 PM   #38
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Thanks also for the additional info.
That constant charge voltage held at 14.5V made me nervous and I am now just going to turn off the battery disconnect switch when on shore power (except if I need that to recharge the battery).

I am wondering if Progressive Dynamics just added the switch for lithium batteries with a different end voltage to be "compatible" with lithium without exactly knowing what they are doing. But I would hate to replace the converter/charger because it is integral to the power distribution and breaker system.

I am even wondering to build my own disconnect system that turns off shore power charging with a solenoid switch when reaching 14.5V and turns it back on when the battery voltage drops below e.g. 13.2V . If there is sufficient solar panel output it would never turn on.

Or is there something on the market that does that already?
I'm not aware of anything on the market that would do this but haven't looked. I'm in the same boat, but in three years since I bought the PD, I have never used it. If I ever do, it will be a manual operation to get the batteries up to 50%; hoping for better solar the next day.
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Old 07-01-2021, 07:58 PM   #39
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I'm not aware of anything on the market that would do this but haven't looked. I'm in the same boat, but in three years since I bought the PD, I have never used it. If I ever do, it will be a manual operation to get the batteries up to 50%; hoping for better solar the next day.
Same here, I did not find anything for this purpose. Oh well, got to do it myself. I bought a 250A relay and a few other components and will hack it together next week. It should turn off charging at 14.4V and turn it back on when below 13.2V .

It is simple enough except the window comparator IC (MIC841) is a teenie-tiny SC70 component that I can barely see, much less solder and my eyes are not that great anymore. I will report back when (or if) I get that working.
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Old 07-05-2021, 07:08 PM   #40
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So I got that thing to work...
Not exactly pretty but as good as my eyes (need new glasses I guess) and my shaky hands allow. That tiny comparator chip almost killed me. I ruined 4 of them before I got one soldered that worked. Fortunately they are only 90 cents each.
This gadget measures battery voltage and turns off the big fat relay to the converter/charger at 14.0 volts and turns it back on when the battery voltage drains to 13.0 V. Some pics and slightly modified schematic below. I would love to do this in a more professional way on a custom PCB but there is no point for a one-off.
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Old 07-06-2021, 12:21 AM   #41
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So I got that thing to work...
Not exactly pretty but as good as my eyes (need new glasses I guess) and my shaky hands allow. That tiny comparator chip almost killed me. I ruined 4 of them before I got one soldered that worked. Fortunately they are only 90 cents each.
This gadget measures battery voltage and turns off the big fat relay to the converter/charger at 14.0 volts and turns it back on when the battery voltage drains to 13.0 V. Some pics and slightly modified schematic below. I would love to do this in a more professional way on a custom PCB but there is no point for a one-off.
Well done! Takes me back to my ham radio days seeing a truly home brew project.
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Old 08-07-2021, 10:06 PM   #42
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I am running the PD 4655L and 2x 100ah Renogy Lifepo4 and I have had no issues. The Renogy battery has a BMS which manages the charging and balancing of the batteries. If you haven't already purchased the battery monitor I can recommend it. The BMS accepts the 14.6 but steps it down and manages the voltage and current to charge the cells. I also built a 105ah Lifepo4 power center for my trolling motor and around the campsite power needs and installed a very similar BMS into the case that I can access via bluetooth and modify parameters to fit the cell characteristics and use requirements. It is far more capable than a cell balancer that I use for my RC electric airplanes and drones.
The manual for the 100 Ah Renogy LiFePO4 battery says:
"The battery management system (BMS) provides comprehensive protection to the battery and manages the charging and discharging process"
This sure sounds like the BMS does what you say. But, if you read down through the protection table on page 15, you will find that like virtually all BMSs, the Renogy BMS does not "manage" charging or discharging, it simply disconnects the battery if charge current exceeds some threshold and re-connects the battery when charge current drops below another threshold. Of course, this makes no sense because the charge current can't drop below some threshold if the battery has been disconnected. It might possibly mean that if charging current is re-applied and goes above the lower threshold, the battery will be disconnected again. While this is conjecture on my part, it is clear that the Renogy BMS does not limit charging current to a safe level. Like all others, it simply disconnects the battery when charge current gets too high. I would not call this "managing" charging. The tech writer that prepared the manual clearly did not understand the BMS and did not spend enough time with the engineers.

It's interesting that the specified charge current is 50 amps but the BMS disconnects only at 100 amps. Though there is a warning at 60 amps. Maybe the warning is what Renogy is calling "managing" charge; i.e., telling you it's too high so you will reduce it.

And, of course, the BMS can't "manage discharge" since that would imply shutting off your appliances to keep discharge current within bounds. Again, as the table indicates, the BMS simply disconnects the battery if the discharge current gets too high. Typical of all BMS designs.

There's also mention of "standard charging" of a 100 Ah LiFePO4 being 20 amps in the Renogy manual (and discharging as well). I think that's a carryover from lead-acid technology that really does not apply to LiFePO4 because losses are so low in a LiFePO4 battery.

It sure would be nice if the battery makers did a better job with their manuals. I've read a lot of manuals, and every one of them has errors of one kind or another.
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