Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 15-09-2013, 17:25   #361
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Quote:
Originally Posted by OceanSeaSpray View Post

No, you are completely wrong here. The cell stabilised open circuit voltage is the only one relating to SOC, and it takes at least 30 minutes for it to stabilise after a disconnect.
Refer to the research paper from a Chinese University I posted a link to earlier. It contains a huge amount of fundamental data about that chemistry and cell behaviour. It also highlights that cell voltage only reliably relate to SOC in three specific parts of the curve.

It is very important to get this right. I see a lot of references to "charging to 98% with low voltage cut-off limits etc" and there are issues with overcharging etc. Overcharging would need to be over 100% SOC.
Exactly. What I said , you've said. " three parts of the curve " in general using voltage in Li is not a good indication of charge. You can use it near full and near empty , but the rest of the time its not that useful, particularly in large capacity configurations.

Dave
__________________

__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 15-09-2013, 17:51   #362
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by OceanSeaSpray View Post

It will lose accuracy indeed. I think your best bet would be tracking internal resistance during charging instead and you are one of the few who can actually do it because your system provides you with voltage and current and has the ability to perform calculations.

There is some info about the rise in internal resistance as charge approaches the end in a table at Li-Ion BMS - White Paper - Balancing cells by parallelling.
This rise in resistance is quite sharp and causes the "knee" in the charging curve. Problem is that the knee is only clearly visible in terms of voltage when there are no restrictions on power.
With a solar system, I am hoping to detect the knee by watching internal resistance. It could also provide that reset point for an energy monitor, albeit not a reset at 100%.
I have not tried in the BMS, however, the PowerLab I use for cell testing measures IR. The figures on the 30-40 cells tested so far are not consistent enough to use IR. The high and low cells vary from Pos 1 to 4, depending on the test, so It's probably right. Normal variation is not far from the low cell indicating 1/2 of the high. I have discovered 10 times the low equals a poor terminal connection....

Thanks for pointing out that Chinese presentation again, I missed it the first time around. I am not yet convinced that OCV or rested voltage is the only accurate voltage measurement to estimate SOC. One significant difference is in general our discharge/charge rates are fractions of EV's, the values tested. 1/8 or .125C is about the highest rate in my system. Smaller banks will see .3C - .5C bursts, but that's about it.

What was interesting is the OCV is different by about .02V at the same SOC, depending on if the last action was charging or discharging. I'm going to run a few tests tomorrow and see how true that is at low rates. Although the difference is about .1V between charge and discharge at .3C at the same SOC.

I can post some graphs tomorrow of IR during charge from my PowerLab data sets if it would help you.
__________________

__________________
ebaugh is offline   Reply With Quote
Old 15-09-2013, 18:05   #363
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by goboatingnow View Post

Exactly. What I said , you've said. " three parts of the curve " in general using voltage in Li is not a good indication of charge. You can use it near full and near empty , but the rest of the time its not that useful, particularly in large capacity configurations.

Dave
It is however what you have to work with after 10 days without a full charge/reset of your SOC accumulator. It's not going to be perfect. I'm sure I will be able to tell 30% from 70%. Hopefully though I can get within +/- 10%.

Part of the problem is counting amps using a 200A-500A sensor for measuring average rates closer to 25A, and even harder for "efficient" boats more normally at 5A, and near zero with the fridge off. But you have to handle at least 200A for the occasional inverter load.
__________________
ebaugh is offline   Reply With Quote
Old 15-09-2013, 19:29   #364
Marine Service Provider
 
OceanSeaSpray's Avatar

Join Date: Aug 2013
Location: New Zealand
Boat: Custom 13m aluminium sloop
Posts: 416
Re: Lithium Batteries (for the rest of us)

Quote:
Originally Posted by goboatingnow View Post
Exactly. What I said , you've said. " three parts of the curve " in general using voltage in Li is not a good indication of charge. You can use it near full and near empty , but the rest of the time its not that useful, particularly in large capacity configurations.

Dave
What I said is "only on disconnected, stabilised cells", this is the difference. Otherwise, the only clear-cut indication is current drop-off at target end-of-charge voltage, i.e. 100% SOC.
If you don't complete the charge cycle, you can't make assumptions about the SOC reached, because it depends on the current-voltage combination you were using.
In order to understand this relation, people would need to charge at different C-rates and termination voltages, disconnect, wait, measure and report.

We don't have this data. It would be a big task to get it.
__________________
OceanSeaSpray is offline   Reply With Quote
Old 15-09-2013, 22:08   #365
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by OceanSeaSpray View Post

What I said is "only on disconnected, stabilised cells", this is the difference. Otherwise, the only clear-cut indication is current drop-off at target end-of-charge voltage, i.e. 100% SOC.
If you don't complete the charge cycle, you can't make assumptions about the SOC reached, because it depends on the current-voltage combination you were using.
In order to understand this relation, people would need to charge at different C-rates and termination voltages, disconnect, wait, measure and report.

We don't have this data. It would be a big task to get it.
Even if you had it, in an operational system, you won't ever see that. Normally you are either charging, or discharging. I already published data for .3C and .2C. If I can get my charging timer issue resolved, I will do .1 and .05C.

This is better data for an operating BMS. If you were charging new Forzatec cells at 15-35 degrees C at a constant .3C, the numbers were repeatable in a small window and you could get accurate results. Just don't ask about cell aging or cells from different manufacturers. I don't yet have a handle on those variables. But give me any one of the 160 cells in the box, and I can charge it at any rate from 20-30A to whatever SOC you want +/- 5% on voltage alone.
__________________
ebaugh is offline   Reply With Quote
Old 15-09-2013, 23:20   #366
Marine Service Provider
 
OceanSeaSpray's Avatar

Join Date: Aug 2013
Location: New Zealand
Boat: Custom 13m aluminium sloop
Posts: 416
Re: Lithium Batteries (for the rest of us)

Quote:
Originally Posted by ebaugh View Post
Even if you had it, in an operational system, you won't ever see that. Normally you are either charging, or discharging. I already published data for .3C and .2C. If I can get my charging timer issue resolved, I will do .1 and .05C.

This is better data for an operating BMS. If you were charging new Forzatec cells at 15-35 degrees C at a constant .3C, the numbers were repeatable in a small window and you could get accurate results. Just don't ask about cell aging or cells from different manufacturers. I don't yet have a handle on those variables. But give me any one of the 160 cells in the box, and I can charge it at any rate from 20-30A to whatever SOC you want +/- 5% on voltage alone.
No, you won't see it indeed, but it doesn't mean it is not relevant. The useful range of voltage vs SOC at the upper end all takes place more or less within 0.1V like you pointed out earlier, this is why it takes fairly good gear to sense it reliably.
If you always follow the same routine, same current, same end voltage, you will achieve repeatable SOCs, but it won't be that easy if the main source of energy is solar.

Are you able to charge a cell with 3.4V regulated until it no longer accepts current basically, disconnect, wait and measure it? It would provide an idea of what maximum SOC is achievable at that voltage. Because fully charged open circuit is about 3.42V, it should be impossible to reach 100% SOC this way anyway. Once you disconnect, the polarisation voltage will blow away.
__________________
OceanSeaSpray is offline   Reply With Quote
Old 16-09-2013, 04:58   #367
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Lithium Batteries (for the rest of us)

Amp counting on Li , is very problematic, for the reasons outlined , ie the lack of a known SOC at the termination point.

Few chargers are going anywhere near the knee point, so few are going to get to 100% SOC, after that you cannot really determine the SOC based on a given cutoff and current to zero.

What you can do , however becuase of an alomost complete lack of Peukerts effect, is to merely assume a capacity ( refined upon experience) and perform manual resets on that assumption.


Using open circuit rest voltage is very little use in a "wired-in" battery environment anyway. My own experience with LI, is the rest voltage varies very little.

It is is worth pointing out that in high C discharges you do get a voltage slope which can be used as some form of "gas gauge"



DAve
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-09-2013, 05:18   #368
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by OceanSeaSpray View Post

No, you won't see it indeed, but it doesn't mean it is not relevant. The useful range of voltage vs SOC at the upper end all takes place more or less within 0.1V like you pointed out earlier, this is why it takes fairly good gear to sense it reliably.
If you always follow the same routine, same current, same end voltage, you will achieve repeatable SOCs, but it won't be that easy if the main source of energy is solar.

Are you able to charge a cell with 3.4V regulated until it no longer accepts current basically, disconnect, wait and measure it? It would provide an idea of what maximum SOC is achievable at that voltage. Because fully charged open circuit is about 3.42V, it should be impossible to reach 100% SOC this way anyway. Once you disconnect, the polarisation voltage will blow away.
First, there is a fallacy about LFP using a CV phase to achieve additional charge, at least at the upper end of the range. I watched the cells reach their target voltage of 3.6V, the charger ramped down to maybe 5A, back up to 10A, then back off to a couple of amps in about 30 seconds. It's in the mA by a couple minutes. I can look at the data in the shop, but the additional charge is on the order of 1/4 to 1/2 Ah.

I can follow your profile, but I don't think I need additional data? In this chart, the charge data was derived by starting with a fully discharged cell. Then the cell was fully charged at a fixed rate controlled by the PowerLab and monitored for the Ah accepted and cell voltage every few seconds. The table was derived by calculating the percentage of total Ah accepted until fully charged to the voltage reading at that time. There is lots more data, I just cherry picked every 5%.

There is some variation in the data because in the posted tests, the full voltage range was different, full discharge was 2.85. 2.5 or 2.0V, depending on the test. Full charge was to 3.6 or 3.65V depending. You can see that value at 0% on the discharge and 100% on the charge. The "charge" starts at a higher voltage, that represents the recovered voltage since the banks had anywhere from one hour to overnight to recover.

The actual test data has information on 4 cells, but since every test started with a top balanced bank, the "weak cell" always fully discharged first. Then the remaining cells were individually discharged to the same bottom charge, leaving the bank bottom balanced. Finally the bank was fully charged, and the same weak cell always finished first, and it is the only cell whose data is presented here.

But take a NEW Forztec cell, charge at 30A for a single 100A cell, or .3C for 10 in parallel, stop at 3.39V and you are at 95% charge, +/- the accuracy available to mere mortals. 3.36V for 70%. This assumes you can calibrate your charging source to my test rig....I have three meters, plus the test rig that all read to 3 digits and none are the same. You can see that either age or chemistry changes the numbers for the GBS branded cells. Those are mine, with one year in service. Some of the difference may be explained (or not?) by their capacity is no longer 100Ah, so the 30A rate is more like .33-.35C, versus .3 for the Forzatec.
Attached Thumbnails
Click image for larger version

Name:	image-3345424702.jpg
Views:	48
Size:	332.5 KB
ID:	67274   Click image for larger version

Name:	image-2263537199.jpg
Views:	49
Size:	141.3 KB
ID:	67275  

__________________
ebaugh is offline   Reply With Quote
Old 16-09-2013, 15:58   #369
Marine Service Provider
 
OceanSeaSpray's Avatar

Join Date: Aug 2013
Location: New Zealand
Boat: Custom 13m aluminium sloop
Posts: 416
Re: Lithium Batteries (for the rest of us)

Quote:
Originally Posted by ebaugh View Post
First, there is a fallacy about LFP using a CV phase to achieve additional charge, at least at the upper end of the range. I watched the cells reach their target voltage of 3.6V, the charger ramped down to maybe 5A, back up to 10A, then back off to a couple of amps in about 30 seconds. It's in the mA by a couple minutes. I can look at the data in the shop, but the additional charge is on the order of 1/4 to 1/2 Ah.
Looking at manufacturer published curves, the higher the charge rate, the more absorption left at CV at the end of the cycle. Charging at small C-rates would result in exactly what you describe here, no more absorption left. This is the case of relevance for solar.

The interest is in the relation between terminal charging voltage and SOC achieved, whichever way you measure it. It would be nice to have this in terms of stabilised open circuit voltage, because this relates directly to research/manufacturer data. If the charge rate is "low enough", whatever that means, and the last bit at CV becomes pointless then the data you collected becomes much more relevant.

Just need to be careful with assumptions and measurement methods. OCV has been benchmarked, accumulated charge energy vs capacity from the previous discharge cycle etc is more of an approximation. I did wonder where you were getting the SOC figures from.
It certainly sounds like you are very well set up for carrying out this kind of testing.
The table I would be interested in is terminal charge voltage vs SOC in % of a fully charged/absorbed cell. Terminal charge voltage vs stabilised OCV for a few C-rates would be the ultimate. The profile below the terminal charge voltage doesn't really matter. Just capacity achieved for end-of-charge voltages of 3.6V, 3.55V, 3.5V, 3.45V, etc. The cell shouldn't need to do a full cycle, just low enough so it recharges at full current for a while.
__________________
OceanSeaSpray is offline   Reply With Quote
Old 16-09-2013, 19:40   #370
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by OceanSeaSpray View Post

Looking at manufacturer published curves, the higher the charge rate, the more absorption left at CV at the end of the cycle. Charging at small C-rates would result in exactly what you describe here, no more absorption left. This is the case of relevance for solar.

The interest is in the relation between terminal charging voltage and SOC achieved, whichever way you measure it. It would be nice to have this in terms of stabilised open circuit voltage, because this relates directly to research/manufacturer data. If the charge rate is "low enough", whatever that means, and the last bit at CV becomes pointless then the data you collected becomes much more relevant.

Just need to be careful with assumptions and measurement methods. OCV has been benchmarked, accumulated charge energy vs capacity from the previous discharge cycle etc is more of an approximation. I did wonder where you were getting the SOC figures from.
It certainly sounds like you are very well set up for carrying out this kind of testing.
The table I would be interested in is terminal charge voltage vs SOC in % of a fully charged/absorbed cell. Terminal charge voltage vs stabilised OCV for a few C-rates would be the ultimate. The profile below the terminal charge voltage doesn't really matter. Just capacity achieved for end-of-charge voltages of 3.6V, 3.55V, 3.5V, 3.45V, etc. The cell shouldn't need to do a full cycle, just low enough so it recharges at full current for a while.
I think we are talking semantics here? If I test at 20A or 30A, first top off the cell, charge to 3.6 then discharge to 2.85, then discharge to 3.6, the measured Ah out and in are nearly identical. Almost exactly 1% more on charge, which is expected and in line with other people's test data.

At what charge current would you terminate the constant voltage portion of the charge? I've seen specs of C/10, C/24, C/100 and finally when the current tapers to near zero. If you hold anywhere from 3.4 to 3.6V long enough, the cell will eventually fully charge. Yes, 100%. The CV portion at 3.6 or 3.55 at C/3 tapered to C/100, with new cells is very fast. Almost an immeasurable difference.

Any voltage held over 3.39-3.4 long enough will eventually overcharge your cell, according to Jack Rickard, a guy who has more practical time tinkering with LFP prismatic cells than just about anyone. But to find SOC for different low current charge terminations at say 3.4-3.45V will be voodoo magic.....

I ended a 3 day trip once holding 3.35V, started the genset almost right after I shut down the mains, and was only able to add a couple few amp hours, 2-3% SOC. I've since knocked the alternators back even more.

My original goal was to remain below 90%, and so far Ive failed miserably.
__________________
ebaugh is offline   Reply With Quote
Old 16-09-2013, 20:17   #371
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
The break over point is around C/3 for little or no CV period. Definitely above C/2 you begin to see a CV tail period but I fully agree with u ebaug at the types of fractional C charge rates were using there is on need for current measuring as a cutoff. Merely terminating at a particular voltage is sufficient.

This was a point I made previously

Equally there is measurable difference in the tiny CV period for < C/3 , ie no point on attempting SoC graphing

Furthermore your point about eventually charging at low voltages left on for long time ( ie 3.3-3.4 ) is an Indication of the issues surrounding " float" charging using conventional LA chargers in Li situations

Dave
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-09-2013, 20:43   #372
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by goboatingnow View Post

Furthermore your point about eventually charging at low voltages left on for long time ( ie 3.3-3.4 ) is an Indication of the issues surrounding " float" charging using conventional LA chargers in Li situations

Dave
Yes, that, perhaps along with a better understanding of ambient temperature, which I originally believed insignificant, are the 64 million dollar questions surrounding maximizing cell life with LFP.
__________________
ebaugh is offline   Reply With Quote
Old 16-09-2013, 23:41   #373
Marine Service Provider
 
OceanSeaSpray's Avatar

Join Date: Aug 2013
Location: New Zealand
Boat: Custom 13m aluminium sloop
Posts: 416
Re: Lithium Batteries (for the rest of us)

Quote:
Originally Posted by ebaugh View Post
At what charge current would you terminate the constant voltage portion of the charge? I've seen specs of C/10, C/24, C/100 and finally when the current tapers to near zero. If you hold anywhere from 3.4 to 3.6V long enough, the cell will eventually fully charge. Yes, 100%. The CV portion at 3.6 or 3.55 at C/3 tapered to C/100, with new cells is very fast. Almost an immeasurable difference.

Any voltage held over 3.39-3.4 long enough will eventually overcharge your cell, according to Jack Rickard, a guy who has more practical time tinkering with LFP prismatic cells than just about anyone. But to find SOC for different low current charge terminations at say 3.4-3.45V will be voodoo magic.....

I ended a 3 day trip once holding 3.35V, started the genset almost right after I shut down the mains, and was only able to add a couple few amp hours, 2-3% SOC. I've since knocked the alternators back even more.

My original goal was to remain below 90%, and so far Ive failed miserably.
This is interesting. SOC once current has tapered off to zero at lower end voltages would be the data of interest.
There has to be a voltage limit that gives 90% SOC only. 3.35V/cell is impressively low already! Very strong hint that there is no point pushing it.

The idea is identifying the voltage that doesn't result in overcharging the cell, 3.35V seems to stand as a first candidate for an upper limit. You still had a little headroom after 3 days, i.e. there was no current flowing in any more and SOC was <100%.
It would be hard to claim that a cell will overcharge there.
__________________
OceanSeaSpray is offline   Reply With Quote
Old 17-09-2013, 04:33   #374
Registered User
 
ebaugh's Avatar

Join Date: Dec 2011
Location: On the boat
Boat: DeFever 44
Posts: 525
Quote:
Originally Posted by OceanSeaSpray View Post

This is interesting. SOC once current has tapered off to zero at lower end voltages would be the data of interest.
There has to be a voltage limit that gives 90% SOC only. 3.35V/cell is impressively low already! Very strong hint that there is no point pushing it.

The idea is identifying the voltage that doesn't result in overcharging the cell, 3.35V seems to stand as a first candidate for an upper limit. You still had a little headroom after 3 days, i.e. there was no current flowing in any more and SOC was <100%.
It would be hard to claim that a cell will overcharge there.
Hmmm...the results from that trip are not consistent with my charge data unless there is a long period of CV at 3.35. Thinking about it, I'm now curious.

I can't duplicate the temperature, or the current profile exactly like that trip. My alternators are not finely regulated, ie. loads like the microwave force a discharge, then they catch back up. But I am interested enough to explore 3.35V in the lab so to speak. I will let you know what it looks like. I will also recheck the cell voltage readings on my Elite Power BMS. As I recall it indicates a little low, so 3.35 might be closer to 3.37. Makes no difference at 3.6 or 2.5, but would mean quite a lot here.

I also don't remember the SOC at the beginning of the trip. If fully charged that would have an impact on the observed behavior too. I may have mentioned it in a post here at the time, I will go look. The Forzatecs came out of the box at 3.25-3.3 and were over 95%, don't ask me why they shipped them at 100%, I don't know.
__________________
ebaugh is offline   Reply With Quote
Old 17-09-2013, 05:47   #375
Marine Service Provider
 
Maine Sail's Avatar

Join Date: Jan 2007
Location: Maine
Boat: CS-36T - Cupecoy
Posts: 3,060
Re: Lithium Batteries (for the rest of us)

Quote:
Originally Posted by ebaugh View Post
Hmmm...the results from that trip are not consistent with my charge data unless there is a long period of CV at 3.35. Thinking about it, I'm now curious.

I can't duplicate the temperature, or the current profile exactly like that trip. My alternators are not finely regulated, ie. loads like the microwave force a discharge, then they catch back up. But I am interested enough to explore 3.35V in the lab so to speak. I will let you know what it looks like. I will also recheck the cell voltage readings on my Elite Power BMS. As I recall it indicates a little low, so 3.35 might be closer to 3.37. Makes no difference at 3.6 or 2.5, but would mean quite a lot here.

I also don't remember the SOC at the beginning of the trip. If fully charged that would have an impact on the observed behavior too. I may have mentioned it in a post here at the time, I will go look. The Forzatecs came out of the box at 3.25-3.3 and were over 95%, don't ask me why they shipped them at 100%, I don't know.
3.35V will give me, on my Winston's, a longer current taper if I am using a large charge source......
__________________

__________________
Marine How To Articles
Maine Sail is online now   Reply With Quote
Reply

Tags
paracelle

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
I Only Have Two Batteries - Which Batteries Should I Use? LifesAnAdventure Electrical: Batteries, Generators & Solar 11 04-06-2014 19:29
Eliminating a Battery Isolator R_C Electrical: Batteries, Generators & Solar 21 19-09-2013 03:42
Lithium Batteries in Handheld VHF Sets Dockhead Marine Electronics 10 26-10-2011 21:52
Killed Batteries ? Dockhead Electrical: Batteries, Generators & Solar 31 27-08-2011 05:14
Voltage drop under load, amps read 99% ?? VVD Electrical: Batteries, Generators & Solar 25 28-06-2011 16:25



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 17:01.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.