Solar charge controller, what for?

[bungawalbyn]

Some people may not realize that what Sunking refers to as Float is not a pure constant voltage charging algorithm. It is a constant terminal voltage algorithm modified by adding an upper bound on the charging current so that a low SOC battery will not see an enormous overcurrent.
Since both the current limit and the voltage limit are constantly in effect, it is called a single stage algorithm, even though the actual voltage at the battery terminals will vary as the SOC increases.
If you look at the current and voltage versus time, you will see a gradual transition from a constant current to a constant voltage regime rather than the abrupt shifts in curve shape that mark multiple stages.

I sort of figured that out in the end. Sunking cheated, he changed the parameters to what would normally be float say in FLA . While current limit may be useful at really low SOC, it's perhaps a limiting setup when the great thing about LiFePO4's is their ability to effectively take all the charge you can give them in a solar setup. That's what attracted me mostly to LFP. With 1000AHRs charging at 100amps is only 0.1C

[Living Large]

I've noticed that often in forums, threads degenerate into acrimonious argument over minor side issues. Hopefully we can avoid that here.

This isn't an argument - simply stating a fact that today, FLA is the gold standard. You may not like them, and *I* have no interest whatsoever in using them though some here strongly encouraged me to.

What caught my eye is an apparent denial that FLA is the gold standard, and then a swerve to something that may be a standard in the future. Hopefully that day will come sooner than later, and a quantum leap in storage can be enjoyed by the average person installing a battery bank for storage - without having to worry that one of multiple "small" oversights can turn a $10,000 bank into rubbish.

Me? I'd have no problem using LFP, like you. But they certainly aren't for the average user. All I was hoping is you would admit that FLA is the standard today. If you can't do that, so be it. We'll agree to not agree.

[bungawalbyn]

Yeah alright, I'll agree they are the standard, never said otherwise, it was the gold bit. Though we could compromise and call them the iron pyrite standard .

[inetdog]

Yeah alright, I'll agree they are the standard, never said otherwise, it was the gold bit. Though we could compromise and call them the iron pyrite standard .

Or maybe the galena standard?

[PNjunction]

This isn't an argument - simply stating a fact that today, FLA is the gold standard. You may not like them, and *I* have no interest whatsoever in using them though some here strongly encouraged me to.

Please - can we leave the pro/con arguments to another thread perhaps designed for that purpose?

The op already has them and has put them to use. Lurkers may have done likewise.

A safety issue arises when the thread is derailed by such a high signal-to-noise ratio, that a lurker may overlook a nugget of operational / safety guidelines and present a danger to himself or others being bored to death with the politics, personal opinions, and so forth from users that have no intention of ever using it.

We're cool - I'm just pleading that to keep lifepo4 *technical* discussions relevant, they need to stay on the tech issues. Otherwise, we turn into every other forum out there derailed by accident or on purpose by the subject.

[karrak]

A little question, I have a 400A (2X200A) Shottky diode in the parts box that I was thinking might be a good idea to put on the solar input line to add extra protection for the panels and to stop any problems should I have a fault in the wiring up on the roof. It's the place where I am most likely to have them. (wind rain condensation etc). All my sensors etc will be on the battery side, so the voltage drop shouldn't affect any thing in the controls. Opinion?

You should have an individual fuse to each panel if you have more than two in parallel so that if one cell or bypass diode in any panel goes short circuit you don't get the current from all the panels going through the panel with the short.

I've got a 400A T class fuse on the way for the battery. This should cover everything running at once with some margin.

Very good, you probably know this will only protect wiring that is rated for 400A. If you have any smaller wiring you will need secondary fuses.

They are still a way above nominal, and if they keep giving like they are, there are MANY hours of useful power. Provided I can keep them from an early death, I think I'm going to be a happy camper.

If your experience is anything like ours, you will be thrilled with these batteries.

I like your term iron pyrite standard for LA batteries, galena is also good. At the moment you probably can't beat LA batteries that are on continuous float for occasional power backup or for starting vehicles etc. but for continuous cycling, if LFP batteries last the distance I think they will become the gold standard for this application.

Simon

[bungawalbyn]

I've been putting some thought into where to set my main charge on/off relay.
From looking at graphs etc and briefly observing that LFP seems to hold voltage, rather than like FLA that drops to 12.6V or 2.1V cell at rest, I was wondering where the nominal cell value of 3.2V comes from? Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?
I'm talking here about at rest. Having not done any charging as yet and only having used them a couple of times, I don't have any experience to talk from, but I have observed some bounce back in voltage after a load is removed (0.01V/cell), though minor compared to FLA. I assume it works the other way with charging. I suppose one should be able to work it out from the internal resistance? Are there any formulae out there for such a thing?

Any thoughts?

[bungawalbyn]

If your experience is anything like ours, you will be thrilled with these batteries.
.

Simon

I like the sound of that.

[inetdog]

Please - can we leave the pro/con arguments to another thread perhaps designed for that purpose?

The op already has them and has put them to use. Lurkers may have done likewise.

A safety issue arises when the thread is derailed by such a high signal-to-noise ratio, that a lurker may overlook a nugget of operational / safety guidelines and present a danger to himself or others being bored to death with the politics, personal opinions, and so forth from users that have no intention of ever using it.

We're cool - I'm just pleading that to keep lifepo4 *technical* discussions relevant, they need to stay on the tech issues. Otherwise, we turn into every other forum out there derailed by accident or on purpose by the subject.

+1,

[karrak]

I've been putting some thought into where to set my main charge on/off relay.
From looking at graphs etc and briefly observing that LFP seems to hold voltage, rather than like FLA that drops to 12.6V or 2.1V cell at rest, I was wondering where the nominal cell value of 3.2V comes from? Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?

Here is a good table I got from this website http://advrider.com/index.php?thread...ifepo4.757934/ which seems to match my observations.
LFPRestVoltages.jpg

3.2 volts is a little low and is probably a figure under load.

I'm talking here about at rest. Having not done any charging as yet and only having used them a couple of times, I don't have any experience to talk from, but I have observed some bounce back in voltage after a load is removed (0.01V/cell), though minor compared to FLA. I assume it works the other way with charging. I suppose one should be able to work it out from the internal resistance? Are there any formulae out there for such a thing?

Internal resistance is complex and will vary under different operating conditions.

In my opinion the best ways to stop charging at a particular SOC are to charge at constant current and cut off suddenly or charge to a lower voltage close to the rest voltage you want and wait for the current to taper off to say less than 0.02C.

The first approach is not much good for charging from solar as the charge from the solar panels is dependant of the sunlight which is not constant. I think the second approach is the way to go.

I have recently been experimenting with what voltage to cutoff at using the second approach and am thinking 3.4 volts/cell at 0.02C will give an SOC of around 90% and not stress the battery too much.

With your simple relay disconnect system you won't be able to control the battery charge voltage, so will have to opt for the first approach. If you cut off the charge at 3.4 volts/cell, I would think the end SOC that you will achieve will vary from around 75% to 90+% depending on the current from the solar panels.

Simon

[jkirkebo]

I've been putting some thought into where to set my main charge on/off relay.
From looking at graphs etc and briefly observing that LFP seems to hold voltage, rather than like FLA that drops to 12.6V or 2.1V cell at rest, I was wondering where the nominal cell value of 3.2V comes from? Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?
I'm talking here about at rest. Having not done any charging as yet and only having used them a couple of times, I don't have any experience to talk from, but I have observed some bounce back in voltage after a load is removed (0.01V/cell), though minor compared to FLA. I assume it works the other way with charging. I suppose one should be able to work it out from the internal resistance? Are there any formulae out there for such a thing?

Any thoughts?

My understanding of nominal voltage is that it's the energy the cell can hold divided by the number of amp-hours it can deliver. So if a cell can output 37Wh until empty and 10Ah until empty the nominal voltage is calculated as 37/10=3.7V.

[PNjunction]

I was wondering where the nominal cell value of 3.2V comes from?

I'll have to ask Prof. John B Goodenough at the Univ of Texas for that. Tip of the hat to him, as he was the pioneering father of the cells we use, (in addition to core memory and more!). Good history and corporate skullduggery afoot brought us what we have today. Otherwise, it would be all proprietary and locked up.. save that for another thread...

Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?

Close, but temperature and the actual materials used can dictate small changes from brand to brand. And we know that voltage is only an approximation. But we CAN get close with RESTED values, and most importantly, an accurate voltmeter! You spent big bucks on your battery, and if we're going to play the rested voltage to soc game, you've got to use a meter you can TRUST or has been calibrated or all bets are off. With lithium, you really should have a STANDARD to calibrate all your other monitoring / metering gear to. Multimeter threads are elsewhere. Lets just say that even though my battery is small, I don't go cheap on my instruments, but only my Fluke is the standard for all else in the system.

Here is a sampling showing the slight variances according to temperature:

http://www.bestgopower.com/technology/documents/temperature-test.html

I don't use their products, but wanted to point out how not *exactly* accurate it can be.

That being said, here are some common values that EV guys use for rested voltages at normal room temperatures for lifepo4 per-cell, and 4S configuration and I use with my large prismatics.

3.4 / 13.6v = 100 % <--- don't sit here in storage too long - oxidation
3.33 / 13.32 = 90%
3.25 / 13.00 = 50%
3.2 / 12.80v = 80% <----- good conservative value to not go below at rest! EV'ers might, but we won't with properly sized prismatics.
3.0 / 12.00v = 90-100% <-- why rest here? If found ASAP, apply a 0.05C or less charge current to *nudge* them gently back to 3.2v, then apply full current charge.

but I have observed some bounce back in voltage after a load is removed (0.01V/cell),

Yes, that is normal and the reason we need to rest them. Preferably for 12 hours, although in a pinch a few hours will do.

WARNING about "pack voltages": for an extreme mental exercise, lets say I ship you one of my fully charged 40ah GBS cells to replace one of your 1000ah cells and take SOC voltage readings. It is obvious that as we discharge the pack, the pack voltage is no indication of the TRUE soc since my weak little runt cell is the limiting figure and would be destroyed in short order if we relied upon pack voltage alone. Fortunately, at "sub-c" rates, and IF your cells are closely balanced in both capacity and internal resistance, AND we are conservative, simple diy measures from those who are willing to put in the time to monitor closely may be successful. Like WB9K points out, this is not realistic for anything but us battery geeks.

Note that if you want to get closer in accuracy, you'll want to do a real measured discharge capacity test. My GBS cells more or less followed these voltages after testing. Yours may differ slightly.

Here's a tip - are you able to obtain 80% of the fully charged rated capacity of the cells during discharge when you reach about 12.7v under load? If so, that's good enough. No need to go into the basement. Allowing for rest, this will rise a bit above to 12.8 or slightly higher *under rest* at a measly .1c rate ....

[bungawalbyn]

Thanks for the info guys,

We seem to be getting more conservative.
13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go. I guess it all gets a bit rubbery with variables such as at rest/ load, temps, charge voltage, current, constant values from mains charging and variable values from solar. And of course there is wether they are left standing at voltage or are in constant use.

Fortunately, my cut off controller is programmable, so I guess the thing to do is start conservative and adjust if necessary. I sized the bank so that I can do this. I'm not too concerned with the low end, under normal usage I don't expect to ever go there.

I guess one can get a good idea about where the knees start on a particular battery by charging into them a bit, and keeping a close eye on how the voltage is going. Like when I was bringing down the balance charge. It was easy to see when the Batteries hit the "Flat"

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.
Also I have one of these from EBAY

RC Lipo Battery Low Voltage Alarm 1S-8S Buzzer Indicator Checker Tester

P309_3_ALL_zpsba55611f.jpg

AU$2.26 free postage. These are programmable for voltage

Simon, is there a link to a description etc of your set up? I'd be interested to see it.

I'm off to town today, hopefully the bits I need to finish off the board will be waiting at the post office and I can actually get to play with my batteries.

[karrak]

We seem to be getting more conservative.
13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go. I guess it all gets a bit rubbery with variables such as at rest/ load, temps, charge voltage, current, constant values from mains charging and variable values from solar. And of course there is wether they are left standing at voltage or are in constant use.

You are right, as with most things the devil is in the detail. Winston still specify 4 volts as the maximum and if my memory serves me correctly the electrolyte will stand around 4.3 volts until the rate of the breakdown chemical reactions will damage or destroy the cell. These cells are primarily designed for high rate charge/discharge usage. Under this regime there lifespan is only around a couple of thousand cycles or 5-7 years. In our application with low rate charge/discharge usage (in my case max charge rate 0.1C and max discharge rate 0.5C) we should get more cycles out of them and we want them to last as long as possible. My understanding is that when charging, higher charge rates and higher voltages will decrease the lifespan of the cells, so my goal is try to keep the voltages down but still charge the battery to as full as possible to get a good economic return. I am not sure if it will make much difference to the lifespan if one limits the voltage to 3.4 volts compared to 3.6 volts or if one limits the SOC to say 90%, but as you can get an LFP battery to ~95% full with only 3.4 volts I don't see any reason to push things.

Fortunately, my cut off controller is programmable, so I guess the thing to do is start conservative and adjust if necessary. I sized the bank so that I can do this. I'm not too concerned with the low end, under normal usage I don't expect to ever go there.

I am interested to know what controller you are using for this.

I agree that the low end is not such an issue, lowest voltage that my battery has been down to since last November is 24.49 volts (3.06 volts/cell) under load. Most months it wouldn't go below 25 volts (3.125 volts/cell) under load. Again it is an issue of economic return as to where you set your end point. I will turn all loads off when any cell gets below 2.8 volts to stop damage to the cell, this has only happened a couple of times when the battery was new due to the battery not being balanced properly by the supplier.

I guess one can get a good idea about where the knees start on a particular battery by charging into them a bit, and keeping a close eye on how the voltage is going. Like when I was bringing down the balance charge. It was easy to see when the Batteries hit the "Flat"

If you act conservatively and stay within the 80%-20% SOC range as PNjunction suggests you probably don't need to do any close monitoring although I think you should still check to see that the batteries are staying in balance over the longer term. If you want to go outside this range for economic reasons you do have to keep an eye on things. I would always advocate some sort of monitoring and alarm system under any conditions just in case of a fault or stuffup.

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.
Also I have one of these from EBAY

RC Lipo Battery Low Voltage Alarm 1S-8S Buzzer Indicator Checker Tester

Something like this should be OK, a couple of things to look for with these sort of cheap devices.
- It needs more than 4 volts to operate so you will need to connect more than one cell to each device.
- Check that the current draw from each cell is the same otherwise in will unbalance your battery
- Being able to calibrate it is a nice feature.

I went for the Cellog 8, as it can be calibrated, the more expensive one will store data that can be downloaded. For use with more than six cells it has a problem which is detailed here http://endless-sphere.com/forums/vie...2&hilit=cellog. With four cells it should be OK.

Regarding calibration, you can check how accurate your multimeter is with a low cost voltage standard from this crowd http://www.voltagestandard.com/, for checking LFP batteries I think accuracy down to 10mV is probably enough.

Simon, is there a link to a description etc of your set up? I'd be interested to see it.

My system is a "Work in Progress" and I would be pounced on if I posted pictures of it. I might post pictures of a system I have installed for a friend when we reinstall it in its new and permanent home in the next few months. Here is a description of my system from a previous post http://www.solarpaneltalk.com/showth...l=1#post154470, One thing not mentioned in that post is that the batteries are from Winston.

Simon

[PNjunction]

13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go.

I don't normally run 13.6v, but no more than 14v. The only time I run up to 13.6v rested is when I'm doing a discharge capacity test to vet the cell performance down to 12.8v rested. It must meet 80% of rated capacity for me. Normally I'm conservative like others.

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.

Precisely the reason we use large prismatics to keep the cell count and balance complexity down! No one-off enthusiast buckets full non-lifepo4 cells pulled from crashed cars or laptops here.

The importance was made of using a voltmeter you TRUST to prove the adequacy of other components, such as those ebay cheapies and calibrate or make chart-conversions for. It really does pay to get a good voltmeter, and one you can trust. Did I mention trust?

Also be careful of stuff hanging off your batteries, which can be potential points of failure or UNbalancing devices if they go wrong. Many guys use JUNSI cell monitors, but to do that right, you run that from it's OWN SEPARATE battery. At 1000ah, even if the cell monitor starts to fail, you may not notice it, but why not do it right from the outset and run it from it's own battery / supply? And oh yeah, the JUNSI or devices like them benefit from being vetted by .... wait for it ... a voltmeter you can trust!