Solar charge controller, what for?

[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

[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,

[bungawalbyn]

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

Simon

I like the sound of that.

[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?

[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

[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.

[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?

[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 .

[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]

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

[bungawalbyn]

Because of the cable size and special element I would be temped to look at getting a cheap 1kW inverter and use a standard 240 volt AC element.


Simon

That's how I'm going (probably), the 1000w element is 240V AC and I have had it operate from a 1500w elcheapo inverter inadvertently. Forgot to turn it off when switching from generator. Most of the time the overload cut out works on them. I've never fried one yet, and I work them hard with powertools etc. I'll give it it's own inverter.

I find the modified square wave inverters to be good value, The only thing I've had that doesn't like them was a cheap fan, and it still worked ok, the motor just buzzed a little. Electronics, no problem. After a few deaths I found the best way to increase their lifespan is don't keep turning them on and off. Leave them on unless you don't need them for days. They use a little more juice than the high end sine wave inverters because they don't go to stanby, but it's not that much and it's a benefit with some thermostats.

[inetdog]

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.

[karrak]

OK, now we are singing from the same sheet on "float" let me run this idea past you.

To use excess charge, slightly before batteries hit the predetermined cut off voltage, 80ish% SOC, I was planning on having another voltage sensor to operate a 1000w HWS element, which is a close match, taking into account efficiency loses etc, to the maximum output of my panels. I'd keep the hi lo points on this tighter than the main charge control sensor, so it acts as a "float" controller.

I have a plan b on this which is to allow the batteries to hit cut off and divert the power direct to a 12V element. My cut of relay is a 120Amp SPDT I sourced so I have the option of load dumping/diversion. The other is the preferred option as cutting charge at full bore I would imagine is a bit rough on the relay contacts, + wire length to HWS and I'd have to find a suitable 12V element.

I intend doing something like this on a friends system I have installed using the controller I gave you the link for.

Because of the cable size and special element I would be temped to look at getting a cheap 1kW inverter and use a standard 240 volt AC element.

Twelve volts does make thinks difficult if you want to use high power devices.

Simon

[PNjunction]

Ok, cool - just had to throw that out that insurance warning for the lurkers ...

It'll be techno paradise when I get this setup running.

I think you'll make it. Most make the mistake of thinking of using the smallest possible battery with lifepo4 once they realize it can go to 80% DOD regularly, but in your case, I have a feeling you'll be operating in a very sweet conservative zone with an oversized battery. If one can afford it up front, that is really the way to go with solar and lifepo4.

[bungawalbyn]

Hi PNJ,
I don't have to worry about insurance. I don't have any. I built my house, it's all steel framed and roofed and nothing to burn. (Rampant termites) And I've saved myself enough in premiums over the years to rebuild if it ever came to that.
I'm nowhere near the grid so I don't have to worry about their regs.
I play safe with electricity, I did a bit of electrician training when I was a youngster and my father was an electrical instrument fitter by trade till he chucked it in and took to chicken farming, so I've grown up with respect but not fear of electricity. Wiring is no problem, but I'm a bit hazy with electronics.

At present my usage is not large, but I'm doing the upgrade so I can increase if I so desire, I'm a bit over having to scrimp on power. I started off with kero lamps and crappy 12V flouros on a car battery. It'll be techno paradise when I get this setup running.