Solar charge controller, what for?

[bungawalbyn]

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.

[Living Large]

Was that Gold standard or Old standard?

If LiFePO4's live up to the claims, and the industry produces specific control gear to suit, then sooner or later, FLA will take it's place alongside the steam engine. Add to that the toxicity of FLA and lead in general, I say good riddance.

As of today, FLA is the gold standard. I could tick off all the negatives I found with FLA as well, which made me willing to take a chance on LFP. Does that make LFP the new gold standard? The new standard? Hardly - there is nothing standard about using LFP in a solar system today. Obviously I am willing to try LFP. That doesn't mean I am ignoring the fact that is an expensive experiment today. Tomorrow? Who knows.

[karrak]

Correct. Last thing you want to do is Float a lithium battery at 100% SOC. But absolutely no problem at 90% or less. FLA batteries start sulfating at anything less than 100%, thus why you want to keep them fully charged at all times. Scaling in Lithium batteries start at 90% SOC and and accelerates from there up to 100%.

I also think that what voltage/SOC to float an LFP battery is a key issue to longevity.

I am interested to know what the mechanisms that degrade LFP batteries are and how they can be avoided, I suppose this is the million dollar question that everyone would like the answer too. Sunking, you say that "Scaling" is the problem, but this only occurs with an SOC above 90%, could you give us more information on what "Scaling" is and why it only occurs above 90%SOC.

Simon

[bungawalbyn]

As of today, FLA is the gold standard. I could tick off all the negatives I found with FLA as well, which made me willing to take a chance on LFP. Does that make LFP the new gold standard? The new standard? Hardly - there is nothing standard about using LFP in a solar system today. Obviously I am willing to try LFP. That doesn't mean I am ignoring the fact that is an expensive experiment today. Tomorrow? Who knows.

The Tesla power wall and the like could well change all that.

[Living Large]

The Tesla power wall and the like could well change all that.

You don't seem to be understanding my point, so I will spell it out.

You appeared to question if FLA is the current gold standard. If FLA isn't the current gold standard, what is? Not tomorrow, not a year from now. What has been adopted, and is the most widely used plug and play battery technology for solar system use?

[bungawalbyn]

It never was "Gold" it was only ever lead. It's crap, always has been. Only reason it's a standard at all is because there has been nothing better, doesn't stop them being crap, hopefully LFP or something even better will gain acceptance and replace it in the near future.
I don't know why you're so defensive of them when you listed their faults and chose LFP. What's that about?

Anyway, this thread wasn't started to argue the pros and cons of FLA. Lets not get hung up on distractions

[PNjunction]

Anyway, this thread wasn't started to argue the pros and cons of FLA. Lets not get hung up on distractions

Heh - rule #1. Do not even mention FLA or other battery comparisons for purely technical threads about lifepo4. It is a guaranteed thread-killer with too high of a signal to noise ratio. Keep that for other dedicated comparison threads.

Balancing. Rule #2 for thread killers. Pick top or bottom and deal. In your case, try them both (not at the same time) and see which one fits your operating environment best. There is no shame switching from doing it one way to the other. They are your cells.

Part of what makes a sensible discussion about lifepo4 difficult, is that they can fulfill a variety of different needs, and each application (EV / RC / sub-c solar etc) benefits from a variety of techniques. Some techniques may not be the absolute best technically, but can work in the real world if one is knowledgeable enough to deal with it.

There are tradeoffs in everything, and this is where most of the trouble begins.

[PNjunction]

I am interested to know what the mechanisms that degrade LFP batteries are and how they can be avoided, I suppose this is the million dollar question that everyone would like the answer too.

It has to do with electrolyte contamination from parasitic reactions at both the top and bottom ends of charge when the anodes and cathodes can no longer support any additional intercolation, ie passing of lithium ions from one side to the other.

Too high a voltage, held for too long a time produces oxidizing materials to contaminate the electrolyte.
Too low a voltage, and the copper, aluminum, and other parasitic products are pulled from the anodes and cathodes into the electrolyte.

The double whammy at the low end of charge, is not only is the structure of the battery being eaten away, and the sei layer contaminated, but dendrites form given enough time, which is why you need to get on top of it asap. The double whammy part is applying a full charge rate at very low soc's, where not only are you dealing with a corroded internal structure, but also the contaminated electrolyte from inorganic pollutants. A full charge current here is dangerous and uneven. Basically at full charge current (less than .01C recommended at very low soc's), there is a traffic-jam of ions not being able to pass through the sei layer fast enough with nowhere to go.

Basically we want to protect the SEI layer from inorganic contaminants. If it weren't for that, the cells would be immortal.

The natural protectant of the SEI layer grows on it's own under the best of conditions. Parasitic non-organic compounds added to that from extremes of heat / charge / usage don't help.

Note that this is slightly different from the organic compounds usually put into the electrolyte on purpose for *normal* cycling. One of the major players here is vinylene carbonate, without which the cells have a very short cycling life. All the manufacturers include this, but add their own additional compounds for different technical results - high heat, ease of manufacturing, charge rate, etc.

This is why statistically it is best to be conservative and run 80-10% DOD or *less*. The further you run away from the top and bottom of charge, the slower the contamination will be (so far it has proven impossible to stop completely). Of course the tradeoff here is your overall desired operational capacity, and of course cost of using larger capacity cells to stay away from extremes.

The quest is to keep your application TIME in mind. For example, EVEN at say a conservative voltage of 3.5v per cell (14v for a 4S / 12v setup), the cells will eventually charge to 100% SOC and there will no longer be any lithium to intercolate. Of course lifepo4 has a very short absorb compared to lead, but it will eventuallly fully charge. Now comes oxidation time.

Those who routinely cycle their batteries may not see the oxidation effects at nearly full charges because they aren't held there long enough for absorb to finish. But for those who want to float, 3.5v would be disastrous overall and *eventually* oxidize once absorb is over - simply because they've been given enough time to do so and pass into the parasitic reaction stage of oxidation.

[bungawalbyn]

Thanks PNJ for the technical explanation.

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

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?

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


An interesting observation. In my efforts to reduce the voltage from the balance charge, I've used the bank to run the house for 2 nights now. The second night caused little drop in voltage over the previous at rest. Like about .02V with bank at 13.21 to 13.25 at rest, Depending on day/night temps. All Cells holding at 3.31V This suggests I have them down on the flat now.

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.

[PNjunction]

All my sensors etc will be on the battery side, so the voltage drop shouldn't affect any thing in the controls. Opinion?

I'm not the guy to ask that from since I don't have a roof install. However, I can say up front that your homeowners insurance needs to cover your DIY install, or a roof fire may leave you penniless and not covered from a professional licensed installation. Just don't want you finding out the hard way.

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

MUCH better than a stereo fuse for sure! Check with the big boys on that, like MaineSail and others here. Remember I'm only running a 40ah battery max!

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.

I don't know exactly what your monitoring is, and how much current you are actually pulling, but just know that around 3.195v / cell *rested* is about 80% DOD. Since you have a lot of capacity to play with, I'd just stop when the first cell just starts to dive under 3.2v, or perhaps when it hits 12.75v pack total.

BUT this is the most important charge / discharge to watch like a hawk for infant mortality. Grab a non-alcoholic beverage, non-conductive chair, and a Fluke multimeter and WATCH to see if any cell is trying to take a dive.

Note that while lifepo4 doesn't really need a break-in, for the first 2 cycles or so, the SEI layer is settling down to a normal depth. So don't be surprised if there are small differences until you get a few cycles in.

[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.

[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.

[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

[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.

[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.