Struggling with battery capacity

[Hedges]

He looked at my data and now thinks that my charge current is too high. I had it set at the default 80A. He said I have a common problem of having way to much solar power available for not a lot of batteries, and I'm "cooking" the batteries by charging them too quickly. His analogy was wetting a sponge with a fire hose.

So I've dropped the charge current to 40A and will see how that goes.

The batteries ramp up to 14.4 V or so in just a couple hours in the morning. I don't understand how I have 24 kWh of batteries drained to 42V, and they charge up to full capacity in just 3 hours, and using max 1500W of solar? (I have 4500W of panels available) Something is not right. I tend to believe the batteries are not actually getting fully charged to capacity, but they are reading 14.4V which should be full charge, right?

24,000 Wh of batteries, 48V bank = 500 Ah ... confirm that's what you have?

4500w of panels delivering 48V would be 94A, about 0.2C ... I would consider that about right for my AGM batteries. Specs for yours may vary.
40A would be 0.08C ... I would consider that low for FLA (0.12C typical I think), maybe appropriate for some Gel.

 

[wgrippo]

I have read through this entire scenario and I believe the only way to get to the bottom of the situation is to monitor the current into the batteries and out of the batteries. They have a number of devices on the market that do just that. With my system the Overkill BMS does a good job of keeping track of the cell voltages (Lifepo4) and also how many amps that the Cell packs have taken in during the day. It also keeps track of the current draw during the night and how many amps are left in the cell pack in the morning. In my opinion, if you are not keeping track of the current going into your batteries and the current that is coming out of your batteries then you will never be able to solve this problem. With all batteries of the different chemistries it is very important to keep the batteries happy. I am not sure of the specs on the AGM batteries that you have but you can look them up and make sure that they are happy. Charge current is important, Discharge current. I believe that you need to occasionally take the batteries to 14.4 volts for a period of time and then drop them down to an absorb voltage. I would become very familiar with the actual parameters if it were my system. When I purchased my Lifepo4 cells, I read everything about the voltages and currents that the cells were to receive. If you exceed the voltage or current then damage will occur. I am not a fan of Growatt but I will not criticize those who have elected to go that route. The All in one concept sounds good on paper, but I suspect that it has some issues which I do not want to deal with. I like the concept of if something goes wrong then replace one component rather than the system. Like if the Charge controller quits working then replace the Charge controller. If the Inverter quits working then replace the Inverter. With the Growatt system that you are dealing with who knows how much power that thing is consuming during the night. For all you know it could be sapping all the power out of your batteries and you would not know. Again enough of the philosophy, in my opinion the only way to determine what the problem is would be to measure the current into the batteries and the current out of the batteries. If the batteries are for example 100 amp batteries, then you should be able to put 100 amps into them and then take 100 amps out of them in a perfect world.

I can measure current on the cable going from the Growatt to the batteries. I just checked now, on "float" at 54V, and it is about 8A.



Edit: this is incorrect - AC current. I have ordered another multimeter to measure DC current.

 

[wgrippo]

24,000 Wh of batteries, 48V bank = 500 Ah ... confirm that's what you have?

4500w of panels delivering 48V would be 94A, about 0.2C ... I would consider that about right for my AGM batteries. Specs for yours may vary.
40A would be 0.08C ... I would consider that low for FLA (0.12C typical I think), maybe appropriate for some Gel.

Yes, confirmed - two parallel strings of four 12V batteries, rated at 250 A-hr each, so 24,000 Wh.

I checked the battery specs, and they say max charging current = 75A (see attached).

I also looked at charging data from the Growatt, and from what I can tell it is only charging below 40A, even if the max charge current is set to 80A.

Another clue that I'm not getting a full charge each day is the Growatt data lists the total solar power for the day. Most every day that number is between 5kWh and 7kWh. If my batteries are running down to 42V overnight, and the capacity is 24kWh, I wonder why I'm only bringing in a fraction of that. It is like the Growatt is switching to float too soon. I'll run this by Ian at Watts247 too.

 

[timselectric]

I can measure current on the cable going from the Growatt to the batteries. I just checked now, on "float" at 54V, and it is about 8A.

View attachment 87723

I can also look at all of the data over time as logged by the Growatt. I plotted the following from one day's worth of data. I believe this is charge and discharge current.

View attachment 87728

In this picture, the meter is set to AC amps.
Set the meter to DC.

 

[wgrippo]

In this picture, the meter is set to AC amps.
Set the meter to DC.

There is only one amp setting on the meter, and that is for AC. I thought amps were amps, although I understand the current flows both ways for AC. Perhaps I don't have a meter capable of measuring DC amps.

 

[timselectric]

There is only one amp setting on the meter, and that is for AC. I thought amps were amps, although I understand the current flows both ways for AC. Perhaps I don't have a meter capable of measuring DC amps.

Press the orange "SEL" (select) button, to switch between AC and DC.

 

[timselectric]

There is only one amp setting on the meter, and that is for AC. I thought amps were amps, although I understand the current flows both ways for AC. Perhaps I don't have a meter capable of measuring DC amps.

Sorry, that meter doesn't have a DC option.
You can't check DC amps with that meter.

 

[timselectric]

I thought that it was the CL390.

 

[wgrippo]

I thought that it was the CL390.

Now I wish it was - I should have looked closer when I bought it, as I do a lot of car / motorcycle work as well and DC amps would come in handy.

 

[timselectric]

It never hurts to have an extra meter. On several occasions I have found myself using 3 clamp meters, at once.

 

[Symbioquine]

Just skimmed this thread and could have missed something you folks have already figured out, but it sounds like perhaps the system just isn't getting enough total hours of charging to catch up with the usage each day.

I wrote up this phenomena a while back, maybe it will be a useful read: https://symbioquine.net/2021-11-29-off-grid-lead-acid-battery-charging

 

[wgrippo]

It never hurts to have an extra meter. On several occasions I have found myself using 3 clamp meters, at once

For sure, thanks for pointing this out - I ordered the CL390 last night. I do a lot of DC work, and I do need to know how much current is going into the batteries.

 

[wgrippo]

Just skimmed this thread and could have missed something you folks have already figured out, but it sounds like perhaps the system just isn't getting enough total hours of charging to catch up with the usage each day.

I wrote up this phenomena a while back, maybe it will be a useful read: https://symbioquine.net/2021-11-29-off-grid-lead-acid-battery-charging

Thanks, these are AGMs, but I will read your write-up for sure!

This has been a long thread, and probably the most difficult problem I've struggled to figure out.

I am starting to lean toward the fact that the batteries are not getting fully charged throughout the day. Twice now (every 4-5 months) I have had two of the eight batteries replaced under warranty (because they showed much lower voltage than the others in the morning). Each time the system starts out fine, battery voltage stays up all night long while powering lights, fridge, two ceiling fans, etc. Then slowly over a few months, the voltage gets lower and lower each night, until I start getting low voltage warnings, and eventually the system shuts down at night due to low voltage.

Instead of blaming the batteries, I think it must be the controller.

4500 W of panels
24,000 W batteries (when full)
Most days the system only brings in 5 to 7 kWh, which is more than my usage (~2 to 3 kWh), but there were some cloudy days in a row a couple months ago, and perhaps the system was never able to recover.

I don't know, but what I can't understand is why my controller is only charging 7 kWh in a day when the batteries are still not at capacity at the end of the day? This has been suggested before here - the controller should be bulk charging longer throughout the day, but it doesn't. Here's yesterday's graph, which is typical of every day:



The peak power is always around 1500W early (8 to 9 am) and then drops. Every day. (The spikes are just from using my skill saw or vacuum). If I have 4500W available, why is it only using 1500W? What's limiting it? I'm bringing a little over 100V from the panels, at 1500W that's only 15A or so - my batteries can take up to 75A - why would it not be bringing in more power? And also why not charge for longer?

There is full sun pretty much every day from sunrise to sunset here in Baja California, which is about 12 hours of sun.

 

[Hedges]

I don't know, but what I can't understand is why my controller is only charging 7 kWh in a day when the batteries are still not at capacity at the end of the day? This has been suggested before here - the controller should be bulk charging longer throughout the day, but it doesn't.

The peak power is always around 1500W early (8 to 9 am) and then drops. Every day. (The spikes are just from using my skill saw or vacuum). If I have 4500W available, why is it only using 1500W? What's limiting it?

What is voltage at battery terminals, and at SCC terminals, when charge tapers off?
It would seem SCC things voltage has reached absorption, and batteries aren't accepting enough current.
Could be voltage drop in wiring, but my first thought is voltage setting of SCC.

Your AC ammeter showed 8Arms ripple. Could be as voltage ripples low and high supplying current to 60 Hz inverter, SCC responds to peak voltage and thinks it is high enough.
It is difficult to filter power, would have been easier to filter sense signals.

Try shutting off inverter for a day, see how SCC performs with zero ripple imposed on battery.

 

[WindWizard]

Well maybe we're not done.

I swapped out the bad battery of one string and I'm just running off that one string of four for now.

I measured the voltages of the four batteries while charging (lots of sun) and the first in the string is at 16.3V! The others at about 13.7V. What the heck?

I disconnected the panels and they all are around 13V.

View attachment 62987

I'm afraid to hook up the panels again - isn't 16V way too high? what could cause this?

The only thing I have seen that causes this is a bad battery.

 

[WindWizard]

Lead Acid batteries are pretty happy as long as you do not abuse them. If you drop them below 10 volts then internal damage can occur to the individual cells. They are pretty happy if you charge them at a constant current until the voltage peaks at 14.1-14.4 volts and then give them a constant voltage of 13.6 Volts.

Then there is plenty of information on the Sulphation that occurs in the lead acid batteries. If you hold them at 14.4 volts for 2 hours it helps to desulphate them.

It sounds like the batteries in your system have degraded since you have installed them. The only outlier is the batteries. Your system was working fine and now you have trouble getting through the night. It keeps pointing to the batteries.

Lead Acid batteries are easy to test and troubleshoot. Charge them up. Monitor how much current you put into them and then monitor how long it takes to discharge them. I would not discharge a lead acid below 12.0 volts.

Plenty of automotive light bulbs around. Most of them tell you the wattage but if in doubt just connect it up and monitor it.

 

[WindWizard]

I don't know, but what I can't understand is why my controller is only charging 7 kWh in a day

A charge controller is basically a constant current supplying current to the batteries. Once the batteries reach a certain voltage as determined by your charge controller then they go into a constant voltage mode. The charge controller thinks your batteries are charged and is going into absorption This reduces the amount of current that is actually going through your batteries because according to the charge controller your batteries are at full capacity.

Maybe your batteries have degraded to 7kw batteries. You probably should do a test to see exactly what the capacity of your batteries are.

 

[Wellbuilt]

The graph in # 93 looks fishy to me for sure , you should be getting a much higher peak then 1600 or so watts .
My system peaks at 2300 watts in February but will see 3000 to 3700 in June with good sun
I have 4500 watt of solar and max out at 3700 or so watts out put
it looks like you are only getting over 1000 watts of power for 2 hours ?
I use 32 to 5200watts in 24 hours , I turn off 2 strings of solar and charge longer all day this time of year .
I have wet cell battery’s
If you turned off your solar panels over night and turned them on at 800am when they are showing there max power
You should jump up to full power and hold there you would need to be way over that 1600 watts
your voltage 56v seams low to me all so and then a 2 hour absorb seams short .
I would think 57.6 would be better with a 4 hour absorb
those are the # I use on my sail boat and they worked for me ?

 

[wgrippo]

Hi, just a follow-up to this thread and conclusion. Thanks for all of the suggestions and help. I ended up picking up a string of 8 golf cart batteries (wet lead acid, 6V), and running them in parallel with these AGM batteries that have been giving me so much trouble. I know not the best setup, I won't be able to equalize the wet batteries while connected to my system, and not a permanent long-term solution, but what a difference! We went from the system rapidly dropping in voltage after sunset to the point of shutting down at 9 or 10pm, to easily making it through the night with the system staying above 51V all night and powering some lights, the fridge, wifi router, etc.

So in the end, these 8 AGM batteries I bought are simply not giving the advertised capacity of 250 A-hr. Once supplemented with this string of additional batteries, they seem to be holding up fine - I have been using this setup for 3 months now.

 

[anadiner]

Hi,

I'm trying to understand why my off-grid system has not been making it through the night, hoping some of you solar sleuths can help me out. I built this system in the spring and it did well for a while with batteries rarely dropping below 75%, but for the past month or so the inverter has been shutting down due to low voltage early in the morning. If we run the generator in the evening for a couple hours it helps, but even then we drop way down in voltage overnight.

I've contacted the seller of the batteries, and he says we simply need more batteries. I contacted Growatt, and they took a look at my settings and say everything is fine. But what I don't understand is why the power we are using doesn't even come close to the amp-hour rating of the batteries.

We have 4500W of panels, Growatt SPF6000 Inverter/Controller, and eight "lead-carbon" AGM 12V batteries. The batteries are rated at 250Ah (at 77°F). I am running two parallel strings of four batteries, so 48V and 500 Ah. I ran the 240V output from the inverter directly into my electrical panel, and all circuits in the house are brand new and working great.

The controller is set to charge to 56V, and then float at 54V. Looking at my data from yesterday, the battery voltage reached 56V by 10:30am, and stayed there for two hours until 12:30pm, when it dropped to 54V. It stayed right around 54V until around 5pm when the sun gets low in the sky, and from there the voltage starts to drop. I'm assuming that when they go to 54V they are full and under float charge...?

Looking at our power usage, from 5pm until 3:30am when the inverter shut down we are using between 200W and 350W, which is two ceiling fans, the TV for a while, the fridge cycling on and off, some lights, wifi modem, and other small electronics. The battery voltage drops from 54V at 5pm to 52V at 6pm, 51V at 9pm, and then rapidly to 46V at midnight. It continues to fall until 3:30am, when it gets to 42.5V and the inverter shuts down.

Attached are plots of battery voltage and power usage from yesterday and this morning.

Now for my calculations. If I have 500 Ah of capacity at 48V, that is 24,000 Wh. I understand that we don't really want to use more than 50% of the battery capacity, so I should safely have 12,000 Wh to use before the batteries drop to 50%, of course a bit less than that due to inverter efficiency, etc.

So I'm using lets say 350W max for 10.5 hours, which is 3,675 W-hr of energy. So why are my batteries dropping down to under 25% when I'm not using anywhere near the energy I have?? I'm a mechanical engineer, not electrical (haha), but the math is pretty simple, right?

I understand this is not an exact science, and battery capacity can vary with temperature and other factors. My battery room is in the low 80 degrees F for most of this time.

Any thoughts? Am I missing something?

Appears you've checked everything. I was going to ask where you were located because temp affects batteries BUT you said the bartteries were in an 80F room; so, hum.
Lead carbon? What is that exactly? Like I use marine lead acid which man ole man I cam not run them down much or have to force charge them...
But I get around it using multiple 24 units running batteries series-parallel.
Has anything changed in your house that might be pulling more? I bought a watt meter off Amazon cheap that you can plug right in that will tell you. You could see at the main or look at each outlet. I'm wondering what is pulling more than usual since you've protected batteries from the cold.