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Growatt 12k has (somewhat) restrictive programming options if using Li-ion Batteries

J.P.

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NOTE: I have changed the title and my premise in this thread as my eyes were opened to some information that I did not have before. Learning is a great thing.


So this might only be of interest to a few since most here do not use Li-ion batteries, but since I did not see it discussed when I looked over the forum posts regarding the 12k before I purchased it, I want to at least state this issue here for anyone else who might be considering it.

I cannot speak for their other inverters, but the Growatt 12k 48V inverter might not be the perfect choice if you intend to use Li-Ion battery packs and want to use the grid as a backup when your battery gets low.

The problem is the limitation in Setting 12: The point in which it switches from (low) battery to utility.

1624542326623.png

Since 14S 48V Li-Ion battery packs have an operational voltage from 35.0V to 58.8V, and Growatt has chosen to set the lower end of this range to 44.0V, it only allows you to use (about) 50% of your batteries capacity.

When I asked Growatt if they could lower that value down to 35.0V, they responded (after several weeks of bugging them) with "We were not able to enlarge the voltage window as your request, it is out of the inverter voltage range."

So then I asked them to at least lower it down to 40.0V since that clearly was in the operational range of the inverter (as indicated by Setting 21) but the response was "We double check with our RD team and was told that 440Vdc is the lowest voltage. Thank you.".

1624541389407.png

WHY? To me, if the inverter will operate reliably down to 40.0V (Setting 21), then (Setting 12) can also be as low as 40V. Am I missing something?

Very frustrating that they say this inverter will work with "Lithium/Lead Acid" batteries but what they really mean is that it will work with "LiFePO4/Lead Acid" batteries, but telling the WHOLE truth seems just like too much trouble for so many companies these days.

Also, to an engineer, not telling me WHY they cannot lower the value below 44.0V is also very frustrating.

Honestly, I have been running this inverter for a few weeks now (and although it has high idle consumption and is loud with fan noise) it seems to be running great except for this one setting that does not allow for me to use more than about 50% of my Li-ion battery bank. It seems something that would be so easy for Growatt to fix if they would just open their eyes and actually listen to their customers when they make requests, ask questions and provide feedback.
 
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I'm long time 18650 (lithium-ion) 14s / 48v nominal. A 44.0v (low) setting is perfectly OK for lithium-ion 48v nominal. That's 44v/14s = 3.1v per pack/cell. 3.1v is past the discharge curve knee for lithium-ion that occurs between 3.5v and 3.2v. There is no significant power below the knee.
For example a 3.3v * 14 = 46.2v low is still higher than 44.0v minimum setting.

Here's an illustrative lithium-ion discharge curve:
1624546466333.png
 
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I'm long time 18650 (lithium-ion) 14s / 48v nominal. A 44.0v (low) setting is perfectly OK for lithium-ion 48v nominal. That's 44v/14s = 3.1v per pack/cell. 3.1v is past the discharge curve knee for lithium-ion that occurs between 3.5v and 3.2v. There is no significant power below the knee.
For example a 3.3v * 14 = 46.2v low is still higher than 44.0v minimum setting.

Here's a typical lithium-ion discharge curve to visually illustrate:
View attachment 53928
Thanks for that. I have my Charge Controller and BMS(s) set up to run the packs b/t 40.0V and 55V (2.86 and 3.93) to add longevity to the packs but really felt like that 44.0V setting was taking a lot (more) away from my packs capacity. Your graph clearly show that to not be the case.

The reason I believed that might be because of how I have my BMS(s) set up.

Question.
My JBD smart BMS(s) have settings that ask where the voltages are at various cell % capacities.
Where would you set the following?

Cell 80% Cap. Voltage =
Cell 60% Cap. Voltage =
Cell 40% Cap. Voltage =
Cell 20% Cap. Voltage =
 
here is another chart that shows why lithium batteries are sooo different from lead acid that most are familiar with and just how tight the voltages are as the Li batteries discharge untilt he end at which time it drops like a rock...
You can see the difference from 40% capacity to 80% capacity is just a half volt hehe
(image snagged from https://www.powertechsystems.eu/home/tech-corner/lithium-ion-state-of-charge-soc-measurement/)

1624547369786.png

While it is always nice to have lots of programming options if possible you may want to "soften" up the title of the thread just a smidge hehe
 
here is another chart that shows why lithium batteries are sooo different from lead acid that most are familiar with and just how tight the voltages are as the Li batteries discharge untilt he end at which time it drops like a rock...
You can see the difference from 40% capacity to 80% capacity is just a half volt hehe
(image snagged from https://www.powertechsystems.eu/home/tech-corner/lithium-ion-state-of-charge-soc-measurement/)

View attachment 53931

While it is always nice to have lots of programming options if possible you may want to "soften" up the title of the thread just a smidge hehe
I have change my title as you guys have shown me it was too harsh. Thank you.

Any ideas as to why they can't lower Setting 12 to be the same as Setting 21.
 
Thanks for that. I have my Charge Controller and BMS(s) set up to run the packs b/t 40.0V and 55V (2.86 and 3.93) to add longevity to the packs but really felt like that 44.0V setting was taking a lot (more) away from my packs capacity. Your graph clearly show that to not be the case.

The reason I believed that might be because of how I have my BMS(s) set up.
There's an old web page from Battery University that I use as a guide to thinking about long life - https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries

It has 2 specific charts that have influenced me. Here's the 1st one - NMC is lithium-ion.
1624551370647.png
And this one later on down the page has become my build design / operational goal.
1624551562042.png


Question.
My JBD smart BMS(s) have settings that ask where the voltages are at various cell % capacities.
Where would you set the following?

Cell 80% Cap. Voltage =
Cell 60% Cap. Voltage =
Cell 40% Cap. Voltage =
Cell 20% Cap. Voltage =
Each battery has a slightly different discharge curve depending on the local situation.
- You could establish yours by charging up 100% and discharging it while you record power output at each tenth of a volt and map out you're own curve.
- You could also try different voltage ranges such as 4.1v -> 3.3v and 4.0v -> 3.5v and record how much power you get. Then divide the max power of your battery into that and you'll get a percent. For example, let's say you get 8kwh between 4.1v and 3.3v and you have a 10kwh battery - then 8/10 = 80%.

For me (alone) I use these types of numbers:
4.1v -> 3.45v = 80%
4.0v -> 3.5v = 60%
3.9v -> 3.56 = 40%

Notice that as I reduce the DOD % goal - I also lower the top (max charge) voltage as well - to stay in the middle of the range. This is due to the influence of the Battery University chart above and my goal is *longest possible life*

However - I have no idea how accurate Battery University actually is... it just seems to be a reasonable guide that many have mentioned over the last few years. Even @Will has graphics from it in the past.

If you'll do some experimentation / understand more about your discharge curve - I think you'll find your sweet spots.

BTW - My oldest 14s120p (260ah @ 48v) 18650 battery just passed 1,000 cycles at 40% average DOD with no detectable power loss. 1,029 cycles as of yesterday. Only 8,213 more cycles (at 11c/kwh grid) to go to get all my money back. :)
 
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