Growatt Inverter and "dumb" Lifepo4 Battery Solar Charge confusion

I have a 3KW/48V Off-grid Growatt Inverter with a “dumb” lithium battery (48V/50AH). This is working great for as a big UPS. The Lithium battery is built using 16x50AH Lifepo4 Cells in Series with a 60A/16S BMS (bought from Lithium Batteries SA). Unfortunately this is not a Smart BMS, so the inverter is set up with a User Defined Battery with Charge, Float Charge and Low Voltage levels set separately.

Adding Solar Panels I have noticed when setting to Solar First on the inverter that the Solar Consumption is way less than the total consumption even though the panels have provided higher watts that I usually draw. Meaning that Battery is depleted rather than using all available Solar Power.

Anyone have a similar setup and solved the issue with inverter configuration?

For Reference, I generally draw 300W during the day (only computers and lights on the inverter) which sometimes spikes to 600W. Solar consistently provides 100-150W less than total consumption, except if I let the battery drain for a couple of hours in which case the solar production is more (I have seen roughly 7A/90V/630W for 30 minutes to charge the battery, and this out of two 455W panels in Series).

Voltages are set to:
Bulk Charge = 58.4 (recommended charge voltage)
Float Charge = 56 (to allow BMS Balancing to still work)
Low DC Cutoff = 48 (highest it can go, would have like to set this at 51.2 which relates to roughly 20% DoD, higher will ben even better like 52.4)

I can’t help much with the Growatt setup, since I have no experience with those, but I can say that the voltages you are using don’t look very good to me. You are likely to over-charge some cells and cause damage. 58.4V is equivalent to 3.65VPC if all the cells are at exactly the same voltage, but they will definitely not all be exactly the same, especially after a few cycles. As far as I understand, 3.65V is the absolute maximum for LiFePO4 cells, above which they could begin to get damaged. So I would recommend reducing the bulk and float settings. If you really must get to 3.5VPC then I guess bulk could be set to 56V and then float a bit lower. I’m not sure how you got to 51.2V being 20% DOD (80% SOC) but I guess you can find that with trial and error since the voltage vs SOC will depend heavily on your load.

I was thinking of lowering the voltages anyways, 3.5V per cell (or 56V) is good for bulk charging (this is also where the BMS will do Balancing, but not lower than this). And then setting the float to 54V (3.375 per cell). Still sucks that I cannot set the Low DC Cutoff (to switch back to Grid) at 20% SoC (forgot that DoD works the other way around).

Thanks for the response!

This may be a silly question, but is there not another setting for the ‘back to grid’ voltage?
I would assume that the low DC cutoff voltage is where the inverter would turn off to protect the battery, and 48V is probably ok for that.

Edit: Found this in the growatt manual:
It seems to say that the back to grid voltage can be set all the way up to 51.2V

I saw that setting, set it to 51.2V so that it does not drain the batteries too much. No idea if this is 50% or 20% SoC because of the linear nature of LiFePo4 battery voltage between 90% and 20% SoC.

I also set the Back to Solar (config item 13) to 55V which is the same as the float charge voltage.

Also don’t know Growatt, but this is exactly what my in laws inverter did, which was an Axpert and thus an Off Grid inverter. Not designed to do what they wanted which is what you want - solar to match the loads.

What happens when the grid is down? Are you on batteries only or is solar still provided?

ok, @johanpre44 the problem is, and @TheTerribleTriplet will join me in this one. That bms you got, is it the daly bms?

Ok, so what is happening is, you don’t have control over the bms and don’t now what sells are doing. so, now you charging at 58.4v volts, you get a sell that is charging super fast/faster then the other, now it hits it’s 3.65v and the bms opens the charging fits that no charging is going it, and so the bms sits and waits and keeps a eye on this cell to drop below the safety of 3.45 and then the charging fits close again to charge, and in this proses the other batteries don’t get full, and your your storage getting lower.

This is why you need a bms where you can connect to it and see every cell state of volts and stuff. You can take a multimeter now and check what you cells is showing.

Best info I can give you, and i’m running my 2 lifepo4 setups with a smart bms for over 18 months now with out problems, pull your charge back on bulk at 54.6v bulk and 53.6v float. Keep this for a few weeks and get a multimeter and check every sell to see there volts. the multimeter must read .000v so that you can see precisely.

Then get a new bms from @TheTerribleTriplet to have proper control over the cells.

Thanks for the info @Gman, I will adjust the charge parameters accordingly and start to manually check the cell voltages. I was thinking of getting a Smart BMS to replace my “dumb” Daly BMS, but the cost is just a bit high (maybe on a couple of months). Think a Smart BMS that can talk to the Inverter will be even better (just need CAN bus or Serial Support).

Yeah, “those times”. I had DC Ripple’s, plural, one after the other. If I dared to run at 58.4v I would have probably lost the inverter. It is scary as shiite when a 4kva switches off because the Dc Ripple is that bad. And I was present when a 250/100 blew because of same Daly BMS.

And I swear the cell I now have that is dicey, stems from those days with that bloody Daly. Bad mojo on a grid-tied solar system, as you need more data per cell … now I’m running at 3.4v and everything is better, but that one cell is still bleh.

This batch we are landing at R2400 per BMS. There may have one spare.
And with the next battery order, we will bring in more.

What BMS are we talking about here?

Johan I would suggest to stop continuing this immediately. You WILL damage some cells this way as @Stanley already pointed out. Before you make a decission about getting another BMS you can run still your system but do the follwing: top balance you cells (important the way you use them). Set your bulk voltage to 56V, 3,5V per cell. Set your float a little lower like 3,45V (55,2V). About you low DC cutoff is this where your inverter goes back to grid? Looks ok to me.

Been there done that … only setting that worked on my old Daly:
Bulk: 54.40v = 3.4v per cell
Float: 53.50v

These ones with Bluetooth AND RS485 port added extra.

What went wrong when you used 3,5 bulk and 3,45 float on this Daly?

Ah! That’s a JBD. I have a little experience with a 60A 14s version and I was happy with it. The one you point to is the one Andy (Offgrid garage YT) is using as well. he has some good reviews on them too.

Edit: here is a vid of him with a QUCC but is seems the same one to me isn’t it?

Problem still is @RikH , his still going to have a sell that runs away and daly bms will stop the charge. You can’t adjust the bms settings so his still going to have the problems. Take it from me, I know what i’m taking about and @TheTerribleTriplet will back me on this. I had have spend some time with this.

You can even go check Solar MD info:
Bulk charge (stop charging) 54.6V

  • Float charge (if applicable) 53.8V
  • Low battery discharge: 48V

freedom won and revov max bulk charge around is 55.4 and 55.6 and float at around 54.4

Had one DC Ripple after the other at <300w with SOC >95% when the BMS disconnects to protect a cell/s shooting out. As mentioned before, also saw a 250/100 getting damaged under same circumstance by same model Daly BMS because of the BMS protecting the bank because of a cell/s shooting out.

When I got a BMS where I could see the individual cells, a whole new world opened up. But I still had a challenge to manage them better. Louis software on the Venus, the last piece I needed.

If cells are out of balance, bad things can happen.

Well if you don’t top balance your cells manually before commisioning I agree with that. But when you do, and your cells are well matched as they should, there hardly any balancing anymore so the Daly should be able to catch up with it wthout disconnecting. If it cannot it is not to blame the Daly but the cells not matching.

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15 or 16 cells?

You do like yanking my chain don’t you :slight_smile:

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Yup. That’s actually one of those engineering compromises, why the balancers often has such pitiful bypass/transfer capabilities. Because in most cases, your big expensive balancer circuit does all the work it will ever do in the first week of ownership… and after that it just sits there, occasionally passing a little current. So the engineers tend to favour cheaper and smaller stuff… and naturally so do the bean counters :slight_smile:

As for the DC ripple thing. Imagine a solar charger connected directly to an inverter… no battery. The only storage capacity on the bus are those big “water tank” capacitors. Because of the very small storage capacity, current fluctuations on the AC side shows up the DC side, and the capacitors themselves cannot damp it down. One such inherent fluctuation is the 100Hz one created when you make a 50Hz AC signal… and that is your ripple.

The ripple is just a symptom of another condition: You are running without a battery. The battery disconnected because it is unhappy about something. This is what I sometimes refer to as “stability”. In a stable system, the battery remains connected. For a stable system, you need a BMS with good margins, and preferably one that can block charge and discharge independently (so it does not have to disconnect completely).

The Daly is a cheap BMS. So the conditions for stable operation is more strict :slight_smile:

One DC Ripple = one DC Ripple.

But when you have 2-3 in like a 10min period … what do you call that? :slight_smile:

I tend to refer to it as an “event”. Perhaps because intuitively I understand that ripples are those circles in a pond… but after I finally understood what you mean by your plural term, I’m now only kidding around with it :wink:

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