Please critique my off-grid setup + some questions

[TinyHouseSolar]

Here are the details of our current tiny house off-grid setup. We’ve been up and running for about two weeks and it is about 90% self sustaining. I have a few questions specific to our battery system as well if anyone can offer insights.

Solar Panels
6 - 250watt Trina poly panels

• Connected in series and peaking around 1000 watts at midday (135v, 7.5A)
• 40 foot run of 10 AWG cables through 15A fuse to inverter/CC.
• Generates 4.5kWh most summer days.
• Oriented horizontally until ground mount is finished.

Inverter
MPPSolar LV6548
Settings below:

• Bulk charge voltage - 58.8v
• Float voltage - 58.8v
• Battery cut off voltage - 45v
• Back to “grid” voltage - 46v
• Back to discharge voltage - 52v
• Max charging current - 60A
• When we go completely off-grid the cut-off voltage will be set to 46v and the back to grid/back to discharge values will be non-applicable.

Batteries
2 - Big Battery Hawk 24v 170AH LiFePo

• Wired in series with 1/0AWG cable with dual Anderson 175 connectors and ring terminals direct to Inverter.

Load
Typically 3.5kWh per day.


Grid
There is a 20A “grid” source we have connected for testing purposes for now. We have tested the system under heavy load to drain the batteries to see when the grid switchover and battery recharge occurs. It properly switches at the cutoff voltage of 46v and begins charging the batteries. Eventually this may be a generator and I'll probably need some advice on types and setup specifics there too, but that's another thread.


Questions

1. There is a 300A fuse built into the batteries but it sits on the negative side of each battery. Presumably this only protects the battery itself. Isn't another fuse necessary on the positive side running into the inverter? I am planning on putting an ANL fuse there. At 1.3 times max discharge rate I’d be looking at 225A so maybe a 200A fuse?
2. Big Battery has recommended setting all charge voltages to 58.4-58.8v. With the system set to charge at 58.8v, I can never get the batteries to display more than 53.5v (26.75v individually). When I charged the batteries individually using a Victron 24v 12a charger set to 29.2v (BB spec) I could get the batteries to settle down to 27.2v without issue after charging. I’ve tested the system with no AC load (only charging batteries) for two solar charge cycles and the 6548 won’t take them higher than 26.7v each. Not really an issue since I don’t ever want them at 100% SOC but I’m still curious what’s going on in the LV6548 that might be affecting that. Especially since the LV6548 is capable of pushing a much higher current into the batteries than the Victron.
3. What user inputs dictate the 0% and 100% SOC battery values on the watchtower app and on the LV6548 display? Does it lock 100% SOC at 54.4v and make 0% variable based on the cutoff and/or back to grid voltages? I ask because it irks me that with 53.5v max from my batteries I can never get the display to show more than exactly 83% full.
4. What does everyone think of my battery charge and discharge settings in the LV6548? I see a lot of people talking about limiting the charge voltages to a much lower level but I’ve spoken to BB about it and they confirmed 29.2v for their 24v batteries and 58.8v for their 48v batteries. They specify that the bulk, absorption, and float should all be set to the same 29.2v or 58.8v figure so that's how I have it right now.

The LV6548 is pretty oversized but it should allow us to grow into with battery and panel upgrades. Any thoughts are appreciated.

 

[mrzed001]

Here are the details of our current tiny house off-grid setup. We’ve been up and running for about two weeks and it is about 90% self sustaining. I have a few questions specific to our battery system as well if anyone can offer insights.

Solar Panels
6 - 250watt Trina poly panels

• Connected in series and peaking around 1000 watts at midday (135v, 7.5A)

From manual: PV Array MPPT Voltage Range 90Vdc~230Vdc.
So 135V is enough, but I would connect plus 1 solar panel to raise it higher. (if Voc is still under 230-250V, check solar panels back for Voc value)

• 40 foot run of 10 AWG cables through 15A fuse to inverter/CC.
• Generates 4.5kWh most summer days.
• Oriented horizontally until ground mount is finished.

Inverter
MPPSolar LV6548
Settings below:

• Bulk charge voltage - 58.8v
• Float voltage - 58.8v
• Battery cut off voltage - 45v
• Back to “grid” voltage - 46v
• Back to discharge voltage - 52v
• Max charging current - 60A
• When we go completely off-grid the cut-off voltage will be set to 46v and the back to grid/back to discharge values will be non-applicable.

Bulk and float V is too high. 58.8 / 16 = 3,675V / cell
Never go over 3,65 with LiFePO4 !
Even more i suggest to set it to 3,45-3,5V / cell maximum.
The ideal would be around 3,45 for bulk and 3,4 for float. In this way you do not float the already 100% charged cells (that cause damage to the LFP)


Max charging current is ideally lower than 0,5C. In your case 170Ah /2 = 85A so 60A is OK. Solar will only produce 21A so nothing is lost.

But discharging current too 0,5C so 85A. 3,2V * 16 cell * 85A = 4352W. Your inverter can produce 6000W but you should not take out more than 4200W from it continuously ! (that is 120V 35A).

Batteries
2 - Big Battery Hawk 24v 170AH LiFePo

• Wired in series with 1/0AWG cable with dual Anderson 175 connectors and ring terminals direct to Inverter.

Load
Typically 3.5kWh per day.


Grid
There is a 20A “grid” source we have connected for testing purposes for now. We have tested the system under heavy load to drain the batteries to see when the grid switchover and battery recharge occurs. It properly switches at the cutoff voltage of 46v and begins charging the batteries. Eventually this may be a generator and I'll probably need some advice on types and setup specifics there too, but that's another thread.


Questions

1. There is a 300A fuse built into the batteries but it sits on the negative side of each battery. Presumably this only protects the battery itself. Isn't another fuse necessary on the positive side running into the inverter? I am planning on putting an ANL fuse there. At 1.3 times max discharge rate I’d be looking at 225A so maybe a 200A fuse?

Yes, fuse to both plus and minus too.

1. Big Battery has recommended setting all charge voltages to 58.4-58.8v. With the system set to charge at 58.8v, I can never get the batteries to display more than 53.5v (26.75v individually). When I charged the batteries individually using a Victron 24v 12a charger set to 29.2v (BB spec) I could get the batteries to settle down to 27.2v without issue after charging. I’ve tested the system with no AC load (only charging batteries) for two solar charge cycles and the 6548 won’t take them higher than 26.7v each. Not really an issue since I don’t ever want them at 100% SOC but I’m still curious what’s going on in the LV6548 that might be affecting that. Especially since the LV6548 is capable of pushing a much higher current into the batteries than the Victron.

You use CC-CV charging. So in the CC part V is lower (like 3,4V/cell) so you only charge with the set 60A max. The charger handles it.
At 3,45V the battery is 90-95% charged. You do not need more. LFP is better used in the 10-90% SoC (or even 15-85%).
If you charge it all the way up to 3,65 then you shorten the life of the battery.
After charging battery to 100% it settles to its relax V ... that is around 3,4V/cell.

 

[TinyHouseSolar]

Thanks for your reply @mrzed001. A few follow up questions.

• Adding an additional panel of the same size would put me at 263 Voc so I think my next upgrade step in the solar department would be to add another array to PV2... or just replacing this array with nicer panels.
• Why does the manufacturer recommend such a high charge voltage? Are they relying on the BMS to handle the charge profile or is a higher voltage needed for top balancing processes maybe?
• I'm familiar with CC-CV charging but not how this is implemented in the LV6548 specifically. Is it automatic? I don't see any options for specific voltage or amperage curves that can be programmed. Will setting lower Bulk (55.2v) and Float (54.4v) voltages allow for a more ideal CC-CV profile? When charging with the Victron, the CC-CV was pretty obvious in the app readout because the source power (120v plug) was constant. With the LV6548 the current and voltage both increase with the sunrise and decrease with the sunset.

 

[mrzed001]

Thanks for your reply @mrzed001. A few follow up questions.

• Adding an additional panel of the same size would put me at 263 Voc so I think my next upgrade step in the solar department would be to add another array to PV2... or just replacing this array with nicer panels.

You have 2 MPPT in the unit. So you can add a separate 6 panel string to PV2.

Also you can parallel PV strings on both MPPTs.
Now you have 6s1p : 6x250W (135v, 7.5A). You can double (6s2p) it on the same MPPT too.

The inverter can handle max 8000W solar power (for safety 7000W) and now you put only 1500W on it.
So you have room for improvement

• Why does the manufacturer recommend such a high charge voltage? Are they relying on the BMS to handle the charge profile or is a higher voltage needed for top balancing processes maybe?

I do not know why they say over 3,65V is good. It is bad. Even 3,65V is bad for battery. You do not need that to fully charge the battery.
Only one situation can be useful. If you want to have a very little absorption phase (charge the battery to 100% as fast as possible ... and you never should charge to 100%).
So almost never.

• I'm familiar with CC-CV charging but not how this is implemented in the LV6548 specifically. Is it automatic? I don't see any options for specific voltage or amperage curves that can be programmed. Will setting lower Bulk (55.2v) and Float (54.4v) voltages allow for a more ideal CC-CV profile? When charging with the Victron, the CC-CV was pretty obvious in the app readout because the source power (120v plug) was constant. With the LV6548 the current and voltage both increase with the sunrise and decrease with the sunset.

Here is the manual for LV6548:
http://www.mppsolar.com/manual/SPLIT PHASE LV/LV6548 split phase manual-20210416.pdf

In the page 42 you can see the charging diagram.
Bulk charge until you reach the bulk charge V. Then battery stays in that V until absorption finishes and A fell (very few Ah goes into battery then)

So not the bulk V is wht changes how fast you charge (until 90-95%) but the charging A !

Check out @Off-Grid-Garage Andy's videos in this topic, like:

Also turn off Battery equalization (if enabled).
You do not need it and it can harm your LFP battery.

 

[TinyHouseSolar]

Thanks again, this is very helpful!

The inverter can handle max 8000W solar power (for safety 7000W) and now you put only 1500W on it.
So you have room for improvement

That's the plan! I'm relatively happy with these SanTan-sourced panels especially for the price but a second array into PV2 would be the next upgrade.

I do not know why they say over 3,65V is good. It is bad. Even 3,65V is bad for battery.

I reset the 6548 to the specs you recommended, Bulk (55.2v) and Float (54.4v). Will the batteries ever go into Float though, since I've never managed to get the batteries past 53.5v with solar alone? Thanks for linking to the manual as well. I must have read that manual a dozen times and somehow the charge profile never seared into my brain.

That video was very helpful as well, thank you. Now I've got a new YouTube subscription to go through

 

[mrzed001]

Thanks again, this is very helpful!


That's the plan! I'm relatively happy with these SanTan-sourced panels especially for the price but a second array into PV2 would be the next upgrade.


I reset the 6548 to the specs you recommended, Bulk (55.2v) and Float (54.4v). Will the batteries ever go into Float though, since I've never managed to get the batteries past 53.5v with solar alone? Thanks for linking to the manual as well. I must have read that manual a dozen times and somehow the charge profile never seared into my brain.

That video was very helpful as well, thank you. Now I've got a new YouTube subscription to go through

Happy to help

Maybe with a shunt you could follow your batteries current SoC.
There is the https://www.renogy.com/500a-battery-monitor/
Or the more expensive: https://www.victronenergy.com/battery-monitors/smart-battery-shunt