TinyHouseSolar
New Member
- Joined
- Jun 26, 2021
- Messages
- 17
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
Inverter
MPPSolar LV6548
Settings below:
Batteries
2 - Big Battery Hawk 24v 170AH LiFePo
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
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
- 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?
- 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.
- 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.
- 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.