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Not Enough SOC in Tiny House

chocoheadfred

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Oct 25, 2021
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Here's my system:

2 x 48V 100AH Water-Proof Bluetooth Battery by EG4 = 200AH
5000W LVM- ES Off-Grid 240V Growatt Inverter with Solar Charge Controller + Growatt Split Phase Transformer
16 x 335W panels, wired in 2 strings, joined in a combiner box

I'm constantly running out of power in my tiny house overnight, with average low temperatures this time of year in NC around 25 - 35.

I'm set at 54 for Float Voltage Point and Battery Undervoltage cut-off point at 46. Any recommended changes to this?

Either way, I am not storing nearly enough energy as I constantly lose power between 2AM - 6AM most every night depending on how cold it is. I've been averaging 5-7kWh Load Consumption yet this is without anyone living in it currently (just fridge and mini split set at 60). I'd expect someone in there full-time would be 2x this. I generate more than enough as I can fully charge the batteries in about 2 hours under full sun so generation isn't the issue. Also, this issue is much more relevant in the winter as I'll be cranking out plenty of power in the summer when temps are closer to 95 during the day.

What is your recommendation to increase storage capacity, add more batteries or run a trickle charge from my barn about 200 ft away? Both options are ~$2k and I'm wondering if one would be better than the other. I think trickle charge may add life to the batteries long-term yet not sure if just adding another battery/batteries today would better prevent running out of power.

For the charge I'd likely get some 4-4-4-4 URD Direct Burial Cable.
 

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Bulk charging should be closer to 58V and float I like about 0.5V below that. Your problem is not enough battery capacity and not high enough SOC. Looking at your chart, you are producing plenty of PV, your system just isn't using it.
 
That's at rest, and gives you (at best) a very rough indication.

What's your bulk / absorb voltage set to?
 
Your peak solar production looks very low ( <1,800 watts) considering you have 5,360 watts of panels. Are your panels not optimally positioned for year round production? Snow? Low sun shading?

With cold winter temps and decent positioning of your panels I would expect to see more power (higher wattage) coming from that size array.

Clearly you need more battery capacity as well
 
That's at rest, and gives you (at best) a very rough indication.

What's your bulk / absorb voltage set to?
Not sure, here are the options at my disposal through the online app. Can you see what you need to answer this?
 

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Your peak solar production looks very low ( <1,800 watts) considering you have 5,360 watts of panels. Are your panels not optimally positioned for year round production? Snow? Low sun shading?

With cold winter temps and decent positioning of your panels I would expect to see more power (higher wattage) coming from that size array.

Clearly you need more battery capacity as well
I think so too. They are optimally positioned for permanent install, with no tilting. 35 degrees in North Carolina. I'd expect they would pick up production in Spring, yet to your point, more battery capacity is needed.
 
You really need to hit at least 3.45v/cell with lfp to charge efficiently. Change constant voltage point to at least 55.2v for a full charge.
 
Tell us more about the mini splits, are they cold weather rated?

What explains the huge jump in load once solar is returned? Did the mini splits kick on resistive heating?

I also second rechecking your charge settings, I don’t think you’re getting those topped up.

Also to conserve load try setting the heat for 50f, no sense in wasting energy when no one’s there.
 
Bulk charging should be closer to 58V and float I like about 0.5V below that.
I disagree with these numbers. Strongly. Floating lifepo4 above 3.5V per cell will overcharge them, leading to damage and premature capacity loss.
I strongly agree with @Wibla, (in fact I think this is broadly agreed upon to the point that there isn't even anything to agree with, I think it is just commonly accepted/known (or should be)). it is detrimental to the health and longevity of the battery to float above 3.45 or even arguably 3.40 Vpc, and you definitely don't want to be floating close to 3.6Vpc (58-0.5 / 16 = 3.59

I have not yet come across any manufacturers of lifepo4 cells, batteries, or charge controllers with lifepo4 specific charge profiles that recommend a float voltage of more than 3.40, with 3.35-3.40 being the most common range of suggested float voltage. I made a post about it somewhere but it might be hard to dig up. This resource on lifepo4 voltage settings and limits may be of value.

This short article "How To Find Happiness with LiFePO4" does a great job of explaining the basics of treating lifepo4 right without getting overly technical.
 
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I disagree with these numbers. Strongly. Floating lifepo4 above 3.5V per cell will overcharge them, leading to damage and premature capacity loss.

56V bulk (3.5Vpc), 53.5V (3.35Vpc) float is safe and will get you to 100% SOC without damaging the batteries.
If you understood how Growatt and MPP Solar charge, you would not disagree.

One thing you didn't notice was I said closer to 58V. If he was using a DIY battery and had a runner, the runner determines where to set charge voltage for the pack.
 
mini split set at 60)
2 x 48V 100AH
LFP will sustain things that lead acid can’t/won’t so you have good demand output for overnights. You can probably gain a surprising amount with the charging parameter changes indicated.

Batteries. You need more storage.
Without doing math or calculating BTU/KWh but wildhat guessing based on heating demands (which are a critical and significant factor here in Vermont) all the rest of the issues discussed pale in my mind.
At 25-35*F if you are not low-E windows and R20+ walls, R32+ceilings then that is part of the issue. 1KW/hr is 3400btu and this time of year you are dark 10+ hours so unless you are super super tight and insulated you will definitely run out of battery at that level. The minisplit I’m guessing is way more than 3400btu
I've been averaging 5-7kWh Load Consumption
So one more 48V battery (50% increase of capacity) and that charge voltage may just get you by for now. Doubling capacity makes some sense.

I wouldn’t bury cable for $2000 myself. Ymmv

Do you run propane cooking appliances?
A 16- 20Kbtu forced air propane RV furnace as backup might make sense. 3- or 4A running at 12V is wicked low watts to run; then battery capacity beyond overnight is less of an issue come three cloudy days… and if there’s a griddown event it’s basically irrelevant.
 
Or you could go cheap diesel heater like they use in vans $120 …. They come with 2.5 gallon tank but could go bigger and bulk. @0 degrees my van used .5 gallon per 24 hours. Not ideal but could supplement.
 
LFP will sustain things that lead acid can’t/won’t so you have good demand output for overnights. You can probably gain a surprising amount with the charging parameter changes indicated.

Batteries. You need more storage.
Without doing math or calculating BTU/KWh but wildhat guessing based on heating demands (which are a critical and significant factor here in Vermont) all the rest of the issues discussed pale in my mind.
At 25-35*F if you are not low-E windows and R20+ walls, R32+ceilings then that is part of the issue. 1KW/hr is 3400btu and this time of year you are dark 10+ hours so unless you are super super tight and insulated you will definitely run out of battery at that level. The minisplit I’m guessing is way more than 3400btu

So one more 48V battery (50% increase of capacity) and that charge voltage may just get you by for now. Doubling capacity makes some sense.

I wouldn’t bury cable for $2000 myself. Ymmv

Do you run propane cooking appliances?
A 16- 20Kbtu forced air propane RV furnace as backup might make sense. 3- or 4A running at 12V is wicked low watts to run; then battery capacity beyond overnight is less of an issue come three cloudy days… and if there’s a griddown event it’s basically irrelevant.
I've increased the parameters a bit to 55.2 and now Growatt is saying I'm getting 9+ kWh of energy a day. I think it just doesn't count the additional energy created if it can't store it/send to load.

As far as recommended parameters, I reached out directly to Growatt which said the below:

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...based on my battery parameters:
1641986356924.png

It sounds like many in this thread would disagree with these recommendations though, and this advice is coming from an inverter company and not a battery company.

My mini split is a pretty small one at 12000 BTU and it has a garage door on one side and a ton of windows at the top so it's fairly drafty I'd say:
1641986636892.jpeg

The forced air propane RV furnace as backup kind of makes sense yet really isn't solving the storage problem. Thanks for this idea. In a real use case, the renters are going to be burning through energy like crazy, leaving the doors open, leaving the TV on forever, opening the fridge for a long time...and also be super pissed when/if they run out of power in the morning.

For the trickle charge, I think I've found a more cost-effective method and it will be closer to $500 now (plus a trencher for a day) instead of $2k so this option is much more interesting now compared to the cost of another $2k battery.
 
For the trickle charge, I think I've found a more cost-effective method and it will be closer to $500 now (plus a trencher for a day) instead of $2k so this option is much more interesting now compared to the cost of another $2k battery.
I’m compensating for winter with a low watt draw charger myself that’s working surprisingly well. That could be a good solution. I’d so much want to use the solar though! And buy a battery.

Nevertheless grid backup is still way less money than solar and batteries per kWh
A lot of hybrid AIOs can be configured to lower charge rate to keep under your UF amp rating. Don’t know if yours can do that.
 
I've increased the parameters a bit to 55.2 and now Growatt is saying I'm getting 9+ kWh of energy a day. I think it just doesn't count the additional energy created if it can't store it/send to load.

As far as recommended parameters, I reached out directly to Growatt which said the below:

View attachment 79358
...based on my battery parameters:
View attachment 79359

It sounds like many in this thread would disagree with these recommendations though, and this advice is coming from an inverter company and not a battery company.

I like to run right into the lower knee area on LFP. This is where you see divergence on cells even if they are matched and top balanced. Those numbers are close, if one had cell monitoring it might be a good choice. I'd monitor individual cell voltages and adjust accordingly. If you see a runner, it will determine where you need to set the pack voltage. Usually back off 0.5V and recheck or work your way up from current setting by 0.5V.

Usually with GW, once bulk setting is hit, the unit won't really float unless there is a load. Cells will settle at the 3.4V area without a load.
 
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