What Would You Do?

[bobdelso]

I'm building a large solar system to run a 700watt minisplit a/c in my RV. I've got 1300 watts of solar panels that are 33v 9amp. I'm going to use a 1500 watt 120v inverter to run a fridge and the A/C. My goal is to power the a/c with solar during the peak sun and maybe a few hours at night. I might get a 2000watt inverter generator to top up the batteries on cloudy days. I was leaning towards a 12v system but it just seems too expensive. A 24v interter and 24v battery with a 12v step down for lights seems best.

I can buy 8, 105 ah cells at 500 shipped.

Or I can buy 4, 280 ah cells at 650 shipped or 8 for probably 1100-1200 shipped.

I'm thinking the 24v 105ah pack would be a good start and then I can upgrade later on with another 24v 105ah pack. Should I just bite the bullet and do a 24v 280ah pack?

 

[chrisski]

Need to take a look at your power requirements and come up with a KWH. When you say AC, I live in Arizona's Desert, so to me that means AC coninously runnning 24/7 this time of year. Your panels and battery packs fall way short of that. For someone in the northern latitudes, that might just means two hours, so that could be adequate.

FOr the battery pack, that depends on your power requirement in KWh. eight 280 AH cells will have 6 kWh, of which you can use around 5 kWh. I believe in building a battery bank to last two days, which means you go to sleep after the batteries have finished charging at night and the next day is cloudy and a no power producing day, and then the panels start producing when its sunny the day after that.

For charging your batteries on a generator, my RV Converter only pushes 15 amps, so if I did discharge 200 amp hours, it would need to run over 13 hours to fill them back up. There is of course higher power converted you can get to replace.

Also, it’d be nice to know if the minisplit you get can be powered by a 1500 watt inverter. They mini-splits are supposed to not have surge wattage should be fine, but I’d do as much as I can to check before I bought it.

EDIT: Corrected battery size

 

[OffGridInTheCity]

I'm building a large solar system to run a 700watt minisplit a/c in my RV. I've got 1300 watts of solar panels that are 33v 9amp. I'm going to use a 1500 watt 120v inverter to run a fridge and the A/C. My goal is to power the a/c with solar during the peak sun and maybe a few hours at night. I might get a 2000watt inverter generator to top up the batteries on cloudy days. I was leaning towards a 12v system but it just seems too expensive. A 24v interter and 24v battery with a 12v step down for lights seems best.

My 7 x 14 trailer conversion with 1" insulation has a 9000 BTU Mini-Spit + 4.5cu ft 120v fridge. Its running 4-6kwh/day to maintain fridge + 80F/27C with outside temps bounding around between 97F/36C to a few 105F/41C with 1 day at 114F. In winter its similar 4-6kwh/day down to 25F/-4C. The mini-split runs in the 400-600kw range but can spike up to 1200w for short periods.

Let's say you want to the battery to supply at least 4kwh thru the night. That's 80% DOD of a 5kwh battery.
At 24v, that's 8 x 209ah cells. At 12v that's 4 x 417ah cells.
*I'm running a 14.5kwh lithium-ion DIY battery bank - 14s88p.

I can buy 8, 105 ah cells at 500 shipped.

Assuming you mean LifePo4 - won't be large enough. 8 x 105ah = 24v @ 2.5kwh battery.

Or I can buy 4, 280 ah cells at 650 shipped or 8 for probably 1100-1200 shipped.

Assuming you mean LifePo4 - won't be large enough. 4 x 280ah = 12v @ 3.36kwh battery.

I'm thinking the 24v 105ah pack would be a good start and then I can upgrade later on with another 24v 105ah pack. Should I just bite the bullet and do a 24v 280ah pack?

You ask for opinion
1) I'd go with 24v@280ah = 6.72kwh

2) You easily can buy 24vdc->12vdc for modest amps but just as an option - I found a 120vac -> 12vdc @ 83a step-down for cheap and that's what I'm using. The conversion losses are not that big a deal for me.

2) With 1200w of solar panels, I've been running 24/7 for over 2 months now (May/Jun/Jul...) burning 4-6kwh/day and haven't needed grid at all.

 

[bobdelso]

Need to take a look at your power requirements and come up with a KWH. When you say AC, I live in Arizona's Desert, so to me that means AC coninously runnning 24/7 this time of year. Your panels and battery packs fall way short of that. For someone in the northern latitudes, that might just means two hours, so that could be adequate.

FOr the battery pack, that depends on your power requirement in KWh. eight 280 AH cells will have 12 kWh, of which you can use around 10 kWh. I believe in building a battery bank to last two days, which means you go to sleep after the batteries have finished charging at night and the next day is cloudy and a no power producing day, and then the panels start producing when its sunny the day after that.

For charging your batteries on a generator, my RV Converter only pushes 15 amps, so if I did discharge 200 amp hours, it would need to run over 13 hours to fill them back up. There is of course higher power converted you can get to replace.

Also, it’d be nice to know if the minisplit you get can be powered by a 1500 watt inverter. They mini-splits are supposed to not have surge wattage should be fine, but I’d do as much as I can to check before I bought it.

The minisplit i'm looking at is an inverter style, so its soft start by nature. Pulls 700 watts on average when running. I'd expect to run it for 4-6 hours during peak sun and to be offset by the panels, so 4.2 kwhrs. The fridge will pull around .2 to .5 kwhrs. The rest of the loads will be small, probably .1 to .2 kwhr.

I only plan to run the a/c a little bit at night, probably would rely on my maxfann more than the a/c. So probably 1.5-2 kwhrs.

 

[bobdelso]

My 7 x 14 trailer conversion with 1" insulation has a 9000 BTU Mini-Spit + 4.5cu ft 120v fridge. Its running 4-6kwh/day to maintain fridge + 80F/27C with outside temps bounding around between 97F/36C to a few 105F/41C with 1 day at 114F. In winter its similar 4-6kwh/day down to 25F/-4C. The mini-split runs in the 400-600kw range but can spike up to 1200w for short periods.

Let's say you want to the battery to supply at least 4kwh thru the night. That's 80% DOD of a 5kwh battery.
At 24v, that's 8 x 209ah cells. At 12v that's 4 x 417ah cells.
*I'm running a 14.5kwh lithium-ion DIY battery bank - 14s88p.


Assuming you mean LifePo4 - won't be large enough. 8 x 105ah = 24v @ 2.5kwh battery.


Assuming you mean LifePo4 - won't be large enough. 4 x 280ah = 12v @ 3.36kwh battery.


You ask for opinion
1) I'd go with 24v@280ah = 6.72kwh

2) You easily can buy 24vdc->12vdc for modest amps but just as an option - I found a 120vac -> 12vdc @ 83a step-down for cheap and that's what I'm using. The conversion losses are not that big a deal for me.

2) With 1200w of solar panels, I've been running 24/7 for over 2 months now (May/Jun/Jul...) burning 4-6kwh/day and haven't needed grid at all.

Thats great advice.

How do you use 4-6 kwhrs in the winter?

I'm thinking 200ah won't be enough to run the a/c much. But for the non summer months it would be more than enough for a fridge and some fans.

 

[kenryan]

eight 280 AH cells will have 12 kWh, of which you can use around 10 kWh

3.2 X 8 X 280 = 7168

If it were me, I would "bite the bullet."

 

[OffGridInTheCity]

Thats great advice.

How do you use 4-6 kwhrs in the winter?

Its a mini-split heat pump - cools in summer and heats in winter. I run it in winter to keep the trailer warm to work in it and keep the batteries from getting too cold.

I'm thinking 200ah won't be enough to run the a/c much. But for the non summer months it would be more than enough for a fridge and some fans.

200ah @ 24v = 4,800watt-hours. That's enough to run a 9000 BTU mini-split https://senville.com/9000-btu-mini-split-air-conditioner-senl-09cd/ (heat-pump) with a 19 SEER (pretty efficient), heat or cool in 100sq ft w/insulation, for aprox 24hrs. The fridge is trivial in comparison

 

[time2roll]

bullet

 

[acdoctor]

I have a 8.5x20 converted trailer. 2 inch foam everywhere and 3 in ceiling. A 1 ton 26 seer cooper and hunter mini split. I have 12 100 watt renogy panels flat on racking 3 inches off of roof. At my lat. 35 deg. In the winter I get as low as 400 watts peak in July I got as high as 850 peak. I have 2 12 volt lifepo4 200 ah parallel. I have an 11 cf fridge/ freezer running. The A/c I run from 9 am to 6 pm on 78 deg. Every day unless it is cloudy with rain and the outside temperature is no higher than 85. Basically it is not occupied but has food and stuff so I don’t want it getting hot. I have 40 lb of pork in the freezer so very important the system doesn’t go off on low voltage. It has not been connected to shore power since mid February. I have charged 2 times with my 2 kw gen. But it needed ran any way. If I had ran air on the cloudy days I would have needed to run generator at least 10 times to total about 40 hours I think. 12 volt I needed 2 60 amp mppts. If I started over I would go 24 volts 300 ah. or larger 1 mppt, smaller wire, breakers and stuff.