Off-Grid Power Requirements

[Arrowshot]

Greeting and salutations. Hope I will get a few responses to my first post.
Not a newbie to this but not the best at figuring out requirements for a off-grid solar system in northern Alabama (above Birmingham) when I am there which is a few days every other week.
Current System

• (Two) Renogy 100-Watt 12 Volt, High-Efficiency Monocrystalline panels (set at correct tilt and orientation for my area)
• EP Tracer 3215RN solar controller (30a and 12v/390-watt max PV input power)
• (Three) Sun X-Tender PVX-1040T sealed AGM deep cycle battery, 12V, 120Ah each (all are several years old now but appear to still be cycling)
• Volfvert 4000W Pure Sine Wave Power Inverter DC 12V to AC 110V 120V

Questions

1. I am attempting to run a microwave rated at 700-watts off the inverter but it runs a few seconds then shuts down. Are my current batteries not strong enough to power it? Is something else possibly the issue?
2. How do I determine how effective my batteries still are?

Thanks in advance.

 

[sunshine_eggo]

360Ah of batteries are healthiest charged at 36A

36A * 14.4V = 518W

Get more solar.

1. A 700W microwave is going to pull 1100W from the inverter. Inverter is < 100% efficient. 1100W/12/.85 = 107A.

Short marine grade 2awg cable is the minimum size you should be using. 2/0 would be better.

All connections between the battery and inverters MUST be of high quality and torqued to spec.

A low quality fuse or breaker in the circuit may present additional resistance.

2. See #1.

 

[E. Cho]

Also list what gauge wire you have between each component as too small wire from the battery to the inverter can also cause this effect, assuming wire gauge to microwave is standard 14 awg or larger 12 awg.

Also, some inverters, like High Frequency inverters, do not run some appliances, especially like microwaves, pumps, some vacuum cleaners or motors with high or longish surge voltages at start up. Search the forums for info on high and low frequency inverters as there are many past discussions. Also there is info in this forums Resource section, use the search function.

This is again assuming the batteries have enough capacity.

Your 200W of solar though at 12 volts is woefully inadequate for a 700+ watt at 120 volts plus conversion losses, meaning that all power has to come out of the batteries, which you don't know the true capacity of.

Your system is under powered IMO, but also check the above and all your connections.

 

[Arrowshot]

Thanks very much for your info/comments.

I used 2/0 cable for connecting batteries together in parallel and the inverter has two strands each of 2/0 wire running to the battery posts. Used high quality connectors and they are torqued to spec. Batteries are showing full charges and inverter shows 13.7 amps when powered up although it dropped when I turned the microwave on to around 13.4 till the microwave shut down. So based on 1100W/12/.85 = 107A shouldn't the batteries be providing enough power to the inverter (4000w) to run the microwave even for a short time provided there is not an issue with the inverter?

Plan to install 2 more 100 watt panels this summer.

 

[E. Cho]

Are your wires pure copper or CCA (Copper Clad Aluminum)? Some cheap inverters use that and double up the wires. Friend had similar trouble with his Xijia? 4000 watt pure sine wave high frequency inverter that would not run his microwave, induction hot plate nor the vacuum cleaner with a brush head.

We swapped out the cheap included wire with 2/0 pure multistrand copper and the microwave worked but we had to swap the high frequency inverter with a low frequency inverter to truly solve the other problems. 1 year later and no other load problems.

My 4000 watt pure sine wave LOW FREQUENCY inverter has handled all loads but one in it's 20 years of use.

 

[MichaelK]

Plan to install 2 more 100 watt panels this summer.

Realisticly, even just 2 more is not enough. Most lead-acid batteries like charging at 1/8th of C, and some AGM batteries like as much as 1/5th of C. Since you have three 120Ah batteries in parallel, that 360Ah of battery that needs charging. You also have to account for the fact that in the real-world, panels almost never put out their rated power. I like to de-rate panels to just 85%.

So, here what the batteries really need...

[(360Ah/8) X 13Vcharging] /85% = 688W of panels. Keep in mind that some AGMs might want even more.

Here's what you can do. Wire the two 100W panels you already have in series to get 36V. Then get two 72-cell grid-tie panels that put out ~36V. Wire all three in parallel to each other. Your 30A controller is not able to handle the output of all three strings at noon, but you have two choices here. Either get a higher amperage controller, or face each string in different directions.

Inadequate charging resulting in a lower state of charge, paired with too thin wire from the batteries to the inverter, is most likely what your problem is.

 

[Arrowshot]

Wire is all pure copper not CCA (only the best). My inverter will run the small shop vac (not sure about wattage) but not with an extension cord.

 

[MichaelK]

Wire is all pure copper not CCA (only the best). My inverter will run the small shop vac (not sure about wattage) but not with an extension cord.

What's the gauge?

 

[Arrowshot]

Wire for extension cord is probably not adequate to carry load if that is what you are asking.

I have already though about new controller as well as new batteries.
What I want to run (although not at same time)

1. LED lights (works with current inverter)
2. TV (LED about 32 inches) (works with current inverter, have run for several hours under existing set up)
3. Small shop vacuum (works with current inverter)
4. 120v small fridge freezer
5. Coffee maker
6. Microwave
7. Water pump for propane water heater

Using your suggestion above regarding tying current panels together with two more what would be your recommended size of my battery bank? Cabin stays fairly cold during winter when I am not there so not sure about using lithium batteries (not sure how long they would take to warm up, cabin is 144 square feet and use propane heater to warm it up after I get there).

 

[sunshine_eggo]

So based on 1100W/12/.85 = 107A shouldn't the batteries be providing enough power to the inverter (4000w) to run the microwave even for a short time provided there is not an issue with the inverter?

That was my next question. What is the voltage at the inverter terminals when running the microwave?

Plan to install 2 more 100 watt panels this summer.

That's still short of what the battery needs. Your 30A charge controller will fall short of whatever you hang on them.

 

[Arrowshot]

Thanks so much for the all the answers. Plan a trip to cabin in a few days to check some of what you asked. Got to call it a night but will be back with some more questions.

 

[Hedges]

• (Three) Sun X-Tender PVX-1040T sealed AGM deep cycle battery, 12V, 120Ah each (all are several years old now but appear to still be cycling)
• How do I determine how effective my batteries still are?

360Ah of batteries are healthiest charged at 36A

some AGM batteries like as much as 1/5th of C.

SunXtender wants at least 0.2C, see page 20:

http://sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

My system delivers CC at 0.2C, followed by CV. That can be done with some DC coupled systems if SCC is made aware of battery current by a shunt.

I have the same battery. It is 104 Ah (24 hour rate), 120 Ah for 120 hour rate.

Sun Xtender Solar Batteries - Photovoltaic Battery

The Sun Xtender Battery line consists of specialty lead-acid solar, photovoltaic batteries, and renewable energy storage batteries manufactured by Concorde Battery Corporation.

sunxtender.com

Mine are about 14 years old and maybe 40% of capacity.
I tested my 4s (48V) bank but feeding inverter and 600W space heater.

After you fully charge the batteries, disconnect them from each other and allow to settle a few hours so you can check them individually. If voltage drops below what manual says it should be (temperature adjusted), you can try the equalization profile SunXtender suggests.

 

[Bud Martin]

BTW:

1. I am attempting to run a microwave rated at 700-watts off the inverter but it runs a few seconds then shuts down. Are my current batteries not strong enough to power it? Is something else possibly the issue?

You say shutdown meaning the inverter goes into shutdown? if it is, did it show any error code or error indicator?
Pictures of your setup?

 

[MichaelK]

I have already though about new controller as well as new batteries.
What I want to run (although not at same time)

1. LED lights (works with current inverter)
2. TV (LED about 32 inches) (works with current inverter, have run for several hours under existing set up)
3. Small shop vacuum (works with current inverter)
4. 120v small fridge freezer
5. Coffee maker
6. Microwave
7. Water pump for propane water heater

Using your suggestion above regarding tying current panels together with two more what would be your recommended size of my battery bank? Cabin stays fairly cold during winter when I am not there so not sure about using lithium batteries (not sure how long they would take to warm up, cabin is 144 square feet and use propane heater to warm it up after I get there).

Looking at your list, items 1,2, and 3 are fairly easy, though 4, 5, and 6 are more of what I would call "big ticket" items. The frig has relatively low wattage demands, but those demands are on and off all day and night, so cummulatively you are consuming a lot of Wh. The coffee maker and microwave both demand a LOT more watts for the short amount of time you need to use them. It's quite hard for a 12V battery bank to output the number of amps required to feed them.

If you are ready to replace both the batteries, and the controller, perhaps I could suggest that this would be a good time to leave 12V behind and upgrade up to at least 24V. This will put less stress on the batteries as they need to output half as many amps to supply the same loads.

Since you need to deal with cold winter weather, I could suggest you upgrade up to at least 6V golf-carts. Trojan makes the T-105 that's 250Ah, double the Ah that you have right now. Costco has a somewhat cheaper grade 6V at 210Ah, but it's only 110$ right now. Four would be just 440$ plus tax and core.

Let's say you got the 250Ah Trojans. Trojan wants their batteries charged at 1/8 of C, or 13%, which will work out to be 250Ah X 13% = 32.5A. To get an honest 32.5A at a charging voltage of ~26V, you need (32.5A X 26V)/85% = 994W of panels. If you got four 250W grid-ties, you could wire them in 2S2P and a 40A MPPT controller will handle them. I'm seeing those on Craigslist for about 75$ each right now. Something like an Epever Tracer4210AN would be fine. Alternatively, keep the two 100W panels, wired for 36V, and get two 300W 72-cell panels as described above. That would make you [(200W + 600W)/26V] X 85% =26A. Maybe though plan for a controller with a greater Amp limit to account for future expansion.

Last, a good budget-priced sine-wave inverter might be Samlex's 2000W model for 650$. It's UL listed, low background consumption, and can even be hard-wired directly into your cabin's main electrical panel (if you have one?).

 

[tinyt]

Thanks very much for your info/comments.

............ Batteries are showing full charges and inverter shows 13.7 amps when powered up although it dropped when I turned the microwave on to around 13.4 till the microwave shut down. .........

You said 13.7 amps when powered up. Sounds like something is wrong with the inverter if this is without load yet.