Cautious Newbie seeking Experienced Eye...

[Ender Cowboy]

Hey everyone, my names Dave. I'm a nurse living in the wilderness of Lexington KY, and after the Rona got rolling I started getting interested in some emergency preparedness. One of the key things I am interested in is solar, hence my showing up here. Hopefully you can help with some questions I have, let me fill you in a little on what I am dealing with.

I recently started acquiring some gear for emergency prep, and one of the things I wanted was backup power for some appliances. Number one being my refrigerator, which is a new energy efficient model. I also wanted something portable, in case we needed to retreat to the back country it would be able to go with us. So think of my system as emergency use/camping. I know some people do this for cost savings, but that isn’t much of an issue for me, so if you have higher cost options then I am open to them.

After some research I decided to go with the MAXOAK EB240 2400Wh/1000W Power Station, mainly because of the combined MTTP and Inverter, as well as the large storage and draw. I know everyone has personal preferences, but since I already have the Power Station, I’m not interested in hearing about getting a different one. My major problem is how to charge it.

I think I have a pretty good handle on the setup I want, but I also know enough to know that I don’t know things, and it’s best to get some experienced advice. The Bluetti has a max input of 500W, and the Open Volt Current is 68V before it throws an error. The Imp is 10A, but I have been told the MTTP will protect from some slight over amperage (not sure if that is correct). What I am trying to find now are some 160W panels, 3 in series for 480W, which can stay safely under the 68V-10A limit. Having never built a system like this before though, I realize I might be missing some vital part of this equation. So if you winced or laughed at my setup description, please tell me what I am missing, I won’t be offended.

Thanks for staying with me through this, I know it’s a lot. Hopefully someone can help me with the intricacies of this kind of setup, because right now I have a lot of options, but a buttload of indecision. If you can help me get a good setup, I promise I’ll repay you with great post-apocalyptic healthcare if it ever comes to that. Lots of alcohol soaked sutures, Percocet, and cauterization, but done by a professional! Thanks a lot, and everybody have a good one.

[Mike90250]

So, the good news is, there is bad news. From the battery specs: 162162mAh/14.8V Which translates to 162ah @ 14/8V

Say you have a super efficient fridge, that consumes 1kwh daily ( the big energy star fridges do this, smaller fridges have less insulation and consume more power ! )

1kwh @ 15V = 66ah, so you could, if starting from fully charged, go 2 days on the battery, but you now need to calculate how much solar you need to power loads AND recharge the battery. How many hours of solar do you have in your location ? 3h, 5h ? few sites get 6 good production hours. You have a max input of 500w into the controller. Can you figure the math, or should we help ?


Built in Top-Brand Lithium Polymer Battery Cell
nice. These are the ones that burn up the easiest. https://batteryuniversity.com/learn/...of_lithium_ion

[Ender Cowboy]

I sort of figured there was bad news, there usually is when you are first learning. Let me start out with my view of what you just laid out.

162ah @ 14.8V equals 2,397.6Wh, which lines up with the 2400Wh advertised for the Power Station. On a fridge that consumes 1KWh at 15V, it's 66.6 repeating Ah, leaving me 96Ah in the tank. That's a 41% draw on the battery, keeping me out of the 50%+ deep draw, hopefully extending the lifecycles of the battery. I need to make up that 66.6Ah during my (let's say) 5 hours of good production (Lexington has more sunny days than cloudy) while also powering the fridge. 66.6Ah @ 15V over 24 hours is a 2.775Ah @ 15V draw every hour, so for those 5 hours I need to make 16.095Ah input per hour. 2.775Ah to power the fridge per hour, and 13.32Ah per hour over 5 hours to regain my lost 66.6Ah. That means I need at least 193.14W coming through for 5 hours on a 12V panel system. I'm aiming for 480W input, on panels that are stated to make 800-1000W max input per day per panel, so I was thinking that would be enough.

Now, I am no engineer, and I only started doing electrical math 2 weeks ago, so that's probably wrong somewhere. I think your response made it clear that it is wrong somewhere, which is helpful, because I need to know if it is. Where it isn't helpful though is that I still don't know what that problem is. I just gave you a bunch of math I did, but I don't know if it is the right math. If I knew the right math, I wouldn't be hesitant about my answers, and seeking some peer review from experts. It's like if someone asked me about time stripping a gravity flow IV, I wouldn't ask them if they could do the math, I'd just explain the process and how we arrive at the correct conclusions.

I mean no offense, I know this is probably pretty basic stuff for you, and I take no offense in you telling me I missed something or am wrong. Indeed, that is what I came here to hear. I just think the delivery of concept to a newb could be better, because I still don't know where I am wrong. Please let me know, because I am eager to learn from someone who obviously knows a whole lot more than me.

[Mike90250]

You are very close with the math, Add another 20% consumed for the inverter losses and another 20% panel losses when hot in the sun. Lead acid batteries would be another 20% recharge loss..

24 hr Loads:
1kwh Fridge
200wh inverter loss

210wh fridge load while charging in that 5 hours

384w from panels for 5 hours = 1920wh daily harvest (actual estimate) (subtract the fridge out

So, rough look, it will run till you have a cloudy day ! And then reality will set in, 5 hours is not enough for bulk and absorb (generally) and you will have to see what the magic of Li Ion batteries does, if it will fully recharge, or If you will have to modify your PV array,
There is some benefit to Virtual Tracking, where - if you had 4 panels, 2 in series facing ESE, paralleled with another 2 in series facing WSW. your harvest would start earlier, but have a lower peak, and then continue charging later, then with just 3 panels facing south.

[Ender Cowboy]

Hey Mike, sorry it took so long to get back to this, been pulling extra duty at the hospital the past couple days. Thanks for responding to me, you gave me a lot to think about! I had known that inverter draw was something, but I actually forgot about it when I was doing the math. You are right though, it draws about 10W when the AC is running, or 240WH over 24 hours, which is right where you said. Is 20% pretty standard for inverters, or just a good average to account for in the math?

I know that panels charge less when they are warm, and more when they are cold, but what is the swing? Is it 20% either way? 20% more than rated when cold, and 20% less than rated when warm? Or is that again just a good average to add into calculations when designing a system?

I do have a small gas generator as backup, in case of extended cloudy weather, but wanted to concentrate most on the solar input as gas is heavy, and might not be readily available if things go bad. I have 20 gallons with fuel saver stored, but only plan to use that if I can't use solar for some reason.

One of the things I don't think I explained well enough when I first talked about my system is my desire to have it be portable, I am actually designing it to be able to be packed up and carted away if we need to move. I am designing a pole frame which will hold the panels, which could be disassembled and stuffed in large military style duffle, then packing the panels in an artists canvas bag (the kind of big portfolio that a painter would carry a canvas in). The Power Station has a handle and is already portable, so a few people could lug everything a couple miles off the beaten path before setting up. Or if we are sheltering in place, could be set up in the backyard if need be. This presents a few different questions though.

I am going back and forth between the idea of three 160W panels in series, or four 120W panels in the same setup you described: two sets of 2 panels in series, hooked together in parallel. I am trying to get as close to the maximum wattage and still not go over the 68Voc and 10A Imp. I'm just not sure how close I can get to all that without running the risk of a surge hurting the system. I have been told that the MTTP will protect it from over-amperage and over-wattage, and thing I need to respect is the 68Voc because if it goes over the Power Station will throw a code and stop charging. Currently I have been looking at some 160W panels with 20.68Voc and 9.06A Imp, which in series would be 62.04Voc. Alternatively I was looking at 120W panels with 21.6Voc and 8.5A Imp, which in two sets of parallel series would be 43.2Voc. I don't know which setup is safer for the Power Station, and honestly I am not sure if I am correct in what I was told about what is safe for the system.

Beyond the safety is effectiveness. I know that the amount of energy generated by the panels is dependent on their angle to direct sunlight, which is why the power waxes and wanes throughout the day, and in North America having them pointed south is the best direction for optimal absorb during daylight. What you said though, about having panels facing both ESE and WSW, is better for having a longer charge period as is presents the best angle for both the rising and setting sun. What I am wondering is would it be best to track the sun throughout the sky? The whole frame plus panels will weight about 60-70lbs, easy enough for two people to move, and I was wondering about moving it to face the sun at certain times. Like on the hour shooting an azimuth to where the sun will be at the half-hour, aligning the array to that so you get optimal power throughout the hour, and then repeating that process every hour. So throughout the day your panels move from ESE to WSW incrementally, and you have a longer period of optimal charging. Is that a good idea, or would it have no measurable effect on the total charge of the system?

Thank you for bearing through all my questions, you really have given me a lot to think about! Hopefully you can help me make sense of all these disparate ideas, and I can finish this project up. Hope you are having a great day, and I can't wait to hear back from you. Cheers!

[bob-n]

"I know that panels charge less when they are warm, and more when they are cold, but what is the swing? Is it 20% either way? 20% more than rated when cold, and 20% less than rated when warm? Or is that again just a good average to add into calculations when designing a system?"

The amount of [pwer lost hot varies by panel. You can often find it on the panel data sheet. For example: Hanwha Q-Cell Mono panels (a relatively common, high-end panel) are specified to decrease maximum power output by 0.37%/degree C https://www.altestore.com/store/sola...-panels-p41041
Panasonic HIT panels (another high-end panel) are specified to decrease maximum power output by 0.258%/degree C
na.panasonic.com/us/energy-solutions/solar/hit-panels/330n-hitr-series
As you estimate temperature, don't forget that panels are roughly 20% efficient. Some of the remaining 80% of incident energy goes into warming the panel to a temperature above air temperature. It's not uncommon to have a panel that is 10 to 15 degrees C hotter than outside air at noon.

[J.P.M.]

"I know that panels charge less when they are warm, and more when they are cold, but what is the swing? Is it 20% either way? 20% more than rated when cold, and 20% less than rated when warm? Or is that again just a good average to add into calculations when designing a system?"


As you estimate temperature, don't forget that panels are roughly 20% efficient. Some of the remaining 80% of incident energy goes into warming the panel to a temperature above air temperature.It's not uncommon to have a panel that is 10 to 15 degrees C hotter than outside air at noon.

An energy balance on the array or a panel will show that the solar energy that doesn't get reflected off the array or get converted to electricity winds up heating the array or panel.

Under high or full sun, that is times of high irradiance, an array's or a panel's average temp. will be closer to ~ 25 C -35 C higher that the immediately surrounding air temp. depending on things like wind and surrounding structure temps., as a check on the PTC conditions and NOCT temps. for a panel will confirm.

[Mike90250]

I'd use the 4 panel system. easier to handle panels
Be careful with the charge controller max volt input. 3 panels in series may just squeak by in the summer, but in winter, at dawn when the panels are cold, they produce more voltage. Use the string sizer chart / caculator for your panels and your historic cold temps. If something happened to one of the 4 panels, will the voltage allow 3 in series ? Consider a spare panel ? Mounted on a roof, panels are pretty tough, but being lugged around, one "ding" and the glass is gone. Remember your panels are like a sail in the wind, anchor them down.

[Ender Cowboy]

Thank you guys for the explanation of the decreased power to temperature measurement, I know I have seen that on the data sheets before, but always been a little confused on the real world application of it. As a reduction of power, is that specifically effecting Wattage, or will that also similarly reduce Voltage and Amperage as well?

I will be going with the 4 panels, thank you! Please help me make sure I've got the setup right here:

I am using 4 RAVPower Flexible 120W Panels with the following specs:

Optimal Power: 120W
Working Voltage: 18V
Working Current: 8.5A
Short Circuit Current: 8.8A
Open Circuit Voltage: 21.6V

Putting them together in an array of series / parallel, I figure:

Optimal Power: 480W
Working Voltage: 36V
Working Current: 17A
Short Circuit Current: 17.6A
Open Circuit Voltage: 43.2V

Now the Power Station Specifies it's PV input as: 42V/160W / OCV 16V~68V / Max 10A. It has an onboard MPPT, so I should just hook the panels directly into it, not use a controller, correct? It says it will protect against over-charge, over-current, and short circuiting. But I don't want to blow it up, so please let me know if I've gotten the specs right.

I have definitely been looking at getting 1-2 extra panels in case of some catastrophic failure of one or more, and if I had to go down to 3 in series, the total VOC is only 64.8V which is still under the 68V limit. The canvas portfolio carrier should easily accommodate the panels, and I'm looking at slotting sheets of open cell foam (cheap work out pads) in between the panels to protect their surfaces. I had thought about the sail effect of a large array standing solo in the yard, and my fix would be driving stakes into the ground at the base, to keep it from toppling. My new design actually includes a rotary piece with a cinch, so you can loosen it, turn the array, then tighten it back down. I'm still hoping the practice of turning the array to face the sun will help to increase the optimal charge time, please let me know if you think it will have any measurable effect. Thanks for all the help guys!

[bob-n]

The reduction in power with temperature on the data sheet is for power.

Voltage and current will also change with temperature. Your MPPT should handle that.

Check the data sheet on the panels you are buying. VOC goes down at high temperature and is highest at very cold temperatures. Panasonic specifies their HIT panels VOC as -0.17V/degree C. These panels have 96 cells. Panels with 60 or 72 cells should have a lower VOC and a lower change in VOC with temperature.

[Mike90250]

> Now the Power Station Specifies it's PV input as: 42V/160W / OCV 16V~68V / Max 10A.

You are beyond the 160w spec for the input. The MPPT controls the power by modulating the tracking efficiency, and pulling the panels to a lower voltage/power point. If your 480w is split east/west, you are still 240W which is over their 160W spec. The 10A is the output amps the controller can manage, and to keep within that range, it has to load the panels down, and is only able to load 160w of panels. Will 80 extra watts cook it ??????