Off Grid Solar Shed - Choosing Batteries - First Post and PV Project

[unt0uchable]

I'll start by introducing myself. I'm David. My father in law and I just built a shed for myself and although he could have easily dug up my yard and ran electricity to it, I chose to make it solar powered instead. This is my first go around with the technology. I'm really enjoying learning about this and he is too (being over 50, he enjoys learning about new technologies, and with myself being 23, and my wife 25, we are always helping them learn the newer age stuff, lol)

Here is the equipment I used (nothing fancy)

-Harbor Freight 45w 3-Panel Setup w/Charge Controller
-Bestek 2000w 12v-120v inverter (I know, overkill. But It was a decent price on Amazon. I don't plan on maxing it out)
-Batteries: That's why I'm here. Read below please.

Here are some photos to show the setup thus far.



Closeup of the panels...(before I tied up the wiring nicely)


I'll point it out now that you're correct if you're saying "boy his panels are laying down quite a bit". They are only sitting at around 22* facing directly South. When we did the measurements with the latititude+15* or so, the angle came out to somewhere around 55* and they were nearly standing straight up off the shed. How much is this going to effect my charging? I live in SE Michigan and the sun hours avg 4, 2.7 on the low side, and winter is approaching.

I'll basically be running lights, and maybe a radio off of this setup. The shed is wired with 6 110v outlets as well which will be powered by the inverter listed above, so I might see how it works with a dremel tool or something. But that will all be trial and error to see what it can handle at this point

The inverter will be here tomorrow. I currently have 1 regular old lead acid car battery waiting on my shelf just to hook up and try the system out with. I don't expect it to hold much energy for me. That leads me to my next question, and ultimately why I joined the forum and posted my project.

I'm on a smaller budget since this is only my work shed and I'm not trying to go over the top here. I'd like to run 2 12v batteries in parallel. So far, I have learned that what I want is a higher aH rating. I've found a couple of low cost options available via eBay that I'm watching currently. I just want some yay's or nay's from the pros around here so I know whether or not I'm wasting my money on garbage.

The first one that caught my attention was a Max Life 12v 35aH Deep Cycle Solar Battery. Two of these can be had for around $130 shipped, which would ultimately give me 12v @ 70aH.

The next one is a UB12500 which is a 12v 50aH Wheelchair Battery. I can get one of these for around $95.

Another one I found was similar to the one above but it's a Group 24 12v 75aH rating instead, still advertised as a Wheelchair battery. These cost $150 each.

I don't think I'll need more than 2 batteries for what I'll be using this setup for. When I am in the shed, I'll be running three 4' shop lights that call for 110v@.8amps from the inverter, and possibly a 12v car stereo that I plan on mounting and wiring into the shed so tunes can be heard around the yard.

Am I on the right track with any of those battery choices? I'd like to spend around $200 on batteries and be alright with the setup until I find a need to grow the battery bank.

Thanks for any tips and help with my setup.

[Naptown]

Those panels and the controller can only support about a 30 ah battery
You will lose 1/3 or more through that controller. So you effectively have about 30 watts to work with
That inverter may use more for self consumption than it will produce for you.
Once you pull more than about 36 watts from the battery the voltage on the battery will start to sag and the capacity will go way down.

Can you return that stuff and perhaps get some real panels and controller?

[unt0uchable]

I do still have the box for the Harbor Freight stuff panel setup and charge controller. I am working on a fairly tight budget so what would you recommend I get in place of these? I'd love suggestions!

[Naptown]

You can pick up a 250 Watt grid tie panel for under a dollar a watt.
The controller will be a bit more but look for a 20-30 A MPPT charge controller.
The operative word there is MPPT
You get what you pay for so beware of a deal that seems too good
Best IMHO at lowest cost is the Morningstar sun saver mppt
That set up although more costly will give you more than 6 times the available power to use in the shed and will support about a 250 amp hour battery

[unt0uchable]

Well, it looks like this system has gone into the redesign process. I've got a 30a mppt controller on the way and a 100w monocrystalline 40x27 panel on order. It was all in all about $30 more than the harbor freight setup with the coupon I had. I'm glad I decided on this. The setup will provide me with closer to 8a of charging instead of 2-3... Now I should be able pin point which ah battery I want to get. Any more suggestions?

Thanks so far.

[Naptown]

Since most folks ruin their first battery I would suggest an inexpensive marine hybrid battery in the 100 ah range. This would be a learning battery to be replaced with better quality when dead and some battery maintenance items become second nature

[unt0uchable]

How can I avoid ruining my first battery?...

[Naptown]

By keeping the water level up
Never discharging more than 20% of capacity
Checking specific gravity with a temperature compensated hydrometer
And applying an equalize charge when the battery becomes stratified and finally by making sure a partially discharged battery is recharged in a timely manner

[Mike90250]

Since most folks ruin their first battery I would suggest an inexpensive marine hybrid battery in the 100 ah range. ....

AKA Costco/Sams/Kmart Marine Battery in the $90 ballpark Leave out the word Hybrid when asking for it. Just Marine Deep Cycle

How can I avoid ruining my first battery?...

95% of folks don't avoid it, so it's best to practice with a cheap one. Often it's a setting on the charge controller set wrong, or using 40ah a day, and replacing with only 35ah, works for about 2 weeks, then you are gone for a couple days, battery charges back up, and has had 1 year taken off it's life in those 2 weeks of deficit charging. And don't forget to add water.

[unt0uchable]

I get it. I'll just pick up 2 of the lower end batteries at Wal-Mart and go from there. My inverter will be in today, so before I tear down the cheap HF stuff and return it, I'm going to hook up this old Autozone battery I have and see what I can do with it. It's been sitting in my garage for a year now. I charged it up yesterday so we'll see how it goes....

Everything else is ordered and on the way. I'm excited, but this put me back another 3 weeks or so on getting the solar power going. At least I have 90 days to return the HF stuff...ha.

[PNjunction]

How can I avoid ruining my first battery?...

Don't worry - we all do at first, so you'll be joining the club sooner or later.

If you want to start out on the right foot, read the stickes and faqs right here on the site! It will save you money and time. As of right now, I'd stop thinking of buying anything more than just a *single* battery for your initial system. One of the things you'll find out is that you don't want to use a car-battery, which is called an "SLI" or starter/lights/ignition battery that is not designed for deep cycling.

[unt0uchable]

Don't worry - we all do at first, so you'll be joining the club sooner or later.

If you want to start out on the right foot, read the stickes and faqs right here on the site! It will save you money and time. As of right now, I'd stop thinking of buying anything more than just a *single* battery for your initial system. One of the things you'll find out is that you don't want to use a car-battery, which is called an "SLI" or starter/lights/ignition battery that is not designed for deep cycling.

The only reason I keep saying that I'll get 2 batteries is because I'd like to have a good reserve bank of energy. Is this system going to struggle to keep both batteries topped off?

Also, I completely understand about the SLI batteries. The ones I'll be purchasing will definitely not be regular old car batteries. I just happened to have an old battery in my garage that has been sitting for a year and I don't even honestly know if it's any good. That being said, I am going to hook that up this evening (got carried away watching the Tigers last night and lost time) and just see if it will even power my shop lights. Then I'll probably head to Wal Mart and grab a deep cycle battery and try using my old one as a core...

Thanks for the tips! I've been bouncing around since I signed up reading the stickied posts and FAQ stuff. Lots of good info around here. I'm lovin' it.

[Sunking]

I just happened to have an old battery in my garage that has been sitting for a year and I don't even honestly know if it's any good.

If it is been sitting a year without a charger it is a boat anchor.

[PNjunction]

The only reason I keep saying that I'll get 2 batteries is because I'd like to have a good reserve bank of energy. Is this system going to struggle to keep both batteries topped off?

You got it!

Consider you purchasing a dual-purpose battery rated at say 75ah at the industry standard 20-hour rate. (If that can't be found, and only an "RC" value on the battery exists, multiply that RC value by 0.6 to get a ballpark ah rating. It isn't the best battery if you can't find a rate, but moving on.....

You want to hang out in your man-cave-shed from 6p to 2am and want to light it up for 8 hours with a 100 watt incandescent bulb. Let's change that to a 25 watt CFL bulb to save energy, but provides nearly the same amount of illumination. How much will be pulled from the battery?

25w / 12v = 2.083 amps per hour. So, 2.083 * 8 hours = 16.5 amp hours pulled total. Since one doesn't want to deplete a battery by more than 50% DOD on average, that means doubling that capacity, so you would want at least at 35ah battery. AGM's are commonly found in this size.

Cool - but the problem is that in winter, you only have about 2.5 hours of solar insolation (If I remember correctly), and your 100 watt panel at the very best would produce 5.5a charge during bulk (100w / 18v). Let's not get picky and say you are using a quality pwm charge controller.

Well, with only 2.5 hours solar insolation * 5.5a = 11 amp hours. Uh oh - you need to put back at least 16.5ah. Actually a bit more since recharging isn't 100 percent efficient.

Welcome to deficit-charge land, where your battery dies prematurely.

So now you need more panel power, BUT you must keep it within a minimum and maximum boundary rating. For FLA batteries, that is usually between C/12 and C/8. For a 75ah FLA battery, that means you need to supply between 6.25A minimum, and 9.375A maximum. AGM's are about twice this, unless they are specialty types.

So now that you know your loads, you size your battery properly to avoid going below 50% DOD during your usage timeframe, and size the panels accordingly so as not to tickle the batteries to death, nor harm them with overcharge. AND, we're only talking about a system with NO extra days of bad-weather autonomy!

Account for some bad-weather autonomy days, and your jaw will drop when you see what it takes $$ to merely light up the man cave 8 hours daily with a 25 watt bulb.

[unt0uchable]

That was a very informative post. THANK YOU for that!

So here is what I ended up with after a trip to Wal Mart:

Everstart Group 27DC Marine Battery(Hybrid, since it still has a CA rating) but it is rated at 104ah as noted on the battery.

I have since ordered my MPPT charge controller, however, for the first couple of weeks I'll be using the Harbor Freight crap until the MPPT arrives along with my new 100w panel.

And to clarify, I have 3 T8 4' Flouescent Lights installed which claim to be 32w each, or 64w for each light. 2 of which are on the same switch, 1 is for shining into the loft area when needed.

So using your math (thanks!) here is what I've come up with. I won't be spending 8 hours out there unless it's a weekend and I am working on something specific. But I'll use 4 hours for my math (even though I think that is overshooting it still)

64w / 12v = 5.333 amps per hour. So, 5.33 * 4 hours = 21.33 amp hours

So with 1 100w panel charging at 18v (how do I come up with 18v if it's a 12v battery/charge controller, etc?) 100w/18v = 5.55amps at 2.5 hours a day in December.

So I'm putting back in 5.55 * 2.5 = 13.875 on average in a day.

I guess I better make sure I give it a few days to charge after a good hard Saturday of use.

I do have a backup plan in case I need to use the lights/plugs for an extended period of time. I can run an extension cord from the house and it plugs into both strings and works at full power so...just in case. lol. I'll make sure I do this in times of serious need.

So what I am thinking is that if this shed starts getting regular use, I will be needing to purchase another 100w panel to keep my battery from taking a crap on me in 6 months.

Then it will be more like 200w/18v = 11.11 amps * 2.5 hours = ~28 amp hours. Effectively doubling my recharging in a days worth of time.

Am I doin that right?


Thanks again for the awesome reply. That math will help me better understand my needs and help me build an overall better setup for my shed and it's (growing) needs for power.