Planning of a small off grid system

Mike M · November 2013

Hello all, I'm new here and somewhat new to solar. Last year I built a small cree LED array for my dad's Tuff Shed using a small 10w panel, controller and battery. I'd like to build one for myself except that I have slightly different uses and would like something with a larger storage capacity and mobile. I'd like to bounce a couple ideas off some of you more skilled folks.

My system will need to be portable but most of the time it will be somewhat idle in my garage. Ideally it would be tool box sized and house most of the components in a box, minus the panel. The box will hold switches, meters, controller, inverter etc. The main use of the power will be to power a 2 meter VHF radio in my garage. At max power it pulls 13 amps but that setting is used very little. Normal use while transmitting is 6-8 amps and most of the time it's just in receive mode. Use in the garage isn't high but if the box and radio are taken to an event it could be used for upwards of 8 hours. I'd say just radio use and 8 hours would require a 10-12 Ah battery. (Yes?)

Another use for the box would be to support small electronics at an event or power outage at the house. Small things like LED lights (DC), charging phones/tablets and maybe running a laptop and router. AC usage would be through an 800-1000 watt inverter but not used as much. Maybe charging a handheld radio through an AC charging dock, running the laptop, 110v LED flood light or lamps.

I haven't purchased anything yet but have some ideas of what I want. I DO have some older (year 1998 ) panels I was given, each rated at 43W that would nice to use. I was thinking I could mount those (probably just one given the size of my system) on the roof of the house and use them while the box is there. I'd get a smaller and more efficient panel for my on the go application but the more I think of it I might be able to just use battery storage and the panels would only be needed to keep the batteries full and needed for extended use periods.

Above is just what is going through my head (no real questions yet but would love some ideas/suggestions or help for my idea). For size and capacity sake what battery would be a good starting point? Remember, needs to fit in a tool box or battery box type enclosure and be able to support my load and take a charge through a solar controller. Also with this load and use is the smaller Morning Star SG-4 controller suitable? Amp and size wise it seems fitting.

I'm not stuck with using the take off panels I have but understand they might not be the best for feeding my battery (not yet spec'd) or matched to the load. I liked the idea because I already have them (2). Leaving one permanently mounted and wired to the house would be great while having the second travel around as needed.

Thanks in advanced for the input.

NorthGuy · November 2013

Re: Planning of a small off grid system

Mike M wrote: »

At max power it pulls 13 amps but that setting is used very little. Normal use while transmitting is 6-8 amps and most of the time it's just in receive mode. Use in the garage isn't high but if the box and radio are taken to an event it could be used for upwards of 8 hours. I'd say just radio use and 8 hours would require a 10-12 Ah battery. (Yes?)

Assuming 8A is the average consumption at the battery voltage, 8A * 8h = 64AH. If you assume a full charged battety discharging to 50% SoC, you need 128AH battery. If you want to do that for 2 days, it's 256AH battery. If this is 12V, then it's 2 6V Golf-Cart batteries, which is questionable for a portable station. If this is 24V, this is 4 Golf-Cart batteries, which is totally out of question for a portable box.

Mike M · November 2013

Re: Planning of a small off grid system

Thanks for the reply. 8 amps is only during transmitting so the draw is actually much less. I've never tallied up talk time but it's like a 5% duty. Googleing HAM go box shows a lot of guy using much more powerful radios, tuners, etc on small batteries and there actually isn't much talk about solar yet I see guys with small panels.

I'm thinking I'm going to try a battery like this: http://www.amazon.com/gp/product/B001VV0318/ref=s9_simh_gw_p60_d0_i3?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=1CCC7197RJB49PGS6CB4&pf_rd_t=101&pf_rd_p=1630083502&pf_rd_i=507846

It's a SLA, 12v, 35 Ah which I'm pretty sure can get me through an 8 hour event. With a panel it will extend the time a bit. While camping or off the grid I'm sure it can support me for a bit especially just charging devices and a light or two at night.

What are the thoughts on this battery? Suggestions on a ball park panel size to keep it happy?

jcheil · November 2013

Re: Planning of a small off grid system

Mike M wrote: »

It's a SLA, 12v, 35 Ah which I'm pretty sure can get me through an 8 hour event.

Since you never want your SOC to go below 50% you would only have 17.5AH of useful power from it. That is not very much.

Mike M · November 2013

Re: Planning of a small off grid system

Yes. Ok, still learning and thank you guys for the input. I looked up some specs on the radio because it seems like the device that I have numbers on. Keep in the mind the most use would be at an event. Typical use would be short periods of time while in the garage. Nevertheless I still need something that can serve as a small back up power.

Radio: Supply voltage 13.8 V DC +/- 15%, negative ground. On receive less than 0.7A, less than 0.3A squelched. Transmit is max 10A on high (extremely limited usage for my purposes). More realistic use is 7A - 5A which is less than I listed above.

Price wise linking two of the above batteries would give me more but at over $100. At that price should I be looking into a different battery?

BB. · November 2013

Re: Planning of a small off grid system

Regarding HAM, the transmitters work out to be around 50% efficient... A 100 watt radio will take about 200 watts of "raw power", etc...

Are you looking at 2 meter, or HF and their 500-1,500 watt transmitters (with their linear amplifiers). Obviously, that is not a very portable unit in itself.

It all works out to how much power you need (times hours of use). A small DC AH/WH meter or Kill-a-Watt AC WH type meter are places to start.

Once you define the loads, penciling out a system is just simple math and assumptions.

AGM batteries are great for surging power--You can run one at a C*4 discharge rate (drain in 15 minutes) with good quality AGM batteries. But if are looking for longer runtimes--Then a simple set of flooded cell golf cart batteries is a good (cost effective) place to start.

Then there are the DC issues... Many radios will only output rated power at 13.8 volts--And a true deep cycle system will run from 10.5 to 15.5 volts or so--And many DC devices really do not like those wide voltage ranges.

Which begs the whole question of just going with a 120 VAC TSW inverter and an AC to 12 volt converter.

-Bill

jcheil · November 2013

Re: Planning of a small off grid system

With 13.8 +/- 15% that puts you in the range of 11.73 - 15.87V. 11.73V is close to the 50% SOC in most batteries so in a way, it would be good if the radio stops working when the voltage gets below that so then your battery will never get below 50% SOC (assuming you are charging it each day).

Always better to have one larger AH battery than 2 smaller ones in parallel. But how "portable" do you want it to be?
I mean, technically everything is portable with the right transport

(2) GC2 style batteries would give you 215AH at 12v. You'd have plenty of power then (for many days actually) but is 110lbs of batteries "portable" to you? That is a call that you are going to have to make.

Just think of it this way...with 17AH available in a battery, it can supply 17 amps for an hour, or 1.7 amps for 10 hours or .7 amps for 100 hours (roughly). I am sure one of the experts here can give you a really detailed breakdown.

Mike M · November 2013

Re: Planning of a small off grid system

Wow, maybe I need to build a dedicated system for the radio. It makes sense I'm not going to have good performance from a system that wants more than 12v to operate at it's peak. A lot of my radio work is super mobile so I use a battery powered handheld but limited to 5 watts of output power.

This first project will probably just be used for the devices listed above minus the radio. At that the 35 Ah battery looks like it should hold it's own with low draw DC items. It looks like an inverter, even with a low draw item plugged in, is going to suck down most anything under 50 Ah. It also looks like my 43W take off panels aren't a good match for such a small battery. Or are they?

NorthGuy · November 2013

Re: Planning of a small off grid system

Mike M wrote: »

Radio: Supply voltage 13.8 V DC +/- 15%, negative ground. On receive less than 0.7A, less than 0.3A squelched. Transmit is max 10A on high (extremely limited usage for my purposes). More realistic use is 7A - 5A which is less than I listed above.

You need to do some estimate of daily power.

Say, 0.7A for 23 hours and 10A for 1 hour. 0.7x23 + 10x1 = 26AH, which will require 52AH battery for 1 day or 104AH for 2 days. That's only for the radio. Emergency power will require more (or you can disconnect the radio in emergency). This still comes way above what you think would work, so this estimate may be wrong.

The best idea would be to measure power consumption of your radio for few days. If you can connect it to AC you can do that with inexpensive Kill-a-Watt meter.

There are several people on the forum who use radios, they may have all the measurements already done and radios running, so they may have a good recommendation for you.

Mike M · November 2013

Re: Planning of a small off grid system

Your estimates seem reasonable. And yes it comes out above what I was thinking before getting some help from you guys. The radio go box might need to be another project as it's requirements are in another ball park.

My thoughts are now on a portable power supply with solar recharge. After looking through some online resources it looks like a 25w panel will be about the size I want to lug around. Hhhmm.

NorthGuy · November 2013

Re: Planning of a small off grid system

Mike M wrote: »

My thoughts are now on a portable power supply with solar recharge. After looking through some online resources it looks like a 25w panel will be about the size I want to lug around. Hhhmm.

25W panel may be able to charge up to 20AH 12V battery. It'll take 6 to 10 hours of sun to charge it, so you either need a very good day or several days. This will give you 120Wh of power. Of that you can run small 3W LED light for 40 hours, or efficient router for 80 hours, or charge 15 cell phones, or run average laptop for 6 hours.

Mike M · November 2013

Re: Planning of a small off grid system

Perfect, thank you.

westbranch · November 2013

Re: Planning of a small off grid system

I confess to knowing anything about Li type batteries other than what other posters here have stated.

They are supposed to be able to deliver up to 100% of their charge, without worrying about getting them recharged right away.
Would this be a better option for portability?

Mike M · November 2013

Re: Planning of a small off grid system

I'm sure it would but I believe price would be an issue. Plus I'm not sure they are that available in the size range I'd need.

Charging via solar, although not necessary, might be a challenge as well.

westbranch · November 2013

Re: Planning of a small off grid system

Look for a burnt out TESLA, they use small Li type batteries...http://forum.solar-electric.com/showthread.php?21501-Second-Tesla-S-fire-in-under-2-months&highlight=tesla

read post #19

newl · November 2013

Re: Planning of a small off grid system

Wouldn't a 2m HT be better for use at an event? It would also be more suited to a smaller solar recharging configuration as well.

Mike M · November 2013

Re: Planning of a small off grid system

My HT is limited to 5 watts and all day at 5 watts would require more than 2 batteries and as well as a way to charge them. Some areas have spotty or no connection to a repeater. Smaller mobile repeaters are set up and crossband repeat is used but a mobile base station takes care of a lot of that. Radio communication at these events can be from logistics, medical to emergencies so a reliable connection the entire time is helpful.

Planning of a small off grid system

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