Newbie question on batteries

Re: Newbie question on batteries

Good Evening John--a little ccccooollllddd up there?

OK, to answer your first question... a 15 watt panel is just about the right size to float two "car sized" deep discharge batteries for 9 months out of the year...

I am guessing that "up north" is north of Belleville, ON--so you have some short winter days up there--so you might be a little short of power then.

Regarding the battery--do not even bother to use a battery designed for starting cars/trucks. In general, the plates are thin and don't take well to discharging any more than 15% of capacity (state of charge 85% left).

Of course, these sound cheap--perhaps if you baby them, you will still get your money out of them?????????

The longer answer is you need to choose devices that are low power (conservation) and you need to know how much power these devices will use (amp*hours or Watt*hours or kWatt*hours).

A Kill-A-Watt meter is a good place to start. You could plug your lights and electronics into one at home--run them for a typical cycle over a few days or weeks--and use the watt*hours of your AC appliances to define how much power you will need.

I am guessing that you will be very surprised at how much power a TV + Satellite receiver will use.

Just a guess..
Lights=4 hours * 13 watt CPFL * 2 lights = 104 Watt*Hours
TV=4 hours * 170 watts = 680 WH
SatRcv=4 hours * 40 watts= 160 WH
==================Total WH====== 944 WHours / day

Assuming that you want ~3 days of power (a good mix of cost+performance for a battery bank design--also takes care of that occasional three day weekend) and you will only use 50% maximum depth of discharge over those three days. And you will be using an 85% efficient AC inverter, plus an 80% efficient wet cell deep cycle battery--the size of your battery bank would be:

Bank W*H=W*H-load*3 days*(1/50% discharge)*(1/85%)*(1/80%)
Bank W*H=W*H * 8.82 (3days+50% batt+efficiencies)
Bank W*H=944*8.82=8,326 WHours or 8.3 kWhrs of storage batteries...

Say you do this with 12 volt battery bank:

A*H = 8,326/12 = 693 Amp*Hours worth of batteries (roughly 7 car sized batteries).

To calculate how much solar panel you will need (your 15 watt panel would be about enough to trickle charge two car batteries--storage, no loads)... Take a look at how much sun you get for your location. Once place to look is here. Looking at Toronto, I see you get about 2-5 hours of sun per day (winter-summer)...

So, depending on which season--lets assume the "nice" months were you get 4 hours per day with a 77% derating factor (fudge factor is the technical term--close enough for us here).

And, lets assume that you want to charge the batteries up over three days (after three days of use with cloudy weather).

Solar panels = 944 W*Hour / (0.77%*0.85 inv eff * 0.80 batt eff *4 hours of sun)
Solar panels = 944 W*Hour / (0.52 * 4 hours of sun)
Solar panels = 454 watts of solar panels for March thru September use...

The above is purely a guess and your numbers will be different--but it does give you some guidance on what you need to know and how to calculate your needs.

Load drives panel and battery sizes. The numbers I used are conservative but reasonable. You could cut the number of batteries by 1/2 and even the panel by almost 1/2 and still have a reasonably working system for sunny weather.

Or, if your TV+Sat+Lights are much less power hungry than I guessed (everyone wants a 32" flat panel in their summer cabin--right?)--your system can be downsized a lot too.

Lastly, look at a small Honda eu1000i or eu2000i generator (Yamaha and others also make similar units) for winter/bad weather recharging of your battery bank. Will also be a nice addition (quiet and pretty fuel efficient).

There are other things to discuss too (type of solar charge controller, pure sine wave inverters, battery types, temperature sensor, panels, panel mounting, etc.). But I will stop for the moment--my fingers are getting tired.

Hope this helps...

-Bill