I've recently put together a very simple solar charging system for the travel trailer that I and a partner have parked in the campground at the Truckee Tahoe Airport. In doing so, I was reminded of how infrequently I have to worry about DC power output and usage.
In the airplane I have a power source: the alternator on the engine. Odds are that it'll keep running for a while, since it was recently replaced after suffering a sudden death in flight.
In the glider I have dual 12 volt 12 amp hour batteries, which will last quite a long time if I leave the transponder turned off. All the other instruments are very lower power. But even with the transponder on, I've got enough juice for any flight I decide to make.
But the camper has no way to generate power. It does have a newly installed deep cycle marine battery. It's a 90 amp hour 12 volt model. So how big of a solar panel did I need to get?
It depends, of course. It depends how much it'll be depleted when we use it and how quickly we'd like it to be recharged.
In order to figure that out, you need to grok the relationship between volts, amps, and watts. Solar panels are typically marketed based on their peak power production, measured in watts. The 12 volt batteries are often measured in terms of "amp hours."
The magic formula is:
Watts = Volts x Amps
So if we make a bunch of simplifying assumptions, the 12 volt, 90 amp hour battery requires 1,080 watts (or watt-hours, to be correct) of solar power to fully charge. The 85 watt Shell SQ-85P panel we bought could do the job in 12 hours, assuming optimal light and various other things that are never all true in the real world.
But if you assume 3 hours of good light per day (that's very conservative in the Sierra Nevada mountains), the battery will be back to normal in 4 days. So even if we were to run it down pretty far over a 3 day weekend and then come back the next Friday night, all would be well.
This concludes your physics/electricity lesson for the day.
I'll document the charging system in more detail if anyone is actually interested.
Posted by jzawodn at August 24, 2006 08:52 PM
Hey are you Jeremy Zawodny the MySQL guy?
Yes, I am interested in your charging system. I am thinking of putting together a solar system to run a furnace for a week in case of a blackout. I can sit in the dark if I am warm but cold and dark would suck. So, anything you could put up would be great!
Units Units Units! You're close, but not quite there.
Volts x amp hours = watt hours
I had to get up close with all this recently charging a Miata battery that had been left too long. In my case, it was about 48 hours at .7-1 amp off of a 15V nominal wall source. But ymmv.
Who needs solar, when you have a Honda generator? Always works, and runs forever on a gallon of gas.
John:
MySQL? Guilty as charged. (Oh, bad pun!)
Eric:
Yeah, yeah... good point. Details matter. :-)
For those who learned all their electrical physics in metric, that formula is known well.
Electrity is often sold in kWh or kilo-watt-hours - which is directly convertable to joules (1 kWh = 3600 J)... Joules is another metric measure of energy used. It compares with BTU (which natural gas is often sold in) and horsepower (Watt is a unit of the same measurement)
Well since you said above that "details matter," I have to complain about the use of "Ohm's Law" in the title. The equation you used is related to it, but isn't actually it. Ohm's law is V = I R (R = Resistance). Don't see any mention of resistance in your posting.
Owen: when I learned about all this stuff in school, it began with Ohm's Law and grew from there. I thought about saying "electrical learning nostalgia" but that seemed to be a bit... long. :-)
On second thought, I'm chaning that. It shortens the title and doesn't lose anything.
Owen's on to something. Amps * Volts only = Watts on simple resistive loads (space heaters, light bulbs). Really it yields a unit called a volt-amp which is like a watt but with no resistance. Resistance is, after all, futile. Anyway, it gets important when you're figuring out the load you're going to put onto a power source; a space heater behaves differently from the power supply in your PC (a variable load). I recently ran a subpanel for an upstairs addition and had to revisit my high-school physics in order to figure out how many amps needed to be on the subpanel and was reminded of this.
But in general, amps * volts == watts.
Luckily, this is DC. With AC, you can get into mumbo jumbo with phase and RMS.
I think I'm going to listen to some AC/DC. I'm on a highway to hell...
Or, instead, you could just plan your trip so that most of it is done on piezo-pads-paved roads:
I am very interested in your charging system for my own very similar camper application.
I would love to use my generator ... but it makes it hard to enjoy the pristine outdoors of the wilds with all that racket! Besides, I don't know what magic fuel Jon Woolley is using, but my generator definitely DOES NOT run forever on a gallon of gas!
I heard of solar panels that can be put on boats, Rvs etc... but the ones i find only charge at a trickle....in milAmps..
Im looking for something to put on a bassboat that will help maintain charge on the electric motor battery, 12v 100amps...
doesnt have to charge it, but at least keep charging during the day..with the ones i've found, at 12v with 185mil amps.. it wouldnt be needed... to slow,,
thanks
"(1 kWh = 3600 J)"
A joule is one watt second, so 3600 joules is a watt hour. A kilowatt hour is therefore 3,600,000 joules, or 3,600 kJ, not 3,600J
Just being pedantic...
I am very interested in your charging system Jeremy for a larger applicationat home. Thanks
Got a 14 watt panel and a 120 amp hour battery.the area is kenya in east africa.Nairobi the city under the sun.i have a 500watt sinar inverter charger that runs a 32" 152 watt samsung lcd for an hour a day. and two 12 volt 11watts energy saver lamps that run for 3 hrs a day.how many solar panels do i need as i figured out the battery would be full in 32 days with the panel i have.all advice appreciated the power grid is not very reliable but improving. Ps usually have to recharge the battery from the mains every 4 days.
I have a 27 foot trailer on a remote site and what you have and how it set up sounds loke just the thing.
My air cond. is a 30 amp system. Have a micro wave, 10 cu. ft,. refridge/freezer, standard lighting trailer scenario, 6 gal hot water heater and a stereo system.
Please advise.
