I'm a Noob... Electrical wiring, Battery, and other questions

System · August 2007

I'm in the middle of doing my first solar power application.

I'm almost done building a sauna and would like it to be powered by solar power, and when I say powered, I only plan on using electricity for lighting, and there will only be 2 lights (12v - 25w)

I need some guidance as a lot of this stuff is very new to me and it's hard to get general help at the local hardware stores since most people working there don't have any experience with much of 12v.

1) Wiring/Hardware - I am planning on using regular 'ol AC outdoor wire 15AMP rating to power 2 - 25 watt (or smaller) bulbs. I purchased standard AC light switches and just wanted to make sure they are OK to use since I can't find any DC switches.

2) Battery - I wanted to purchase a single deep-cycle battery for this and already have a 12v, 6w solar panel. I will only need this battery to last me no more then 4 hours in any given day and not likely more then 10 hours a week for the two light bulbs I mentioned earlier, what size battery would I need? Which type/brand are better to use?

Thanks in advance for all your suggestions.

niel · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

this is try #2 as i lost my more lengthy reply in hitting a wrong key so i'll shorten this somewhat.
1> you can get away with using the ac switches if you keep the current low as in one switch per 25w light and having to replace them more often, but they were not meant for dc switching. this may be good for you costwise as ac switches are usually cheaper than the small dc switches you would buy at the proper current rating.
2> i'm guessing you to need at least an 84ah deep cycle battery(50% dod accounted for and based on proposed loads on a weekly basis) with 100ah as being more commonly available. now this could be much lower if you had specified the maximum per day wattage needed for i don't know if you'll use all of the lights only on day 7. i recommend a controller with a battery temperature sensor to properly charge the battery and prevent overcharging.
my biggest concern is that the small pv you have will not properly recharge a battery of that size(84ah) even though in theory the wattage from your small pv may or may not may work out right for the loads. we like to go with a 3% minimum charge rate with no loads and this is about 2.5amps from a pv and would most likely be a 40w pv or better. in going with a 100ah battery the charge requirements will be slightly higher as 3% of 100ah is 3amps and would be around a 50w pv or better. remember the watthours in must be equal to or greater than the watthours used plus watthours in losses, but in your case you also must meet the minimum charge requirements of the battery. if your daily loads are equally divided over the week then all of this could change and be different as the charge doesn't have to be generated and stored for use on day 7. do remember also that some of those 7 days of the week could be shorter due to it being winter and let's not forget those cloudy days to that need accounted for so i am not going against the weekly recommendations as it would give cushion to your needs.

System2 · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

Thanks Niel, that was very informative--just what the doctor ordered.

Sweet, that's the plan to use one light switch for one bulb and the other light switch for another (2 rooms); However if I do find some Low voltage switches I'll probably replace the AC ones.

As far as the lights use goes this will most likely be a once/ twice a week for a few hours and not even every week. The PV cell I have looks to be too small... too bad and wishful thinking on my part.

niel wrote: »

this is try #2 as i lost my more lengthy reply in hitting a wrong key so i'll shorten this somewhat.
1> you can get away with using the ac switches if you keep the current low as in one switch per 25w light and having to replace them more often, but they were not meant for dc switching. this may be good for you costwise as ac switches are usually cheaper than the small dc switches you would buy at the proper current rating.
2> i'm guessing you to need at least an 84ah deep cycle battery(50% dod accounted for and based on proposed loads on a weekly basis) with 100ah as being more commonly available. now this could be much lower if you had specified the maximum per day wattage needed for i don't know if you'll use all of the lights only on day 7. i recommend a controller with a battery temperature sensor to properly charge the battery and prevent overcharging.
my biggest concern is that the small pv you have will not properly recharge a battery of that size(84ah) even though in theory the wattage from your small pv may or may not may work out right for the loads. we like to go with a 3% minimum charge rate with no loads and this is about 2.5amps from a pv and would most likely be a 40w pv or better. in going with a 100ah battery the charge requirements will be slightly higher as 3% of 100ah is 3amps and would be around a 50w pv or better. remember the watthours in must be equal to or greater than the watthours used plus watthours in losses, but in your case you also must meet the minimum charge requirements of the battery. if your daily loads are equally divided over the week then all of this could change and be different as the charge doesn't have to be generated and stored for use on day 7. do remember also that some of those 7 days of the week could be shorter due to it being winter and let's not forget those cloudy days to that need accounted for so i am not going against the weekly recommendations as it would give cushion to your needs.

mike95490 · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

4hrs x 2 bulbs x 25W = 200 Watt Hours used [max day]
10hrs x 2 bulbs x 25W = 500 Watt Hours used [week]

6W panel x 6 hours good sun = 36 watt hours to recharge. [on a good day] {252WH week}

Add in your battery & charger efficiency losses of at least 30%

You need either a larger solar panel. (much larger) or some 12V LED light bulbs (or BOTH)

niel · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

mike,
i didn't do the calculations and i did say it may or may not be sufficient for the loads, but it was irrelevant because the battery is much larger and needed a minimum rate of charge to it negating if the loads would've been met or not. you did answer the question on his pv though once and for all for those load requirements.:cool::p :roll:

on the bright side he may be able to use that pv in parallel with others to get the needed wattage for his loads, but unless he finds a bargain it will be one larger pv.

System2 · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

Mike, thanks for doing the math I had no idea it was this simple.

You mentioned LED bulbs, I've thought of those, but had a very difficult time finding any good ones (good for sauna) and can't really tell if they're going to be bright enough. the sauna room is rather tiny (5x7 feet). Someone suggested using those LED Rope lights that are seen on the street around Christmas--but I'm a bit skeptical about those.

Do you have any recommendations regarding LED lighting?

PS: the temps will get really high 150-175+ degrees (F) and a long time ago I read about some university successfully converted heat into electricity, NOW that would be a great solution for me, since when the sauna is in use there is heat and consequentially light.

mike95490 · August 2007

Re: I'm a Noob... Electrical wiring, Battery, and other questions

senixon wrote: »

PS: the temps will get really high 150-175+ degrees (F) and a long time ago I read about some university successfully converted heat into electricity, NOW that would be a great solution for me, since when the sauna is in use there is heat and consequentially light.

You will need some sort of a sealed fixture, as you don't want moisture getting from the sauna into the bulb socket & wires. may be as simple as gluing a chunk of polycarbonate plastic to the ceiling, and shining a bulb thru that.

I don't have any really good bulb sources, but look for a reflector PAR bulb that uses a Luxeon 1 or 3 watt LED emitter should work really well.

I'm a Noob... Electrical wiring, Battery, and other questions

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