# Solar battery charger question???

Discussion in 'Shop Talk' started by goggleye57, Jan 23, 2005.

1. ### goggleye57Active Member

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Location:
Indiana
I'm don't know a lot about electricity. I want to buy a solar collector unit to charge up a 12 volt deep cycle battery when I'm off the grid. In good sunlight how long would it take to charge -say an 80 amp.hour battery if the Solar unit has say 20 watts on capacity?

I was thinking Watts = Amps x Volts so Amps = watts/volts

Amps = 20 watts/12 volts = 1.67 amps

Is that 1.67 amp/hours going into the battery? Would it take 80 amp/hours divided by 1.67 watt/hr to take about 48 hours of good Sun to charge the battery if it were drawn down completely?

Please let me know the error of my thinking. It shouldn't take long to find some!

2. ### BobBoyceWell-Known Member

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Aug 21, 2004
Location:
SE TN/SW NC
Solar panels are typically rated at their maximum power point, usually near 17 volts. So in order to get maximum power out of them, you would want to use a MPPT (Maximum Power Point Tracking) type charge controller.

In simpler terms, that 20 watt rating is not directly into a 12 volt battery.

Also, batteries are not 100% efficient, so in order to get 80 AH of usable power, you would have to charge the battery with more, like 90 AH or so, depending on battery age, chemistry, temperature, ect.

Of course, with lead acid batteries, in order to have your battery last, you only want to discharge to 80% SOC (State Of Charge) before recharging. So you would either limit your consumption to 16 AH (20% of 80 AH) or add more batteries. If you were to draw your battery down completely, it would be damaged, would quickly lose the capacity to recharge fully, and would not last very long at all.

See how complicated things can get? And all you wanted to know is how long it would take to charge an 80 AH battery from a 20 watt panel. :haha:

There is no simple answer, because there are a lot of factors to consider. It depends on how much power you consume from the battery, how many hours of sun do you get per average solar day, solar insolence (sun brightness) at your location, ect.

That doesn't mean to give up. Just try to read all you can and educate yourself so you will be able to make wise choices in equiptment purchases.

Bob

3. ### mightyboobooWell-Known Member

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I would say that panel produces about 1.1 amp an hour under ideal conditions.I base that on my 75 watt panel putting out about 4.5 amps.Everything being just right would be about 5- 6 AH/day.That sound about right Bob?

4. ### BobBoyceWell-Known Member

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Sounds about right if there's no trees or other things to shade the panel during the daylight hours.

Bob

5. ### mightyboobooWell-Known Member

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LOL,Im thinking turning the panel every 15 minutes on a cool June day with bright sunshine! Im thinking absolute ideal to get 6 amps,more like 4 or 5 in the real world with a perfect day. :haha:
Still love those panels though,what can beat making your own power? :worship:
Besides THAT?
BooBoo

6. ### BobBoyceWell-Known Member

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313
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Location:
SE TN/SW NC
I have some panels I would love to see the power from.

They're so huge I still have them in storage because I haven't figured out a way to mount them on a pole mount where they wouldn't be too much of a wind load during thunderstorms. :haha:

I don't have anyplace to mount them until I can get a driveway up to my mountaintop where I can set up a good solid ground mount for them.

Bob

7. ### mightyboobooWell-Known Member

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Location:
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BIL(off grid) made a 60 degree frame from 2x4s,then a plywood front,to which he mounts (I think?) 4 120 watt Kyoceras per frame.Not pretty,but works.He is now moving into 180 watt panels,one or two at a time.Last time i checked he had 25 amps at 24 volt configuration..Nice the modular aspect,by the time he retires he sure wont have any electrical bill at the rate he's going.Im Jealous.

BooBoo

8. ### BobBoyceWell-Known Member

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313
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Aug 21, 2004
Location:
SE TN/SW NC
My main problem is the location of this home. I had it put just a couple hundred feet from the road. We wanted to build a site built home near the mountaintop out back where, we could take better advantage of the terrain, and it would be out of sight from the road. This was supposed to be a temporary residence for us until then, then we were going to rent this one out or sell it.

At the time we ordered this home (singlewide), I did have some things added for limited alternative energy use here, like an exra electrical breaker panel in a dedicated equiptment closet for the battery bank and other equiptment. I had extra supports added under the floor under the equiptment closet to support the weight of the battery bank.

Unfortunately, I became terminally ill not long afterwards, so the site built home plan kinda vanished. Since my health has stabilized for the time being, I've started to revive the plans, but I don't know if I'll be around to finish it.

I guess what I'm trying to get at is this, the location of this home is not ideal for solar power. It is on the northern slope of a mountainside, nestled between ridges to the east and west, both covered with heavilly forested land. I only get about 4-5 decent solar hours a day on a clear day, and to get that my array has to be on top of a pole 16 feet in the air.

Since this is a manufactured home, I was more interested in protection from the extreme elements than solar hours. I had always planned to add microhydro to the mix, since I have a year-round creek farther down between the home and the road.

Bob