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Bzzzzzt
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# Posted: 10 Jun 2012 10:39pm
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For some of you people that have figured out all the DC to AC connections and such for solar power I have a couple questions to ask.
As you guys know, I'm an electrician, but I don't deal with batteries and inverters at all so I've guessed at this math and I'm wondering if I'm right.
Since 120V ÷ 12V = 10 then I'm assuming that any amperage ratings for 120V appliances would need to be multiplied by 10 to get the 12V amp-hour draw off the battery. Then I've seen somewhere that there is a little bit of power loss because you're stepping up the voltage so I am guessing that to be 20% give or take. In any event I came up with this formula:
1A (@ 120V) x 10 x 1.25 (1/80%) = 12.5A (at 12V) so for every 1 amp at 120V you will have 12.5 amp-hour of draw off of your battery bank. A 100 AH battery would theoretically last 100 AH ÷ 12.5A or 8 hours. Then 4 amps at 120V would last 2 hrs through the inverter assuming the 100 amp-hours. Does this math look right?
The second question I have is how do you ever figure how long X wattage of solar panel will fully charge an Y amp-hour battery? Is there a formula to say that if you have 100 watts of solar panel working at 50% efficiency they will charge a 100AH battery in 12 hours, or something to that effect? I have never needed this sort of information in my day to day electrical work before.
What I did this weekend was that I purchased a 45W solar kit from Harbor Freight to play with and a 70AH marine battery from Batteries plus. The first night all we ran off the battery was 4-26W CFL lamps for a few hours and a small 28W fan most of the night. This worked fine so the 2nd night we ran all of that plus a larger fan that pulls 2.1A and it ran the battery down in about 2½-3 hours. All this was at 120V for the wattage and amperage. I let it charge some today, but I'm not staying there tonight so I took the whole setup down and brought the battery home with me but left the solar panels down there, but put up in the cabin for safe keeping. We were really happy to have the lights on without the generator going so we're pretty satisfied with the setup. I think I'll get a better charge controller as that seems to be the biggest complaint customer reviews had about the harbor freight kit.
Any thoughts or input would be appreciated.
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Rob_O
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# Posted: 11 Jun 2012 12:02am
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I'll throw out a few things here that may be helpful...
Battery life is proportional to depth of discharge. Lightly loaded batteries live longer and if you drag your battery down to 10-20% available charge on a regular basis you will be replacing it every season. 50% or less of the available AH rating is the most you should be using
Inverters are not 100% efficient, if you have a cheapie figure about 80% efficiency, same with your charge controller and then there's the losses in your batteries. Expect about 2 watts output for every 3 watts you put in, watts are watts and if your 45 watt panel makes 30 useable watts in one hour then you can use 30 watts per hour for every hour of useable sunlight.
Do the math and you will see you are using a lot more power than you are producing, and need to make some changes
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VTweekender
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# Posted: 11 Jun 2012 07:42am - Edited by: VTweekender
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Ummmm..not so easy for me to type into words...easier for me to give you this link that answers you question...lot of reading but its there in the second section of the page I think.....I think you may have been close with your calcs... http://www.marxrv.com/12volt/12volta.htm
For those that want to learn more about kinds of batteries themselves, 12v vs. 6v etc..etc..and how to hook them up...click on the "return to previous page" link at the bottom of that page I linked to.
Kev
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Bzzzzzt
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# Posted: 11 Jun 2012 12:17pm - Edited by: Bzzzzzt
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Thanks for the link VTweekender. That was the exact information I was looking for as far as draining batteries is concerned.
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Bzzzzzt
Member
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# Posted: 11 Jun 2012 05:44pm
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I'm going to read through some of those links on that site and see what knowledge I can glean.
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Bzzzzzt
Member
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# Posted: 12 Jun 2012 09:48pm
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Okies, so what I get out of this was that AC watts ÷ 12V * Efficiency (which I figured at 80%) 1.25 gets DC amphour draw
so 120Watts @120V ÷ 12V * 1.25 = 12.5 Amp Hours
and my formula was AC Amps * (120/12) * 1.25
So 1Amp @120V * 10 *1.25 gets me the same answer of 12.5 DC Amp hours.
Therefore, 1 amp at 120V will draw down a 70 Amp Hour battery in approximately 5.6 hours
Now the only question I have left (and I think this may be covered in that site some as I haven't read it all ) is how long will it take to recharge a battery with solar cells. I know that more wattage of solar cells will equal faster (or more efficient) charging, but there must be a formula to figure this.
My site only has eastern exposure and I can rearrange my solar cells to catch sun up to about 4pm then the trees block the suns rays. Maybe I should get a second harbor freight set and have 90 watts but point one east and one west so as to capitalize on the sun during the biggest portion of the day.
Things that make you go "hmmmmmmm."
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VTweekender
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# Posted: 13 Jun 2012 01:18am
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BZZZZZT, your most welcome....your 45 watt panel will output around 2 amps per hour in good sunlight....you may be getting around 3 good hours of sunlight on average......giving you 6 amps per day, so for your 70ah you are looking at a dozen days or so to go from full discharge to fully charged without any draw during charging.....if the inverter is hooked up that will draw some just being on line, about 5% loss there.....in my state of VT the average good sunlight is 2.2 hours per day...it varies per state..you can google this info "good solar sunlight per state"........a smaller system like that really will only handle a couple lights efficiently if used on a daily basis....Keep in mind that with that battery type you only want to draw it down 50% before recharging...constant draw below that will toast that battery in no time....deep cycle and marine batteries are not designed to be drawn down fully and recharged constantly......6V golf cart batteries ARE designed to do that , although you still don't want to do it all the time, the golf cart batteries will withstand it and last tons longer ....
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MtnDon
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# Posted: 13 Jun 2012 09:24am
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A few things to keep in mind to make batteries last and to end up being satisfied with a PV system...
... aim for no more than a 50% DOD (depth of discharge) as measured with a hydrometer. A maximum of a 25% DOD is even better. [the less the DOD, the longer the batteries last]
... a normal single days worth of sunshine should recharge the batteries fully. The charge controller should always end the day in float charge mode, indicating a fully charged battery.
... hot batteries die sooner than cold/cool batteries, although hot batteries deliver more power then cold batteries.
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Bzzzzzt
Member
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# Posted: 13 Jun 2012 09:52am
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The reason I went with the deep cycle battery is because that was what the guy at batteries plus steered me towards for my intended purpose. I asked him specifically about golf cart batteries being better suited and said that was not true. He said deep cycle batteries can be discharged and recharged just as effeciantaly and at a fraction of the cost. I have to plead ignorance in this area becauwse I just dont know. Thanks so much for the input.
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VTweekender
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# Posted: 13 Jun 2012 10:11am - Edited by: VTweekender
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Deep cycle batteries are designed to be discharged down as much as 80% repeatedly, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are solid Lead plates - not sponge. Unfortunately, it is often impossible to tell what kind of battery you are really buying in some of the discount stores or places that specialize in automotive batteries.
Many Marine batteries are actually "hybrid", and fall between the starting and deep-cycle batteries, while a few are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. "Hybrid" types should not be discharged more than 50%.
Chances are that you bought a hybrid. You may be able to google the battery type and come up with specs..
And I think what MTNDon is referring to when he suggests one day to fully recharge, is how the system should be sized overall, panels and battery bank, an efficient system would have enough panel array to fully charge your calculated sized battery bank in one sunny day.....so in your case with a 70AH battery, you would want enough panel to recharge 35 amps in one day since you only want to discharge at 50%....lets use 5 hours of good sunlight in one day as example...about a 100 watt panel would be efficient to a 70 AH.
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