# Math question,,,,,,,,how long will it run?



## budgetprepp-n (Apr 7, 2013)

I know an assessment is an educated guess and that's all I'm looking for.
I am going to be running a light from dusk till dawn it pulls 13 watts I'm going to be using a 100 watt solar panel
and the battery that you see the top of. I know it's not a true deep cycle only battery but that's what I'm using.
So this battery and a 100 watt solar panel is that enough for all night running? over and over 
Keep in mind the short dim winter days we have sometimes at 40 degrees latitude.


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## Slippy (Nov 14, 2013)

Paul S or Hunting Hawk are pretty smart old boys, they will know.


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## budgetprepp-n (Apr 7, 2013)

Slippy said:


> Paul S or Hunting Hawk are pretty smart old boys, they will know.


 Yes our foil hat mathematicians


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## jro1 (Mar 3, 2014)

Use Led lights and it will run all night!
you need to know your amps, remember 1amp = 1hr of run time with out charging
So, 12watts/12volts=1amp
correct me if I'm wrong, but at 13watts that gives you roughly 8.5 hrs run time on 114 amp hrs if the battery is fully charged and well maintained, full charge is 12.67Volts, 11.9Volts is considered completely discharged


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## Oddcaliber (Feb 17, 2014)

A lot of RV sites have threads about battery life. It's worth a look.


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## paraquack (Mar 1, 2013)

Definitely go with LED lights. Slower draw the better.


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## jro1 (Mar 3, 2014)

And 2 deep cycle 6 volts will last way longer than one 12 volt!


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## Jeep (Aug 5, 2014)

This is where I sit back and scratch my head. I can not figure electricity out


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## jro1 (Mar 3, 2014)

Jeep said:


> This is where I sit back and scratch my head. I can not figure electricity out


 Nor can I, But I do know how to read and recite the verbatim quite well, while looking extremely educated as the end result, Ha
But in the end, I still know Jack Shit!!


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## Jeep (Aug 5, 2014)

Are there places to buy empty batteries, I mean if I build a solar array, can I buy like blank or empty but new batteries ?


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## 1skrewsloose (Jun 3, 2013)

I think I would do a dry run on the battery and see if it holds up. Leave solar out of it, a week of cloudy days and solar may not re-charge. Depending on temp of battery and length of run affect amps. I'm far, far away from being an expert, just using my electronics/electrician backround. Folks here who would put me in beginners class. just my .02.


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## budgetprepp-n (Apr 7, 2013)

1skrewsloose said:


> I think I would do a dry run on the battery and see if it holds up. Leave solar out of it, a week of cloudy days and solar may not re-charge. Depending on temp of battery and length of run affect amps. I'm far, far away from being an expert, just using my electronics/electrician backround. Folks here who would put me in beginners class. just my .02.


 Yea a dry run might be in order,,,,,, I did something like this before but I didn't have a volt meter.
I think I will run one of the deep cycles down to 12.00 to see how long it takes.

The cloudy days don't bother me as much as the short days that are coming.
I was surprised how much charging power I get when it is overcast 
I'm still learning and picking up things about solar. (lots of surprises)

Another thing that bothers me is that in the middle of (Dec.21) winter I only get about 9 1/2 hours
of daylight and that may be gloomy I'm wondering how hard that may be to over come.

I was hopping Hunting Hawk or Paul S would chime in.


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## dwight55 (Nov 9, 2012)

There are a lot of variables involved in this question, . . . one being the voltage. I assumed you are anticipating using a 120 volt bulb, . . . necessitating a power pack to boost the 12 to 120.

That said, . . . if you do it that way, . . . your amperage draw will be slightly over 1 amp per hour, . . . the battery lists it as 114 hours at 1 amp draw, . . . considering the power loss etc that is inherent in boosting the voltage, . . . I would anticipate about a 100 hour run with no problems.

But again, . . . that is just an old electricians ROT guess (rule of thumb).

Theoretically, you should be able to run the 13 watt bulb for 7.6+ hours for each hour it is being charged at the solar charger's full 100 watt capability. You are putting into the battery 100 watt-hours per clock hour (optimum) and only taking out 13 watt-hours by burning the bulb. Dividing the outgo into the inflow gives you the length of time it takes in hours to theoretically run out that charge.

May God bless,
Dwight


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## 1skrewsloose (Jun 3, 2013)

So succinct, thank you. Been said if you cannot explain something simply, you don't know what you're talking about. Got guys at the plant that can't explain how to tie a shoelace, but know everything.


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## budgetprepp-n (Apr 7, 2013)

dwight55 said:


> There are a lot of variables involved in this question, . . . one being the voltage. I assumed you are anticipating using a 120 volt bulb, . . . necessitating a power pack to boost the 12 to 120.
> 
> That said, . . . if you do it that way, . . . your amperage draw will be slightly over 1 amp per hour, . . . the battery lists it as 114 hours at 1 amp draw, . . . considering the power loss etc that is inherent in boosting the voltage, . . . I would anticipate about a 100 hour run with no problems.
> 
> ...


 Yes I agree with you on this. On the 12 volt side the converter pulls .2 amps or 2.4 watts the bulb should pull about 10 watts so 12.4 should be our total draw. 
Tomorrow if I have time I'll hook up the converter to a deep cycle battery and set the timer and see how long it takes to get down to 12.0 volts


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## Old SF Guy (Dec 15, 2013)

13 watts at 12V draws 1.082 amp... your 114 amp hour battery will last 105 hrs. SO roughly...4.5 days and considering your recharging the battery during the day...with 100 watts.. lets see 13 watts consumed for 12 hours....balanced against 100 watts for 4 good hours or 50 watts average for 8 hours= 400 watts added with 156 watts consumed (13 watts over 12 hours)....I think your good for an indefinite period even considering cloudy days.


and this math is only good if your using 12 volt bulbs...if you are using 110/120 then your in the dark fairly quick buddy.


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## LDSreliance (Sep 2, 2014)

Unfortunately, battery labels are a bit misleading. The run rates displayed on most battery labels are an attempt at a standardized way to compare batteries. However, this battery does not conform to the normal 20 hour run rate. So we have to make one of two assumptions: the run rate either means that it will take 114 hours to discharge the battery @ 1 amp to 0.0 volts OR it will take 114 hours to discharge the battery @ 1 amp to 12.0 volts (which is fully discharged for a car/marine battery like this). If we make the wrong assumption we risk destroying the battery by discharging the battery past 11.9 volts.

Another piece of information that is needed is your wiring. How long is the wiring from the battery to the inverter? What gauge wire was used? There will be some line loss in a 12v system which could be significant in these types of calculations.


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## Maine-Marine (Mar 7, 2014)

are you using a dusk to dawn solar controller? 
Does it have load disconnect?


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## LDSreliance (Sep 2, 2014)

I didn't see any mention of a charge controller for the battery but that may not help from killing the battery unless you can program the low voltage disconnect at a specific voltage. Charge controllers are meant to be used with a deep cycle battery and usually will cut the load on a 12v system at 11.9 volts or sometimes even lower. That would still kill a car/marine battery.


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