# Solar Panel Help Me!.

Discussion in 'General Electronics Chat' started by syaf921, Jan 12, 2013.

1. ### syaf921 Thread Starter New Member

Dec 21, 2012
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What period of time is needed to fully charged a 12v battery used 3 watt 12v solar panel? And how long will the battery can last while lights a 15W 240V lamp?

2. ### mbohuntr Active Member

Apr 6, 2009
413
32
More information is needed, What type of battery? Are you converting the battery voltage to AC? Can you use a different light? LED for example?

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3. ### syaf921 Thread Starter New Member

Dec 21, 2012
18
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Yes, I convert the battery to VAC. Use lead acid batterry 12v, 7.2AH. I want to use 240v lamp.

4. ### mbohuntr Active Member

Apr 6, 2009
413
32
I have limited knowledge, but here goes. Someone please catch my math errors
Regular starting batteries shouldn't be discharged more than 25% , so you have 1.8 amp/hours to work with. A 3W solar panel at 70% rated gives you around 2.1W (Panels usually won't produce full rated output)
2.1 W/ 12v = .175 A

1.8 Amp/hours / .175amps = 10.29 hours. Not going to work well....

Your light... 15W 240v not including losses in converting...

15w/240v = .0625A 15w/12v = 1.25A, your light will use up your battery in about an hour.
Deep cycle batteries have more capacity, but your panel is badly undersized. Also, you don't want to charge that battery faster than c/10, or 720ma I would recommend looking at LED's if you are able to use them for what you want. Someone PLEASE check my math, my noggin is dusty.

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5. ### syaf921 Thread Starter New Member

Dec 21, 2012
18
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What kind of battery is suitable to turn on the light at least eight hours? Any suggestion?

6. ### mbohuntr Active Member

Apr 6, 2009
413
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A very big one! Why can't you use LED's? They use just a fraction of the power your light uses, will last longer, and do not need the converter!!!

7. ### mbohuntr Active Member

Apr 6, 2009
413
32
You can push the charging limits a bit, but your running the risk of heating the battery, and needing a more complicated charging system to operate safely. I am not an engineer, and not able to make reliable calcs for you, but I think you need to change some of the parameters.

8. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,436
1,626
Seems you want 15 watts for 8 hours.

A lead acid battery has a voltage of 12.6V fully charged, and 12.3 volts at 70% charge. Let's use the lower number.

The current for 15 W is 15W / 12.3V = 1.2A. For 8 hours that's 1.2A * 8 H = 9.6AH (amp-hours), and that's 30% of the capacity (see post 4 above) you need a 32AH battery.

To recharge you need to take into account batteries are only about 75% efficient in accepting energy, so you need to add back in 9.6AH / .75 = 12.8 AH every day.

Next comes your charger. Two basic types, PWM is about 70% efficient, and MPPT which comes in at about 97%. PWM is used as it is much cheaper then MPPT.

How long do you have sun on the shortest day of the year? Say that is 5 hours: to get 12.8AH in 5 hours takes 12.8AH / 5H = 2.56 A.

Using a PWM you need 2.56A / .7 = 3.66 amps... and for a 12V panel that's 3.66A * 12V = 44 watts.

Of course, a 32AH battery and a 44 watt panel are minimums. Buy larger and you will be happy.

Last edited: Jan 12, 2013
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9. ### mbohuntr Active Member

Apr 6, 2009
413
32
Thanks for the help Ernie, I have no problem sitting on the shore with a cane pole and a bobber, You guys are driving the Time Bandit on the Bering Sea! I got what little battery smarts I used from an RV site on solar charging.

10. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,436
1,626
I got my battery smarts researching some changes to a lighthouse I volunteer for. Needs a bit of help as some very creative amateurs got into it and build lots of stuff for "lighting."

Oh, and I'm an engineer too and do calculations like breathing.

And I sure do miss fishing during the winter time.

11. ### mbohuntr Active Member

Apr 6, 2009
413
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Hoping to try some hard water fishing for the first time this year. I live north of syracuse. Howdy neighbor.