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Old 15th July 2012, 07:41 PM   #1
matros is offline matros  Denmark
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Default Question about Solar panel charging for my boombox

Hey guys.
I am thinking about buying two of these for my miniature boominator with Monacor SP-60.

7.5V 5.4W Solarpanel Solarmodule 12V MONOKRISTALLIN 6V | eBay

I have a 12V 12Ah (CT Leader CT12-12) battery in it.

If I put those in series I will get 15V which is very close to the maximum charging voltage for the battery. Is it possible to limit it with a voltage divider made from two resistors?
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Old 15th July 2012, 08:45 PM   #2
cbdb is offline cbdb  Canada
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Not really because the output impedance of the panels changes with sunlight and the resistors burn power that should be used for charging. The easiest way to limit the voltage is with a high power (20 watt) zener.
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Old 15th July 2012, 08:48 PM   #3
cbdb is offline cbdb  Canada
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And I hope you know that 5.4 watts is in full hot sun at the proper angle, the nominal power will be closer to 1 or 2 watts
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Old 15th July 2012, 08:51 PM   #4
matros is offline matros  Denmark
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And with two in series?
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Old 15th July 2012, 09:35 PM   #5
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Hi,
Just put a diode in series that will drop .7 V. Bringing the voltage down to 14.3 V. Max charging voltage of the battery it is 14.7 V. Also it will block the discharge of the battery back to the solar panel when the voltage drop from the solar panel.
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Old 15th July 2012, 09:48 PM   #6
cbdb is offline cbdb  Canada
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A diode isnt on or off, it will still pass some current at .2v
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Old 15th July 2012, 10:13 PM   #7
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There's no danger in putting them in series. In fact you have to.

Total power output of the panels in series is lower than 1/10 of the battery capacity. This is normally considered safe for permanent connection.

The voltage will not be 15V at any point either. The internal resistance in the battery will pull that down to about 13.8V absolute max, even when fully charged.

You will need a diode though, otherwise the battery will discharge itself into the solar panel which will get very very hot and probably take permanent damage.
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Old 15th July 2012, 11:00 PM   #8
matros is offline matros  Denmark
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Thank you Saturnus, I was hoping you would see this thread and give me your input, as my boombox is made out of your design, but with SP-60, and I didn't wanna ask in your thread

Will any diode do? Like a 1N4007 ?
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Old 16th July 2012, 05:52 AM   #9
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Quote:
Originally Posted by matros View Post
Thank you Saturnus, I was hoping you would see this thread and give me your input, as my boombox is made out of your design, but with SP-60, and I didn't wanna ask in your thread

Will any diode do? Like a 1N4007 ?
Just use the one that is included with the panel
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Old 16th July 2012, 06:35 AM   #10
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Quote:
Originally Posted by matros View Post
Hey guys.
I am thinking about buying two of these for my miniature boominator with Monacor SP-60.

7.5V 5.4W Solarpanel Solarmodule 12V MONOKRISTALLIN 6V | eBay

I have a 12V 12Ah (CT Leader CT12-12) battery in it.

If I put those in series I will get 15V which is very close to the maximum charging voltage for the battery. Is it possible to limit it with a voltage divider made from two resistors?
Well the answer is yes to both questions...you will get 15 volts in full sunlight, and sure a voltage divider can be made with resistors, but the resistors have to be large enough to dissipate the heat.
The charging of a battery is better when done slowly, the faster the charge rate, the hotter the battery gets, the shorter the battery life will be. Slow charging prolongs the battery life.
And as many have found, overcharging the battery (too much current) can actually make it overheat and leak, or explode. Batteries do not like to be heated when charged.
And then as you already know, if there is less than full sunlight, the voltage drops.
This is why many have used a current regulator, to control the charge rate as the voltage rises and falls---instead of just resistors.
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