PV Gap

Shawn Said:

How long will it take to charge a 12v 5amh battery with a 5 watt solar paneL?l?

We Answered:

My answer is at least 12 hours of sunlight. 12 hours is the minimum theoretical time, without taking power losses, inefficiencies and variations in sunlight intensity into consideration, which would have the effect of adding to the actual time required.

Here's how I came up with my answer:

A 12Volt 5 amp-hour battery will ideally produce 12volts of electricity at a rate of 5 amps for 1 hour.
Since Power in Watts = Voltage X Amps, that means the battery should be capable of producing 12 X 5 = 60 watts for 1 hour = 60 watt-hrs. So, ideally, the time required to fully-charge it would be 60 watt-hrs/5 watts = 12 hours.

Of course, in reality, it's going to take a more time than 12 hours to charge it, due to inefficiencies, heat generation, and electrical resistance. Since you said perfect light conditions, I am assuming the sunlight remains constant during the entire charging process, when in reality the sunlight will wax and wane with its peak intensity near midday.
I am also assuming the solar panel output is 12V DC, and it may or may not be. If it is not, in order to charge a 12 volt battery, its output voltage should be converted to 12V, and there would be an additional inefficiency in that conversion.

Bradley Said:

I want to use a 12v 5 watt solar batt charger to power my sirius boombox. How can I do this?

We Answered:

You would be best served by using the solar panel to charge up some NiCad batteries instead of directly connecting the panel to a boom box. (unless you need this for a demo on solar power)

The solar panel is not going to output a stead voltage. It will vary from 0 to more that 18volts depending upon the amount of light. To charge a batery you need more voltage that the battery to push current into the battery. Normally solar charging systems use a regulator to protect the battery (and panel) http://www.harborfreight.com/cpi/ctaf/di… This regulator wold protect a car batery & panel from over current and you could then use a 12V inverter to power 120VAC appliances or another battery charger.

You can use a solar cell to directly charge a small NiCad as this is done all the time in those cheap solar lights. So you can scale this up a bit and manually charge some NiCads for the boom box passing the voltage vrom the solar panel through a series resistor and diode. the resistor would limit the current to the batteries during charging to their recomended quick charge (5hr) current. in the case of the harbor freight batteries this is 450mA and the diode is used to prevent a backflow of current from the batteries to the solar cell if there is little or no sun.

If you wanted to limit charging current a little more reliably you would use a linear regulator wired as a current source to limit current flow automatically to a consistant value. (This just becomes a variable resistor that adjusts for variations in solar output)

And while your at it don't forget to wire in a fuse to protect everything.


EDIT
Assuming that you will spend upto $100.00 you would save the need to use 12 sets of disposable batteries. (Assuming a little over $8 per set. I am also assuming that you need D batteries.) While a NiCad set charged off regular house hold current @ 0.05 per hour is your alternative. Using Five cents and 5 hours charging per day you would break even in 400 days of daily Boom box use(alternating between two sets of batteries) . In other words you save 25 cents in electricity each time you recharge via solar. You can tweak these numbers with your electric rates to compute payback.

Arlene Said:

Will a 1.5 watt solar panel keep a battery charged?

We Answered:

On it's own, no. You'll also need at least a rudimentary charge controller, to avoid over-charging. The simple "voltage regulator" from a car will do, if you are talking about a 12V battery