Solar question - Not quite a robot

[Bobsta]

Hey guys,

Its not quite a robot question but you guys have a world of knowledge I crave to have in my cranium!

So I lost my beloved car a few years ago and never got round to buying another one, so i stripped the sound system out one day in the hope i could make a portable one..

Yes I did succeed & with perfect results! So im running a car sound system out of a box with a 12v battery powering it.
I recently acquired a medium sized solar panel from a mate that was just lying around in his shed..

I was thinking.. can I use this solar panel to charge my portable sound system during the day (as i use it when we go camping and such) ?

So here is what im dealing with:

Car sound system
- Head-Unit
- 4x Speakers

12V Battery
Details:
Vision
CP 12120 12v 12AH
Valve Regulated
Rechargeable Battery
Constant voltage charge
Cycle Use: 14.5-12.9v
Standby Use: 13.6-13.8v
Initial Current: Less than 4.9A

Solar Panel
- No idea the specs of it but its just a medium size panel not to big not too small..
- I hardly know much about solar power, all i see is the panel and a coper filled wire coming from it.

So i looked up on the net to see what i could find out about solar power and charging the battery.
What I came up with pretty much confused me.. so help me out here.

The panels absorb sunlight and convert to electrical energy but during low light / no light the panel can leak energy. – So i cant connect the panel directly to the battery (also to prevent overcharging?? )

So to solve this i connect a 'diode' to the wire from the panel to the battery..
- Do i use a certain type of diode, a big one a small one a what one? :S

Then that doesnt solve the overcharging ?! How will i stop that?

So I think i covered everything, I have had a little experience in robotics so id rather construct and build what ever i may need than buy it premade if its possible.

So whats your brains say guys and gals??

Cheers
- Andy

[Soeren]

Hi,

So I lost my beloved car a few years ago and never got round to buying another one, so i stripped the sound system out one day in the hope i could make a portable one..

I'm sory I didn't strip mine, when it went up the river - played wav's and MP3's from SD-cards and USB flash drives and had lots of other smart functionality 

12V Battery 12AH

Solar Panel
- No idea the specs of it but its just a medium size panel not to big not too small..
- I hardly know much about solar power, all i see is the panel and a coper filled wire coming from it.

You NEED specs!
Here's how to get them:

You need:
 1 DMM
 1 very sunny day (2 overcast days won't be a proper substitute)
 1 pencil
 A notepad

Setup the panel perpendicular to the sun, so that it gets the maximum possible amount of light (i.e. at a 90° angle). Setup the DMM to read 20V DC (or 200V DC if it over ranges).
Try slight angling of the panel in each axis until you get the maximum voltage reading.
Jot down the voltage reached.
Change the DMM to read current, start on a 2A DC range and switch towards a larger sensitivity until you are on the most sensitive range that can still get the reading.
Jot down this short circuit current reading.
Repeat a few times to be sure you get the maximum readings on both counts.
Serve numbers (here) while hot.


Now you can either calculate the function or draw an X/Y diagram with current [A] on the vertical (Y) axis and voltage [V] on the horizontal (X) axis.
This will reveal how much current is available at any given voltage.

Now since you didn't give the size of the panel, it's hard to judge the possible output, but to compare, I have a panel which is 30cm (1') x 10cm (4") and it gives around 100mA max. at 1000W per square meter (which you'll never really get in real life) and another panel of 30cm (1') square giving 275mA under the same conditions.
They're both intended to connect to car batteries for trickle charging (diode included in the panels).

A current of 100mA won't overcharge your 12Ah battery if the voltage is OK

If you post the numbers measured, it will be a quick matter to decide whether it would work for your 12Ah battery or not. If it's indeed made for car trickle charging, all you need is a diode thet can handle the max. current. Best is a Schottky diode, as it has the lowest voltage drop, but a simple 1N400x could probably be used in a pinch.

[Spoil9]

This is a common thing to do (at least from what I can tell)

As I posted in another thread, solarpowerforum.net is a place that I like to look when it comes to anything solar powered.
Two threads that I found that may help you are below:
http://www.solarpowerforum.net/forumVB/do-yourself-discuss-your-projects/3630-good-solar-project-beginner.html

http://www.solarpowerforum.net/forumVB/solar-energy/4797-diy-portable-12v-solar-charger-questions.html

Hope this helps.
- Will