Solar cell phone charging?

[BatteryCharger]

I've got a "12 volt" solar panel that is rated at 18 volts and 180ma max. Does anybody know if it would be possible to hook up the 12v charger I have for my cell phone to that? The charger is just one of those cheap ones on ebay that are basically the same for every phone. First of all, do you think it would ruin my phone or the charger with 18v? Second, the charger normally uses like 400ma I think. Would it work at all with 180ma?

 

[RussH]

The solar panel could be as high as 21.5v open circuit. Most phones charge at 4.5v slow or 6v fast. All the 180ma rating means is that it would charge slower than the 400ma would. You probably won't get 180ma, I'd expect 100-130ma under normal conditions, and 5-10 hours for a full charge depending on what size (900mah?) batteries you have. The phone MIGHT have 16v rated electronic components in it, although I would expect 25v ratings. If it's a $20 phone, I wouldn't worry about it, since the voltage will drop quite a bit under load. Thats why they use panels of 33 cells (18v) or 36 cells (21.5voc) to get the nominal 12v. A 6v panel would be a better choice, however.

 

[Steve K]

If you are worried about the voltage damaging the charger, just put a 5 watt, 12v zener on the output of the solar panel. This will limit the output to a safe level.

Steve K.

 

[RussH]

A zener is probably a good idea. Some of those phone chargers are regulated - I have some with circuits and one even has a LM317 voltage regulator. But the newer phones mostly seem to have built in protection/regulation for the lithium battery. Older pre lithium chargers with regulators are not the ones commonly available for 1 or 2 bucks. I really think the phone manufacturers made allowances for voltages above 16v in automobiles, there are often spikes above that, and a number of common malfunctions might produce higher voltages. If you were to hook the solar panel to a 12v gell cell, it would hold the voltage down to a level that would not hurt the phone. However, with just the phones small batteries, it might get close to the open circuit voltage when the batteries are fully charged. It probably wouldn't hurt even 16v components tho. YMMV, caveat emptor, etc...

 

[BatteryCharger]

What exactly will a zener diode do? Obviously keep the voltage down, but how? If I used a 12v diode would that keep the output at 12 volts? Seems too easy. /ubbthreads/images/graemlins/smile.gif I'm not really worried about damaging the charger, as I said they are only a couple bucks on ebay; however, I am worried about the phone, as it cost me $200. /ubbthreads/images/graemlins/grin.gif

 

[fivemega]

I would connect solar pannel to 12 volt small (2 Ah) gell cell before hook up to charger. It will limit forwarded voltage to charger and ofcourse you will have normal speed charging of cell phone. Gell cell can be recharge after cell phone is fully charged or disconnected.

 

[RussH]

There's a quick & easy explanation of zener regulators here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html
along with a design program - just plug your numbers in. -RussH

 

[Steve K]

A zener regulator normally requires a series resistor between it and the voltage source. Fortunately(?), solar panels have a large internal resistance. This is easily demonstrated by watching how much the output voltage drops as the load current increases.

This is basically how a lot of satellites charge their batteries; by using a shunt regulator that has the same function as a zener.

If you have the batteries, you could also just hook the panel up to a 12v nicad or lead-acid, and connect the charger to the battery. The battery will handle the panel's current while maintaining a fairly regulated voltage. This arrangement of solar panel and battery is how I power one of my radios.

Steve K.