Using Solar Power to Recharge Flashlights

I've got a question for some of you experts.

I do a lot of camping without electricity.

And I often have the need to recharge my flashlights (I have several that come with both ac and dc chargers.

I hooked an old rechargable spot light up, directly up to a 3.2 watt solar panel ... and it seemed to charge it ok.

When I hooked up my trusty old Maglite to the 3.2 watt solar panel ... the LED on the charger didn't light up.

So I hooked the Maglite up directly to a 13 watt solar panel ... and bingo ... the charge light illuminated.

Both of these panels are 12 volt panels. The 3.2 panel outputs something like 170 mA ... the 13 watt panel outputs .74 mA.

Measuring the "no load" current of each panel can run as high as 22 amps.

My question ... before I hook up my more expensive rechargeable lights ... do the "built in" or "cradle" chargers have the intelligence and ability to be recharged directly from a solar panel?

Or am I going to fry something.

To hook up the flashlights, I connected a female cigarette socket to the solar panel leads ... which allows me to plug in the rechargeable light's male DC cigarette plug.

I put this in the "homemade" area, because I'm "rolling my own" with these solar panels.

If need be, move it.

First, let me say that I love to play around with solar panels, especially for charging batteries. Even as I type, I have two 1 watt panels charging up a 7.2v nicad battery, which is used to power a radio. This works because the solar panel provides a loaded voltage adequate to charge the battery (the panels are nominally 12v), and the total current is less than C/10 (i.e. the battery has a 4A-hr capacity, and the panels can only put out about 200mA max).

In your case, the biggest concern is that the unloaded voltage from the solar panel might damage the charger. If the charger is designed to work with xx volts, then you might just use a zener on the solar panel output to limit it to xx volts plus a small margin. If the charger is designed for 6v, I'd try a 7.2v zener. Note that the zener must be rated for the full power of the solar panel! Might want to use a few 5 watt zeners in parallel when using the 13 watt panel.

You may also be having trouble because the small panel just can't provide enough power to run your charger, especially if the charger wants to do a quick charge. The small panel might be well suited to trickle charging your battery via a direct connection.

good luck,

Steve K.

your mag charger is a constant current reg.
about 200ma
the small panel cant deliver that much.
i dont think you will hurt it with the larger panel.
if you do lm317's are cheap.thats whats inside.

If it's a normal LM317, the upper voltage limit should be about 37v. Unless Mag got a bad batch (or used an oddball variation of the lm317), I doubt if the open-circuit voltage of a '12v' panel will hurt it.

An open circuit voltage of 21-22 volts is normal for a 12v (36 cell) panel. Those older spotlights are mostly 6 volt. I don't know why your little panel wouldn't charge the maglite. Maybe the current/voltage is too low to pass some of the other electronics to light the LED.

I use a nominal 15w (it's 10" square, that's not enough area for 15w), 12v panel to directly charge (ocassionally)most of my batteries. I figure max current is 600ma from this panel. I've done this with 6v & 12v gell cells, and various combinations of NiCADs or NiMH batteries from 2 cells to 8 cells. And when I say direct charge, I mean just that, no diode, no zener, no voltage regulator, just direct connection. NiCADs can take awesum charge currents as long as they don't get too hot. 600ma isn't too much for AA NiMHs, although I wouldn't want them on there too long after they reached full charge.

The solar panel wouldn't be harmed even with a complete short. Most of these battery combinations will pull the panel voltage and or current down to safe levels without any other regulation. I need to check the voltage and current again with 2 AA NiMH - that would be the worst combination. It might be a problem if the NIMHs were left on too long. And I don't do this with lithium batteries such as in my cell phone although the internal (built into the battery pack) regulation should work OK, AFAIK. I also would regulate larger panels to the right charge voltage.

Is it possible to directly charge a 12volt gel cell from a 14 volt solarpanel? VW of North America, is using these very cool solar panels in all new VWs...basically to keep a trickle charge into the battery. I obtained a couple and have been playing around with them. I want to set up a solar panel to charge the gell cell to drive a series of leds at night to illuminate my house numbers at the end of my drive. Are there any simple circuits to do something like this?

What kind (brand) of panels are you guys using?
Does anyone care to recommend a low-cost source for solar panels?

There is no such thing as a low-cost source for solar panels. Even used they are at least $3/watt (try Ebay). I just got lucky and found my "15w" panel surplus at an unually low price. The next couple of panels I bought here:
http://www.solarsolar.com HTH, RussH

unclearty - you need to measure the open circuit voltage of those panels in good sun (no shade or clouds) with a DVM. You will also need to know the amperage it will put out to see if it will work for your application. You should be able to do what you want, tho. Are you looking for a circuit to turn the LEDs on at night, or what did you have in mind?

The most efficient solar panels to be looking for are those using mono-crystalline silicon solar cells. These are the ones that do NOT look "flakey" (panels with flakey looking cells are poly crystalline, are cheap and have much lower efficiency), mono crystaline panels have shiny uniform colored multiple cells with a grid pattern on them. These panels produce the LONGEST solar day for charging! The mono crystalline panels produce useable output long before their counterparts(output at amazingly low light levels) The least efficient cells are the thin film deposited types, these look wonderful and are smooth and a uniform color but produce less voltage per junction and require more light and produce less power per square cm than the mono"s do.

This is just a note so as to help those avoid buying useless cells at high prices. I do not believe you will find many mono crystalline cells at $3/watt but in my opinion polly's at $3/watt is a waste of money. Thin film serves for applications requiring a great uniform look but their efficiency is far below that of the mono crystalline panels.

Don't fall into the TRAP, two panels, both rated at say 55 watts at 1000 watts/square inch of solar radiation. One unit is polly th other mono crystalline. THESE ARE NOT THE SAME! The Mono crystalline unit in a 12 hour dawn to dusk day will produce useable energy for about 10.5 hours per day where the polly crystalline will only produce useable energy for about 7 hours of the solar day at best. Without taking into account dawn and dusk fade, the mono crystalline will produce 33% MORE Solar Day Watt Hours than the Polly Crystalline panels.



NOTE: take notice of all the highway side web cams and traffic aids, you will note they almost exclusively use mono crystalline panels.

I extensively use solar cells to power remote communications and telemetering sites where there is no commercial power available. The solar technology is becoming so good that I am powering sites where commercial power is available and just don't want to pay for it!

These are just a few notes of experience that I am posting to the forum for you use as you see fit.

Regulation, it is industry standard on small arrays to use shunt regulation, It would be proper as RUSSH says to use a zener dioe to limit maximum voltage produced by a solar panel. When using GEL, AGM or Wet cells it is advised that one use a temperature compensted charge regulator such as the commercially available ASC (Specialty Concepts) with temperature compensation. A temperature sensing probe is provided to attach to the cell to control the "finish" charge. Remember Lead Acid cells terminal voltage is dependant upon temperature, this is VERY critical.

On the Nicad's, NIMH, and LiIon, I would just clamp the voltage to a safe level below the maximun input voltage limit for the appropriate SMART CHARGER. Go with dedicated smart charging chargers for the premium cells. Most of these cells require a modified or "multistage" constant current charge.

Remember, these cells, especially LiIon are very expensive and "DUMB CHARGING" will only cost you money in failed cells.

When you go above one or two watts of solar panel they are no longer toys! A high efficiency 15 watt panel as talked about CAN and WILL damage a battery from over-charge when left un attended and make someone very sad. /ubbthreads/images/graemlins/frown.gif

BY THE WAY ... DON'T FORGET, ALWAYS PLACE A BLOCKING DIODE BETWEEN THE BATTERY AND THE PANEL !!! The battery will discharge through a solar panel when there is insufficient light to produce power. Best place to do this is in the positive lead of the panel.

SOLAR PANEL -->|-- BATTERY.

(ASC charger for Lead Acid technology has diode built in)

Just my two cents worth as an avid solar user.

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