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[on:]

Here's what I had laying around and I was hoping it would be useful for battery recharging. Something bought off of eBay.
http://www.ebay.com/itm/12V-Solar-Panel-Power-Battery-Charger-for-Car-Auto-RV-SUV-Truck-Boat-Motorcycle-/140780578252?pt=LH_DefaultDomain_0&hash=item20c72d25cc

WIth the rates of
Power: 1.55 watt
Voltage: DC12V/100mA

Can I just lop off the cigarette lighter plug and connect the ends to one of those AA battery adapters from Radioshack?

[Reply #1 on:]

Can I just lop off the cigarette lighter plug and connect the ends to one of those AA battery adapters from Radioshack

Do you have a good fire extinguisher or insurance?

[Reply #2 on:]

It will work.
A pretty inefficient way of doing it , but as long as the AA batteries you are charging are more than 2000 mah capacity it will charge them, albeit very slowly.

[Reply #3 on:]

It will work.
A pretty inefficient way of doing it , but as long as the AA batteries you are charging are more than 2000 mah capacity it will charge them, albeit very slowly.

You would also need to disconnect the AA rechargeables when they get warm. Basically watching the kettle boil

[Reply #4 on:]

The batteries wont get warm.
Although the Solar Panel is rated at 1.5 watts, the most it can deliver is 100 ma .
Into a AA battery, either nimh or nicad , thats only 140 milliwatts.
Maximum charge voltage for a AA nimh is 1.4 volts.
Its a very inefficient way to charge AA batteries.
Those types of chargers are intended to trickle charge car batteries, which essentially stops them going flat
if not used for a long time.

[Reply #5 on:]

Its a very inefficient way to charge AA batteries.

By "inefficient" you mean that there energy is lost somewhere, or it's just a slow?

[Reply #6 on:]

Its a very inefficient way to charge AA batteries.

By "inefficient" you mean that there energy is lost somewhere, or it's just a slow?

In real-world designs energy is always "lost", even running current through copper wire uses a small amout of energy.

But seriously it would be slow, even though a nominally 15 V_DC solar panel could charge 9 (possibly 10 in strong sunlight) AA batteries at a time.  If the AA batteries have a 2500 mAh capacity, and the average current from the solar cell is 100 mA; it would take at least 25 hours to fully charge the batteries (in real-life it would probably be somewhat slower).  Given that most places on Earth have less than 12 hours of sunlight during the majority of the year, that's about three days total...  Unless of course you only plan to use this charger within either the Arctic or Antarctic Circle during their respective summers, then it would be just over a day.  

[Reply #7 on:]

Although the Solar Panel is rated at 1.5 watts, the most it can deliver is 100 ma .
Into a AA battery, either nimh or nicad , thats only 140 milliwatts.

You are making a lot of assumptions about how the original poster might be hooking this up.  Maybe we should ask things like:

1.  What is the battery technology being used?
2.  How many batteries will be charged and in what configuration?
3.  What conditions will the solar panel see?  (outside, unobstructed view of sky?  indoors?)
etc.

[Reply #8 on:]

Given that most places on Earth have less than 12 hours of sunlight during the majority of the year, that's about three days total.

If the device powered by the said batteries draws only 30mA, for example, in a few days, the batteries will become fully charged, but the solar panel will still provide 100mA. Aren't they going to become overcharged and heat up?

[Reply #9 on:]

If the device powered by the said batteries draws only 30mA, for example, in a few days, the batteries will become fully charged, but the solar panel will still provide 100mA. Aren't they going to become overcharged and heat up?

It really depends on the type of battery.  In my reply I was assuming NiMH since that type was mentioned earlier in the thread.  You can use trickle charging to maintain a full charge on NiMHs, the usual recommendations are a trickle current between one thirtieth (1/30) to one fortieth (1/40) of a battery's C rating.  Since 2500 mA/30 = 83.333... mA, if you put the load circuit and the battery in parallel you might be able to do it safely (over stressing NiMHs will produce hydrogen gas which is of course flamable and explosive if it builds up in a small space).  Although you might want to consider adding in a current limiting resistor in series with the battery (probably after the diode used to keep the solar cell from discharging it at night) or use a dummy load (i.e. one or more resistors) in parallel with your actual load and battery to siphon-off even more of the current.

[Reply #10 on:]

My point was that there can be a situation when the batteries are fully charged, but the solar still pumps more current into them. What's going to happen then?

[Reply #11 on:]

My point was that there can be a situation when the batteries are fully charged, but the solar still pumps more current into them. What's going to happen then?

As perviously stated, it depends on the battery type and capacity! 

Some, like NiMH and Lead Acid batteries (automotive and gel SLA) it won't be a problem, provided the current is kept small enough to function as a trickle charge otherwise it can damage the battery and at high enough currents lead to the production of hydrogen gas (always very flammable in normal air and potentially explosive if sufficiently concentrated).  Other battery types you'd have to have a way to automatically electrically disconnect a fully charged battery from the solar panel to prevent it from being overcharged.  LiPos, for example, are easier to damage and have a lower current threshold for self-ignition or explosions.  With only 100 mA of overcharging the more spectacular failure modes are still possible (especially with small capacity LiPos and Li-Ion), but the far more probable issues will be faster than normal degradation of the batteries and/or total failure where physical damage is limited to the battery itself. 

In any case, even if you are using the cheapest possible recharagable batteries, it's usually more cost effective, and always a bit safer, to prevent minor overcharging than to live with its consequences.

[Reply #12 on:]

Might be easier to just buy a ready made solar charging setup.

http://www.harborfreight.com/portable-household-battery-solar-charger-68690.html

[Reply #13 on:]

Or easier to add an LM317M linear regulator (for about $3.00... including a 220 ohm resistor and a 10K pot) to lower the solar panel's voltage to about 3V output. and forget about ruining the batteries.

Doc

[Reply #14 on:]

Well, I am sure glad I asked you awesome folks here on the Arduino Forum about this. Thanks for the links and the info. For $3 I'll go with Doc's suggestion since the pre-made one online costs $14 + shipping. The purpose was to use something I already had laying around in order to reduce cost. Yes, 2500mah AA rechargeable batteries.

Thanks!