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Recharge rechargeable battery with a led battery powered lamp? Answered

I am making a lamp cordless using batteries and low poer 12v led strip. My question is can i somehow connect a solar cell so as the light hits it recharges the rechargables batteries that will actually power the lamp. So I dont have to take batteries out and recharge them every few days or week? 


If this is a desk lamp that directs all the light down, you could put a small solar panel on top and any ambient light would be used to charge the light during the day. It would be like a Malibu lawn light. So you could set the lamp in front of a window (preferably one that gets some direct sun light) and offset the need to recharge at least a little bit. I recently made a portable reading light using a sealed lead acid battery that was taken out off one of our UPS (computer battery back up uninterruptible power supply) units at work, It can no longer reliably power up a computer but works great for powering 4 leds in series with a current limiting resistor. In this application the connections to the battery are still accessible from the outside world for the purpose of recharging. I have been running this desk lamp for over an hour a day for over 2 weeks and the voltage has dropped so little that I do not anticipate having to recharge it for a few more weeks. When I do recharge it I will be charging it off of a 5 watt panel that is on my roof. I am running the current through a 7812 regulator and a 1N4002 diode. The point is that to make your light more convenient to use you may want to modify it so the batteries can be recharged without tasking them out and possibly using a sealed lead acid battery also. A 7 amp hour sealed lead acid battery is so over kill in my application. The circuit draws only 20 milliamps. 12 leds (3 sets of 4 leds in series) would draw only 60 ma.


I would like to put the solar panel inside teh shade with at least 10-15 led depending how big of lamp shade it is. i have 2v 50ma solar panel i can try to put in . just want to make sure it would work before i do it all

That would be really awesome, wouldn't it? Unfortunately, it won't work.

Well, let me be careful. It won't work the way you have described it. If you have the lamp in a completely dark room, and turn it on, the light output will not be sufficient to recharge the batteries which are driving the light.

The basic problem is a combination of conservation of energy and energy conversion efficiency. Whenever energy is converted from one form to another, some of it is "lost" to some unusable form (sound, resistive heating, friction, whatever). The conversion efficiency is always less than 100%, and usually much less.

In your conceptual system, you have a battery (B) which drives an LED (L) , which drives a solar cell (S) to charge the battery (B). A small difficulty in analyzing the system is that different units are used for "efficiency" of the different stages, so I will try to work through the details to keep everything straight.

1) LEDs typically use 20 mA of forward current. You've specified a 12V system, so your batteries will be draining at 240 mW output.

2) Commercial LED lamps have luminous efficacy (light output vs. power) around 50 lm/W. (The current record for a laboratory system is 208 lm/W for a white-light LED.) With your batteries, we can assume an output of about 12 lumens.

3) Light output has a bunch of complex units. Lumens is the total output, over all directions. LEDs generally emit only into one hemisphere (2pi steradians), so 12 lm/2pi = 1.9 cd, which corresponds to 2.8 mW of light power.

4) Your solar cell is presumably not completely surrounding the LED (it wouldn't make a very good lamp). Let's assume a 5 cm square cell, 45 cm below the LEDs. That is an area of 25 cm2 vs. a full hemisphere (2pi sr) area of 12,723 cm2, or 0.001964. The solar cell is going to receive just 5.5 microwatts of light!

5) The best available solar cells (the GaAs ones in use on the Mars rovers) have an efficiency of just 44% for converting light to electricity. If you get your hands on those, then you'll get 2.42 microwatts of power output from your LEDs.

If you recall point (1), your batteries are putting out 240 milliwatts to power your LEDs. So your solar cell is going to give you back 1/100,000 of what the batteries are putting out.

Ok, I was just wondering because I already have the battery powered lamp and it works ,,just thought maybe if I put solaor cells under shade they would get more light directly to charge.. Thank you for your quick repsonse and detailed info it saves me alot of time and extra money

If you lined the lampshade with solar cells, they would cover a larger area (and a larger solid angle) but you still can't get back the energy you used to run the lamp, because of the conversion efficiencies.

I used the LED light strip they sell on amazon for 13 bucks and it has sticky backing and stuck in a circle around inside of shade. Wonder how long it will take to drain

That's cool. Do you mean the 5 m long spools? If you used the whole spool, it will draw 2 A (24 W), which means your batteries are likely to run out fairly quickly.

Those spools are configured so that you can cut off shorter strips, the current will be proportional to length (so, for example, 0.5 m will draw 1/10 the current, or 0.2 A).

You can figure out how long your batteries will last by looking at what their total capacity is rated. A 1 amp-hour battery (= 1000 mA-h) can deliver 1 A current for one hour, or 2 A for half an hour, and so on.

i think it is 300 leds spool but didnt use them all maybe 50 at most...I havent used it alot yet since I didnt know if I was done building or not..i left it on for probaly ten minutes and it was fine. Most likely was a waste of time to even do it but was bored and wanted to try something out .lol

Learning something is never a waste of time! This sounds like it was a cool project, and your idea of using solar cells for charging was great. It'll still work, but you have to keep the lamp out in the open (sunlight, even indoors, is much brighter than your lamp).

Very true, thanks for all your help.

I guess ill just have to take batteries out and charge them . thank you very much for the info..It was exactly what I looking for..