View Full Version : Solar LED garden light as night light
Slo-go'en
03-21-2009, 12:42
I found a solar powered LED garden light at Wal-Mart for $4.00 the other day. I bought one mostly to see if the solar cell would be useful (not really, made of glass and hard to remove).
But then I got to thinking, the thing is only about 4.5 oz so it would not be totally insane to carry the thing. It could be fun to have a night light outside the tent or to mark the path to the outhouse or just for the comments it would get from other hikers.
Once the trees leaf out, it would be hard to keep it charged, but then the novelty would have worn off and could put in someones yard as you walk by to town. I just might put one on my pack when I head down to VA in a couple of weeks...
I'm not at home so I can't get the picture of it, but I was thinking of selling these to ultralighters... I've taken a small watch battery and soldered one of the leads of an LED to one end and a hooked-dabble of solder on the other end of the battery. When you want a nightlight, just take the other lead and "hook it" and it's the simplest and lightest electric nightlight possible.
The Old Fhart
03-21-2009, 13:16
Totem-"I'm not at home so I can't get the picture of it, but I was thinking of selling these to ultralighters... I've taken a small watch battery and soldered one of the leads of an LED to one end and a hooked-dabble of solder on the other end of the battery. When you want a nightlight, just take the other lead and "hook it" and it's the simplest and lightest electric nightlight possible."Unless the LED you're using has built in current limiting, connecting it directly across a battery will probably fry it instantly. Also, DO NOT solder directly to a battery. Doing so will obviously heat the battery and the increased internal pressure can cause the battery to, as they say in the biz, "vent rapidly", or in layman's terms, explode.
FAQs from the web
"2. Can I apply heat to lithium batteries or dispose of them in fire?
If heated to 100°C or more, plastic materials inside the battery such as the gasket and separator may be damaged, causing leakage. The heat generated by a short circuit inside the battery may lead to rupture or fire. If disposed of in fire, batteries may burn violently.
Do NOT dispose of lithium batteries in fire.
3. Can I solder directly to lithium batteries?
If solder is applied directly to the battery, materials inside the battery may be damaged due to overheating. This can cause leakage or an internal short circuit. Heat generated by a short circuit inside the battery may lead to rupture or fire. Even if no abnormality takes place immediately after soldering, the resulting leakage or other damage may harm the components in the battery resulting in safety or performance problems.
Do NOT solder directly to lithium batteries.
No need to solder. Just pinch the leads of the LED across the battery. As for current limiting, most small cells are fairly high resistance. A common red LED, which is probably best for a night light, has a forward voltage of about 1.5V to 1.7V, if I remember correctly. So it will barely light with a 1.5V cell, and will probably not be damaged by even a 2V high internal resistance cell. Ok for intermittent use, like trying to find your tent after a nocturnal emission.
Also easier to pack away without accidentally making a connection and craining the cell. Good idea though.
Note that other colour LEDs, especially white ones, have a greater forward voltage, which may get trickier, and probably more difficult to arrange with a single LED-Cell combo. Also more expensive.
The Old Fhart
03-21-2009, 16:12
GGS2-".... As for current limiting, most small cells are fairly high resistance. A common red LED, which is probably best for a night light, has a forward voltage of about 1.5V to 1.7V, if I remember correctly. So it will barely light with a 1.5V cell,
That is incorrect. LEDs are current operated devices that basically have a constant voltage threshold (like 1.7v, depends on color) below which no current will flow. This voltage drop across the LED remains somewhat constant constant no matter what current flows through it. This is different from an incandescent bulb with a filament (i.e., a resistor) that draws a somewhat constant current no matter what voltage flows through it. If you have a LED that requires 1.7V and you connect it across a 1.5V cell, no current will flow and there will be no light-period. Google any source like this one (http://www.kpsec.freeuk.com/components/led.htm)and they will all tell you the exact same thing.
GGS2-"....and will probably not be damaged by even a 2V high internal resistance cell. Again, incorrect. If you exceed the maximum current rating for an LED it can be burned out instantly. The battery chemistry determines the voltage output and 2V cells are lead-acid chemistry, like a car battery. Most "coin" or "button" cells are probably 1.5V or 3.0V depending on chemistry, alkaline or lithium. The internal resistance of almost any cell (http://en.wikipedia.org/wiki/Internal_resistance) is nowhere near high enough to prevent burning out an LED.
The bottom line is if you don't understand what you're trying to do, don't do it. Small "photon" style lights are so reliable and cheap that it makes jury-rigging some mess impractical and potentially dangerous. If you've ever bought a replacement cell for a watch or small cheap flashlight you' have probable found the replacement battery is more expensive than buying an entire new flashlight with battery.
Fhart: My White-LED coin-battery rig has seen hours and hours of use. And a drop of solder on a battery isn't dangerous.
The Old Fhart
03-21-2009, 17:23
Totem-"Fhart: My White-LED coin-battery rig has seen hours and hours of use. And a drop of solder on a battery isn't dangerous."I'm sure you think you know more about this than the manufacturers of batteries and the countless engineers and sources that ALL say it is dangerous to solder directly to batteries. I've presented hard evidence (read the links I've supplied) that says it is dangerous, can you supply any hard evidence from a reliable source that says it isn't? (anecdotes aren't real evidence) There are hobbyists that do solder directly to batteries but to claim it isn't dangerous is totally false.
If you want to solder to a battery, don't do it directly to the cell. Buy a battery with thin solder tabs (http://www.allelectronics.com/make-a-store/item/LBAT-50/3V-LITHIUM-COIN-CELL-W/-SOLDER-TABS/1.html) designed to prevent this known hazard. Also make sure the LED you're using either has internal current limiting or put a resistor of the proper value in series with the LED to protect it as described in my previous links. (http://www.kpsec.freeuk.com/components/led.htm).
Again, the bottom line is if you don't really know what you're doing, don't do it. I've presented a good technical and easy to understand description of the dangers of what you're doing and whether you choose to believe it or not is up to you.
That is incorrect. LEDs are current operated devices that basically have a constant voltage threshold (like 1.7v, depends on color) below which no current will flow. This voltage drop across the LED remains somewhat constant constant no matter what current flows through it. This is different from an incandescent bulb with a filament (i.e., a resistor) that draws a somewhat constant current no matter what voltage flows through it. If you have a LED that requires 1.7V and you connect it across a 1.5V cell, no current will flow and there will be no light-period. Google any source like this one (http://www.kpsec.freeuk.com/components/led.htm)and they will all tell you the exact same thing.
Again, incorrect. If you exceed the maximum current rating for an LED it can be burned out instantly. The battery chemistry determines the voltage output and 2V cells are lead-acid chemistry, like a car battery. Most "coin" or "button" cells are probably 1.5V or 3.0V depending on chemistry, alkaline or lithium. The internal resistance of almost any cell (http://en.wikipedia.org/wiki/Internal_resistance) is nowhere near high enough to prevent burning out an LED.
Yeah, TOF, I know about LED and cell characteristics too. The device described is for intermittent operation, and the LED is not likely to fuse very rapidly. The life of the cell is another question. Most likely it won't last as long as in a watch or a hearing aid, but that's to be expected, isn't it? If you want to go to the trouble, match your LED to your cell. Most button cells are around 1.5V, and most small red LEDs are also. If you get a 1.7V LED, it's probably a high current device, and it may well run at 1.5V for this service. As for running any red LED at 2.0V, it will be quite bright and shouldn't be run for more than brief moments, to see what is in from of you. Better to go the 1.5V route.
I wasn't referring to a car battery with the 2V cell. Only button cells. Most of them are alkaline or silver, and run at ~1.5V. I there a few that run at 2-3V (Li-ion), but they are not as cheap or common.
Batteries are commonly spot welded together. That means a short, high burst of heat. A solder joint is worse, with a lower heat for a longer time, but it is all a matter of degree. A skillful solderer with a sufficiently hot iron can do it with minimal internal damage. Still, it does assume a degree of skill, and the right equipment.
To get technical for a moment, have a more detailed look at the characteristics of both: LEDs are by no means perfect diodes. They do have an internal resistance. Same with power cells, expecially when they are somewhat run down, and especially when they are of small current capacity. This subject isn't worth more debate than this, but the systems described will work, with a little tinkering to get the right components. Will it be good enough to send to market? No, I don't expect so, but who cares? The LEDs cost a few pennies, and the proper cell would be a few bucks. What's the problem?
Please climb down a moment and consider what we are talking about. Also, if you do it the way I suggested, you will feel the heat of the cell discharging, and you will want to let it go before it does any serious damage. Best in any case to just use it for flashes of a couple of seconds duration, which is why I would prefer the loose assembly version. If you blow an LED, carry a spare. They only weigh a couple of grams.
To anyone else who is still reading, never mind what us engineers say. This is not very dangerous stuff, but if you don't know what you are doing, don't do it.
MintakaCat
03-22-2009, 07:45
I would agree that soldering directly to a lithium battery is a bit risky, not that I think it would blow up, but I think it would reduce the reliability of the battery over time. I’m thinking mostly of the seals in the battery as being the issue.
Why not use a lithium battery holder? Those little suckers are cheap and it saves you the risk of soldering directly to the battery. Plus, the end user can change out the battery as needed, (a little added value to the product.)http://www.whiteblaze.net/forum/images/icons/icon7.gif
This whole discussion reminds me of soldering the legs of ham radio crystals in the old days. I used to do that all the time. It was like rolling the dice but it did allow me to use those newer metal case crystals in place of the older (and harder to find) FT-243 crystals. Most of the time it worked, but every now and then one would go belly up after soldering it.
Old timers would tell me “Don’t do it!” I did it anyway, LOL. I would get on the air with the crystal that I just soldered, those guys would be POed for about a month with me. http://www.whiteblaze.net/forum/images/icons/icon10.gif
Anyway, good luck on your idea for the UL crowd, that just might be what they are looking for.