Powering a Light Bulb With a Battery
Most recent answer: 10/22/2007
- marci a
D. A. Dorsey Ed. Center, Miami, FL
Its important to select a bulb which matches what your battery can put out. If the battery has too low a voltage, the current flowing through the bulb will be small and the bulbs filament wont get hot enough to visibly glow. If the battery has too high a voltage, so much current will flow that the filament will get too hot and vaporize.
Standard bulbs are designed to work with a voltage of around 120 V, which is an unusual range for batteries. Ordinary flashlight bulbs are designed to work with about 3V, easy to obtain with two batteries in series. Bulbs from cars are usually designed to work with about 12V, the output of a car battery or of eight standard battery cells in series.
You might think that using a lower voltage would only slightly dim the light, but actually the effect is much more severe. First, the heating power in the bulb goes as the square of the voltage, at least until the voltage gets big enough for the bulb to heat up and increase its resistance. Second, the amount of visible light produced in the bulb is virtually zero until the filament temperature gets close to the standard operating temperature. Thus using one fourth of the power will give much less than one fourth of the light output. If you use a bit too low a voltage, the bulb will glow orangeish, because it can still put out some colors of light but not the blue part of the spectrum.
You yourself generate about 60 W of heating power, the same as a 60 W bulb, but Ill bet you dont visibly glow very much. Thats because your temperature is too low to give off visible light. The light you do give off is infrared, which can be detected but not directly by our eyes.
Tom and Mike
(published on 10/22/2007)
Follow-Up #1: battery powered bulbs
- Ryan (age 8)
Fort Worth, TX, USA
The headlights in a car have exactly the parameters you desire -- about 50 watts per, and there are two of them, driven in parallel on a 12V car battery.
You can easily find 40W halogen bulbs at most hardware stores that run on 12 volts of electricity (AC or DC). They may come in higher wattages, too.
Ordinary alkaline cells may not supply 75W for very long at all -- a good car battery will last much longer.
(published on 10/22/2007)
Follow-Up #2: Switches and batteries
- johnny mcgimak
(published on 02/23/2008)
Follow-Up #3: battery-powered CFL?
- Chris (age 18)
Windsor, Ontario, Canada
Perhaps you'd do best using some LED lights. LEDs intrinsically work off dc power. You can directly get plain LEDs and power them off the batteries. This is safe, because there are no high voltages at any point in the circuit.
According to a Wikipedia article, new white-light LEDs are about as efficient as fluorescent bulbs. It's important to match the power supply voltage to the optimum operating voltage of the LEDs. An alkaline cell's voltage depends on how much current it's supplying, so the voltage can be adjusted a bit depending on how many parallel LEDs are being driven. By adjusting the number of LED's in series and parallel and the number of batteries in series, you should be able to get an efficient combination. It may be convenient to start with a commercially available lamp driven by 12 Vdc, which could be supplied by about 8-10 alkaline cells. You can measure the actual voltage to check.
(published on 06/01/2009)
Follow-Up #4: efficient lighting
- Tyler (age 23)
Compact fluorescent bulbs will work fine if you run 115 V ac wiring out there. However, the battery system avoids any safety problems.
(published on 06/08/2009)
Follow-Up #5: How many batteries does it take to...?
- Gregory F. (age 16)
Bay Area, Cal., USA
(published on 06/10/2009)
Follow-Up #6: battery and LED
- Ryan W. (age 17)
San Diego, CA, USA
Odd, we've suddenly gotten a rash of questions on this. Anyway, it's pretty simple. A wire goes from one battery terminal to one side of a single-throw single-pole switch. The wire from the other side of the switch goes to one lead from the LED. The other LED lead goes to the battery. It matters which lead is which on the LED. One should be marked + and one -. The + goes to the + side of the battery (maybe through the switch) and the - goes to the - side of the battery.
That's about it.
(published on 06/11/2009)
Follow-Up #7: wire leads
- Andie S. (age 19)
Portland, Oregon , USA
I think most of the LED's sold for home use have some sort of rectifier etc to adapt to 115 V ac house electrical power. There are bulbs intended for automobile use that work well with 12 V auto batteries. These should need no internal converter, since that's just right for about 5 LEDs in series.
(published on 06/12/2009)
Follow-Up #8: Battery size for auto lamp?
- Anthony (age 27)
So you need a 12 Volt battery, or two 6 Volt batteries wired in series.
That would work, but for camping I'd recommend an LED-based light, because it would be more efficient, so you could use less bulky batteries. Mike W.
(published on 07/24/2009)
Follow-Up #9: snowboard light
Kalamazoo, MI, USA
(published on 02/08/2010)
Follow-Up #10: go kart lights
- graham stoughton
It's hard to use our system to draw the circuit.
(published on 05/10/2010)
Follow-Up #11: Lighting up a body?
- Alexander (age 22)
(published on 10/30/2010)
Follow-Up #12: 6.3 Volt bulb on a 9 Volt battery
- Amber (age 31)
(published on 06/28/2011)
Follow-Up #13: 12v LED
- chris (age 20)
staten island new york
(published on 12/11/2011)
Follow-Up #14: How to wire an LED
- jim buchan (age 42)
(published on 08/18/2012)
Follow-Up #15: blowing bulbs
- Frank (age 12)
Brockton, M.A, U.S.A
(published on 05/16/2013)
Follow-Up #16: Using Blue T5 bulbs
- Juliet (age 26)
Houston, Tx, US
I looked up those bulbs. In the picture that went with the ad on Amazon you can clearly see a filament wire inside the bulb, so it's not an LED. It's an ordinary incandescent bulb. It looks like you could get some blue LED bulbs with the same base that use only 0.5W, much more efficient.
With your bulbs, any 12V battery will work. A car battery would work for a long time. You can get smaller 12V batteries for motorcycles, etc. A simple switch in one of the wires going to the battery will work fine. There's a nice picture of the circuit here: http://www.school-for-champions.com/science/dc_circuits.htm.
If you decide to replace them with LED lights, you have to make sure that the positive terminal from the battery is connected to the positive terminal of the LED. For your incandescent bulbs, you can connect either way.
(published on 09/10/2013)
Follow-Up #17: battery-powered lights
- Susie (age 35!)
Leesburg, VA, USA
If that's a standard old-fashioned incandescent screw-in bulb, it's designed to be powered by a voltage of about 120 V. It's a nuisance to put together a battery to power it. I'd recommend getting a 12V LED light and powering it with a single car battery. It's safer and more convenient. I just checked online and you can get an outdoor LED bulb like that for $14 that puts out more than half the light of a 100W incandescent and only uses 10 W. If you get two of them, you end up with more light, less cost (counting the batteries), longer running time, more lightweight batteries, and no dangerous voltages.
(published on 07/02/2014)
Follow-Up #18: Battery vs Mains?
- Brad (age 27)
Theorethically, it is possible to arrange a battery system; but I do not think you would find it practical. Based on your location, the mains is 230V AC; a typical car battery is 12V DC. You would need about 19 batteries connected in series (or design some step up electronics) to reach the same brightness. But the worst part is the need to replace/recharge them. With some overestimation, a car battery will be 100 Ah, this corresponds to 1200 VAh = 1200 Wh. But you have 4*60 = 240W power in the circuitry, so whatever you do, a battery will need to be replaced every 5 hours. Smaller batteries will require even more frequent changing. All this was assuming you had an incandescent bulb as its high power consumption suggests. Switching to LED would increase your performance dramatically. You can get LED bulbs specifically designed to run off 12V batteries, about as bright as an incandescent that draws 5 times the power.
(published on 02/02/2015)
Follow-Up #19: Stepping down the 250V potential to 10V?
- Michael Paquette (age 41)
First of all, the power (#W, stands for Watts) on the equipment will not be directly related to your problem, because the voltage and the power are related to each other by P=I2R in a circuit with batteries. That power rating is probably the maximum the socket can handle. Likelwise the 250V rating is probably the maximum the socket could handle. Your electrical power is more likely the U.S. standard 115 V or so. Your little bulb could easily be replaced with a CFL or LED bulb.
Tunc +Mike W.
(published on 02/05/2015)
Follow-Up #20: What about LED with batteries?
- Linda (age 65+)
Compare and contrast: LED is such an efficient technology that using batteries in this case is far more feasible than the incandescent cases above! A low power LED system around 5W would probably suffice for this. However, a single AA battery would have insufficient energy storage. A high quality rechargable battery is around 2.5 Ah (=2500 mAh). Since the output voltage is about 1.5 V (actually slightly lower than this), you have 3.75Wh (1.5V*2.5Ah), so you have 45 minutes of pleasure per AA. But the good news is that a G4 lamp base is designed for bulbs that run at 12V, so you can save yourself the task of building a circuit by purchasing a 5W G4 LED replacement and attach to a 12V battery directly. Putting a car battery (80Ah and 12V) under the table would mean 8 full days of power (80Αh*12V/5W = 192 hours). But another good news is that the potential of a battery depends on the electrode types and the solution inside, but not on the size of the system. So, you could also utilize a smaller battery, say an 8Ah one, if beneath the table is reserved for your dog.
p.s. You can also buy LED bulbs that screw directly into ordinary light bulb sockets, if that's what your lamp has. Some of these screw-in bulbs are also designed for 12V supplies. You then have to connect the lamp's power cord to the batteries, making sure that the + and - connections are right. /mw
(published on 02/15/2015)
Follow-Up #21: Why is more power supplied by a cell than is delivered to a bulb?
- Daniel (age 18)
In a circuitry, the energy is conserved. That is, whatever energy is dissipated must be provided by the battery. An incandescent bulb is a resistor dissipating energy squarely proportional to the current passing through. In the ideal case, this wiull be the only power output. But the cables and the cell itself are not perfect conductors, they also have somewhat small resistance causing an additional energy loss. Similar losses also exists for other circuitry elements such as capacitors or inductors.
(published on 05/14/2015)
Follow-Up #22: battery for LED
- Chris (age 38)
Pittsburg, KS, USA
We seem to have answered this question in here: https://van.physics.illinois.edu/qa/listing.php?id=574. As you will read about its details, wattage of a bulb is independent of the voltage but is specified by manufacturer seperately. We cannot judge its feasibility with the given information, but had made some estimations for car battery case before.
Thank you for your interest in the Physics Van.
(published on 05/14/2015)