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Q & A: Powering a light bulb with a battery

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Q:
How can I power a lightbulb with a battery?
- marci a
D. A. Dorsey Ed. Center, Miami, FL
A:
Just connect the positive terminal of the battery with one electrical contact of your light bulb and the negative terminal with the other electrical contact of the bulb. Many bulbs have one electrical contact with screw threads on it, with the other contact as a round dot on the end of the base. Other bulbs will have metal prongs sticking out. It's notoriously hard to get good electrical contact on batteries and bulbs by soldering wires on. The spring contacts in flashlights work much better (but even they are troublesome from time to time).

It's 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 bulb's filament won't 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 I'll bet you don't visibly glow very much. That's 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

(republished on 07/13/06)

Follow-Up #1: battery powered bulbs

Q:
If I wanted to power a 75 - 100 Watt bulb with a battery, what size battery (or how many batteries) would I need?? Has it been done before, and will it work??
- Ryan (age 8)
Fort Worth, TX, USA
A:
Sure, any arrangement of batteries that provides the right voltage (about 120 V, if itís a standard household bulb) will do. The batteries will provide dc voltage, not the ac provided by wall outlets, but the effect in heating the bulb filament is the same. Here's the problem, though. Stringing together 120V of batteries (say 9 or 10 car batteries in series) leaves a very dangerous voltage, easily enough to kill someone, without the relative safety provided by standard wiring outlets and plugs. I would instead find a bulb that works at lower voltage- say 15V or 25 V.

Mike W.

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.

Tom

(published on 10/03/06)

Follow-Up #2: Switches and batteries

Q:
if i connected a flashliht bulb to a 220 light switch and then to a battery would it work?
- johnny mcgimak
la,Ca,usa
A:
A switch is simply a go-nogo device.  It interrupts current flow.   As long as the switch is not connected a 220 Volt supply as well as the battery you are okay.  It should work.

LeeH

(published on 02/26/08)

Follow-Up #3: blowing bulbs

Q:
Is it posible to burst a light bulb with batteries?
- Frank (age 12)
Brockton, M.A, U.S.A
A:
Certainly. If you use a high-voltage battery, you may blow out a bulb designed for low voltage. You could also burst the bulb by throwing the battery at it, but that\'s probably not what you meant.

Mike W.

(published on 02/12/09)

Follow-Up #4: battery-powered CFL?

Q:
Now I have a project I'm working on, and I was looking to power one of those new fluroescent lightbulbs that use only 13W and produces the same light as a standard 60W bulb. My question is would it be possible - and safe - to use a number of C batteries to power the bulb, as i need it to be mobile on the end of a pole/tube. Is there any possibility this could work?
- Chris (age 18)
Windsor, Ontario, Canada
A:
Those compact fluorescent lights are designed to be used with ac power, not the dc power supplied by batteries. A dc-ac converter (inverter) would add substantially to the weight.

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, http://en.wikipedia.org/wiki/LED 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.

Mike W.

(published on 06/06/09)

Follow-Up #5: efficient lighting

Q:
We have a 12 by 12 rabbit shed in the back yard. Every night she goes out to feed and water them in the dark with a flashlight. I want to hook up a car battery up to a compact flourescent bulb -- or two. Would I need an inverter or buy a certain kind of bulb? Am I okay just wiring them up to a switch and light socket?
- Tyler (age 23)
Lynchburg, VA
A:
You could use an inverter, but that would be inefficient. It might make more sense to get an LED bulb designed to work directly off 12 V, and skip the inverter. I just googled and found that you could get a pair of these bulbs for $10. Oddly, the sockets (just standard auto parts) also cost $10.

Compact fluorescent bulbs will work fine if you run 115 V ac wiring out there. However, the battery system avoids any safety problems.

Mike W.

(published on 06/08/09)

Follow-Up #6: How many batteries does it take to...?

Q:
Hi! I was wondering how many batteries it would take to power a 120 or 230 volt bulb? Also, how would you go about wiring the power and also installing a switch to turn it on and off?????? Thank you for the help!
- Gregory F. (age 16)
Bay Area, Cal., USA
A:
If you insist on using a 120 Volt bulb then you would need 10 12-Volt batteries in series to light it up. It's better and simpler to use a single 12 Volt battery and hook it up to a 12 Volt bulb.  For example the electrical system of most automobiles is 12 Volts.  Automobile head lamps are also rated at 12 Volts and are pretty bright.   A simple on-off switch, available at a hardware store or at Radio Shack, should be wired in series with the circuit.

LeeH

(published on 06/11/09)

Follow-Up #7: battery and LED

Q:
HEllo! I was wondering how to wire he circuit of a 12 volt LED bulb and a 12 volt battery with a on and off switch?? thank you!
- Ryan W. (age 17)
San Diego, CA, USA
A:

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.

Mike W.


(published on 06/12/09)

Follow-Up #8: wire leads

Q:
I was looking at follow up #7 and i was wondering what you mean by lead???? Also, is that formula compatible to a LED bulb used in a home lighting furnishing??? If so, how? thanks
- Andie S. (age 19)
Portland, Oregon , USA
A:
Whoops, here a 'lead' is just a wire coming from a device- a switch, etc.
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.

Mike W.

(published on 06/13/09)

Follow-Up #9: Battery size for auto lamp?

Q:
I like to camp, so just for kicks I want to make my own lantern. If I were to use a car head lamp bulb, what size battery would I need to power it?
- Anthony (age 27)
NY
A:
Most automobiles run on a 12 Volt system so most of the bulbs are designed for the same.
So you need a 12 Volt battery, or two 6 Volt batteries wired in series.

LeeH

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 07/26/09)

Follow-Up #10: snowboard light

Q:
I am trying to hook up a 250w work light to a car battery for some late night snowboarding. will i have enough power if car headlights are only 50w? if not how can i accomplish this?
- Adam
Kalamazoo, MI, USA
A:
Our best recommendation here is to get some of the 12V LEDs designed to work off car batteries. That way you get something light, simple, sturdy, and efficient, all of which are useful properties for these outdoor portable applications.

Mike W.

(published on 03/12/10)

Follow-Up #11: go kart lights

Q:
I was wondering what I should do I'm building a go kart for a rally and I need to put headlights on it I was wondering what I should use I have no clue what I'm doing and it needs to include a switch thanks Graham
- graham stoughton
mason oh
A:
You probably want something light-weight. Maybe a smallish battery with some LED headlights would work best. The headlights can go in parallel with each other, in a circuit where the switch and battery are in series.

It's hard to use our system to draw the circuit.

Mike W.

(published on 05/18/10)

Follow-Up #12: Lighting up a body?

Q:
would like light some LEDs on a personís body. I was wondering if I could use a battery and some conductive gel? Paint the gel to each side of the LED to get power to it. (A art project.)
- Alexander (age 22)
VA
A:
Sounds like an interesting idea!  It might  work, it might not.  Be sure and try it out on something else than a real body first and be sure not to use any higher voltage than a battery.

LeeH

(published on 10/31/10)

Follow-Up #13: 6.3 Volt bulb on a 9 Volt battery

Q:
How does the mA of the bulb fit into the equation? I have a 6.3 volt bulb that I am trying to light with an interrupted circuit attached to a 9 volt battery. It will work fine for a day or so but after that it will light but then quickly fade. I haven't seen a 9 volt bulb. Is it better to have a bulb a little over the voltage of your battery or a little under? Thanks
- Amber (age 31)
Shreveport, LA
A:
Look at it this way:  if you hook up a bulb with resistance R to a battery with voltage V1 then the power consumed is P1 = V12/R.    Hook it up to a different battery with voltage V2 the power consumed is P2 = V22/R.  The ratio in your case the ratio is (9/6.3)2 ~ 2.  So this naive model would predict that the 9 Volt battery would last half as long if the two batteries had the same amount of stored energy.  Now this is probably not correct because the resistance of the bulb would probably increase a bit as it grew hotter with the 9 Volt battery.  It might be worthwhile to connect a small resistance in series with the bulb.  It will dim the bulb somewhat but the battery will last a bit longer.

LeeH


(published on 06/29/11)

Follow-Up #14: 12v LED

Q:
I bought led wheel well light and it says it connect to a 12volt power source.. would I also be able to connect it to a 12 volt battery and if so how.. the control box for the lights has a positive red wire and a black ground wire. Also if I connect it to a voltage higher will it burn out the insides? Thanks
- chris (age 20)
staten island new york
A:
I don't see why there'd be any problem directly connecting the LEDs to your battery, so long as you're careful about plus and minus. I wouldn't use a significantly higher voltage, since it could indeed burn out the LEDs.

Mike W.

(published on 12/11/11)

Follow-Up #15: How to wire an LED

Q:
I,m trying to wire a 4v led does this mean I'll have to wire it with ahigher voltage battery say 6v and add resistors or can I wire it with a lower voltage battery say 3v I've got very limited space for the led's switch and batteries
- jim buchan (age 42)
uk
A:
It's unlikely that at 3V you'll get enough light to be useable. Going to 6V is overkill, since you'll need a big enough series resistor to have about 1/3 of the voltage drop (and hence 1/3 of the power) wasted heating the resistor. Why not use 4.5V, and a small series resistor?

Mike W.

(published on 08/19/12)

Follow-Up #16: Using Blue T5 bulbs

Q:
I bought these blue bulbs for landscaping( not sure if they are LEDs) but wanted to use them for a project: Phillips 4w/12volt Blue T5 with a wedge base. My question is what batteries can I use to light them up and also include a switch to turn it on and off. Please help... If you can will you provide drawings due to my dyslexia its hard for me to grasp.
- Juliet (age 26)
Houston, Tx, US
A:

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.

Mike W.


(published on 09/10/13)

Follow-Up #17: battery-powered lights

Q:
Iíd like to power a 100W bulb for 90 minutes using batteries. Based on a previous answer (Ryan, age 8), Iíd need a car battery. Mike talked about 120 V and Tom about 12 V. How would I figure out how many volts I need? Thank you!! (Go Illini! : ))
- Susie (age 35!)
Leesburg, VA, USA
A:

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.

Mike W. 


(published on 07/07/14)

Follow-up on this answer.