Hello
Any chance someone could help me with a simple lighting circuit calculation.
I want to power 4 x 12v/20w halogen bulbs from a battery.
All to run in series
How do I calculate what battery I need to power the circuit?
Its part of a fancy dress costume and my school days physics seem a long time ago.
Many thanks
Are you sure you want them in series? You need them in parallel if you want them to work how they normally would or use 48 volts to power them in series.
To continue what HD said, IIRC from high school, if you run the bulbs in series, each one will be dimmer then the one before it.
No, this isn’t true.
As for the OP - you are talking about 80W - that’s a HUGE amount of power for a portable application. Maybe LEDs would be a better idea?
yes look at LEDs.
in a costume 4 20W bulbs would be warm and the batteries heavy.
How long do you want/need these bulbs to stay lit? What level of dimness (as the batteries run down) is acceptable?
Watts = volts X amperes, thus a 20W bulb draws ~1.7A. You’ll need a power source capable of delivering ~7A @ 12V* for however many hours.
To get 12 volts you’ll need eight 1.5V cells, four 3V cells, or two 6V lantern batteries.
There are many choices for 1.5V (AA, AAA, AAAA, C, D) and many choices for 3V (various “button” or “coin” cells).
If you google on the battery/cell type and “ampere hours” you can get an idea of which ones might do the job well enough for long enough. Then you can work on the logistics of wiring and wearing the ones you choose.
ETA: If you’re strong, or can incorporate a little wheeled cart into the costume, you can use an automotive or marine battery. That will kick butt. 
*This is in the neighborhood of what a car headlight draws. I have some serious doubts that small, easily wearable cells are going to fit the bill.
Upon further thought, you may be able to find a 12V gel battery (e.g. a motorcylce battery) that would do the job and still be small enough to be wearable. It’s still going to be heavy, but conceivably would be manageable.
If you can use LED’s, as suggested, then coin-type cells would probably work fine. They’re a whole lot easier to carry than the other options.
Another vote for the LED. They are harder to find than 12 volt halogen bulbs. I don’t know if Radio Shack stocks them or not. You may have to go on line to some place such as www.mouser.com. You could also tear apart some of those LED flashlights becoming popular.
The voltage for any electrical devices in series is the sum of the voltages. As above, four 12 volt lights would be 48 volts. The total watts will be the same series or parallel.
Note, Most Christmas tree lights have been series since energy crisis 1.
As others have noted, those are some seriously big lights.
The lights are designed for 12 volts. If you want to put four of them in series, you need 48 volts. Finding a portable 48 volt battery isn’t going to be easy, so you’ll probably be best with four 12 volt batteries also in series. As was previously mentioned, this is probably easier to wire them in parallel. Then all you need is one 12 volt battery.
These are 20 watt bulbs, which means they draw 20/12 = 1.67 amps. That’s a lot of current. Your wire will need to handle that much current, so you’d be best using something no thinner than about a #18 wire. You’ll also need a battery that can supply that much current. Your basic Joe Average flashlight cell won’t work. They aren’t designed to crank out that many amps.
If you parallel the lights, you’ll need 4x1.67 or 6.67 amps. Lesser voltage, more amps, same amount of total power.
The proper way to figure out how long the battery will last is to use the discharge curves from the battery manufacturer. The “close enough” quick and dirty way is to just take the amp-hour rating and divide it by 2. If you powered these from a 12 volt motorcycle battery rated at 12 amp-hours, you could therefore expect your lights to last about an hour before the battery runs out (12 amp-hours divided by roughly 6 amps = 2 hours, divide by 2 and you get 1 hour).
As others have noted, you can get the same amount of light for significantly less current (and therefore a significantly smaller battery) by switching to LEDs.
12 volts also isn’t high enough in voltage that you really need to worry about electrocution. 48 volts though is getting up into the range where that amount of voltage can be dangerous if you touch it. No matter what you use, you are dealing with enough current to potentially melt and/or catch things on fire if you have a short or a bad connection. Be careful. Heat from the bulbs may also be an issue if this is something to be worn.
Menards has 12 volt compact florescents. IIRC, they are the equivalent of a 20 watt incandescent.
Firstly thanks for the responces, really helpful.
Im building an Optimus Prime costume, the lights are to go on the upper chest and will only be turned on for effect. I have the bulbs already. I imagine a total of 5 mins intermittent use.
I also have indicators (blinkers) in the feet which I will make later.
The whole thing has kinda got out of hand with extras being added but its been a long time since I had a project.
I think I will source a 12v battery to use.
The next question id could I use a battery like this:
http://www.buyabattery.co.uk/car-alarm-key-fob-battery-27a-12v-battery-c-757_768.html
Or does it have to be a large one like this:
I really dont want to have to carry a large battery
Any ideas how to calculate which batter I need?
Many thanks once again
Thought I should add that I am now going to have these in series.
Im thinking of getting a battery box which holds 8 x AA batteries
This would give me 12V and 19Amps (?), do you think this will be OK ?
Many thanks once again
The 1600MAH battery in your second link will run the lights for around 15 minutes (probably a lot less, actually, but that’s what the calculations show). The 7.2AH battery will run them for around an hour. Don’t wire them in series - wire them in parallel, otherwise you will need 4 of the batteries above.
No, it won’t ‘give you’ 19 amps.
The 4 lamps in parallel are a 12 volt, 80 watt load, so they will draw about 6.7 amps from a 12 volt source.
The typical Alkaline AA battery can supply about 2700 mAh (milliamp hours) or 2.7 amp hours In A Perfect World.
This is not a perfect world application. The actual capacity of an alkaline AA cell at the discharge rate you are talking about would be more along the lines of 300 mAh due to the internal heating and other characteristics of the battery stack. Alkalines (nor carbon-zinc, nor any similar battery chemistry) just don’t perform well at high discharge rates. At that, the AA stack would have a maximum usable output for somewhat less then a minute, and noticeably dim until the bulbs are dark in approximately 2.7 minutes.
These are just off the cuff calculations, but you should look more towards a sealed gel-cell type battery… such as this one which is rechargable, and should provide useful output at your required loading for about 15 minutes total before starting to noticeably dim, with final darkness at somewhere around 30 minutes.
Your OP already said you’d have the lights in series. Are you repeating yourself, or are you now talking about batteries (cells, actually) in series?
Regardless, do you understand that running the lights in series will require 48V for full brightness? That run in series they will all be dim, like 5W bulbs?
I’m curious why you’ve mentioned a series circuit. To the rest of us, parallel seems to make much more sense.
Sorry my mistake
I will be running these in Parallel
Is it not a good idea to use the AA battery pack?
Thanks once again
This battery will be much too heavy for me, the costume is already cumbersome with that extra weight I wont last the night. Im also struggling for space on the inside of the costume
If its not possible I wont make it, it would just make a nice little something different.
You have four 12-volt 20-watt bulbs, correct? If connected in parallel, that’s a total of 6.7 amps. That’s a lot of current.
It takes 6 AA batteries to get 12 volts. Since they are in series, each one needs to supply 6.7 amps. I don’t think alkaline batteries can do that. Maybe NiCd batteries can, but it would last about 8 minutes.
Not only do halogens require a lot of power (and current) to operate, but they can get *really *hot. Might be a fire hazard on a costume.
As others have mentioned, why not LEDs?
Since you won’t be running the lights continuously, I see some flexibility in battery choice, although I don’t know precisely what battery to recommend for your needs here. I believe, however, that this could work: get two 6V lantern batteries. They’re somewhat heavy, but probably not much heavier than 8 D cells (and it appears that AA cells won’t cut it). They could be arranged one per side rather than having all the weight in one spot. Their terminals are such that attaching wires is simple. I’m speculating that the combination of power, versatility, and weight may be your best bet for making this idea work.
