So how big of a battery should I buy?
Well, that's a little more complicated so read on. In the physical world of electricity, there are laws that must be followed as with every branch of physics. The most basic of these laws is known as Ohm's Law.
 Ohms law in simple mathematical terms is: Voltage = Current * Resistance, or V = I*R
After a small algebraic manipulation,
 Ohm's Law becomes: I = V/R
From this equation, we can see that the amount of current through an electric match equals the voltage across the electric match divided by the resistance of that electric match. This is important since electric matches function by electrical current flowing through them. So to determine how many electric matches you can fire off of a single cue, you must take into account four main things.
 The resistance of each electric match
 The number you wish to fire
 The method (series, parallel, or seriesparallel) in which you wire them
 The internal resistance of your firing system
To simplify matters, we shall assume it's an ideal world (it's not) and there is no internal resistance of your firing system. We shall also assume that you are going to wire everything only in series. To not make those assumptions, the calculations are more complex, and ultimately above the scope of this tutorial. If you are still interested in learning these more advanced ideas, order Issue 20 of the Journal of Pyrotechnics which contains my published paper, "Maximizing the Number of Electric Matches That Can Be Fired in a Single Circuit", on the subject. There you will get all the math, data, and experimental results you desire. Now for an example. Let's assume our electric match allfire current (current at which all electric matches will fire) is 1 Amp. Now let's say we have 10 electric matches. Let's also say each electric match has a resistance of 1 Ohm (Ohms are the units of resistance, not to be confused with Ohms law). So if we tie the 10 electric matches in series, their resistance's will add (not true if wired in parallel), thus giving us 10 ohms of resistance. So going back to Ohms law, when we plug these numbers into the equation, (24v/10ohms) we get I = 2.4 Amps. So we will have 2.4 Amps going through each electric match. This amount of current is much more than most "allfire current"'s given by the manufacturer of that particular brand of electric match. Now, if we were to have 10 of the same electric matches in parallel instead of series, we would only get 0.24 Amps of current through each electric match (math omitted due to elevated complexity). So you can fire more electric matches in series than you can in parallel. Or can you? Believe it or not, with some more math, it can be shown (above the scope of this tutorial, once again order the journal issue mentioned above if your interested) that matches wired in seriesparellel combinations can fire more electric matches than series or parallel alone. I have fired up to 50 electric matches successfully on one 24 volt system. This is not done very often in the world of fireworks, and applies more to ignition of a cluster of rocket engines. Click here to read more on this subject.
Is ohms law the only law we have to worry about?
No. There are a couple other important laws that govern the multiple seriesparallel circuits, namely Kirchoff's Voltage and Current Laws, however they are also above the scope of this tutorial. Look them up in a basic electrical circuits book if you are interested.
What are the advantages/disadvantages of seriesparallel wiring for multiple shots aside from the above mentioned?
Advantages:
Series
 When you check continuity, you know all matches are hooked up correctly.
 You can fire more matches on a single cue when wired in series as opposed to parallel.
Parallel
 If all matches on a single cue are not wired into the terminal properly, at least one will fire if you had continuity.
Disadvantages:
Series:
 If one match fires much quicker than the others in the chain on a multiple shot, the rest of the chain may fail to ignite.
 If something happens to the chain during the show, none of the shots on that cue will fire.
Parallel
 If only one match of a multiple shot is wired into the cue terminal correctly, you will still get "good" continuity. Ensuring that some of your shots will not fire.
Standard Rules We Follow:
Series:
 No more than 5 in series in one cue
Parallel
 No more than 3 in parallel in one cue
 This number also takes into consideration care of the electronics terminals, as to not cram to many wires into a single one.
Other series & parallel issues to worry about
A frequently asked question is, "What happens in a series chain if one electric match fires before the rest have time to ignite?" Nothing if you have an electric match from a good manufacturer. If the manufacturer is not very consistent in the manufacturing of their product, they will have significantly larger differences in the resistance of electric match. From personal experience, I have seen certain electric match brands work flawlessly, and others fail when in series on a regular basis. So if you don't have a large number of electric matches to shoot from one cue, say five or less, then just use a parallel wiring scheme. Just remember to double check your wiring when using parallel since one match in the terminal correctly will give you good continuity, and not indicate any possible errors with your other shots on that cue.
Here is another great electricity 101 tutorial.
