# Is it safe to short circuit 3 x 9 V block batteries?

I'm trying to heat up a thin wire like shown in this tutorial. Since short circuiting one 9 V block battery doesn't provide enough amps to heat up the wire sufficiently.

If I take 3 of those 9 V block batteries and connect them together, will the power be so much that the short circuit causes a reaction like an explosion? Will it make any difference if I connect them parallel or in row?

• Try AA batteries- but alkaline, not rechargeable types or lithium type. Sep 15, 2015 at 17:34
• That is good advise, follow it ! Sep 15, 2015 at 17:40
• youtube.com/watch?v=duoxvqxASak I try to build the same thing like the guy in the video except that my wire is a little bit thicker. 2 x 9V Block Batteries arent able to heat my wire up sufficiently enaugh and wont ignite the match. Sep 15, 2015 at 17:40
• Powering heaters directly off batteries and power supplies is a tried and true tradition. Make sure whatever battery tech you are using sufficiently low impedance that it isn't taking a lot of that short circuit current internally. It may be wise to use a regulator in series with the supply to avoid the issue entirely Sep 15, 2015 at 17:52
• Using a 9v battery sounds good, but since you are going for high current, with low resistances, you'll do better with bigger cells even at a lower voltage. In fact, 40 years ago, you could buy exactly this (electric rocket igniter), and it used six "D" cells. Sep 15, 2015 at 19:23

If you want to heat up a wire, you'll need to know the rated ampacity of that wire. Since you don't care about the survival of the wire, and since you're actively trying to ignite something, you can then exceed that ampacity by several times. But that gives you a ballpark figure to start with.

Suppose we're talking about something relatively thin, 30 AWG. Three amps will heat that to about 90C, assuming typical ambients. Double the current, which quadruples the power, and you'll probably be at 250 or 300C. And that wire doesn't have much mass, so it should get to temperature very quickly. So to get it nice and hot, we need a source good for 7-10 amps.

http://data.energizer.com/PDFs/522.pdf You'll be lucky to get an amp out of a 9V battery.

I'd suggest using a capacitor. An electrolytic cap has (for your purposes) unlimited available short circuit current. All that will limit it is the resistance of the path and the energy in the cap. I'd probably do something like this:

simulate this circuit – Schematic created using CircuitLab

R1 limits the current from the battery to the cap, so there aren't any unpleasant sparks during wiring, and the battery doesn't see much of the current into the load wire. I'd probably use something on the order of 1 kOhm, 1/4W would be fine. Lower resistance means more power.

A 1-ohm R2 would give you about nine amps into your wire until the cap discharged. Discharge time will depend on the capacitance. Suppose you want that current for ten milliseconds, you'd probably want a time constant of 20 mS, so you'd need 20,000 uF of cap, probably rated for at least 12 VDC. That should be pretty easy to come by on Digikey.

These are all ballpark numbers, but it's an approach I would consider. Keeps your battery alive longer, avoids loading it past spec, and should be much more predictable and adjustable to get the results you want.

No, short-circuiting a 9V rectangular battery cannot be considered "safe" under any conditions. You're causing all of its energy to be dissipated in its own internal resistance, and it isn't designed to handle that kind of dissipation. While many batteries may tolerate this treatment without "violent" side-effects, there's no up-side to doing this.

Characterizing your load as a "short circuit" is a bit unfair and misleading anyway. Clearly the wire has a non-negligible resistance, or else it wouldn't be heating up. What you need to do is connect your wire to a power supply and measure its voltage and current (from which you can derive its resistance) when it is operating at the desired temperature in the intended application. Only then can you decide what combination of batteries best matches the load.

I wouldn't say "short circuiting" 9v batteries is a good idea, try using high dicharge lipo batteries, commonly used for RC toys (try on ebay, alibaba, or gearbest)and such with proper voltage regulator and protection circuitry. Those batteries are designed for high dicharge and are more resilient.

• Thats not an answer to my questions but a considerable alternative. Sep 15, 2015 at 17:35
• Don't do this. Short-circuiting a LiPo battery is extremely dangerous. They have a tendency to react violently to this type of abuse, and may catch fire or explode.
– user39382
Sep 15, 2015 at 19:02
• Sorry duskwuff, if I did not make it clear, I said to use an appropriate REGULATOR AND PROTECTION CIRCUIT after the HIGH DISCHARGE Lipo battery. I can share more details about the setup if can share more details about your application Sep 15, 2015 at 20:09