# What does it take to short a battery

I understand that low resistance through a circuit will create a short circuit but what combination of voltage and amps which causes this?

For example, if I had a circuit with just a wire and a battery and it had a high voltage and low amperage, would this cause a short circuit in comparison to a circuit with low voltage and high amperage?

• If it's a high-amperage battery it takes stupidity. May 17, 2020 at 16:24

'Short Circuit' gets used in two different ways.

In the context of a battery (or any power source), we usually mean it to be a load that is far too large for the source. Any battery, whether a high voltage or low voltage battery, will be 'short-circuited' by putting a low or zero resistance load on it.

A short circuit usually produces damaging conditions for the battery, and the load, if maintained for enough time. At best, the battery will be run down quickly. At worst, the battery may catch fire, burst itself or its container, or the load start a fire.

The wiring to a high current battery, like a car battery for instance, will invariably be protected by a fuse, which opens in the event of a short circuit. The wiring to a low current battery may not need protection, if the short-circuit current is low enough for any practical wire. Given this, there may be some sense, hinted at in your question, that for high current batteries, a short circuit is an issue, where it is not for low current batteries. For instance a PP3 or CR2032 battery, while it will be run down by a short circuit, is most unlikely to start a fire as a result.

In circuit analysis, a short circuit is an ideal zero resistance, that will support any current with zero voltage across it.

Outside of formal circuit analysis, there is no universal definition of short circuit in absolute terms of voltage, current or resistance. In most contexts, a short circuit is not a mathematical function or limit, it's just a general term for a type of failure or behavior.

Current tends to prefer the path of least resistance, which can also be thought of as the "shortest" path. If a significant portion of the current is able to take an unintended or intended "shortcut" between the poles of the supply, and therefore never reaches the intended load, then that situation may be called a short circuit.

If the total resistance of the circuit is "too low", or "abnormally low", or "much lower than intended", then you may also be justified in saying, at least informally, that the circuit is "shorting out" or "shorted".

• In the area of formal circuit analysis, a short circuit is defined to be equivalent to an ideal resistor of zero ohms or an ideal voltage source of zero volts. May 16, 2020 at 14:33
• Fair enough, but I would argue that it is not universally applicable, because ideal components are kind of hard to come by. (I'm not saying that formal analysis can't be useful though.) I added a caveat about it to my answer. May 16, 2020 at 14:35
• 'Current tends to prefer the path of least resistance' - by definition. May 17, 2020 at 22:16

For example, if I had a circuit with just a wire and a battery and it had a high voltage and low amperage, would this cause a short circuit in comparison to a circuit with low voltage and high amperage?

To answer this we need to look at what limits the battery's ability to supply infinite current. The model of internal resistance is useful for this.

simulate this circuit – Schematic created using CircuitLab

The short-circuit current of a battery will depend on its voltage, chemistry, size and internal structure. We can usually simplify this to a simple model of an ideal voltage source and an equivalent series resistance. It should be clear from the model that the voltage at the battery terminals will droop with increasing current.

With the values I've made up for Figure 1 you can calculate that the 9 V battery will limit the current into a dead-short to $$\ I = \frac V R = \frac 9 {20} = 0.45\ \text A \$$. Running the same calculation on the car battery will give us $$\ I = \frac V R = \frac {12} {.025} = 480\ \text A \$$.

To recap: the short circuit current is a function of several variables but is mostly determined by the nominal voltage and internal series resistance.

• You need another set of {} around your 12 in the final formula. I tried to add it myself, but "eDiTs MuSt Be SiX cHaRaCtErS". :/ May 17, 2020 at 19:13
• Fixed, thanks. I think the minimum edit restriction goes away after you get a certain rep. I can make single-character changes to posts. I only realised a few months ago that I could do single character MathJAX fractions like \frac V I without the {} around each term. I must have been carried away. May 17, 2020 at 19:36
• Aah that (kinda) makes sense. Gonna be a long time before I can make those sorts of fixes here, then. May 18, 2020 at 20:27

If the positive and negative terminals are connected by a wire then the battery is by definition shorted.

What the voltage of the battery is does not really matter.

If the current is very high then that means that the battery has a very low internal resistance.

If the current is low then that just means that the battery has high internal resistance.

If you can draw and solve a circuit "properly," the line between a short circuit and a really high amperage line is very blurry.

As others have said, there's no official line, but I find the most useful line is the point where the high-current effects in the circuit exceed the modeling of your circuit.

If you have an ideal voltage source hooked up with a wire across both sides, that is a "short circuit" because that model cannot properly describe what really happens. On the other hand, if you have a battery and a wire with a specified resistance, then that is merely a "high current" circuit. That holds up until the thermal effects of that high current start to change the real wire's resistance. So on and so forth.

Put a wire across your car battery, and we think of it as a short because we really don't have a good sense of the resistance of the wire and the internal resistance of the battery. But quantify that resistance, and add a shield of argon, and you have a MIG welder that relies on those properties!