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When cleaning a standard 12 Vdc car battery's terminals, it's a common practice to pour a mixture of baking soda + hot water all over the terminals. The water can spread and effectively short out the battery in two ways:

  • Positive terminal to negative terminal
  • Positive terminal to car chassis (to negative terminal)

Why does this not cause major problems (ex: explosions)?


Background

Quora answer: What happens if a car battery short circuits?

YouTube video: Shorting Out a 12V Car Battery

Indicates that shorting a car's battery can be quite detrimental.

YouTube video: How to clean car battery terminals corrosion Cheap and EASY with baking soda Car maintenance

You can see the water connects the battery's positive terminal to the negative terminal. In my mind, this should create a short circuit, as the water is effectively shorting the positive and negative terminals. However, nothing bad seems to happen. Why is this?


Research

Reddit post: ELI5: Why is it safe to pour water all over a car battery?

Says water isn't all that conductive, but I am not sure if I agree with this vague statement.

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    \$\begingroup\$ "Says water isn't all that conductive, but I am not sure if I agree with this vague statement" For all it vagueness, the statement is more or less right if you talk about 12V. \$\endgroup\$
    – Oldfart
    Commented Apr 19, 2020 at 19:00
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    \$\begingroup\$ Hi! Nice to have you here from your intrastellar exploration! General note: Youtube videos are definitely not a reliable source, and if I had a nickel for every time someone came on here and said "in this youtube video they saidy X, so why X?" and then everyone here was "oooh, X? that's totally wrong!", I would have enough nickels for a while! \$\endgroup\$ Commented Apr 19, 2020 at 19:15
  • \$\begingroup\$ @Oldfart you can certainly get into to the single-digit ohm region with 20cm of water bridge saturated with baking soda. \$\endgroup\$ Commented Apr 19, 2020 at 19:17
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    \$\begingroup\$ @MarcusMüller I was talking of normal water. I did not wanted to digress but was thinking to add that a film of conducting water (e.g. very salty) was likely to evaporate quickly with 20A going through it. \$\endgroup\$
    – Oldfart
    Commented Apr 19, 2020 at 19:22
  • \$\begingroup\$ yeah, at the point where the current becomes problematic for the battery or bystanders, the water will be gone. \$\endgroup\$ Commented Apr 19, 2020 at 19:29

2 Answers 2

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Definition of short circuit – according to IEC – is accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal to or close to zero.

You can short the battery with a wrench or copper wire, but not with water. Tap water is so poor conductor (about couple of tens of millisiemens/meter) that it does not cause a short circuit, only a little electrolysis bubbling (or corrosion if the terminals are wet a long time).

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  • \$\begingroup\$ My question in the comment above was: what should we call an unintended current flow in a circuit? Based on the definition of a short that you cited above, it is unclear to me how to describe the current that flows through the water, other than perhaps, leakage current. What is the problem with describing all unintended current paths as shorts? \$\endgroup\$
    – Qubit1028
    Commented Apr 19, 2020 at 20:32
  • \$\begingroup\$ @Qubit1028 The problem is, that they have different definitions. Short circuit is defined "accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal to or close to zero" Leakage current is defined "electric current in an unwanted conductive path other than a short circuit". So, because the current is so low that the potential between terminals is not zero, this is a leakage current. Always check the IEC definitions first :). \$\endgroup\$ Commented Apr 19, 2020 at 20:38
  • \$\begingroup\$ For clarification purposes, assume that a very thin copper wire is connected between the battery terminals that has a sufficiently high resistance to only drop the voltage by about 10%. Now, this does not meet the strict definition of a short, but it seems reasonable to actually call it a short, at least to me it does. The 10% reduction in voltage due to the unintended current through the copper wire could cause the designed circuit to behave improperly. We need a way to describe a situation like this. If we don't call it a short, what do we call it? \$\endgroup\$
    – Qubit1028
    Commented Apr 19, 2020 at 20:50
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    \$\begingroup\$ ... and I think the "short" term means the current is taking an easy "short-cut" home and deviating from the approved path. \$\endgroup\$
    – Transistor
    Commented Apr 19, 2020 at 20:56
  • \$\begingroup\$ @Qubit1028 Connecting battery terminals with very thin copper wire to form a short that droops the battery voltage by 10% is impossible. As the car battery is a 12V source with low enough internal resistance to give out say 300 amps, any thin copper wire would vaporize in microseconds. It would require several amps, if not tens of amps to bring down the voltage by 10%, so it would not be a thin wire, and loads of only tens of amps are common loads for the battery so it would not be considered a short but normal loading. \$\endgroup\$
    – Justme
    Commented Apr 19, 2020 at 21:57
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Shorting means connecting with something that conducts electricity well.

Water does not conduct much. So it does not short it.

(At least in the context of it being a 12V battery)

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    \$\begingroup\$ It does short in the technical sense of the word. Current will flow via electrolysis of the water/cleaning solution. The combination of two factors keeps the short current very low: 1) low conductivity, as you pointed out and 2) low electrode surface area. \$\endgroup\$
    – Qubit1028
    Commented Apr 19, 2020 at 19:13
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    \$\begingroup\$ Shorts as much as a 1 megaohm resistor. I would not call it a short, even if in the technical sense it connects the terminals. But plastic, dirt and air connect the terminals too, so technically they would be shorting the terminals too then? \$\endgroup\$
    – Justme
    Commented Apr 19, 2020 at 19:20
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    \$\begingroup\$ @Qubit1028 No, it is not a short in the technical sense of the word. A short circuit is a very low resistance connection. \$\endgroup\$ Commented Apr 19, 2020 at 19:30
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    \$\begingroup\$ Let's stop arguing and see the IEC definitions: * Short circuit = "accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal to or close to zero" * Leakage current = "electric current in an unwanted conductive path other than a short circuit". \$\endgroup\$ Commented Apr 19, 2020 at 20:41
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    \$\begingroup\$ Kiitos Vesa. My thoughts exactly. Water, or water saturated with baking soda, does not short cirtcuit. \$\endgroup\$
    – Justme
    Commented Apr 19, 2020 at 20:50

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