# Why are capacitors more dangerous than batteries?

It seems decent size capacitors can electrocute you when it's charged. Somewhere I read capacitors don't have internal resistance therefore, current is NOT limited unlike the batteries with internal resistance. But two plated of the capacitors have infinite impedance, so, is that conclusion correct? So, why are capacitors more dangerous compared to batteries

• The amount of current needed to kill you is tiny, internal resistance isn't going to save you. Enough AA cells in series will do the job if applied in the right place. Dec 7, 2023 at 20:55
• I guess is it better to say that you can build a lot small capacitor to kill a person than a battery?
– nmr
Dec 7, 2023 at 21:02
• I think the question must be preceded by a different question: "Are capacitors more dangerous than batteries?" And that question would still need to be qualified. Dec 7, 2023 at 21:58
• One factor is that before you carelessly pick it up, seen from a distance, you'll probably recognise whether a battery is a low voltage one. With a capacitor, whether it's charged or not is impossible to tell, and it might be difficult to intuit its voltage rating. A common wheeze in our lab was to solder a 100 nF 400 V capacitor inside a tobacco tin, the type with the rubber seal all round the lid that insulates the top from the bottom. Charge it with a brief contact to mains, and leave on a desk. Oh what suspense listening for the yelp! Dec 13, 2023 at 13:50

Capacitors are not necessarily more dangerous than batteries. It all depends on the specific capacitor and battery, and the voltages present on each.

Capacitors have internal resistance called ESR, equivalent series resistance. It can be in the low milliohm range, or much higher.

Batteries also have internal resistance. Not enough to save you from shock, or the battery wouldn't be very useful to provide energy to a load.

Considering that a few tens of milliamps can cause a significant shock, the voltage on the capacitor or battery and the impedance of the person and their connection to the terminals are typically much more relevant than the internal resistance of the cap or battery.

Just because a capacitor is made of 2 plates with an insulator between them does not mean that there's an infinite impedance to current flow out of a charged capacitor.

https://blog.cambridgecoaching.com/capacitor-confusion-basic-pointers-to-salvage-your-sanity

• given the same size can a capacitor be more dangerous than a battery
– nmr
Dec 7, 2023 at 21:03
• @nmr Probably. Batteries would need a lot of series cells to reach a dangerous potential, while you can get a fairly small high-voltage capacitor. But now I'm worried about why you're asking this. Dec 7, 2023 at 21:08
• @nmr It is easy to have a small capacitor, smaller than a sugar cube, charged to, say, 300 V. You will not be happy if you short that with a fingertip. Dec 7, 2023 at 21:08

Capacitors have much higher power density than batteries, because the latter have comparatively higher internal resistance. If that wasn't so, chances are capacitors would be obsolete in many scenarios.

Batteries on the other hand have much greater energy density.

To kill a person you need quite a bit of instantaneous power but not for long, hence not so much total energy.

Therefore, given a certain size, a capacitor with suitable ratings can be more lethal than a battery of the same size.

• How can you say that capacitors have more power density? Is there a way I can think about the power density of a capacitor vs a battery?
– nmr
Dec 7, 2023 at 23:33
• @nmr I dont get your question, sorry. Are you asking what power density is? It's the maximum outputable power of a thing normalized by its size Dec 8, 2023 at 5:41
• sure... makes sense. I was just looking for a power density equation for capacitor and a battery. Now for parallel plate capacitor it's easy to derive one but probably not for a battery.
– nmr
Dec 8, 2023 at 15:28
• @nmr It's (maximum current) * (voltage at max current) / (volume), for both the capacitor and the battery. Dec 8, 2023 at 16:58

I think that the "capacitors are dangerous" fear largely comes from vacuum tube days, where most circuits used 200 - 300 volts, and capacitors could hold that voltage for some time after the equipment was switched off.

We do still have that concern in AC power supplies, but outside the power supply, most things now operate on much lower voltages like 5 or 3.3 volts, where any voltage remaining on capacitors is not a significant concern.

• Although very large capacitors that aren't properly stored can be dangerous--there's a reason the biggest film capacitors ship with a shorting bar across their terminals. Dec 8, 2023 at 17:00