A reader writes:
Though I didn't follow all of the details, I did enjoy your writing about electrocution and car batteries.
Do you know the odds of getting electrocuted if one is standing in a wet shower with wet skin using a cordless (battery-powered) sander? I don't know what kind of power I'll need to work on residential showers for hours at a time, but the electric chorded sander I WAS using (until I decided that I'm tired of risking electrocution) says it's a 120 Volt, 10 Amp model.
There's probably no danger, but there could be some.
Cordless tools all run from low-voltage DC, although the voltage has risen in recent tools that use one or another flavour of rechargeable lithium battery. Higher voltage is better, from the tool-makers' point of view, because a given power from a higher voltage requires less current. This means thinner wires, less beefy switches, and generally speaking a cheaper, lighter tool with the same power.
Cordless tools are also, in general, significantly less powerful than corded versions. It's normal for corded drills and saws and sanders and such to draw peak power of at least several hundred watts. The ten-amp 120-volt rating of the sander you mention makes it a 1200-watt unit (so I presume you're talking about a belt sander, not an orbital one), though it'll only draw that much when it's working hard. You can expect even big heavy cordless tools to have no more than half the power rating of a similar corded tool.
Discovering exactly what that rating is can be difficult, partly because cordless tools can have a larger range between their "spinning freely doing nothing" and "working so hard it's barely turning at full power" power consumption than corded tools do. Mainly, though, cordless power ratings are harder to find because consumers think more watts are always better. So a cordless tool that costs three times as much as the wall-powered version, yet has a third the power rating, won't sell well, unless the manufacturer conceals that latter number.
I'm telling you all this just to explain my original wishy-washy "possibly dangerous" statement. If you're using a 12V tool then you probably won't be able to do yourself any electrical harm with it, even if you smash the thing on the wall until it breaks and then smack yourself in the chest with the pointy bits.
A 36-volt tool, on the other hand, is edging up toward the kind of voltage that actually can harm you, if only indirectly. (Direct harm: Current through your heart stops it, you die. Indirect harm: Current through some other part of your body causes you to spasm and dig a tool into yourself, fall off a ladder, flop out of the shower recess and smack your head on the toilet, et cetera. This sort of secondary injury following a non-fatal shock is a lot more common than injury or death caused directly by electricity.)
In the real world, even crappy bargain-basement cordless tools have enough plastic between you and the wiring battery terminals that no matter what voltage they run at, you pretty much have to make a specific and deliberate project out of killing yourself with one. Working in a wet environment is still dangerous, but only because it makes it easier to slip and then drill, saw or sand yourself instead of the workpiece.
Brand-name tools are generally safer still, and adding water to the situation may ruin the tool but is unlikely to hurt the user. Even the commonly-recognised-as-lethal "dropping a hair-dryer into your bath" situation is actually not terribly likely to kill you, though I don't recommend you try your luck.
If it's possible to electrocute yourself with cordless-tool gear in any way at all, here is I think your best chance of doing it without specifically running wires from the inside of the tool to nails driven into your chest. There are plenty of battery designs with exposed terminals of one kind or another, so suppose you eject the battery from the tool by accident, and then somehow grab that battery with both, wet, hands, so positive is touching one hand and negative is touching the other.
Even then, the resistance of human skin is way up in the tens of thousands of ohms - I found the resistance between two closely-spaced points on my tongue to be 70,000 ohms. So even with a 36-volt battery it'd be surprising if one whole milliamp managed to flow across your chest, and not all of that would go through your heart. I think you'd be an easy order of magnitude away from enough current through the heart for there to be any risk at all.
(I'm sorry to say that I'm not about to conduct heart-stopping experiments on myself. I have, however, previously zapped my arm for science.)
If both of your hands had bleeding cuts on them then 36 volts might be enough to at least give you a shock you could feel and it might have cardiac consequences, but this is really pushing it. And any sort of work gloves not made of chain-mail would erase the risk completely.
And, of course, back in the real world it continues to be downright difficult to actually touch the positive with one hand and the negative with the other. If you just grabbed both terminals of a 36-volt battery with one wet bleeding skinless lightly-salted hand then it'd sting like a bugger, but once again the only real health risk it'd present would be if the pain startled you enough that you then hurt yourself in some other way.
I won't be surprised if cordless-tool voltages rise further, though. There are already cordless mowers that run from 48-volt packs, for instance. So it's possible that a few years from now there'll be cordless tools running from voltages high enough to pose real electrocution risks.
It'll still be a lot less dangerous than it was in the olden days of corded tools, though, when casings were still commonly made of shiny cast aluminium. Then, the user's life was in the hands of the manufacturers and electricians who're meant to keep earth wires connected, and prevent live wires from touching the tool chassis.
With modern plastic casings and other construction improvements, even a theoretical 96-volt cordless tool is not likely to be an electrocution risk, even if you use it in the rain or, more realistically, get all hot and sweaty while working.
There's a lot of energy in a cordless-tool battery, though, and they definitely can hurt you if that energy is released very quickly because of, say, a short circuit...
...or severe over-charge...
...or physical damage...
The reason why drills and laptops and iPads aren't exploding all over the place is that the naturally excitable personality of lithium-ion technology, in particular, is kept calm by strong casings and protection circuitry ranging from simple fuses to smart current limiting:
If one of your cordless tools manages to puncture the battery of another, though, your life may still become quite exciting.
So I suppose I've allayed your fears of one kind of injury and then given you a new one to worry about.
There's no need to thank me.
And then commenters will, I hope, correct at least the most obvious flaws in my answer.