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Will a person get electrocuted? Is it possible to protect against this? Some rules set or protection devices?

For i.e. we have a power bank of 16 18650 3400mAh 30A batteries. What if it's case will be damaged and people accidentally touch the batteries contacts?

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Parallel connection is a way of connecting batteries that maintains voltage but increases capacity and current providing capability.

18650 is a lithium ion cell that's usually around 3.7 volts although it could be LiFePO4 which is 3.2 volts. If you connect 16 of them parallel, with each having 3400 mAh capacity and 30 A capability, you will have:

  • 3.7 volts (or 3.2 volts if LiFePO4)
  • 54400 mAh capacity
  • 480 A capability

3.7 or 3.2 volts is not a high enough voltage to cause any problems to humans. Consider this: cars use 12 volts usually (although some large trucks may have 24 volts), and even then people work with the system without any special precautions other than avoiding making a short circuit.

Your 3.7 or 3.2 volt system is safe provided that you don't create a short circuit. There's no way 3.7 or 3.2 volts would even hurt, even if your hands are wet from salty sweat. Don't make a short circuit, though, as you wouldn't want a 480 ampere short circuit to happen.

However, if the batteries are connected in series (and you made a mistake by saying they are connected in parallel), then you will have:

  • 59.2 volts (or 51.2 volts if LiFePO4)
  • 3400 mAh capacity
  • 30 A capability

In that case, the voltage is large enough that I wouldn't touch it with sweaty hands. Especially if you touch the positive with a sweaty left hand and the negative with a sweaty right hand, you could have a current going through your heart. It's direct current so not as dangerous as alternating current (AC can disrupt the rhytmh of the heart in ways DC can't), but still -- I wouldn't want that current going through my heart. Also once the current starts flowing, it can cause your muscles to contract, making it impossible for you to remove your hands from the current source. Actually DC can cause far more problematic muscle contractions than AC since the current flow doesn't reverse many times per second.

About 60 volt direct current is still relatively safe but you have to understand that it can be touched only with dry hands. Also short circuiting should be avoided too, as interrupting an about 60 volt arc with 30 amperes going through it may not be easy -- and there's nothing to guarantee the current will stay below 30 amperes. The 30 ampere rating of the batteries means you are not allowed to use larger currents, but a short circuit will do exactly that. So use a 30 ampere fuse and insulate well any conductors that could allow you to bypass that fuse accidentally and create a short circuit.

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  • So, correct me if I wrong. It is safer to have a 12V 300A battery than 60V 60A, right? So, 3.7 volts can't hurt me even if it will be hundreds of amperes, right?
    – Robotex
    Aug 18, 2022 at 12:18
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    It's the voltage that causes current flow through a human. 100 milliamperes (0.1A) kills, but to cause that current, you need high voltage. 60V could maybe cause at least painful shock with sweaty hands, but 12V, never. The 300A / 60A part does not matter at all, because it tells what the battery provides if you have low resistance. Humans never have that low resistance. 3.7V battery is never high enough voltage to cause hundreds of amperes through a human. So 3.7V 300A is the same as 3.7V 0.1A for lethality purposes -- neither is lethal.
    – juhist
    Aug 18, 2022 at 13:45
  • In my opinion this answer could be improved by explicitly mentioning Ohm's law and some ballpark numbers to make it clear what the reasoning is. As you say yourself it's the amount of current which determines damage done, the batteries can deliver ample current, so there's a missing link for why that current cannot flow (basically: skin resistance being way too high). electronics.stackexchange.com/questions/189691/… for example explains it pretty well.
    – stijn
    Aug 21, 2022 at 18:48
  • What is the limit between safe voltage and dangerous?
    – Robotex
    Oct 20, 2022 at 13:39
  • Extra-low voltage (en.wikipedia.org/wiki/Extra-low_voltage) is DC 120 volts or less, or AC 50 volts or less. You can get shock from highest extra-low voltages as well, but they are relatively safe voltages with dry hands, so the shock shouldn't kill you unless you do something stupid such as touching 50 volt AC with left and right hand, both hands wet. But to be completely safe, you should halve the voltage to AC 25 volts or less, or DC 60 volts or less.
    – juhist
    Oct 21, 2022 at 14:31

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