DC and safety.

Discussion in 'Electronics & Electrics' started by Dezza Bot, Jan 1, 2016.

  1. Dezza Bot

    Dezza Bot Member

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    So I'm just toying around right now with a bunch of 18650 Lithium-ion cells that I've liberated from old laptop batteries. Individually they do not pose a significant electrical risk; they can be fully charged and you can touch both ends with you hand and not feel anything.

    However I've currently got about 60 (and growing) of them which I'm in the process of testing discharge rates and capacities since most of them are unlabeled.

    Aside from the obvious danger of overheating and fires with regards to charging and discharging A thought just occurred to me that if I hooked all these fully charged in series I'd get about 240Vdc. Even at a pretty conservative 2A discharge rate that's 480W. At what point do I need to start worrying about personal safety when handling a pack of these cells? Information on AC dangers is pretty easy to find on the net, but I'd be pretty ignorant if I just used that data to make assumptions about DC.

    I take it there's a bit more involved than my knowledge goes, I mean your average car battery can discharge over 300A at 12V and touching both terminals doesn't kill you despite that being more watts than a household power socket.

    I assume there's more than a few people doing DIY solar setups these days too, where I imagine safety would be a significant concern.
     
  2. mtma

    mtma Member

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    Generally 48VDC nominal (about 60VDC max) is considered to pose a very low risk of electrocution. If you puncture the skin or connect your tounge and another orifice it's a different story.

    Whilst up to 120VDC (important: DC voltage) is classified by standards as 'Extra Low Voltage' and is categorised as posing a low risk of electrocution, certainly the discharge energies typically increase therefore other risks such as explosions and burns become significant considerations.

    Also importantly, 24V, 48V, etc. are different potentials and there are design considerations needed to be made, depending on what you are doing. For example if you have an 80VDC system 24VDC rated interconnections are inappropriate.
     
  3. ShadowBurger

    ShadowBurger Member

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    don't batteries have a limit on how many volts can be run through them?
     
  4. heydonms

    heydonms Member

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    One other things to be aware of is that AC will cause your muscles to spasm, if you touch a live AC rail your hand will latch onto it but you will probably let go. DC will cause your muscles to clench, you may not be able to let go until power is removed.

    No. It doesn't really make sense to talk about "volts through something". You have volts across something, or amps through something. Perhaps you are thinking about Equivalent Series Resistance? That limits the current through a battery.
     
  5. @rt

    @rt Member

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    That was one of Electroboom’s practical demonstrations. Welding with car batteries in series.
    https://www.youtube.com/watch?v=ZxBF7WC0TQk
    I think the thing to take out of it is that you can burn red hot or explode
    some metal in your hand with power that you won’t conduct yourself.
    An example one could come across is manual home electroplating!
     
  6. Life_Essence

    Life_Essence Member

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    This ^^ Volts isn't your problem, you might get a very slight singe like an extreme 9V battery licking if you touch it with wet hands or something like that (not that you should be doing that anyway) but the problem comes when it's going through another conductor, wire, spanner, screwdriver whatever. I doubt a laptop battery could do this but I know those reasonable sized RC Car lithium batteries can still put out a butt load of amps, not enough to melt steel like a car battery can but certainly enough to heat it up a fair amount.
     
  7. alch

    alch Member

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    OPs post is stupid. Thats near telephone line voltage.. Meeeeeeemories of my dog chewing the cables under the house. :( .. memories of me having to half ass connect them again and get a bit of a buzz.

    RIP Jesse.
     
  8. OP
    OP
    Dezza Bot

    Dezza Bot Member

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    This was a great real world example on DC and what I'd call 'practical safety' (ie. does this kill you/does this hurt)
     
  9. RobRoySyd

    RobRoySyd Member

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    The bold part is where you need to be careful with batteries. A 18650 Li-Ion cell is typically rated at 2AH. As a rule of thumb the fault current from a battery is taken to be 10x the rated capacity. So the humble 18650 cell could deliver 20A. That's still not enough current to cause a typical piece of hook-up wire to explode in a fireball of molten metal so I'd not see 60 18650 cells in series as posing a serious electrical hazard. Start building series / parallel banks of those cells like the DIY EV crowd do and it does start to get potentially nasty.


    The two challenges DC poses compared to AC are the difficulty of interrupting the current flow and corrosion. When a switch opens electrons flow through the air gap. With AC because the current stops flowing 100 times per second the arc doesn't have much opportunity to grow very long. Also because on average each time the contacts open the current is flowing in the opposite direction the arc doesn't move metal in the same direction i.e. less damage is done to the contacts.

    Corrosion is a potential issue with DC for much the same reason. Any water bridging the positive and negative of a DC potential will cause ions to migrate in a constant direction moving metal from one place to another.
     
  10. Symon

    Symon Castigat ridendo mores

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    That's actually a common fallacy. It is the other way around, organic tissue behaves differently to what you would expect it to. AC shocks tend to give the "can't let go" effect. DC shocks with a large ripple will have a similar effect, but ripple free DC does not.

    You may get a slight tingle off the 48V DC supply on the line, but you'll really know about it when 90V AC gets applied when someone tries to ring the phone.

    Anyways, as has already been said, above 120V DC is considered to present a hazardous shock risk, but there are other things to keep in mind such as protection and insulation ratings.
     
  11. 2xCPU

    2xCPU Member

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    errr ... no the OPs question is not stupid, but your comparison to a telephone line is either uninformed, or possibly 'stupid'.

    A telephone line is inherently current limited, from both what the exchange will supply and the limit the cable resistance imposes. You can't expect more than about 50mA from a phone line which is orders of magnitude lower the what the OP is concerned about.

    2.
     
  12. RussellK

    RussellK Member

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    In practical terms, I'd be inclined to keep to 48V. It's a common DC voltage and there's very little chance of shock, the main concern is energy release from a dead short, which can be addressed by including a fuse near the battery.

    Once you get up towards 100V there is real risk of shock, and you need to take many more precautions, not only against short circuit, but inadvertent contact with live conductors.
     
  13. OP
    OP
    Dezza Bot

    Dezza Bot Member

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    Thanks, this is the sort of pertinent information I need to be aware of. My intention from this project is to gain the practical knowledge required to build a larger pack for a potential EV project and/or an off grid/UPS type of setup. I'd like to gain more experience in the area before I build a pack that is capable of turning me into a small pile of dust, and then putting that pack into a 1 tonne moving box of steel which I'm strapped into.
     
  14. RobRoySyd

    RobRoySyd Member

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    Obviously you'll find a heap of info on the net. All I can add is as you'll at some point be working with live DC is to develop a lot of respect for it. Clear mind and clean work area. Don't allow anyone or anything to distract you.

    As for the risk of having to deal with a lithium fire; if you've got the budget invest in a Class D extinguisher, any other extinguisher at best will be useless and at worst make things even more spectacular. There's a Class D extinguisher specifically for lithium fires. I dread to think what these things cost though. Got to play with every type of extinguisher but them too expensive! Failing that a bucket of dry sand is pretty good. Make certain everyone in the dwelling knows you've got bulk lithium batteries in the place and if there is a fire to tell the fire fighters when they arrive. Also check insurance policy and government regulations.
     
  15. aXis

    aXis Member

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    Best post so far.

    One of the most dangerous things with batteries is not their voltage, but their ability to dump massive amps into short circuits. It's very easy to be complacent when dealing with low voltage DC and accidentally causing a short is pretty common. This can easily cause injury or a fire.

    Safety rules:
    - Keep voltages below 48VDC
    - Put DC rated fuses/breakers as close to the battery terminal as practical.
    - Fuse ratings should take into account both your expected load and the cable's capacity.
     
    Last edited: Jan 4, 2016
  16. Symon

    Symon Castigat ridendo mores

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    Also insulate your tools and any terminals, prevention is just as important.
     
  17. mtma

    mtma Member

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    /scans DIY solar thread one forum over for pics...

    Yeah I don't know why people don't bother with this.
     
  18. RobRoySyd

    RobRoySyd Member

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    The DIY solar people and the DIY EV people are taking different approaches.
    A typical DIY EV battery is hundreds of 18650 cells wired in parallel by soldering directly to the cells, then those batteries are wired in series.

    It's impossible to insulate a soldering iron. What'd worry me the most is soldering directly to the cells, too much heat and they can internally short. Tesla spot weld to the cells but doing that's beyond DIY.
     
  19. SLATYE

    SLATYE SLATYE, not SLAYTE

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    You can use a gas iron. No connection to any electricity, and they get very hot so you can solder the tabs reasonably fast (unlike, say, a 15W iron designed for PCB work).
     
  20. 2xCPU

    2xCPU Member

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    I've slipped a length of silicon rubber tube over an iron tip to prevent damage working in really tight locations. A larger diam. over the barrel could be used to insulate a temp controlled iron.
    Not really. There are plenty of DIY plans for DIY spot welders if you search for cordless drill battery repacking.
    Type 1
    Type 2
    (these are just the first two that came up in my search).

    2.
     

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