Dangerous voltage

[tomv]

I'm trying to figure exactly what electric shock dangers are in ebike battery packs. Sofar I've found

- Danger to humans is from amps not volts. 50mA and up are dangerous. 1mA is when you start feeling tingle
- Lots of sites put 50V-60V DC as dangerous voltage. Somehwere I've found that legally 42V and up are considered lethal voltages.
- Skin resistance varies greatly. I've been poking my fingers with multimeter and pretty much consistently get 1 megaOhm which would require lightning to do any harm. I suspect multimeter is not showing the whole picture here.
- Sweaty moist skin could be as low as 1ohm resistance.
- I've read somewhere of a guy who electrocuted himself with 9V when hi poked electrodes through skin.

This is all bits and pices from the internet, rules of thumb and I've found no thorough discussion. Did you ever got shaken by your pack?

Let's consider imagine this situation: I'm riding my bike hard and sweating. Small rain starts and shorts the wires. All of a sudden I get shocks while bike is going at high speed. Can this happen?

 

[EMF]

This is all bits and pices from the internet, rules of thumb and I've found no thorough discussion. Did you ever got shaken by your pack?

Let's consider imagine this situation: I'm riding my bike hard and sweating. Small rain starts and shorts the wires. All of a sudden I get shocks while bike is going at high speed. Can this happen?

Nope. First thing, if your bike shorts due to moisture and the bike has enough mass, it will blow a fuse and all you will feel is the decelleration as your bike coasts to a halt. But then, lets say for some reason the frame was still "hot" and you set your wet foot on the ground, when the bike stopped, you might get a shock.

The resisitance factor is important in your tests, BUT if you put your body in series with a battery and a good ground, this is when you bet shocked. If your bike frame was electrified somehow, but lets say you had a rubber tip on your kickstand, your bike would be isolated from the ground. With a good pair of rubber soled shoes, you could touch it an probably not feel a thing. If you went up to it dripping wet in your bare feet, you would get a jolt. Or, grab your bike with one hand and then a metal post, like a fence or something with the other and you will get shocked. It would probably kill you as it would take the shortest path of resistance which would be across the old ticker.

When working with electricity, I always avoid this type situation.

The electricity will always take the path of least resistance. It also has no friends.

 

[TylerDurden]

Let's consider imagine this situation: I'm riding my bike hard and sweating. Small rain starts and shorts the wires. All of a sudden I get shocks while bike is going at high speed. Can this happen?

Not likely while riding... more conductive stuff would short the pack first.

More likely when messing with the pack after riding or if you stall and start to fiddle with wires. Dry off, wear gloves and sneakers in a dry spot.

8)

 

[Ypedal]

Heh heh.. yeeeah...

I've had a few " experiences " so far..

as TD said above, i went for a rip on the e-BMX, with 48v LiFe in the backpack. Got home slightly sweaty, so i get the bike back in the house, take the backpack off. take the coat and toque and gloves and boots and all that stuff off....

Open the backpack and grab the packs from the top..

Now.. these LiFe prismatic cells are shrink wrapped, but the bottom of the cells are exposed, and yes.. the - and the metal case are common !!!!!!!!!!...

You ever get a shock .. and jump back.. then wonder if you just imagined it ?

Well.. dume me picks it up again.. and Bzzzzzz... tingly pain feeling all down my arms.. i put the pack down quickly.. like.. vERY quickly lol..

so 48v will give you a jolt !

 

[Dr. Shock]

I've been shocked enough not to worry too much about it. That said, it doesn't take much amperage to kill you if it does the wrong way. As DrunkSkunk notes, as long as it doesn't go through your heart, you'll probably live. Try not to let both hands touch anything conductive at the same time. Wearing surgical gloves is probably a good idea when fooling around with battery packs over 36v. I know I don't get shocked from 36v, if my fingers are dry. I've been mildly shocked by 24v when my skin has been wet.

 

[tomv]

Interesting stories, keep them comming! Real life examples give better idea of what to expect/what to avoid then abstract info on the net I've found sofar.

I've built 36V - 48V packs and must have touched wires with full potential difference many times with bare hands, but never noticed. The only time I've "felt" electricity is sticking 5V to my tongue. Nice and pleasant tingle But I'm not trying that with 48V wires.

It seems wet skin/dry skin is a deciding factor here.

Another thought is that 12V automotive batteries are sold with bare terminals, so it must be that nobody ever got killed/seriously shocked by 12V. So 12V - safe. 120V - fatal. Everything in between

 

[mcstar]

We were taught the 50V rule of thumb too, but I think it would take an extreme of circumstances for that to do you in (think open heart surgury with 50V probes touching opposite sides of the heart. SOME people have survived lightining strikes to the head, and others have sucumbed to its affects. The idea of the stepping off a shorted bike to make contact with ground seems highly unlikely to me. From what I understand, "rubber" tires are very poor insulators due to large amounts of carbon and other impurities used in their construction. Most likely your shoes and handle bar grips are better insulators which means you won't be the best path to ground. I may try measuring my bike frame's resistance to ground just see for myself. Personally, I've been bitten pretty hard by 120V a few times and once, it went from one hand to the other. It knocked me right out! Of course this was when I was small. I've put fingers across my 50V pack a few times and didn't feel a thing with dry hands. I haven't had the nads to do it wet yet

 

[tomv]

Heh. Open heart is a big sidenote actually in many of the pages talking about electric shock It takes microamps to stop heart if it goes straight to the right place. Static electricity spark from a finger is plenty. This is a big concern for people with external pacemakers apparently.

 

[Link]

I've only been shocked once. I was putting the last two wires on a small 48V pack. I reached behind it to grab my wire strippers and accidentally touched my arm to the electrodes. Felt kinda like being poked with something sharp. I kinda thought that was what happened, but I touched them again to make sure 8).

Danger to humans is from amps not volts. 50mA and up are dangerous. 1mA is when you start feeling tingle

I hear it's like 8mA according to MythBusters. I think 50V is the more or less "official" line between low and high voltage.

- Skin resistance varies greatly. I've been poking my fingers with multimeter and pretty much consistently get 1 megaOhm which would require lightning to do any harm. I suspect multimeter is not showing the whole picture here.
- Sweaty moist skin could be as low as 1ohm resistance.

No, that's about right. I can get between about 2MΩ and 150kΩ depending on how dry/wet my skin is and how hard I squeeze. I once tried a dab of electrical grease (I was bored, okay?) and was able to get like 100kΩ, so I'm pretty sure it would be nearly impossible to get a path that goes through your heart to only have 1Ω of resistance. Even patches of skin on the same hand when wet have more resistance than that.

-I've read somewhere of a guy who electrocuted himself with 9V when hi poked electrodes through skin.

I'll believe that if he was 80, had a pacemaker, and the electrodes were steak knives in his chest.

Let's consider imagine this situation: I'm riding my bike hard and sweating. Small rain starts and shorts the wires. All of a sudden I get shocks while bike is going at high speed. Can this happen?

It would be very hard. Like a few others have said, the fuse/breaker should if it's a really bad short and cut the battery out of the circuit alltogether. And you'd still have to make a path through your body from the positive terminal to the negative terminal. Given that the whole bike is in electrical contact with the rest of it, there would be no way to touch just the frame and get shocked.* You would have to be touching the frame as one electrode and something else that happened to be touching the other side of the battery. In other words: Unless you're trying really really hard, I'd bet my ebike that you couldn't get shocked in that situation.

*I take no responsibility for anyone who happens to have a frame that is two pieces electrically.

 

[fechter]

100k would be a good average for typical skin. If you jab two needles through the skin, around 10k is typical (I've measured it). Really sweaty skin might be close to 10k. Your palms and fingertips have a higher resistance than thinner parts of the skin, like your arms.

8ma can kill you if you have a weak heart. Most people can survive 6ma, but you will definitely feel it. So, worst case, 10k resistance, 8ma, gives you 80v.

I've touched a 90vdc circuit before, and it definitely got my attention.

You'd have to try pretty hard to electrocute yourself with 48v or below. As pointed out above, your body has to complete a circuit from the battery positive to negative. Normally the frame of an EV is isolated from both battery terminals. This is a good practice for many reasons.

Getting burned from a piece of metal (watch band, ring, wrench, etc.) or starting a fire is much more of a danger, even at 12v. It's always best to disconnect the batteries when working on the wiring.

 

[Drunkskunk]

you are going to get as many oppinions as there are people on here.

None of us are correct. Not even what I'm about to tell you. But there's little bits of truth in all of it.

A while back, I would grab the bare ends of my 48V battery to put the meter on it before and after a ride. every once in a while, i might feel something, but it wasn't enough to make me stop. Since going to 72V Lico, there's more danger from the batteries exploding if I short them, so I don't test like that.. often. when I have, I wasn't shocked. But all thiongs considered, its not something i would recomend anyone doing. But thats fairly low voltage. Imagine the average Taser, or electric cattle fence. High voltage, but non lethal. Taser Tag is fun! Ever rub your feet on a carpet and shock your wife/girlfriend/parent/kid? Have any idea how many volts you dumped through them? It would have been a few thousand times more than the 42volt so called safe limit.

and yet 1mv at 1ma can be lethal. Less than that probably is too.

The trick is to treat all electricity with respect, and know that any of it can kill you if it gets out of hand, but its going to be pretty hard to get hurt from a battery powered bike.

 

[PJD]

I have routinely tested the condition of ordinary 9 volt batteries by touching them to my tongue all the time. A strong burning-tingling means the battery is fresh.

Touching my 48 or 60 volt pack terminals with my fingertips, even damp ones, has never produced anything I can feel. But touching a terminal with the underside of my forearm or other thinner skinned part of my arm definitely produces discomfort of the forearm. I know, closing a circuit with each arm is not the best way to test-shock yourself because the circuit passes through the heart.

I need a fast way to sort out the weak batteries from eight lead acid cells I've got. So, I'm planning to connect them in series and hook them to a toaster for a load, and look for the bad batteries with a voltmeter. That's about 106 volts when freshly charged so i need to be careful. BUT, like others wrote, you need to complete a circuit between the terminals so I will have to be terribly careless. Touching a copper rod driven in the ground, plumbing, or anything else, does nothing.

This is not true with AC of course, where the worst thing you can do is contact the hot 120 volt conductor while standing barefoot on a damp garage floor. I'm sure a lot of people get killed that way. Must be more deadly in Europe.

This is an electrical newbie question, I have always been confused at why AC produces a current to earth ground but DC from a battery (or presumably from a rectifier too) doesn't. It sure works with lightning, and with the electric charge in carpet or cat on dry cold winter day, though. Can someone explain this?

 

[fechter]

AC electrical distribution systems have the neutral tied to ground at some point, usually the point where it enters the building. This makes the ground connnected to one of the conductors.

I'm not really sure why they don't completely isolate it, but it's always been this way.

 

[Dr. Shock]

Tying the AC neutral to the ground means that all of the voltage changes have to happen in the hot wire. I have tried to imagine what is actually taking place in the wires. The best I can come up with is that at the beginning of an AC cycle, electrons rush out from the generator through the wire until they reach an electrical "pressure" of around 120 volts in relation to the ground, then the electrons are sucked back through the generator and pushed into the ground until the hot wire has an electrical "vacuum" of 120 volts in relation to the ground. This happens at 60 cycles per second in the Western Hemisphere. An opposite cycle happens on another hot wire paired with the same ground, so that the moment of highest pressure on one hot is the moment of highest vacuum on the other, giving a voltage of 240 volts between the two hots, and 120 between either hot and the ground. This is household current, of course, not 3 phase AC used in industry.

As Fechter notes, the ground/neutral is usually tied to your cold water main pipe. It is always prudent to shut off your house's main breaker when you do plumbing repairs on the main line. I had to repair a hole in the main once, and cut through the pipe to put in a patch. As soon as the pipe was cut I got a good shock. I had just cut the house's ground and all the electrons were looking for somewhere else to go. A fair number of them found my skin, which was wet from the residual water in the pipe.

 

[tomv]

As soon as the pipe was cut I got a good shock. I had just cut the house's ground and all the electrons were looking for somewhere else to go. A fair number of them found my skin, which was wet from the residual water in the pipe.

This is actually very relevant to ebikes I've been pondering how to do battery power switch with FETs. Easy solution is to put N-FEt on ground wire. But what happens when the switch is open? Everything that's supposed to be ground becomes hot, right? This is what happened with the water pipe.

I have a similar "incident" when trying to see what charger does. I have four components
- USB datalogger with analog in and ground wires. Let's call them ADC0 and PC_GND
- SLA battery pack with B_PLUS, B_MINUS
- Current measurment shunt of 0.05ohm, SHUNT_PLUS, SHUNT_MINUS
- Charger - C_PLUS, C_MINUS.

So i hookup everything
C_MINUS, SHUNT_MINUS, PC_GND - one wire
C_PLUS, B_PLUS
SHUNT_MINUS, ADC0, B_MINUS

I turn on the charger, and poof, all lights blinking on my data logger. I pull it apart and jab in my multimeter and see that there's 40V between C_MINUS and PC_GND!! I'm just happy that i didn't see smoke coming out of my PC

So looks like charger is switching on ground wire, the easy way to do it. But I'm undecided if this appropriate for BMS power switch...

 

[PJD]

But what happens when the switch is open? Everything that's supposed to be ground becomes hot, right?

Thais what Iam tryng to understand. I believe the only ground a positive termanal of a battery pack "recognizes" is the negative terminal of the same battery pack. There is no potential betwen the positive termanal and anything else.

I'd like to be corrected if I'm wrong about this.

 

[Drunkskunk]

But what happens when the switch is open? Everything that's supposed to be ground becomes hot, right?

Thais what Iam tryng to understand. I believe the only ground a positive termanal of a battery pack "recognizes" is the negative terminal of the same battery pack. There is no potential betwen the positive termanal and anything else.

I'd like to be corrected if I'm wrong about this.

You're exactly right. A battery will only release an electron if it is recieving one on the oppisite side.

 

[Dr. Shock]

"This is an electrical newbie question, I have always been confused at why AC produces a current to earth ground but DC from a battery (or presumably from a rectifier too) doesn't. It sure works with lightning, and with the electric charge in carpet or cat on dry cold winter day, though. Can someone explain this?"

AC doesn't necessarily produce a current to earth ground, i.e. the actual planet. House wiring is tied to the physical earth, I believe, as a way of minimizing lightning strikes. If the house has the same electric potential as the ground, there is no greater chance the lightning will be attracted to it than to any other point on the planet's surface. You could also tie a DC ground to earth, and you can have an isolated AC circuit.

 

[Link]

House wiring is tied to the physical earth, I believe, as a way of minimizing lightning strikes.

Actually, it was originally done as a way to prevent static. When the AC line wasn't using the earth as a ground massive static charges would build up in the lines and occasionally cause 10 inch sparks to fly from the outlets to your fingers. It wasn't fatal, but it hurt like hell. Tying the line to earth eliminated this, though it introduced electrocution hazards. At first, you'd have to touch both sides of the line to get shocked. Now, you only had to touch the hot side (or have a short in whatever appliance you were working with) of the line and be grounded in any number of ways. They established some codes and made one side of the socket bigger than the other. Then there was something about adding a ground for safety and a little on ground fault interrupters.

I found an interesting page about why electrical outlets (at least in North America) are shaped like they are. Of course, I can't find it now. :?

 

[fechter]

Actually, it was originally done as a way to prevent static. When the AC line wasn't using the earth as a ground massive static charges would build up in the lines and occasionally cause 10 inch sparks to fly from the outlets to your fingers. It wasn't fatal, but it hurt like hell.

Cool I guess that would be free electricity.

 

[Dr. Shock]

Yes, it sounds like Tesla's idea of wireless electricity. No cords, just turn the appliance on and it picks up current from the ether.