The electricity supply in your house is measured with respect to ground, isn’t it? So the potential difference (RMS) between the live and the ground is 240V.

But if you have a high voltage battery, say 300V like the one in my car, the voltage is the PD between the two terminals. So what’s the PD between one of the terminals and the earth I am standing on?

That’ll depend on the resistance between the car and the ground it’s standing on.

AC/DC

Thunderstruck!

Got a volt meter?

Dunno, but has anybody ever put a 9v battery on their tongue?

bsims

Member
Dunno, but has anybody ever put a 9v battery on their tongue?

Pffft how about a car battery to your testicles

Also molgrips what sort of car has a 300V battery? I’m assuming it’s fully electric but I’d have thought that’s still multiple batteries arranged to give a total pd of 300 not a single battery?

It should just be across the positive negative terminals which, effectively should be the chassis to positive though that shouldn’t work.

The mains in your home is only relative to ground, because the neutral is connected to ground near/at the final step down transformer.
It’s also worth mentioning the actual peak voltage is nearer 340v (rms is the peak voltage divided by the square root of two).

With regards a battery, the potential difference between the battery and ground can’t really be measured, as they’re not part of the same circuit. If there is any measurable voltage, it’s more likely to be due to some kind of static build up.

i would imagine that one of the terminals would be approximately at ‘ground’ just to simplify the switching circuitry, so I assume you would see 300V to ground on one of them.  I can’t imagine that it would just be allowed to float.  In the bad old days you used to have these conductive strips you would hang from the back of your car to prevent static build up but as these are no longer around I would assume that tyres are slightly conductive

bsims

Member
Dunno, but has anybody ever put a 9v battery on their tongue?

Pffft how about a car battery to your testicles

The posters underneath seem to think it would be fine, but I’m not trying it the 9v was fizzier than sherbert.

Ok this might take me a while. And I might occasionally be wrong but her goes;
As someone said since ground is strictly not part of the circuit, the voltage at either of the battery terminals to ground is undefined or unknown/don’t care.
Imagine the battery floating. Literally, but also technically as ‘floating’ would be the term for a voltage that is not in circuit with ground in any way. Imagine it floating in a vacuum. We know there’s 300v between the two terminals, but nowt about either of those to ground. You are standing on the ground, so your at 0v rel to ground. You touch the -ve terminal of the battery. Suddenly that terminal of the battery is connected to ground by you, you are about 10k ohms. So that terminal is quite soon at 0v to ground. The other terminal is at 300v to ground. Now let go, and grab the +ve terminal. The terminal you’re not holding is at -300v to ground. Let go again.
When you’re not holding the battery it could be at any voltage it likes relative to ground, which could be millions of bolts due to static charging, but you’ll quickly dissipate that’s charge to ground without noticing much.
Grab hold of one terminal in each hand, and you will feel what 300v dc is like, which is death. (You might have got away with 300v Rms of ac, but def not dc!)

Dunno, but has anybody ever put a 9v battery on their tongue?

It tastes hurty

Now back in the real world, I suspect one of the battery terminals is connected to the chassis, which is grounded via the tyres, so it’s at 0v.

And finally a word on terminology; 300v or higher is a typical battery voltage for an electric car. It will be composed of multiple cells of lithium. Each cell will be about 3v. A ordinary 12v car battery is composed of multiple lead-acid cells, which I think are about 1.2v? An alkaline AA battery in yer remote control is not a battery, it is a cell. But even I call them batteries cos I’m not that much of a …

@leffeboy some vehicles do still build up static, it’s just that manufacturers have learnt how to minimise it, so it’s not as blatantly obvious.

My Custom gives a very slight shock when I get it out it, but it’s no where like some old vehicles that would give a very noticeable shock.

edit: tx @mc.  I’m a bit surprised by that really but thanks for the info.  My  Googling suggests that car tyres do now contain some conductive material but that info. holds the same weight as everything else on the internet 🙂

I think it is something to do with the vulcanising process that makes the tyre rubber a bit conductive.  As said previously, the 0V bit of the car (chassis) will be loosely connected to earth via the tyres making the 300V bit approx 300V from earth as well.  That will give you a nasty belt as there is a hell of a lot of current (amps) available in the battery.  Plenty of voltage to punch through your resistive skin and reach the conductive salty bits underneath.  It then takes just a few 10’s of milli amps current flowing across a healthy heart to kill you.

Assuming one terminal of the battery is tied to the cars chassis.

I ride with some Merc technicians and they dread it when a hybrid battery doesn’t “safe” itself as they have to get a specialist in with special insulated tools, rubber matts and an insulated suit to manually “safe” the battery before the car can be worked on.

The fact tyres contain a fair bit of metal (in the tread reinforcing belt, and around the bead – sidewalls are typically just Kevlar on modern tyres), and the rubber compound itself contains a fair amount of carbon, it’s makes sense tyres are not perfect insulators.

@goldfish24 ‘s explanation is pretty good. One of the key things to understand, is potential difference between points in a circuit. It’s potential difference that makes things happen, not voltage.
It’s like you can quite happily go and sit/hang on a high voltage transmission cable, provided you can get on to it without touching anything else. The voltage is irrelevant, until you complete the circuit, at which point the potential difference is what will cook/kill you.

It’s potential difference that makes things happen, not voltage.

<ultrapedantmode> A Volt IS the unit of measurement for potential difference. <\ultrapedantmode>

But you’re absolutely right other than that.

I should of added ‘alone’ to the end of that sentence.

In my defence, I’m just back home from France and only got a couple hours sleep last night!

as I was taught as an apprentice, its the volts that jolts and the amps that cramps.😁

At some power stations you used to be able to hold the output leads from the turbine alternators at approx 18kV but you were on a well insulated stand so no path for electricity to flow.  It was usually a photo op for politicians and the like with presentations etc.

You can have a high voltage shock, think static electricity or Insulation Resistance tester (don’t do it, it hurts) which will tingle but as there is little current being carried its not harmful.  However if that high voltage has a lot of current available (think railways overhead line) you will fry.

Similarly you can hold lots of current flowing through bare copper but if the voltage is low then you’ll never get a belt as your skin resistance saves you.  Think standard 12V car battery, currents in excess of 1000 amps are able to flow if its dead shorted with a spanner but I’ve not heard of anyone dying from a 12V car battery electric shock.

That’ll depend on the resistance between the car and the ground it’s standing on.

The current that would flow depends on that, but the potential difference does not.

Having thought about it a bit more – a flat battery is neutral and hence 0V to ground. If you charge it up you are moving electrons away from the anode and towards the cathode. This must mean that a the cathode of a 1.2V cell would be -0.6V with respect to ground, and the anode +0.6V, I think.

The battery in this case is from a Prius MkII and is NiMH. It’s currently on my bench in the garage, safe, and it has one bad cell that I have identified. I am still alive.

The negative side of the battery is not connected to the chassis of the car. This would make it dangerous, I reckon, because any fault of the +ve wiring to the chassis would complete the circuit and cause a big boom. Also if you happened to touch the +ve whilst working on it and touching the car, you’d be in trouble.

There are lots of youtube videos about removing the battery, and it’s designed to be made safe as you handle it. But as you open it up to check the cells, there’s a point where the bus bar is still connected where touching terminals at opposite ends of it would kill you. Better order some insulated gloves.

Most stuff has been covered but worth mentioning that static buildup on the car will be tens or hundreds of thousands of volts. As a rough estimate it’s 3000V per mm to get a spark in air.

So if there is any static buildup at all it will dwarf the battery voltage.

Also if you happened to touch the +ve whilst working on it and touching the car, you’d be in trouble.

Haven’t the time to respond in full but please be aware that if you touch any terminal of this battery you are liable to be in serious trouble.

The difference with the voltage due to static charge is there’s not enough charge behind it to cause a deadly current to flow. If you find yourself in any sort of circuit with the battery there is the potential for thousands of amps to flow and that is deadly. Be careful with big batteries please!

Can you setup a live internet feed so that we can watch please?

I suspect one of the battery terminals is connected to the chassis

That won’t be the case in the OP’s vehicle. As I understand it, the standards are such that in a system over 60V, both terminals must be sufficiently insulated from ground (hence a lot of recent research into 48V hybrid systems, which is the highest nominal voltage you can get away with without the extra weight/cost of the insulation).

Edit: what molgrips said.

Haven’t the time to respond in full but please be aware that if you touch any terminal of this battery you are liable to be in serious trouble.

Well yes and no, as I’ve mentioned above.

The question is about what would happen (not that I want to try it) if one terminal of battery was grounded. How much current would actually flow?

How much current would actually flow?

Arguably none should, to prove the point take any* battery you wish and connect the +ve to a ground via a multimeter. You should get a 0 reading. Don’t think I’d want to try it with a 300v battery mind as the control circuits could well have a whacking residual, especially if one of the cells is borked.

Iirc from many years ago… A cell works by moving ions from anode to cathode, if you don’t complete that loop there is no movement (as you’re not stripping electrons to create +ve ions to move about) so 0 current. There is no theoretical pd between either terminal and earth, the anode/cathode is to all intents just a lump of metal dipped in a solution** without its partner.

*I’m thinking aa size…
**our whatever depending on the battery

A cell works by moving ions from anode to cathode, if you don’t complete that loop there is no movement

Charge flows wherever there is potential difference and a conductive path. In a charged battery there is a PD between the terminals, but charge carriers moving from one electrode don’t have to go to the opposite electrode in the same cell – they can go wherever.

There is no theoretical pd between either terminal and earth

If there is 300V PD between each terminal then there can’t be zero PD between BOTH terminals and ground.

between BOTH terminals and ground.

Very true, but both is not either, you’re asking about *a* terminal.

but charge carriers moving from one electrode don’t have to go to the opposite electrode in the same cell

The movement of charge in a battery is from anode to cathode, there is no pd where there is no movement of charge between those two within the cell, that’s how those little plastic dividers between batteries for shipping/storage in devices work. It doesn’t matter if its a pd of 0.3v or 3Mv. (obviously you’d need a big piece of plastic to separate a couple of 3Mv batteries.
(bear in mind the movement in the battery is physical not [purely] electrical)

The high voltage traction battery in a modern battery electric vehicle operates at DC potentials of typically between 300 and 800v, as measured between the batteries positive and negative terminals. The battery is not chassis referenced, and ideally would be completely (galvanically) isolated from the chassis, ie there would be no potential difference between the batteries positive or negative terminal with respect to the chassis itself. In practice, it’s just not possible to achieve a complete galvanic isolation, especially when we consider AC effects such as parasitic capacitance and inductance etc.

Therefore, a compromise is required, and that compromise is based on the system voltage, as this effects the potential (sic) for injury to anything that breaches that isolation. Typically, most EVs include an near real time isolation monitoring system, working on top of a system of physical interlocks and isolators, that prevent anyone from accessing a “live” system (without significant work with tools to say dissassemble the battery itself).
The battery will include isolating contactors on it’s positive and negative outputs, that must be explicitly powered to connect the DC link to that battery, and all plugs and sockets include “tamper” connections that are designed to break before the main conductors are accessable, and hence open those contactors before there is any risk of contact (and on top of that, all connectors are coloured orange and are “finger proof” ie you cannot actually stick a finger in and touch any conductor). At power up and power down, contactors are monitored for contact welding to ensure that the disconnect is physically active.

The isometry system measures the impedance to the vehicles chassis, pretty much in real time, and calculates the resistance to that chassis. Should that value fall below typically 100KOhm, the battery contactors will be opened and the system safed. In addition, a pirotechnic “fuse” is used to permanently sever the battery mid string connection in the event of an SRS deployment. Reasons for a low impedance to the vehicle chassis are numerous but generally actually fairly unlikely, including water ingress, physical damage to conductors or wiring or internal damage to the battery cells etc.

Because the battery is isolated wrt the chassis, you CAN touch a single battery conductor (either +ve or negative) without receiving a life threatening shock. You may feel a minor shock or tingle, as capacitive and high impedance resistive paths do exist, but you won’t be at risk of significant injury or death. Usually, the middle of the battery is deliberately tied to the chassis via a high value resistance (in the megaohms) and a low impedance AC shunt (ie capacitively coupled). This is to prevent static buildups between these parts, and to act as a shunt path between them for high frequency AC current (usually measured in the milliamps) that occur because of the way the motor and inverter in these cars work.

A final point is most EVs include the Service Disconnect Switch” which depending on architecture of the battery is a way of disconnecting the battery to ensure that no fault state can exist (like welded contactors or shorted cells) that can prevent an accidental high potential where none would be expected. Once pulled, that switch requires the use of the service diagnostics system to be reset, to prevent accidental re-enablement of the battery.

BTW the “100kOhm” shut off point is set because of the way current flows damage our bodies.

The worst case is a current flow through our bodies that crosses our hearts, and that interrupts the normal firing of the nerves that drive the heart muscle ( and cause Atrial fibrillation).

Grabbing conductors of high potential with either hand is really bad, because the current path is straight across your chest from arm to arm. Touching a single conductor whilst your feet are the ground is less bad, as the path is away from your heart (esp if you are right handed)

Tests were done to determine the level of current that results in differing levels of “shock”. For non critical muscles, like say in your arms, you can withstand 10’s of milliamps without serious risk (it will hurt, you will probably swear, you may even punch yourself in the face as your arms spasm, but you won’t die!), compared to something like 4mA across your heart being pretty much fatal (unless immediate defibrillation is administered)

Pretty much all commerical EVs run below 1000 vdc, so a resistance of 100Kohm, results in a maximum current of 10mA in the case of a “dead short” ie a zero ohm short. However, us humans, despite being pretty low resistance internally (as we are mostly salty water…) have a layer of insulating skin, and generally we wear clothes. All that increases our own resistance, and that ranges between 1 MegaOhm and around 50kOhm depending on things like dampness, contact pressure, and the exact area of contact on our bodies (our feet have thick dry skin, so have more resistance than our fleshy fingers for example)

I’m still ordering insulating gloves.

Handling the battery as a whole is safe because the isolating plug you remove breaks the series circuit. However when you open the battery up to test the individual cells, there’s a point when the bus bar is still attached that, if you touch opposite ends of the battery, you’ll still die. With the bus bar removed I reckon it’s safe.

I’m still ordering insulating gloves.

The kit dealers have to have to work on Electric cars is quite extensive, insulating mat, insulating sheets to lay over body work, huge gloves, goggles etc. I’ve just been helping the wife by filling in online forms for dealer workshop tool audits, know the part numbers off by heart now!

if you touch opposite ends of the battery, you’ll still die

300V DC, unlikely / unlucky to die unless you’re hands were sopping wet – probably some burns….

DC is less damaging to humans that AC and 240v AC isn’t that unpleasant (I’ve had loads of shocks over the years which started aged 3 when I stuck my finger in a live light socket with no bulb in it and burnt the end of my finger off).

If you’re working on car system, most dangerous part is using a socket set to undo bolts, the metal wrench can short on the bodywork and then you’ll be shorting out the battery with a metal wrench with your hand on it – there will be a massive bang and some pretty impressive sparks. Might blind you if you’re unlucky…

I do love a good maxtorque post 😎

DC is less damaging to humans that AC and 240v AC isn’t that unpleasant (I’ve had loads of shocks over the years which started aged 3 when I stuck my finger in a live light socket with no bulb in it and burnt the end of my finger off).

Dangerous anecdotes.

240V can be an unpleasant jolt or it can be fatal depending on where the current goes and how damp/conductive your hands are. Varies a lot.

If you’re working on car system, most dangerous part is using a socket set to undo bolts, the metal wrench can short on the bodywork

On a traditional car setup yes, that’s why it always says ‘disconnect negative lead’ before doing any work because it isolates the battery. But this is a HV hybrid traction battery si is not earthed to the chassis as detailed above.

I’d have thought that’s still multiple batteries arranged to give a total pd of 300 not a single battery?

Surely a single battery isn’t a thing, that’s a cell isn’t it? At least that’s how I remember it from CSE science.

Dangerous anecdotes.

240V can be an unpleasant jolt or it can be fatal depending on where the current goes and how damp/conductive your hands are. Varies a lot.

Possibly, but I’ve had 240v L & N in each hand (as in across my chest) a few times and still here. So, it’s no where near as lethal as people make out.

Surely a single battery isn’t a thing, that’s a cell isn’t it?

Strictly speaking yes, but battery has been used to mean ‘cell’ for decades, eg no one buys a 1.5v D type cell, they just ask for a D type battery…

Surely a single battery isn’t a thing, that’s a cell isn’t it? At least that’s how I remember it from CSE science.

Per foot flaps the terms are fairly (however inaccurate) interchangeable.

But in this case I do mean battery – I’d have genuinely expected a number of (25x12v for instance) batteries (eg a collection of 6 cells) however arranged to give a total pd of 300v not a single battery,if only for the sake of making it easy to maintain and keep the size and format fairly standard.

bsims

Member
Dunno, but has anybody ever put a 9v battery on their tongue?

Just don’t put your tongue on a scalextric track and pull the trigger.
I don’t know why I thought it was a good idea to try, but I didn’t do it again….

But in this case I do mean battery – I’d have genuinely expected a number of (25x12v for instance) batteries (eg a collection of 6 cells) however arranged to give a total pd of 300v not a single battery,if only for the sake of making it easy to maintain and keep the size and format fairly standard.

Most EVs seem to construct the battery unit out of smaller batteries. I guess partly manufacturing simplicity and also fire protection, you can isolate each battery sub unit and have less risk of the whole thing going up.