Energy Stored in Inverter Capacitors [SOLVED]

[NoobMaster]

Hello All,

This may be an obvious question to the Electrically Initiated..
But I’m a humble petrolhead trying to understand the basics

But I’ve looked through the forum and can’t see an answer to my question so please can someone offer some wisdom?

My question is: Inverters and EV sub systems such as AC chargers and DC-DC converters have link and filter capacitors that can handle a fair amount of energy.

When you want to use an EV and connect the High Voltage DC traction pack to the Inverter, it’s safe practice to use a pre-charge circuit (relay and resistor) to limit the current to the capacitors in the inverter so that they can build up charge slowly and not cause large in rush currents across the Main Contactors (which can weld them).

What happens to the energy stored in the capacitors when the contactors are opened again suddenly? For example if the car is in an accident, the Contactors open isolating the HV DC pack and make it safe. However there could still be dangerous to touch levels of energy stored in the capacitors for emergency responders attending a Traffic Accident?

[SciroccoEV]

They're fitted with 'bleed' resistors to discharge the capacitors.

[NoobMaster]

They're fitted with 'bleed' resistors to discharge the capacitors.

Ok.. thanks for your reply..bleed to where though? The charge is dissipated as heat?

[MattsAwesomeStuff]

Ok.. thanks for your reply..bleed to where though? The charge is dissipated as heat?

Yep, just wasted as heat. It's a relatively small amount, you just don't want it dumping instantly.

[NoobMaster]

Ok thanks for the info.

[johnspark]

Resistor value may be designed to waste the energy in a minute or two.

[PacEmaker]

From memory, Toyota/Lexus recommend giving it 10 minutes to discharge before messing with it. Personally I like to check what's left 'in the tank' before getting too deep into such things...i.e. get the multimeter out.

[NoobMaster]

How do mechanics work on a hybrid or ev race car then, that pulls into the pits? They certainly don’t wait a period of time to jump onto a car..

Do they plug in some device into the dc bus to instantly discharge the caps?

[Kevin Sharpe]

Do they plug in some device into the dc bus to instantly discharge the caps?

No, they design the capacitor discharge rate to match the requirement.

[SciroccoEV]

How do mechanics work on a hybrid or ev race car then, that pulls into the pits?

Unless they're actively working on the electrical system, why would they do anything?

If you're working on a fuel system you know to depressurise it first, but you're not going to do that whilst you're swapping drivers or changing tyres.

[arber333]

How do mechanics work on a hybrid or ev race car then, that pulls into the pits? They certainly don’t wait a period of time to jump onto a car..

Do they plug in some device into the dc bus to instantly discharge the caps?

Actually there are discharge resistors built in which have sufficient performance (25W to 150W) to discharge system capacitance in sufficient time to allow for safe work after using main HV system disconnect. This is connected to certification requirements which we obviously dont see much .

You can use 160R (Volt inverter) to 1kohm which discharges main cap fast enough for DIY. Warning resistor is constantly connected and it heats up!

[P.S.Mangelsdorf]

How do mechanics work on a hybrid or ev race car then, that pulls into the pits? They certainly don’t wait a period of time to jump onto a car..

If anyone else has watched some of Hoonigans' videos about the Ford Mustang Mach-e 1400, Ford is really trying to make it seem like an EV race car is a nuke or something. Rubber mats, aprons, face shields, arm length gloves, a rubber hook to pull the person away from the car, all just to plug in the car to charge. (see Hoonigan Autofocus video from a day or two ago).

Not sure that's helping anyone's perception of EVs a practical cars/racecars, but that my two cents.

[NoobMaster]

How do mechanics work on a hybrid or ev race car then, that pulls into the pits?

Unless they're actively working on the electrical system, why would they do anything?

If you're working on a fuel system you know to depressurise it first, but you're not going to do that whilst you're swapping drivers or changing tyres.

How about re-fueling? Is there a possibility for a Hybrid (or any race car for that matter) to generate a significant enough static charge from airflow over the car at speed for it to be an ESD ignition risk around fuel vapours?

[arber333]

How about re-fueling? Is there a possibility for a Hybrid (or any race car for that matter) to generate a significant enough static charge from airflow over the car at speed for it to be an ESD ignition risk around fuel vapours?

Now you are stretching it. What on earth would an EV be doing around petrol? Heh... Maybe fueling up webasto heater?
If you remember from physics rotating/rubbing parts create static charge which has to be discharged. Usually this is done because of Alu Steel galvanic reaction not because it would throw up sparks.
Leaf motor has carbon brush system inside that carry static from shaft to motor case.

Greater hazard would be a dude lighting a cigarette nearby.

Edit: sorry I saw you are referring to hybrids? There is a document about hidrogen hybrid certification. ECE R100 rev2. You can read all about the requirements of charging system...

[NoobMaster]

Actually I was thinking about LMP1 race cars and F1 where they pull upto a pit box and the first action that is taken before anyone touches the car is to make sure is Electrically safe to do so.

I just wondered how this is achieved in a General sense. It’s an extreme example yes granted. But as time goes by, more and more Hybrid and Full EV cars will enter into club level motorsport (which I Marshall for). So I’m just trying to learn a little knowledge into safety features etc.

Thanks to everyone for their input.

[P.S.Mangelsdorf]

But as time goes by, more and more Hybrid and Full EV cars will enter into club level motorsport (which I Marshall for). So I’m just trying to learn a little knowledge into safety features etc.

When it comes to racing safety, its probably good to break it into 3 categories: General EV stuff, Production EVs, and Conversion EVs.

General Stuff
-Probably a good idea for tracks to begin buying class D lithium battery extinguishers
-Probably a good idea for tracks to get their personnel basic training on EVs (discussed more below)
-don't touch the orange wire.

Production EVs
-lots of OEMs have at least some training modules/media available for first responders regarding their hybrids and EVs. Don't cut here, do cut there, this is where the disconnect is. Probably worthwhile for tracks to at least review with personnel.
-HV cables are routed to be safe in typical accidents, likely the car is safe in most on track accidents, barring some really insane damage.

Conversion EVs
NEDRA and NHRA put together a decent rules package, as did Pikes Peak. Probably worthwhile reviewing.

A danger with conversions (to track personnel) is that cables may have been routed for convenience rather than safety, so tech inspection needs to catch that and make sure cables are away from the car perimeter.

In my opinion, any conversion going on track should have a rear mounted kill switch and a dash mounted kill switch. This should disable 12V power, including 12V power to a contactor used for this express purpose. This is required for any EV conversion on a NHRA track.

In my car, I will be adding a third switch between the drivers seat and drivers B-pillar, next to my outside shoulder, so that I can slap it, or crews opening the door can slap it. All three switches are run in series, so any switch that is off shuts down the entire car.


What About Battery Damage?
Now all of that is great and fine but it doesn't address one item, which perpetually bothers me because there is not a great answer yet: What if the battery compartment is damaged in a wreck?

First off, as we build cars we should strive to build robust enclosures, and I know the OEMs attempt to as well. Despite this, a big enough wreck could damage the batteries.

If batteries are damaged they can cause issues. Dousing them with a class D extinguisher can stop the reaction, but moving the car (i.e. off the track) can restart it (as I understand it).

This, to me, is the biggest challenge still facing EV racing. Beyond just building as beefy an enclosure as we can, I'm not sure what the answer is.

[NoobMaster]

That is a heroic answer The first question I asked the BTCC on social media when testing a hybrid touring car for the 2020 season was “where are the lithium fire extinguishers” everyone treated me like I was stupid!

I also wondered about damage to the battery. Especially since in a race car they are often stored in or as part of the floor Structure.. and often damage to the floor pan on a race car -riding the curbs on the corners for example -goes unseen.

Thanks so much for taking the time to answer P.S. Manglesdorf

[P.S.Mangelsdorf]

I also wondered about damage to the battery. Especially since in a race car they are often stored in or as part of the floor Structure.. and often damage to the floor pan on a race car -riding the curbs on the corners for example -goes unseen.

The OEMs put some pretty heavy armor on their packs, but I could see wear being an issue over time. I'm drag racing focused, so if I'm hitting something there is already a problem happening. My assumption for road racing would be this: in an OEM with packs in the floor, the standard armor should provide enough protection for the typical track day from wear and tear, however repeated use may develop an issue. Maybe that's something the tech inspectors need to be looking for when a car arrives on site ( and periodically if a multiday event).

My biggest concern for battery safety is a big crash. I'm thinking a Tesla that's spun out and then T-boned in a big incident, maybe where a wall means the car can't just slide away. I'm not sure what the protocol should be for that type of event. That's a lot of force going into the pack.

It also comes down to us as builders and owners to a) build and maintain safe cars, and b) do everything we can to educate track officials in a way that is simple, concise, and clear. They aren't dumb, but they've got a lot to worry about, make sure you're not one of those things.

- Push button. Pull me out.
- Don't touch the orange.
- If this box/part of the car is damaged, use the special extinguisher.

[arber333]

Actually I was thinking about LMP1 race cars and F1 where they pull upto a pit box and the first action that is taken before anyone touches the car is to make sure is Electrically safe to do so.

Well... When I was licenced aircraft engineer for refueling we had to connect one end of a cable to the aircraft and the other one to a dedicated earth point. That was a metal stake in the ground. We were not allowed to refuel anywhere else. I imagine it is similar for F1 carbon body racer. It had a dedicated discharge probe and they connect it to GND before doing anything else to it.

Then on EV one woul disconnect 12v power if needed. But really If you would have to remove battery pack to refule you would rather use manual fast fuse disconnects and so split HV packs into more manageable pieces.

If your mission would be to recharge the existing battery really fast, then you would use an external charger and turn inverter system off for safety somehow. Maybe even charge umbilical proximity would remove 12v from Traction system for charging...