PWM shunt regulator

You might be surprised. Have you done an enrgy audit on the

If there are things that normally consume a portion of that 'kilowatt', then the 48V buss might be a good candidate for an energy storage node.

If nobody wants the energy, then that's the point where you start dumping (or storing) it.

If you add two '48V Buss' terminals to the box, you allow for possible later improvements to energy management in your lab.

RL

Above my budget for sure.

Actually, no. High power tubes need lots of forced air and often need finned heat sinks or water cooling. Expensive, fragile, unreliable, not available in surface mount.

Not to mention needing kilovolts.

Resistors are the best way to dump power.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Of course.

A customer could plug in eight 8-channel load boards, 64 loads in a 3U chassis, over a kilowatt to dump.

On bigger load modules with fewer channels, we use a copper CPU cooler on the board. But that wipes out a lot of board area.

I wouldn't ask customers to do that.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I once had to make a set of replacement resistors for the starter unit of a 5 kW blower motor that took quite a while to run up to speed because of its high-inertia load. I had some slate bars about 2" x 2.5" and 18" long cut to order by a slate quarry; then hand-wound them with resistance wire .

The original wire had broken into short pieces when the unit was destroyed by arcing but there was one piece long enough for me to calculate its resistance using a four-terminal measurement. When I contacted the wire manufacturers, they supplied the exact replacement which was still in production despite the unit having been made in 1919.

The windings were interesting, with several tappings and one section wound two-in-hand to handle a higher current. The control gear was connected to the air outlet of the blower and, as the motor speed increased, a cast-iron chamber was pressurised. This caused a leather bellows inside the chamber to collapse at a rate controlled by lead weights hung on a lavatory chain. The belows pushed a rod which slid a set of brushes over some brass contacts on an ebonite panel and progressively cut out the resistors.

The motor was interesting too. It was a repulsion-start, induction-run motor, which is quite rare nowadays but was popular in 1919, before reliable starting capacitors were available. It needed a major rebuild, including skimming the face commutator. The shaft was so long it overhung the tailstock end of my lathe, so we had to support it on wooden 'V' block bearings and feed it with a constant supply of oil.

A very challenging and satisfying job.

John doesn't like designing or paying for purpose built wound components, and he's corresponding unwilling to the work required to find out how much they would actually cost.

They are certainly the cheapest, not least because you can let them get very hot - hot enough to drive vigorous natural convection and a useful amount of radiation. A careless designer can be embarrassed if that heat ends up in the wrong place.,

Let's do some sums:

Assume the fault propagates at 10mm per second and begins at one end of a typical cell. The cell is 70mm long so the entire cell will be involved in 7 seconds. Videos of vehicle battery fires show one cell exploding every 10 seconds, which is roughly in agreement with this figure. The maximum discharge rate for a lithium battery is around 1C, so it would take an hour to discharge the battery.

The battery capacity of cars, on average, is about 70 kWh. This means a resistor capable of dissipating 70 kW continuously is needed to discharge the battery in one hour.

I'm sure cars with a red-hot bedstead of resistance wire on the roof would soon catch on.

First show where you got your numbers from.

I've snipped out that bit of bizarre speculation.

In reality, the problem is picking up the increased rate of self-discharge long before you get to the point where thermal runaway is likely - the battery has to get above 120C before this can get going.

You'd dump the excess energy slowly into the motor, letting it rock the car rapidly back and forth by about a foot or so to generate a little extra air circulation. It would take a while to discharge the battery, but it would get it done.

It would be a emergency solution - the driver would get told that the battery needed attention long before this would be justifiable, and in our brave new world the battery condition monitor would probably have it's own mobile phone to rat out the inattentive owner to the local fire service.

Your enthusiasm for impractical solutions is noted.

In more detail: the delamination of the seperator occurs at 25 metres per second but the thermal runaway reaches a peak of 600 mm/sec and then falls to 80 mm/sec according to Franson, Pfaff et al. "Exploring thermal runaway propagation in Li-ion batteries through high-speed X-ray imaging and thermal analysis".

For their experiment, they initiated the failure by penetration with a nail, but the same propagation could equally well be started by failure of a very small area of a separator. The nail penetration was near the casing and this sometimes resulted in a hole melting in the casing and relieving the excess internal pressure. A separator failure away from the casing could well result in much higher pressures and greater spreading of incandescent materials.

They measured the propagation time between the initially-failed cell and an adjacent cell to be about 4 minutes but various videos of lithium battery fires show cells exploding at a faster rate than this, once the fire has taken hold.

If we take the 4-minute figure as a reasonable approximation, this is the time in which a 70 kWh battery must be discharged to prevent a failed cell from setting off the others. That is more than 1 megawatt to be dissipated in something the size of a car.

A typical cell holds around 80 Wh of energy but less than 1 watt could easily heat a small area of separator to over 120C without the temperature rise or the discharge current being detectable outside the cell. if you think you know a way of reliably detecting the failure of less than a square millimetre of separator in a battery containing 500g of materials, including about half a square metre of separator, the car industry would be glad to hear from you.

If you don't know of such a system, your assertions that lithium batteries are safe as long as the designer has done his (or her) job properly, and they can be discharged before a failure become catastrophic, are based on nothing more than wishful thinking.

It is probably just as practical as having a car start rocking backwards and forwards for hours on end to discharge the battery.

An even better solution (in a Brave New World) would be to have it drive itself to somewhere where it can't cause any harm, as quickly as possible. Perhaps every Local Authority should have a designated place, downwind of the town, where cars with faulty batteries could be programmed to drive themselves and burn out in relative safety.

Gee, maybe some small person could figure out a propulsion system where the oxidizer and fuel wouldn’t be in such intimate contact. Maybe it could even use air!

Cheers

Phil Hobbs

I wish you wouldn't use mathematics here. It confuses some people.

They would have to drive like Indy drivers, out of garages and parking lots and parallel parking spots on streets, to get to the designated burn zone in a minute or so. Ignore the baby buggies. Well, if the sensors provided a minute of warning.

Better idea: sense a cell runaway (somehow) and disassemble the battery pack. Use explosives.

Or put water-cooled steel separators between cells to limit the flame spread. Replace flammible tires with tank treads.

I took a taxi yesterday. They are almost all hybrids here. A few are electric, but have to be recharged mid-day to manage a full shift.

I wonder what kind of electric car Sloman has.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

"I have a 48 volt power supply that can be back-driven."

Can, or 'will be but must not be'?

Also, why a killowat power supply if it's only powering load modules? Control power consumes . . . .

Has to be some thin'in' going on somewhere.

RL

Impractical. You'd waste too much energy compressing and storing the oxidized reactants.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Typo I guess. It can't be back driven.

My typing is bad in a newsgroup, where I don't triple-check. I never learned to type.

The chassis can host power supplies (ac or dc) or loads (resistive or resistive/inductive) and some other things.

The more the better!

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Sounds like fun.

I had to dump about 8 KW once. We used a garbage can full of water and a coil of copper wire. Had to refill it now and then when it boiled.

Plastic garbage cans get wobbly when they are full of boiling water.

A PCB with etched or dremeled traces might be a good dummy load, with forced air or water cooling.

There are HVAC things, nichrome duct heaters with fans, that are about the cheapest dummy load you can buy.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Some Haas CNC machines use standard stove coils as spindle brake resistors

load testing a generator when OSHA isn't looking,

This wasn't just an experiment, but rather a piece of fossil carbon industry propaganda generated by simulating a remarkably improbable incident.

This is roughly equivalent to insisting that petrol-engined car won't catch fire if you shoot a bullet into the petrol tank.

That depends on your temperature sensing. Any useful battery condition monitor has have at least one temperature sensor inside the battery, and another one for external temperature. 120C is a lot hotter than ambient anywhere you are going to use a car, and the battery as a whole would start warming up fast pretty much instantly. Getting millidegree sensitivity out of a temperature sensor is trivial.

They wouldn't. Anybody who understands the physics involved could tell them.

Not really. The grand-daughter of one of my old friends married a guy who'd done his Ph.D. on lithium batteries and is now a lecturer at the University of New South Wales, and I got to talk to him about the subject from time to time. The marriage hasn't lasted and the grand-daughter - who works for Microsoft like two of her uncles and a couple of her cousins - is moving the Microsoft headquarters in Washington state, so I won't get any extra information form there.

It's perfectly practical as an "if all else fails" solution. It should never be need to be invoked, but there are plenty of silly people around and we do have to cope with their inadequacies.

My car, which was bought new in 2011, has little radar sensors fore and aft that could tell it how far it could go back and forth without hitting anything.

Cars are frequently garaged in closed spaces. They don't tend to be delivered with the hardware that would let them get themselves out. Mobile phones work inside garages. There are garages and car-ports in Australia that lie beyond the reach of the mobile network, but the fire service out there aren't all that local.

There is one, and it's in popular use. Internal-combustion-engined cars catch on fire rather more frequently than their electric counterparts, if the car insurance statistics are to be believed. The fossil carbon industry propaganda machine doesn't highlight that particular statistic.

You know perfectly well that I've got the same Mercedes 180B that we bought in 2011. It burns gasoline. If I needed to replace it I would probably buy an electric car, but I don't drive much and the little Merc is perfectly adequate. My wife bought it largely because it has a four-way adjustable passenger seat. She'd got European Lyme disease around 2005, and needed that sort of adjustment to get comfortable.

There are a lot more internal-combustion-engined cars and they are, on average, much older. Most of the fires in cars originate in the electrics and most of the electrics are the same in electric cars and internal-combustion-engined cars, so the means of propulsion isn't the reason.

If you want a fair comparison you should separate the causes: compare spontaneous battery fires with spontaneous fuel tank fires (petrol=some, diesel = none). Compare refuelling fires: forecourt with home electrics. Compare non-fuel engine fires with electric motor fires and compare electrical system fires with electrical system fires.

Then look at the ease of extinguishing them, the pollution they cause and the severity of collateral damage.

Then weight the whole exercise to account for the difference in number of vehicles.