Sasa asked in Science & MathematicsEngineering · 7 years ago

# How does vacuum affect electrical conductivity?

We have a vehicle battery (Zebra battery) that is constructed in the following way; the the cells are all inside one container stainless container. Plus poles are in centers of the cells, minus is outer casing of each cell and stainless container. There is also an outer stainless container that is connected to ground. The vehicle uses plus and minus dc for dc circuitry and the ground for ac that is created from dc to drive a three phase ac motor. Factory made battery uses some silicate thermal insulation material between inner case (minus) and outer case (ground). Insulation material is in vacuum. According to technical documentation of the battery I cannot find the value of the vacuum. The vacuum creates thermal and electrical barrier between those two shells. As a result of a collision the shell lost the vacuum and the battery management system recorded an insulation error between the inner and outer case. The battery case has been repaired. I presume that the recorded error is consequence of humidity of air.

With our equipment we were able to make a 760 mBar underpressure in reference to atmospheric pressure, but I do not know is it enough.

The question is, what amount of vacuum should we apply to the case to create the dielectric and thermal properties of the factory made product? I understand that there may be unsufficient amount of data to make that conclusion, but some comparation of thermal and electrical conductance between two stainless boxes one inside another at distances of 1cm or 2cm, and in situations with atmospheric pressure, approx 240mBar pressure and absolute vacuum could be very helpful. On the other hand, a few formulas that could help me calculate the conductance differences in various conditions by myself could also help.

Thank you

Update:

Just to clarify we are now at atm -760mBar, that's around 1/4 of am atm. The voltage of the fully charged battery is 371v dc between plus and minus, but between minus and ground should be much lower voltage (nowhere is specified that voltage or the potential of the minus pole in relation to the ground)

Update 2:

@Mike: Yes, I know. That's the background story, but the information that I really need is how varies the electric conductivity between two concentric stainless boxes in function of air pressure (atmospheric, vacuum and between these two values) and in a function of distance between the two boxes.

I may be able to calculate the conductivity between two plates in theoretical vacuum and air (it's Column's law if I remember correctly), but with boxes and with with underpressure I do not have the formulas needed and dielectric constants of underpressured air.

All the other presumptions I can make on my own latter. The manufacturer does not exist nearly five years, there is a substitution company that makes similar batteries based on the same technology. The standing solution for this kind of problem is to recycle the old battery and buy a new one. Because of the price (approx 8000eur) I am willing to experiment and I am aware of the dangers. Your answers would only help me to

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• 7 years ago

371 vDc between positive and negative? Unless there is one heck of a lot of plates of anodes and cathodes in that circular battery (even a lithium ion battery is only good for about 3.5V per plate) that "battery you are claiming is starting to appear more as a super capacitor.

Batteries use an electrolyte between anode and cathode. There is no dielectric needed because the whole idea of a battery is to assist the flow of ions between the charged plate and the uncharged one. By having a vacuum and a stainless steel shell (which stainless is very strong for resisting vacuum pressures) makes this appear even more as a capacitor type battery and not a typical electric battery at all.

You would never pull a vacuum on an electrolyte, even for heat transfer reasons, because a vacuum would make the electrolyte boil much faster than ambient pressures would. Boiling is bad because it creates cavitation in the electrolyte which decreases contact area and would give less electrical and chemical activity. If anything they would RAISE the pressure in the cell to squeeze any gas bubbles formed at the cathode or anode so chemical activity on the plates would be at a maximum.

You got to find the manufacturer of this battery and find out what kind of battery this is. It definitely doesn't sound like a typical electric car type battery and sounds more like a Toshiba super capacitor style battery

• 7 years ago

The zebra battery is a molten salt battery that operates around 300 degrees Celsius. The vacuum is there as a thermal insulator since vacuum is the best thermal insulator, many times better than anything else you can buy to keep the heat or cold inside your home. I guess the stuff in the vacuum chamber is a fail safe in case that the vacuum is lost to still protect people from the 300 celsius inside the battery. The molten salt inside your battery probably froze and it might have ireversably damaged the battery. The molten solt battery's are a very good promise for future cheap medium and large scale battery's. The drawback is that you have to keep them above 250 degrees celsius. And a true table salt battery above 500 degrees Celsius.

• Gary H
Lv 7
7 years ago

Well... a vacuum does not conduct electricity (not unless the voltage exceeds the dielectric strength of vacuum).

However, there is no perfect vacuum, at least not on earth. In fact, as you lower gas pressure, you get to a range where it is quite easy to ionize gas molecules (glow discharge). You do NOT want this condition because, once the gas ionizes, you will get an arc and, if the arc burns for very long, your battery will catch fire and/or explode (the temperature of an arc plasma is about 10,000°C).

Atmospheric pressure is, nominally, 1013 mBar (760 millimeters of Hg) so, if your vacuum gage is accurate, you are at ~ 3/4ths of an atmosphere (a really poor vacuum). Consult the mnfr.

• ?
Lv 7
7 years ago

Your question deals with something that is impossibly complex and I wouldn't trust any answer to it found on Y!A unless you also knew the qualifications of the engineer who answered. If you can't find this data from the manufacturer or on the internet in the open, you are going to have to hire a qualified engineer - I doubt 1 person in a thousand has ever heard of a battery that uses a vacuum as an insulator.