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Stray Capacitance

Posted: Wed, 28 Feb 2018, 22:07
by jonescg
Sometimes when building a battery, we like to put it inside an aluminium enclosure. We go to great lengths to ensure there is a non-conductive surface between the enclosure and the cells and hope that nothing is touching.

But we measure the voltage between a terminal and the chassis and we see a voltage - sometimes in the tens of volts - which rapidly decreases as the resistor in the multimeter drains it.

I notice this on my first bike, which had A123 pouch cells held inside an aluminium enclosure. Old video here:

As a complete battery, fully charged to 117 V DC, I could often measure 100 volts between the chassis and the positive or negative terminal. It would give me unpleasant shocks leading me to drop tools where I shouldn't. From this point on I started to use polycarbonate as an enclosure material and never had an issue, even with a 700 V battery on Voltron.

But recently I've been looking at soaking heat out of the battery pack using a good thermal conductor like aluminium and the issue of capacitance comes up. I built this battery, due to be finished shortly, using machined ally plates. It will be 117 V when fully charged. But even with Kapton, electrically insulating, thermally conductive epoxy resin and sizable gaps between conductors and the case, I measured a few volts between the terminals and the enclosure.
Kapton in.jpg
Kapton in.jpg (201.99 KiB) Viewed 2198 times
Nearly done.jpg
Nearly done.jpg (234 KiB) Viewed 2198 times
Fortunately it's not that high, and I don't think it will be an issue in terms of safety or electrical interference, but I got to thinking about how some battery designs just don't lend themselves to aluminium enclosures due to stray capacitance. Any voltage between two conductors has the potential to build charge. And when a solid aluminium case makes a conductive path from one end of a hundred-volt battery to the other, it's possible that charge will accumulate. Perhaps the worst case would be a battery like this:
Capacitance.png (20.02 KiB) Viewed 2198 times
High potential, close proximity between conductive plates, and a dielectric that may or may not be all that helpful. Basically, there is a good reason Voltron's battery enclosure is made out of polycarbonate, as the charge build up would be significant!

What do you think? Any design features that would help alleviate the issue of charge build up on your enclosures?

Re: Stray Capacitance

Posted: Thu, 01 Mar 2018, 08:35
by Rusdy
Put a mega-ohm resistor between 0V terminal and chassis? Will that violate any regulation?

Re: Stray Capacitance

Posted: Thu, 01 Mar 2018, 10:32
by T1 Terry
Would a drain resistor between the aluminium cases and chassis with the chassis connected to the 0v terminal work?
Does this sort of idea work with static discharge build-up as well?
The reason for asking. I'm guessing it is static build up but quite often our Prius will give a good bite when anyone makes a circuit between the metal body and mother earth after sitting in the seats for a while. My assumption was the nylon in the seats working with the nylon in the clothing worn at the time. Would an earth strap from the seat to the metal body discharge the build up?

T1 Terry

Re: Stray Capacitance

Posted: Thu, 01 Mar 2018, 13:12
by Richo
Hmm not too sure if capacitance is the problem.
I'd start by trying the same test on an antistatic desk mat and with a better multimeter.
You can do it here in Welshpool if you want...

Every material will build up a potential at different rates for different reasons.
Shove it in a big antistatic bag?!?

Re: Stray Capacitance

Posted: Tue, 27 Mar 2018, 13:28
by digsys
Yes it is definitely a capacitance issue. I seen it a lot, especially with Formula SAE-E builds (Universities), plus my own battery packs.
Because the HV section must be isolated, usually with differential "earth leakage" to vehicle 0V, there is no point "earthing" the Al case to
the vehicle chassis. IF you do earth the Al case, you now have well enough HV leakage to trip the safety. HV earth leakage is actually quite tricky
to design, as both -ve and +ve are non-referenced, and you have to somehow work out where in the pack the "leakage" occurs, but that's
another story :-)
If you do need a metal enclosure, distance and Insulation is your friend. Don't use raw metal, a hardened powder coating is 1st line of safety.
The next insulation should be HV rated AND very resistant to being punctured ie not a simple plastic. 3rd level would be the minimum spacer.
eg A heavy / thick rubber padding. One helpful assist is to wire a say 100pF + 0.1uF 100V caps + 1Mohm 1W resistor from case to vehicle 0V.
This doesn't interfere with "earth leakage" and helps with charge build up. Report back if you have questions.

Re: Stray Capacitance

Posted: Wed, 28 Mar 2018, 13:00
by Richo
Ahh if you are going to put a cap from the pack to chassis I'd suggest a Y1 safety cap 1000pF.
They are designed to blow open in the event of a failure.