I'm working on a design where the rechargeable Li-Ion battery is placed closely (attached with a magnetic connector) to a body-worn contact. The concern is that in an event of a defibrillation, the voltage could arc to the battery contacts on the connector and cause an issue. One of the ways to ensure this doesn't happen is with an air gap of 4mm or more (for 4000V). However, since this device is intended to be small, a 4mm gap is a huge hit so I am looking for an alternative solutions. The device doesn't have to survive the pulse, just has to prevent the battery from getting damaged / exploding. I've thought about using PTC fuse to limit the current to the battery along with some ESD diodes but am not entirely happy with that solution. If you've dealt with something similar or have a better approach, I would love to hear it. Thanks!
2 Answers
Seems defibrillator currents are relatively modest (a few tens of A) so maybe you could limit the voltage with a gas discharge arrestor or TVS, and having the voltage limited could then limit the current with something like a pair of MOSFETs.
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Surrounding a battery with a conductive layer of anything (as suggested in another answer) will absolutely result in failure. Do not do it. If your Defib arcs to the conductive layer it than has a direct plasma connection (so your whole conductive layer is now at 4kV) and will look for the next gap to jump which is INSIDE your so called protection layer. The only way to prevent this failure is to ground the shield to the output of the Defib, but even doing this grounding is iffy depending on current flows and cable inductance.
The 4mm you quote is for the arc length in dry air ….and air is NOT a good insulator, it's just cheap.
You can cover the battery with an insulator that is much much better than an air gap.
As an example you can use E/Fusing self fusing Silicone tape, which is only 12 mil thick and this will withstand 4800V on a single layer. Two layers will withstand 8kV.
Read here and here and perhaps watch this .
E/Fusing tape is readily available at ridiculously low cost, you can even get it at Amazon. 3M also make equivalent tape types (I've never used them) but I'm sure they would work too.
If you do go down this path, just make sure that you don't have any wiring from your protected product exposed. Make sure you have TVS clamps such as this on all wiring, and clamp at the lowest voltage possible for you application.
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