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From digest.v6.n872 Fri Jul 11 15:48:33 1997
From: Raymond G Vacha <>
Date: Thu, 10 Jul 1997 23:35:25 -0400
Subject: Airbag Capacitance

>Surely D/C'ing the battery would help, but may not prevent airbag
>discharge. I don't know about BMW's, but I would suspect that the
>engineer's on many vehicles built in capacitance discharge systems so that
>the airbag would deploy in an accident even if the battery power was
>discontinued as a result of earlier damage.

Your assumption is correct - the designers use a large bulk capacitance to store enough energy to blow the airbag even if the battery becomes disconnected. While I myself have not designed such a system, I have helped many companies in their designs. As a general rule, the capacitance should be
able to blow the bag several seconds, to maybe 10 seconds after disconnection.
The control electronics is running at the time and helps to discharge the capacitance in this short time. Keep in mind, however, that when you repair your vehicle, the electronics is most likely off at the time you disconnect your battery and as a result, the capacitance may stay charged for very long periods of time. For your own safety, you must assume that the capacitance is
capable of blowing the airbag, even if the battery has been disconnected for HOURS.

Ray Vacha
97 540i Six Speed Aspen/Aubergine
BMWCCA #143366, Northern Ohio Chapter

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