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Message: <blockquote data-quote="alohart" data-source="post: 72127" data-attributes="member: 1439">The ETX18L battery is designed to be a power sports battery unlike an AUX18L that is designed to be an automotive auxiliary battery.  The manufacturer of the OEM battery in the U.S., East Penn Manufacturing, included the below FAQ on one of its AUX battery brochures:Q. The AUX battery looks like an ETX battery.  Are they the same?  Can the AUX be substituted with a Power Sports battery?A. No.  The AUX battery has a different design.  One major difference is in the venting system.  Do not substitute an ETX or other Power Sports batteries for an AUX!  The ETX has a single point venting system that works great for Power Sports applications. However, the AUX has an Individual Cell Venting System that optimizes valve-regulation to maximum efficiency in auxiliary power support functions.The ETX18L doesn't have a cell venting tube that connects to the tube that exits underneath an i3.  If an overpressure condition in one of its cells caused the cell valve to open, flammable gasses would be vented.  With an ETX18L, that flammable gasses could accumulate in the frunk which could cause a small explosion if ignited.  With an AUX18L, the flammable gasses would be vented to below an i3 so that they would escape to the atmosphere rather than into the frunk.  This is certainly an unlikely occurrence, but one that BMW decided to deal with by installing an AUX18L rather than an ETX18L battery in an i3.Many i3 owners have installed ETX18L batteries with no reports of problems that I'm aware of other than the need to modify the terminals as you have described.</blockquote>

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