Replacing 12V battery with 3d party

Hi,

The 12V battery finally gave up the ghost and needed replacement. But the $277 price from the local BMW parts department was too much. So I found a 12V substitute, Deka ETX18L, for $136.00, in stock at "The Battery Store" (256-5361845 or Facebook 'TheBatteryStoreHuntsville".) After one modification to the battery, the car started up and appears to work just fine.

The BMW and Deka batteries are identical shape with the exception of a gap between the lead base and a raised, hollow, plastic wall toward the center. The BMW gap is ~0.5 inch (~12-13 mm) but the smaller Deka gap was ~0.25 in (~6-7 mm.) Using a box cutter/rasor knife and Dremel tool, I removed lead from the Deka positive terminal to make a BMW sized gap. See the battery image from this URL:
https://www.batteriesplus.com/produ...H-EfkKIyOhcIoQVxf6GRi6jJubksHOoRoCIbMQAvD_BwE

The separate, BMW brass, positive terminal has a screw-on, center post surrounded by a circular gap and raised, key-shaped outer wall. The red plastic, encased cable clamp has two 'sensors' but no exposed electrode to touch the brass ring. What these two sensors do is unknown so lead removal on just the positive base allows the brass terminal to fit. I suspect one sensor is a temperature and the other a 'presence' sensor... a mystery. The negative clamp has no such sensors so a simple post was used. Note that I will try to use Google drive to hold viewable images and/or a YouTube video.

The takeaway is the BMW and Deka ETX18L batteries are identical except for the size of the gap on the lead, positive and negative electrode base and hollow, plastic wall. Only the positive terminal clamp appears to have two sensors that may use the key shaped outer, brass ring for unknown purposes. Removing part of the soft lead allows the BMW brass center post to mount. The term 'over engineered' springs to mind.

For the next couple of days I will re-torque the 10 mm bolts and hex-head screws as well as continue tests. I do have a Duracell replacement on order but will inspect before purchase.

Bob Wilson

ps. The YouTube video "UK BMW i3 - 12 volt battery replacement ..." shows the brass terminal fittings.

pps. If I do it again, I will trim the positive brass fitting so it will fit on any of the equivalent batteries.

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.

Thank you! We needed the technical details.

My wife's car failed on Saturday and we were down to sharing one car, the Tesla. So speed of repair was important. With both BMW and Batteries+ reporting it would not be in Huntsville AL until Friday, it was going to be a long week. Regardless, here is an executive summary of what I did and image:

• "The 12 V battery in our BMW i3-REx began to fail. BMW wanted $277 but this is how I replaced it for $134.
  The BMW battery core comes from Deka, EXT18L, who added a side terminal anchor (see photo.) This changed the size of the electrode mount so the BMW terminals would not fit. However, removing the useless side tab lets it fit."
  
  https://www.facebook.com/groups/720...cently_seen&multi_permalinks=1805052386516851

Bob Wilson