I was going to start a thread of my own but figure out I might as well post here as I just took appart one of my keys. The goal will be to add some pictures and advice about the battrey replacement and also about the button replacement.
I have one the two keys which I received with my car that is very lazy on locking the car. I am used to play with a soldering iron so decided to open the first key, confirm the situation and order the batteries for both keys.
Part 1: Opening
As far as opening goes, I started at the back end, took multiple passes with a scalpel and then with a cutter. I then used a plastic pick and then a spoon (was easily available at the time) to start separating the two halves while finishing the cut. I would only recommand taking your time to avoid slipping and either damaging the case or a finger (one of them heals at least
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Part 2: Diagnosis
I wanted to confirm battery and buttons status. Measure battery sitting at 3.00V which is not bad BUT when pushing buttons it fell to 2.65-2.7V telling me it is likely struggling to deliver current and at this stage replacing the battery is not hard so I might as well do it. One thing I noticed though is that the lock button did not produce such effect. So I decided to measure resistance across each button whitout and with pressing on them and realised the lock button does not seem to do anything. In order to confirm it is indeed the button itself functionning I shorted two of its legs while measuring the battery voltage and measured a voltage drop telling me the button is in fact dead.
Part 3: The issue
I have seen kits on ebay with a battery, 3 buttons and a blank case/key (I guess for people who want to refresh the case too?). My issue is all the kit seem to include 3 of the same buttons, long ones with a leg at each end (blue circles). For some reason, my key has the lock button being square with a leg at each corner(red) and not pads for soldering a long button instead:
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As far as I have seen in pictures online, all the diamond keys seem to have the long buttons while other BMW keys have 3 square buttons but none of them have this mix. Also, my key has some glue on the charging coil which I also have not seen on any other.
I ended up ordering two batteries on their own and a kit of Surface Mounted buttons on the side. The kit has the two type of button I found in the key and a lot of other sizes so it will complete my collection of handy electronics.
Next part should be in about a week once I receive everything. I already mounted a fine tip on the soldering iron so it's ready to go.