My BMW M3 E36 glovebox flashlight has not worked for years, so I decided this weekend to do a teardown and repair. BMW is the only car manufacturer that I know that puts a flashlight in the glove box. It is a nice idea, but the simple design guarantees that it will need replacement in 3-6 years. When do you ever need a flashlight in the car? However, when you do, you probably want it to work. In this post, I'll show the teardown, the circuit description, how I repaired it and put it back together.
To get into the case, use a small flathead screwdriver and pry open the case down the middle starting from the end that plugs into the glovebox. Also, use a plastic pry tool (like the ones for iPhone Screen Replacements) to work the opening down the length of the case until it comes apart.
Carefully take out the two cell coin battery from the enclosure taking note of the copper switch and the L-shaped steel foil
Now comes some fun stuff taking apart the battery to get to the copper switch piece and the steel upper L-shaped steel foil that the blue ground is attached to. Use a set of pliers to break the metal welds to the battery enclosure for these two pieces. You won't need the battery enclosure so don't worry about bending it.
The circuit is quite simple and took all of two seconds of video to draw. The Kicad schematic tool a little longer. You can download the schematic from GitHub.
When the car is on, the two NiMH batteries are charged by through the 1.47k resistor giving a trickle charge rate of 6.5mA. This constant trickle charge is one of the main reasons the batteries failed. A similar 250maH NiMH coin cell battery from Varta, the original manufacturer, shows that with a constant trickle charge, the lifetime is 3-6 years depending on temperature.
Because the circuit is attached to the 12v battery, I'm sure that there were several design challenges for this charging circuit. 40v Load dump, reverse voltage protection, all need consideration. Since the charging port is always on, the lifetime profile is not more than 6 years. While I don't recall the exact time it failed in my E36, I don't think it was much past 6 years.
Two new 200mAH cells and a roll of copper foil were purchased from Amazon. As will be shown below, copper foil is useful during the reassembly.
Links to the replacement battery and Cooper tape:
During reassembly, it is important to keep the two coin cell stack up as flat as possible. This is where the thin copper foil comes in to keep the connection between the two cells flat. A powerful iron also helps to keep the solder flow as thin as possible.
step 1: Solder the copper switch to the positive terminal base of the coin cell
I made the mistake of taking off the green insulation on one of my NiMH batteries. This insulation helps to ensure when stacking the coin cells you don't short the positive and negative terminals. You can easily remove just a portion of the insulation so that you can solder the copper foil, but try to leave the rest. You will see later where I had to add some electrical tape (can you see it below?).
Step 2: Use copper tape to take the top of one cell and bottom of the other cell
Remember to stack them so the positive terminal of the top coin cell is connected to the negative terminal of the bottom terminal. The positive terminal has a "+" stamped on the end. Do you see the black electrical tape to ensure the copper foil did not short the bottom NiMH battery. Try to keep the green insulation on and this will not be an issue.
Step 3: Solder the copper foil to the coin cells.
You may not need to solder, just using the conductive tape. But I did this step to maximize reliability in case of oxidation. Inspect and work to ensure the copper foil is as flat a possible
Step 4: Wrap the L-Shaped steel foil in copper tape.
With even a super hot iron, I could not solder to this L-shaped steel foil. By wrapping it with copper foil, the connection should be very low impedance and not the assembly can be soldered to the top of the coin cell stack.
Step 5: Fold the tape connecting the coin cells so they are stacked.
Step 6: Solder the L-Shaped steel foil to the top of the stack.
Step 7: Reassemble the battery into the enclosure
Step 8: Test the operation and the fit with the second half of the enclosure.
By testing, you can adjust the copper switch to provide a solid connection to the bulb. Also, use two rubber bands to see if the enclosure can completely close with no air gaps around the edges. You may need to make the battery enclosure thinner by inspection.
Step 9: Glue the enclosure together with plastic adhesive.
I used Plas-T-Pair adhesive along with the rubber bands to allow the glue to dry 20 minutes.
Extending the Lifetime of the Flashlight
A detection circuit that only charges the device when needed can extend the battery life past 10 years. Another approach is to only charge the device when the car is on. The duty ratio in this scenario could extend lifetime way past 10 years. With the current design, the flashlight is always being charged. As shown on the datasheet, this continuous trickle charge will reduce the lifetime of the battery.
In any case, a simple and fun repair. It is nice to have it repaired in the unlikely event I may need it. I will need to do this in another 5 years unless I can hack something to extend the battery lifetime. Of course, now I have a blog entry with step by step instructions. 🙂 Let me know your comments and questions below.