Glovebox Flashlight – ElectroSchematics.com

You never know when you will be marooned in darkness! Always keep a flashlight in your car’s glove box that is bright enough to illuminate under the car hood. Let me share my thoughts on making a compact rechargeable flashlight you can use in your glovebox.

Hello BMW

As far as I know, BMW is the first car manufacturer who puts a flashlight in the glovebox. It serves mostly as a light source for the Glovebox duty but can be detached easily to find out something buried under the seats. It is an ingenious idea indeed!

It took a bit of Googling, but I’ve found that, starting in the late 90s, BMW provided a pre-wired charging socket in the glovebox of many models that recharges the glovebox flashlight, but the flashlights were discontinued as standard equipment after 2008 Anyway, note that the devoted charging socket is merely a rectangular dock with two electrical contacts (a pair of male prongs). The rechargeable glovebox flashlight fits in the charging socket on the glovebox perfectly. Below you can see the snip of an eBay listing.

Following is a touched-up photograph of the ubiquitous BMW M3 E36 glovebox flashlight. Thanks to https://surfncircuits.com, and http://fastwagon.blogspot.com.

The BMW glovebox flashlight pictured above has a built-in two-cell rechargeable NiMH button battery pack which is constantly being charged while the glovebox flashlight is plugged into the E36 glovebox dock.

The inside circuitry of this ‘1996’ electronics design is quite simple. When the car is on, the two-cell NiMH battery pack (2.4V/250mAh) is charged by through a single 1.5KΩ resistor giving an approximative trickle charge rate of 6.4mA.You can see the schematic (prepared by me) below.

This is the quick technical specifications of a similar NiMH battery pack, I found online:

  • Nominal Voltage: 2.4V
  • Nominal Capacity: 240mAh (250mAh typical)
  • Normal Charging: 24mA for 14-16hrs
  • Accelerated Charging (20℃): 48mA for 7-8 hr
  • Fast Charging: 120mA for 3 hr
  • Trickle Charging: 7.2mA
  • Life Expectancy (typical): Trickle Charge (20℃) – Up to 6 years

Frankly, there are some anomalies in this circuitry, but there are always some design challenges for a simple battery charging circuit running constantly on an automobile battery. Thus, leave it as is!

BMW Glovebox Flashlight Hack

Maybe you’re looking for a clever idea to modify your rechargeable BMW glovebox flashlight by tweaking its internal electronics through a bit of extra hardware. If so, this simple hack is for you!

The proposed hack is nothing but the addition of a simple circuitry to charge the flashlight battery pack only when needed. I guess this approach can extend the battery lifetime. The trick used here to extend the lifetime is to turn off the charge once it is fully charged ie the battery pack is charged with a feeble current and the terminal voltage is monitored. When the voltage reaches 2.75 volts (1.375v per cell) the battery charging current is reduced to zero, so it will not be at full (100%) charge. I shamelessly borrowed the basic idea from a note published by Mark Smith (surfncircuits).

See below for the overall schematic. Solder the components on a flake of veroboard according to the schematic, and trim it down to fit alongside the existing battery pack of the flashlight. Once the new circuit is installed, confirm that everything is in order before bond the enclosure back on. That is all for now!

As you can see in the above schematic, the cheap and dirty hack is centered on a precision programmable integrated shunt regulator TL431. To see the 1TL43 in action here, first assume that it is not, hence the charging current flow from the power supply through the 1.5ΩK resistor and 1N4007 diode into the battery pack. The voltage across the battery terminals naturally increases, the battery pack charges, and the voltage in the middle of the voltage divider increases proportionately. When it hits the 2.75V threshold, the TL431 turns on, robbing the charging current and flipping the charging process simultaneously.

Even better you may consider substituting the 1.5KΩ resistor with a bit lower value one, for example 560Ω, to deliver a higher (but still safe) charging current to the battery pack.

Universal Glovebox Flashlight

In this session, you can find one of my design ideas for a universal fit (ie not vehicle specific) compact glovebox flashlight/work light perfect for automotive use (convenience or emergency lighting).

First off, remember that the genuine BMW glovebox flashlight will only fit in cars which have the glovebox charging socket located in the glove compartment. Likewise, you cannot use a homemade glovebox flashlight there unless you have the proper flashlight enclosure – a new one replicated by your 3D printer or an old original one jumped off the attic. Therefore, I am not sure this universal glovebox flashlight model meets all the requirements, but it is a cheap, compact, and adaptable auto light. Talking about portability, you can simply plug it into the cigarette lighter socket of any car to charge it. Moreover, if you do an online search carefully you might be able to find a suitable auxiliary power adapter to plug this universal flashlight easily into the charging port of the BMW glovebox (see below). The adapter takes the place of the standard glovebox flashlight, and lets you charge your universal flashlight comfortably through the glovebox charging port.

Now to the construction details of the universal glovebox flashlight. This is designed to be significantly brighter and last a lot longer than your stock glovebox flashlight, while keeping the overall construction cost within a minimal budget.

This is the adaptable schematic of the universal glovebox flashlight. As you can see, it is nothing but a heedful blend of a Li-Ion battery and a Li-Ion battery charger. Furthermore, a powerful white LED strip is employed as the light source.

The LTC4054-4.2 is a standalone linear 1S Li-Ion battery charger IC from Linear Technology. You can retrieve one related project from https://www.electroschemics.com/economic-led-emergency-light/

And, this is my white LED strip (Aluminum-alloy plate + COB LEDs). Use the search key “4V 5W 3 COB LED Lamp” to get it from your favorite web shop.

After gathering all the components, the electronics assembly needs to be made smaller to fit into a rectangular flashlight case or into a prototype enclosure. You will likely need to make some fine adjustments in the basic scheme to make it perfect. In case the recommended light source is not within your easy reach, you can go for another 4V white LED plate based on your preference but make sure it fits. I also made a single light source (8mm white LED + 8.2Ω resistor) using older searchlight parts lying around – just a narrow beam light source (look below).

Finally…

It is great to keep a universal rechargeable glovebox flashlight in your car. Before getting out of the soldering iron and making the electronics permanent, toying with a couple of ideas will lead you to better constructs. Since not a lot of thought went into each part of the universal glovebox flashlight design, I would be glad to talk about if you have specific questions.

Satisfaction of making your own electronics: Priceless!

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