Many BMW E36 enthusiasts know the flashlight mounted in the BWM E36 glovebox sucks.  With a constant trickle charge of 6.5ma, the flashlight will fail in less than 6 years.    In the case of my E36, It failed around 5 years. For the next 15 years, the flashlight was just a decoration in the glovebox of my BMW!

NiMH batteries out of my 20-year-old E36 glovebox flashlight.  Crusty!!

I did eventually get around to fixing it in a blog entry Teardown and Repair of the Simply Designed BMW E36 Glovebox Flashlight.   As I finished the blog, I realized I was setting myself up to fix the device again in less than 6 years.  A suggested solution from a reader, Preston,  got me thinking how to extend this well past 10 years.   In this blog, I’ll show a simple hack to extend the flashlight lifetime, guaranteeing you won’t need to replace this anytime soon.  A more advanced faster-charging circuit is also shown.  A set of simulation models confirm the flashlight won’t be damaged under load dump  (Car battery = 40v ) and reverse battery (Car Battery = -12) conditions, important specs for any 12v powered automotive component. 

What is the big issue?

The big issue is that the BWM Flashlight, while plugged into the E36 Glovebox port, is constantly being charged, shortening the lifetime to just six years.   Check out a similar datasheet to the 200mA Varta NiMH battery that the flashlight uses.

A constant trickle charge guarantees a lifetime less than 6 years!

A Simple Trickle Charging Hack

The simple way to extend the lifetime is to turn off the charge once it is fully charged, However detecting a fully charged NiMH battery is quite difficult.   The best methods use temperature and monitor the change in voltage of the cell.    For a simple solution, in this hack, the battery is charged with only 6.5ma and the voltage is monitored.   When the voltage reaches 2.75 volts (1.375v per cell) the current is reduced to zero, diverting all the current through the TL431 reference IC.    With this method, it won’t be at 100% charge. However, experiments show it will be greater than 90% and now the trickle charge damage is eliminated.   The schematic and simulation of the circuit are shown below and also available for download on Github.

The TL431 in the circuit operates like a precise zener diode.   Once the reference pin reaches the internal 2.5v reference when the battery reaches 2.75, an internal NPN transistor will turn on diverting the charging current from the battery to keep its voltage at 2.75v.    The normal 10%/month loss of charge on NiMH will also be compensated by the circuit keeping it right at 2.75v, while not over charging and lowering operating lifetime.

A Simple Charging Hack :  See Github for the files
The Battery Current (Blue Trace) Drops to zero when the Battery Voltage reaches 2.75v

The simulation file can be setup to verify operation at 40v (Load Dump) and if the Car battery is reversed (i.e. -12v).  In both cases, the voltage of all devices is protected.

Fast Charging Hack

In this improvement, the charging current is still below C/10 (C=200maH), but 2.5 times faster than the trickle charge above.     Keeping the charging current below C/10 will ensure that one cell of this series connected NiMH battery string does not get overcharged in the case they are mismatched.  A higher charging current should monitor the voltage on each cell to avoid overcharging a cell.   At a charging current below C/10, a maximum battery lifetime is still guaranteed.    The circuit below will charge at 16mA, then drop to zero after the voltage reaches 2.75v.   The schematic and simulation of the circuit are shown below and also available on Github.

Similar to trickle charging hack,  the TL431 will monitor the battery voltage.  However, before the battery reaches 2.75v the Q1 transistor is fully on, supplying 16ma through resistor R3 to the battery.  This will allow the battery to charge quicker than the circuit above.   When the battery reaches 2.75v, the TL431 will draw current, dropping the gate of Q1 and dropping it current to zero.    Also, like above, the normal 10%/month loss of charge on NiMH will also be compensated by the circuit keeping it right at 2.75v, while not over charging and lowering operating lifetime.

A Fast Current Charging Hack :  See Github for the files
Simulation showing the 16mA charging current drops to zero when Battery reaches 2.8v

The simulation file also has an option to simulate with a “.step” Input voltage of 12v, 40v, and -12v, showing compliance to a Load Dump and Reverse battery connections.

Battery Charging Experiments

Experiments showed that a charging current shut off detection voltage of 2.75v was appropriate for the NiMH batteries that I purchased to replace the original Varta NiMH batteries.   With the trickle charge of 6.5ma we get the following charge profile for two different initial charge voltages.

A Threshold of 2.75v will provide good charge percentage but won’t over charge battery

The Tickle Charge Hack

Here is the BOM for the trickle charge lifetime improvement hack.

  1. TL431 Shunt Regulator
  2. 100k 1% 1/4 watt resistor
  3. 10k 1% 1/4 watt Resistor10k 1% 1/4 watt Resistor
  4. Prototyping board

Solder the three devices on a breadboard according to the schematic and TL431 datasheet (see Github), add the power and ground leads. Then trim it down with a hacksaw or Dremel tool to fit along side the battery as shown.

Trickle charging hack (three components on proto-board) added to the flashlight to extend lifetime

Once the new circuit was installed, I confirmed the maximum charge voltage was 2.75v before gluing the cover back on.

Thanks again for the suggested solution from Preston.   A good idea I extended here.  I hope you enjoyed this simple circuit to increase the lifetime of the BMW E36 flashlight past the original design 6 years.  Time will tell.

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