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Motorcycle Low Battery Over Discharge Protector Circuit

Motorcycle Low Battery Over Discharge Protector Circuit

A simple motorcycle battery over discharge protector circuit is explained in the following post. The circuit will prevent the battery from over-discharging by the motorcycle headlamp whenever the mo-bike alternator is not enabled or is idling in the neutral mode in which case the battery is normally subjected to excess loads via the headlamp bulb.

Increasing Motorcycle Battery Efficiency

Motorcycle batteries are normally a lot smaller with their sizes and ratings compared to the vehicle and the usage. The main use of the battery present in mo-bikes is for enabling electronic start through a press of the given start button.

However this small sized battery also has to undergo further stresses while operating excess loads such as the horn, the indicator lights, the tail light and the brake light.

Even though the above loads mostly tend to depend on the motorcycles battery power, these do not affect the battery charge level significantly.

The one that truly affects the battery is the motorcycle headlamp, which when switched ON starts drawing huge current via the alternator and the battery in a shared manner.

This is the reason why we normally see the headlight intensity vary with varying mo-bike speeds.

At higher speeds the alternator shares the load to fair extents, but in cases when the vehicle is not moving or is idling in the neutral mode, the lamp starts consuming substantial amount of battery power, depleting it's charge to dangerous lower levels, and this may happen within minutes if not switched OFF.

The proposed circuit of motorcycle headlamp low battery discharge protector circuit is intended exactly to tackle this issue automatically.

It's nothing complicated, it's a simple low battery cut-off circuit set to switch of the link between the battery and the headlamp whenever the battery level falls below some set predetermined level.

The circuit may be understood as follows:

How it Works

The opamp 741 IC is configured as a comparator here.

It's pin#3 is referenced at a fix voltage determined by the connected zener voltage. Pin#2 of the IC executes the function of the sensing input and keeps the output of the opamp pin#6 low as long as its potential stays above the reference value of pin#3.

The above condition is held in position as long as the battery voltage is above the set safe threshold level, which in turn keeps the output pin#6 at a low logic level.

The low at pin#6 ensures that the p-mosfet is allowed to conduct and illuminate the attached headlamp.

Therefore the headlamp is allowed to receive the required power through the mosfet as long as battery voltage continues to be above the safe threshold.

Now suppose the battery level begins falling beow the set level, this would mean pin#2 potential going down below the reference level at pin#3, or in other words pin#3 reference getting higher than pin#2 potential.

The above situation instantly prompts the output of the opamp pin#6 to pull at the supply level or at a high logic.

A high at pin#6 means the mosfet is now inhibited from conduction, switching off the headlamp.

The above situation continues in a flip flop manner until the battery voltage is no longer able to rise above the safe threshold when the lamp is permanently switched Off.

Circuit Diagram

motorcycle low battery over discharge protector preventor

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

17 thoughts on “Motorcycle Low Battery Over Discharge Protector Circuit”

  1. Hola, gracias por tan buenos aportes tengo una inquietud: si yo conecto en paralelo al bombillo del faro un relé para controlar las luces de posición y que cuando se apague el bombillo se me enciendas estas luces es posible

    • Gracias por querer mis publicaciones, ¿podría mostrarme el esquema? sin diagrama puede ser difícil para mí sugerir. ¡puedes subir el esquema a cualquier sitio de imagen gratis y proporcionar el enlace aquí!

  2. Hey sir
    I have a street light led project that's going to use v 12v 4.5AH battery. Can i use the above circuit to prevent the led's from over discharging the battery when power is bot around,
    What will i be required to change in order to make this circuit work under this conditions

  3. Hello Mr. Swagatam;

    İ built this circuit but it doesn't cut the low voltage for battery? i used for 24V battery, could you kindly help please. when i try to adjust POT, both voltage at PIN2 and PIN3 are equal. what i mean is at PIN2 if i fix the voltage at 15 PIN3 also adjuting itself to 15. could you help please.

    • Hello oktay,

      do the setting in the following manner.
      apply an external 23V to the points where "battery" is printed.
      connect a lamp at the shown points.
      now adjust the preset until the lamp just switches OFF.
      the circuit is set now, you can connect it in the application area.
      but in actual use te mosfet will not cutoff the lamp, it will switch ON/OFF rapidly at the threshold point.
      if you dont want this you can add a 100k resistor across pin6 and pin3

  4. Hi, thanks for making this circuit, i like the explanation, which more usefull while creating this circuit, btw can we use a relay instead of P-channel mosfet? Its to hard to find here, since im living in small town 😀

    • you are welcome! Relay will keep on chattering at the threshold level, that's why I employed a mosfet here, you can try using transistors instead.

      OK, a relay can be made to work by associating a delay network at the base of the driver transistor. This will control the chattering problem.

      Replace the mosfet with an NPN transistor/relay driver stage and connect a 470uF/35V capacitor across the base/ground of the transistor.

      Wire the relay N/O with the headlamp and battery positive.

      In this case the pin#2 and pin#3 positions will need to be swapped, this is important.

      The 10k and the zener at pin7 of the IC are not required actually, so they can be removed and pin7 directly connected with the positive.

  5. Hi Swagatam,

    The Batt. does not get fully charged when bike is in motion. and similarly I have a question regarding to Head lamp of non-Y2K slpendor, as I have one. The illumination of Headlamp is not so Bright, even I have changed the Headlamp Console to new, which has a Halogen Lamp fitting 12v/35w, but it has of no use, the lower filament light is still Dim…so regarding this can I connect Voltage Doubler Circuit at the rectifier ???
    or how do I get Bright Light from Halogen same as all todays new bikes have.


    • Hi PP,

      Voltage doubler circuit won't work because here voltage is not the issue rather it's the low current which is causing lower illumination.

      For better and consistent illumination, the only way of achieving it is to upgrade the battery to some higher level, meaning replace with a bigger battery.
      In Place of halogen lamp you can try a LED lamp which would consume a lot less and produce equivalent lights.

    • Hello Sir,

      By replacing Halogen lamp, Do you mean I should break halogen lamp and connect high wattage LED's after removing Upper and Lower Filament ???


    • Hi, PP
      Did your motorbike headlamp using power from altenator which call fullwave rectifier? Or still halfwave? If your motorbike using halfwave rectifier then you must change to fullwave rectifier, this mean your motorbike headlamp now using power from battery, that will bright like new motors, and this circuit will help you (very usefull) for manage voltage battery 🙂

    • Hello pp,

      you may have to replace the halogen module with the LED module inside the headlamp reflector.

      The leds will need to be equipped with all the necessary current limiting circuitry before it can be powered.

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