About: I am a retired Electronic Systems Engineer now pursuing my hobbies full time. I share what I do especially with the world wide student community.

I drive a 1985 model car, no MPFI, no electronics and a vibrating-contact voltage-regulator/cut-out system. Additionally, my usage is also limited and practically restricted only to weekends. I wanted a simple device which could monitor the health of my car 12V battery and the charging system.

This Instructable presents a compact device I built based on the Texas Instruments LM3914 LED Dot-Indicator integrated circuit. The circuitry is housed within a defective car mobile charger case and needs only to be plugged into the car cigarette lighter socket.

A unique feature of this device is the provision of a socket which connects to the battery through a fuse permitting trickle charging of the car battery while monitoring the charging voltage.

Check out this Video:


This simplified schematic shows how a typical car +12V bus is formed.

The +12V battery with the -ve connected to body is the primary power source which is augmented by the AC alternator when the engine is running. The alternator output is rectified and feeds the bus, its output is regulated based on a feedback connection. Different loads are switched onto the bus when lamps and other electrical systems are operated.

In operation battery power is used to start the car and then the alternator powers the bus while charging the battery. Under heavy load conditions both the alternator and battery share the power output required and on alternator failure or malfunction the power is again primarily from the battery.

The indicator I have built is connected directly to the bus and is powered by the battery before the engine starts up. While the engine is running it indicates the bus voltage and when there is a malfunction of the alternator it indicates the bus voltage while drawing power for its operation from the battery.

Step 2: THE CASE

I used the case from a defective car mobile charger. This had the advantage that it would be compact and could directly plug into the cigarette lighter socket. The added advantage is the the LED indication would easily view-able.

After removing the existing circuitry from within the case it is ready for introduction of the modified circuitry.


I have used the LM3914 bar/dot display driver chip, a few resistors and some LEDs, to implement a simple expanded-scale voltmeter. I choose the resistors so as to create an input voltage range of 10V to 15V compatible with the open-circuit and voltage-on-charge for a 12V lead-acid battery.

I connected three red, three green and three yellow LEDs to indicate the low-battery/low-bus-voltage, healthy battery/bus and battery on-charge conditions respectively.


I wired the circuitry into one half of the mobile-charger case. I had to adopt the crude but effective method of 'sky-wiring' where all connections have been made directly on the device leads without any PCB. I found that this was the only way to cram everything into the limited space available. It's a bit difficult but doable with care.

I used the existing fuse from the mobile-charger case to provide safety to the overall system in case of any fault. I chose a rating of 750 mA for this fuse.


I had previously built a 12V Lead-acid battery trickle charger www. which can now be used for in-situ trickle charging the car-battery.

The battery charger output plugs directly to the socket on the device and I use it to top-up the charge on the battery whenever required.



    • Water Contest

      Water Contest
    • Metalworking Contest

      Metalworking Contest
    • Tiny Home Contest

      Tiny Home Contest

    9 Discussions

    Plz share the clear parts list and does it change of any connection if i directly connect to the battery as portable battery tester while bonnet opened to test the battery charging and stregnth

    1 reply

    Yes, you can use this circuit to directly check the voltage status on the battery terminals.

    Here is the BOM:


    Great idea, Thanks for sharing...

    very nice..if the battery is full ..will all led turn on or only the last yellow get on...i am thinking of power consumption point of view

    2 replies

    As it is a dot indicator only the last LED will be ON. Power consumption is small, only the chip and one LED <15 mA.

    dear ajoy..this is then wonderful..i will consider this in my ic based solar charger...thanks