Simple battery charger circuit and battery level indicator with low battery recharge alarm

This is a simple 12V rechargeable smart battery charger circuit. You can use this best battery charger circuit as car battery chargers, Inverter battery charger, Emergency light battery charger etc. An automatic indicator alarm circuit also comes along with this battery charger schematic. The main advantage of this indicator is that a buzzer informs us when the battery needs recharge. This circuit schematic definitely helps for your daily life battery charging applications.Also Read:

Circuit diagram of battery charger

Batery+charger Simple battery charger circuit and battery level indicator with low battery recharge alarm
Click on the image for enlarged view

Components required

  1. Transformer (230V to 15V or 110V t0 15V)
  2. Bridge rectifier (1N4007 x 4)
  3. Capacitor (470µF, 50V)
  4. Voltage regulator IC 7815
  5. 12V rechargeable battery
  6. Diode (1N4148)
  7. LED Zener diode 9V
  8. Transistor (BC547 x 2)
  9. Resistors (10kΩ, 1.5kΩ, 100kΩ Each ¼ Watt)
  10. Buzzer (12V)

Working of battery charger circuit

  • The charging circuit is build around voltage regulator IC 7815 and two transistors BC 548.
  • The main supply 230V or 110V is stepped down using a step down transformer, and then it is rectifiered and filtered out.

Read:  Full wave bridge rectifier circuit with working explanation

  • That DC voltage is then fed to the voltage regulator IC 7815; the output will be regulated 15V.
  • 12 volt rechargeable battery is connected at the output of voltage regulator and it charges when main power is available.
  • This circuit also indicates the charging status, that is the LED1 is glows when the battery is charged (Above 10.5V).
  • When battery voltage goes below a particular value, LED1 stops glowing and the buzzer produces sound indicating that the battery has been discharged and it needs recharge.

Pin out of 7815, Diode, BC547, LED

57 thoughts on “Simple battery charger circuit and battery level indicator with low battery recharge alarm

  1. I am interested in using this circuit to simply buzz and tell me to turn on my charger…when the battery gets down to 12.4 volts.
    What would I modify to get the buzzer to go off at 12.4 V instead of the 10.5 V noted in the narrative?
    Can I simply omit everything to the left of the battery (the charger) AND can I also eliminate the unnecessary LED? I only want a sound indicator.

  2. Hello everyone,
    Am pretty new here and need some help on how to integrate a battery charging circuit to an inverter. That is, i’d like the produced power to be continious. Same AC produced by the battery will be used to charge it up for a continious supply of Ac power.

  3. Hi .
    I made this circuit, but i didn’t achive indication . In fact buzzer sounds continously whether battry low or fully charged.
    Can you please tell me the exact value of pot so that Led glows at the level of >10Volts.
    Thank you

  4. when the batteries power come downs then mz charger controller buzzer warns which i do not want, how can i staff the buzzer warns,
    thank from waiting to your reply

  5. have you ever made and test this circuit sir? i am very interested to your circuit and i just want to know and check if this circuit will not blow down. thank you

    • Hi
      For extra protection use a 10 ohm 20 watt resistor in series with the regulator output to limit the charging current.

  6. What Is The Ah. Of The “Transformer”, May I Charge 12V 9Ah Battery With The Circuit.
    Pls Reply ASAP..

  7. Sir I m very interested in doing this on my own but i want to know if i make a battery circuit to charge and inverter circuit to produce output of 230v/50hz as different circuits then tell me every time i must charge the battery separately when it is charged off am i right? or cani use relay for this to switch immeditely

  8. Dear Sir,
    Good day.
    I am planning to use this circuit in monitoring a 48 VDC battery, can you kindly provide circuit modification.

  9. I guess this circuit is good so if possible u may email me the limitation of this circuit at my email iradyregis@yahoo,com because I will use it and gimme more explanation abaut it and some summarized notes abaut a buzzer and variable resistor so thanks so much for creating dis web to help students .

  10. I have 3 basic qquestions.
    1. Can I connect this circuit in parallel with an inverter circuit.
    2. Whats d charging current of this circuit. What formula can I use to calculate it. And how can I increase it. Or better still reduce d charging time of my battery.
    3. I hope dis circuit can charge a 12v 100 or 200ah battery. Will appreciate a reply asap

  11. Hello. Please i want you to help me with a 24VDC to 230AVC inverter circuit schema integrated with battery charging circuit. And a guide on how to connect battery full and battery charging indicators. Thanks

  12. I am going to make ds circcircuit for my major project of diploma
    I have a question
    The connection of 4.7k resistor and 9 volt zener is confusing and i can not understand it
    Kindely help me soon as m wating 4 rply
    Thnxx in advance!
    Aditya :)

  13. sir,
    if we are using a 9-0-9 transformer,and we are required to charge 12v ,7 A, li ion battery
    then wt chnges should i make in the above circuits
    tell me soon plz

  14. Yes jaspreet u can use 9-9-9 transformer by eliminating its center tap
    Infact 230-15 or simply 0-15 transformer will not work as the ic lm7815 works above 18 volt so u need 2 supply above 18 volt.
    U cn check its datasheet

  15. I am trying to find information on formulas used to calculate capacitance required for a given circuit. Not a circuit in particular, just in general how to figure out what value should be used.

    • Try this one to find out capaticance of series and parallel capacitors:

      To find out how much energy (and for how much time) can be stored by a capacitor, try this calculator:

      The calculator give you the value of Time Constant, which is the time it takes for a capacitor to acquire 63% of the voltage beeing supplied to it, if its starts with zero volts. To keep the story short, the capacitor needs about 5xTC to get fully charged.

      So with these in mind you can roughly calibrate the capacitance required by the loads in the circuit and knowing the time gap you would come across by the help of the capacitor.

      Anyhow, for simply analog circuits it is rarely needed to calculate the exact value of capacitance. Just use a probably big enough one, and it almost always will be fine. Also, data sheets gives you recommended values of capacitors to use with for example the LM78xx series. Just make sure you use at least the given value of capacitance.

  16. lm7815 terminals are little confusion
    v reg means terminal 3 ( out put )
    com is gnd or terminal 2 in the practical am i correct or not ?

  17. can u pls explain the zener diode’s coonection with the 4.7k which is not given in the component list


    • Yes, definitely.

      However, a 200Ah depleted lead acid battery or battery bank is about at 70% of its capacity. So you need to feed 60A current to fully recharge it. As LM7815 can prduce 1A, it theoretically can recharge your depleted battery in 60 hour.

      If you battery is a deep cycle type solar battery (, than it can be safely depleted to 30% of it’s rated capacity. In your case, to about 60Ah. At that point you need to pump 140A into your battery to get it fully charged again. And this will take 140 hour, or almost 6 whole days.

      What if you would charge your battery faster? Than use more of the above circuit in parallel (probably without the LED and buzzer, as one low voltage warning circuit will be enough). This way the other LM7815 regulators will shorten the charging time.

      How much LM7815 you will need?
      As we know that charging current should be about 10% of the Ah rating of battery, in your case the max charging current is 200Ah/10 = 20Ah. So you will need 20 (!) pieces of this LM7815 based circuit to charge your battery as fast as it can be charged.

      As linear regulation produces much heat, 20 of them wastes a huge amount of power. It could be good in winter time, if you utilise it as a heat source. ;)

      Anyhow, if your battery is not so depleted then charging it with 20A will make it bubbling and boiling. So in reality you really don’t want to charge it constantly with 10% of its rated capacity.

  18. sir can u please explain… how does the indicating circuit detail..???

    it will be a greater help for me.

  19. Be aware of LM7815 requires 18V or more to produce a stable 15 Volts. With a 15V output transformer it probably produces some 13.5 Volts, which is hardly sufficient to effectively charge your 12V battery.

    Also this is a 1 Amper charger as the LM78xx chip can handle this amount of charge. 1 Amp is enough for a conditional charge of large (more than 30 Ah) capacity batteries, but will be way too low current for a depleted solar battery bank.

    Anyway, the circuit is impressively clear and simplified, can be built with a few components anf for cheap. So this is a great schematic. Thanks for publishing!

  20. Pls sir, can I use a 230, 12 0 12 center tap transformer. Is it going to work out. Thanks for ur time. Kindly acknowledge my mail

    • Hi Anthony,

      Bad news, you can not build a 12V battery charger from a 12V AC output transformator.

      – The rectifier diodes drops the voltage as a side effect by about 1.2V. So after you make DC from your 12V AC transformer, you would measure just about 10.8V DC output. That is the voltage level of a very depleted 12V battery, so it is totally inappropriate as a charger.
      – LM7815 only can reduce voltage to a stable 15V from a higher voltage, at least from 16.5V
      – An LM7812 would also fail to produce 12V stabilized voltage with an input less than 13.5V
      – To charge a lead acid 12V battery you will need at least 13.2V

      To build a 12V charger you need a 15V or 18V AC transformer. (15V is ideal as the linear voltage regulators like the LM78xx series produce quite much heat while dropping the voltage level. So the same circuit with a 18V input rather than 15V produces much more heat to dissipate. Just get a 15V transformer for this circuit, or a switch regulator rather than linear regulator, if dissipating the heat is an issue.)

  21. hey, nice circuit btw.

    just simple question, what is the function of the potentiometer and the zener ? and how do they work ? Thanks in advance.

    • Hi Akbar,

      > what is the function of the potentiometer and the zener

      In a nutshell: with the potentiometer you can find a certain voltage level when the LED and the buzzer turns on.

      The 9V zener with the variable resistor determines the voltage point when enough current flows through the zener diode to the base of Q1 tranzistor. At that voltage level current starts flowing through Q1, turning on the LED. Also, Q2 getting connected to ground through Q1, and the buzzer also get alive.
      R5 is just a pull-up resistor to ensure Q2 is off when Q1 is off. R4 belongs to the LED to protect it from burning out.

      • Ooops… I misunderstood the schematic. Let me correct!

        So, when the zener diode let current flow to the base of Q1, the LED emits, indicating that the voltage is above the level has been set with the potentiometer. In this state the base of Q2 is negative (because of R4 and mainly for D3), so the buzzer is off.
        When the voltage drops so the zener offer no more current to the base of Q1, the LED goes blind and the base of Q2 gets positive through R5, which turns on Q2 and makes the buzzer loud.

    • Just one note: you probably can leave out Q2, D3 and R5 safely by simply connecting the buzzer’s negative terminal between Q1 and R4. A typical small buzzer eats about 10 to 20 mA, just like an LED.

      So a small transistor (Q1) rated for max 100 mA can easily feeds the buzzer along with the LED (20-40 mA all together).

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