48 24V battery charger (simple and automatic)

This is a simple 24V battery charger that you can build. The good thing is, you can charge your battery at a high current, 1A up to 20A max if properly constructed (50A is doable when you replace Q4 with higher power capacity). The charger is also automatic, meaning it turns off automatically when battery is fully charged, thus protecting it from damage.
charger schematic diagram
Part List:
R1 - 20 kilo ohms 1/4W
R2 - see R2 vs charging current table below for more info
R3 - 2 kilo ohms 1/4W
R4 - 10 kilo ohms 1/4W
R5 - 2 kilo ohms 1/4W
Q1, Q2 - 2N2907, CS9012, 2N4403 or similar PNP transistor
Q3 - 2N2222, CS9013, 2N5551, or similar NPN transistor
Q4 - FQP27P06 or any P-channel Mosfet with higher capacity
Dz - 1N4750A, or any 27V zener diode
DC - 30VDC, 20A max AC-DC converter or DC power supply
Part List diagrams: click image to enlarge
PMOS pins
2N2907, CS9012 pins
2N2222, CS9013 pins
AC-DC converter
AC-DC converter Parts:
Transformer - 220V to 22V-0V-22V center tap 600VA
D - D2020L or any rectifier diode rated min 20A
C - 2200uF or 3300uF electrolytic capacitor rated 50V

You can increase or decrease the maximum charging current of your battery by changing the values of  resistor R2. The approximate formula for current is given by the equation:
Current = 0.7V / R2  or  R2 = 0.7V / Current. See table below for the approximate result.
R2 vs charging current table

If you have noticed the table, for you to have a maximum charging current of 1A you need 0.7 ohm R2 rated 1W. Since R2 is not available, you can parallel two resistors to obtain 0.7 ohm value. For example, 1.2 ohm and 1.8 ohm in parallel has an equivalent resistance of 0.72 ohm. For more possible values, you can use the parallel resistor calculator to make calculation easier.

Note: You can increase the value of charging current as high as 50A if you replace the PMOS Q4 with higher power capacity and replace the value of R2. Don’t forget to add a good heat sink.

Other useful chargers:
6V battery charger - good for 6V lead acid battery
12V charger - automatic type, current and voltage controlled
Universal charger - charge any battery voltage form 1.2V up to 12v
Mobile phone charger - very simple charger for phones

48 comments:

  1. How can i modify this circuit to charge 12v 70ah battery?

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  2. The modification of the 24V charger is in 12V battery charger

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  3. Hi, great protucts. Need help, where can I connect a LED for full charged indicator on this circuit?
    Than you!

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  4. Just try replacing R1 with 2 kilo ohms and LED (connected in series). Just decrease the value of 2K resistor to increase the brightness of the LED.

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  5. Can this charger be used for All types of Battries, like Li-ion, lead acid etc.

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  6. This charger is for Lead Acid battery only

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  7. i want to connect 2 Sealed lead acid batteries(12v , 7AH) in series. can i use this circuit to charge them?
    Thank you

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  8. You can but i dont recommend doing it.
    It is only good if the batteries are of the same internal resistance and voltage per cell. In reality, these batteries are not the same specs(even same brand or type)
    I suggest try using 12V charger to charge your battery. you can build two for your batteries.

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  9. i think if two batteries have a bit different internal resistances then we can connect them in series.

    because we mostly connect the different value of resistances in series and get their combined effect. i think by connecting 2 batteries in series will give us a combined effect of one battery.

    charger will only need a path to circulate a current through the battery and i think it is not so intelligent to detect either one battery is connected to its terminals or two :-)
    what you say about that?
    Thank you

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  10. Please also tell me the formula/technique to charge a specific battery, because i want to design by my own.
    i.e; how much current and voltage is needed to charge a battery effectively or in desired amount of time?
    secondly what should be the rating of Transformer that should be used to charge the battery?
    hope you will tell me.
    Thank you

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  11. and what happened to the ampere hour rating of the batteries when we connect them in series or parallel? is this rating added when we connect the batteries or remains the same?

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  12. to Qaisar Azeemi,
    about the 2 bat in series, I agree with you and you are right you can charge it that way. My concern actually is if two bat rated 12V (open circuit voltage) each but 1 has 1ohm (value is for illustration purposes only) resistance while other has 0.1 ohm. If charging current is 5A. Since batteries are connected in series, the other battery is 4.5V higher (or lower) voltage than the other one ((1ohm*5A)-(0.1ohm*5A)) that might cause some problem to the battery. Since our overvoltage protection is across the series combination of two batteries,even voltage across is 29V(assumed full charge value), the other battery has voltage of 16.5V while the other is just 12.5V.
    The illustration above is purely simple analysis and may or may not happen in actual.
    Actually if i have two batteries ill charge it in series (same as your idea) but monitor it from time to time.

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  13. to Qaisar Azeemi,
    ampere-hour in series is the same but in parallel is doubled.Just make sure the two batteries are of the same specs when connected in parallel (series also)

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  14. to Qaisar Azeemi,
    How to charge the lead acid battery?
    * over-voltage protection
    * over-current protection
    * temperature control
    i suggest just read the Automatic 12V Battery Charger, it will give you idea how to design simple automatic charger for lead acid battery

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  15. Hi;
    i've implemented this circuit to charg my 24v battry(two 12V seald lead acid battries in series) but it is not charging the battry.

    i just connect a blue LED in series with 3.9k ohms resistor at the collector of Q3(im using C828 NPN) the voltages at the collector of Q3 are 37.4V(as 42.3V is the DC in) the base of Q1 has 41.6V and its collector has same 42.3v(DC in) and also at collector of Q2 and the base of Q2 is also 42.3V. the collectors of Q1 and Q2 are at 41.4V that are feeding at the Gate of MOSFET Q4(im using IRF9610) the source of Q4 is at 42.3v(DC in) while drain is consistently delivering 20.8V at output(either battery is connected or not). my batteries are also discharged and giving 20.8V at output.

    i check my circuit on bread board many times but i m unable to find the fault. hope you will help me.
    Thank you

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  16. i just checked the current. only 0.25mA current is flowing to the batteries, which is approximately nothing. i can say no current is flowing to charge the batteries.

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    Replies
    1. Dear Qaisar Azeemi,
      I have the same problem. Have you found how to solve this problem or any ides?
      Thank you.

      Delete
  17. Don't you think that the drain and source should be swapped in this circuit???

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  18. How much mast be the voltage across the R1 and the output pins?

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  19. I dont know about that but the voltage across the series combination of R1 and Blue LED is 42.3V-37.4V=4.9V - 3.8V = 1.1V

    where 3.8V is across Blue LED and 1.1V is across R1.

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  20. Problem. It always burns up the resistor R5 and is just on power without battery on the board. All connectios are fine. I built it 3 times now and I have the same problem.

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  21. For the LED, i suggest dont use high intensity LED such blue or white since it requires more current and higher forward voltage. The input DC must be around 30VDC.

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  22. what should be the voltage at the output terminals of a charger?

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  23. I think there is a problem on this circuit. I built it many times without any result. All worked fine but no voltage on the output. Finally I thought to connect a voltmeter on the output terminals and supplied the circuit with a variable power supply. I started from Zero volt and went higher. While I went higher the output voltage went higher also. But I observed that when I reached the 12.7V on my power supply the output voltage went straight down to 400mV, then I went back to 12.6V and the output measured 11.45V. I think it is something like over-voltage on one of the Qs. Maybe this circuit is only for 12v batteries or need something else to be 24V charger.

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  24. Dear DH-gm,

    Good Day.

    Can we use this circuit by rectifying 230VAC to 30VDC??? Will it need Pure DC source?? Kindly support me.

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  25. Dear Sir,

    Kindly respond...I have rectified AC voltage to DC and given..but the battery is not charging through this circuit....may i know why this is happening??

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  26. 30VDC input is enough and the circuit should be working fine. Use 27V rated zener diode.

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  27. Sir,

    Our requirement is to charge the battery through 10A.I have constructed the circuit as given above and choosed the resistance also. It is charging. But, before i connecting the battery i have checked the output voltage of the circuit in Scope.There am getting 4V ripple voltage. How to minimize this ripple voltage?? After bridge rectifier i have used 2200uf capacitor(As per the calculation). I have checked after the capacitor and am getting 31VDC with 1.9V ripple.

    Kindly support me how to reduce the ripples? The circuit is working fine. Now i need to reduce the ripples. Please support me ASAP.

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  28. Sir,

    If i charge with that output voltage (Having ripples) the battery life may go right???? Kindly assist me how to make pure DC(with minimum ripples)???

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  29. To reduce voltage ripple, add a very large capacitor across your DC source. Adding large capacitor may reduce or eliminate ripple voltage but does not improve charging.
    Another thing is some commercial charger uses "dirty DC" or pulsating DC since it improves charging and removes sulfate deposited from the plates and return it to the battery acid.
    What I am trying to point out is you don't have to worry about the ripple voltage since based on studies it improves battery charging and can prolong battery life. 1.9V ripple is okay.

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  30. Dear Sir,

    Thank you so much for your reply.

    One more doubt i had.Why we require Q3 there? what does it actually do??

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  31. Initially, the Q3 is off. When the battery is fully charged, the battery voltage is higher than the zener voltage. At that instant, the Q3 turns on and it will trigger Q1 to turn on. When Q1 is on, Q4 is off so is the charging.
    In short, Q3 will turn the charging off when battery is fully charged.

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  32. Dear Sir,

    Sorry sir, I thought to ask what is the purpose of Q2 but by mistake i typed Q3 dont mistake me.. We know that Q1 is there to switch off Q4 when the battery is fully charge. Kindly let me know what is the purpose of Q2.

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  33. Q2 is the one who limits the maximum charging current to be delivered to the battery. The maximum current is equal to 0.7V/R2, where this 0.7V is the base-emitter voltage of transistor Q2. Try Constant Current Circuit to know more about the use of Q2

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  34. Sir,

    Thanks once again. So, this circuit will give constant current output.Sir,When the battery is charging will the current gradually reduces as battery Voltage increases? Will the current comes to zero once battery is fully charge?(Without Zener Diode)

    One more Doubt,What will be the output voltage for this circuit?

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  35. The Q2-R2 combination will just limit the maximum current. If the DC in is high or has enough power capacity, the current will be constant. theoretically, as the battery voltage increases it is expected that the current will decrease. But using this charger, it could be:
    1.The current decreases and when battery has reached its full charge voltage, charging is off.
    2.The current is constant all throughout and when bat the charging process and when the battery reach its full charge voltage level, charging is off.

    Do not remove or replace the zener diode, it plays a critical part in the charger. Removing the zener diode or replacing with much higher voltage value , charger will not shutdown even battery is fully charged (can damage your battery).
    Replacing the zener with much lower value makes your charger not to charge at all.

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  36. Sir,

    Thanks a lot..

    What will be the maximum output voltage for this circuit?

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  37. The maximum output voltage is controlled by the zener diode. Since the zener diode is 27V, we can expect that the maximum output voltage is around 27.7V (0.7V is the Vbe of Q3). but this output will drop to 23V or 24V once the battery is connected.

    Another is if no battery is connected you can experience no output voltage. That is normal, once battery is connected there will be an output around 24V.

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  38. Dear Sir,

    Even my DC source is pure DC am not able to filter the ripples at output of this circuit. I dont know why the ripples coming out from this circuit even my DC source is pure DC. Kindly sugggest me..

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  39. Dear Sir,

    When i connect capacitor across drain and ground am getting pure dc. Can i connect like that? please support me

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  40. You can connect capacitor in parallel to battery (source-ground) or drain-ground.

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    Replies
    1. I have constructed the charger and it is not working.
      I have taken it to a professional electronic workshop and together we have tried to make it work but no success.To cut the story short I have spent several hours not only to construct it but also tried to locate any fault and make it work but with no positive result.
      I have constructed several electronic projects without failure. From what I have read in the comments from other people who tried to construct this project have the same problem with me.
      Please check your design and circuit diagram and let us know in order to make it work

      Regards
      Tsitsios

      Delete
  41. Trying to build or convert a charger to charge a16v agm battery large capacity not sure specs but its starts a high power engine and runs a race car for up to three hours

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  42. Dear Sir
    Can I use this circuit to charge two dry cell batteries in series each one with 12V, 60AH
    please, reply to me ASAP
    Regards;

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  43. Hello.
    Can I use a IRF9540 in place of Q5, FQP27P06?

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  44. Just a note. Q4 (P-MOSFET) has a VGS of >25V when the battery is not fitted or the charger has turned off. This exceeds the rating of this device.

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  45. thanks for the circuit above, please i want to build a charger that will charge batteries of 50amps and above, do you know any mosfet that can do thst?

    regards

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  46. will out put dc 25.8 volt and 15 amp..will charge battery coz my output is reading 25.8....

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