This one transistor cheap battery charger circuit is designed to automatically switch OFF the supply to the battery as soon as the battery has reached its full charge level.
A very simple single transistor automatic battery charger circuit is described in this article, which uses just a single transistor for the voltage detection as well as for automatically disconnecting the battery from the supply when it gets fully charged.
Contents
Circuit Operation
As shown in the diagram we can see a straightforward configuration where a solitary transistor is connected in it’s standard operating mode. The circuit functioning may be understood with the help of the following points:
Considering the battery to be charged is a 12 volt battery, we know that it is advised to charge the battery until it reaches between 13.9V to 14.3 volts.
The transistor base voltage is adjusted using the preset P1, such that the transistor just conducts and operates the relay at around 14 volts.
How to Adjust the Cut of Thresholds
This adjustment becomes the high voltage trip point of the circuit and is used to switch OFF the charging voltage to the battery when it gets fully charged or its voltage reaches around 14 volts.
The lower trip point of the circuit cannot be adjusted as this circuit is too simple and does not incorporate the low voltage detection feature.
However the transistor is itself equipped with a switch OFF feature in case its base voltage becomes too low.
Typically a general purpose transistor like the one which is shown (BC547) when adjusted to switch ON at 14 volts may have the lower threshold of around 10 volts, when it might get just switched OFF.
This wide voltage difference between the high set threshold and the lower natural threshold is because of the involved big hysteresis with the design. This acts like a natural hysteresis in the design.
The lower threshold of 10 volts is dangerously low and we cannot wait for the circuit to restart the charging process until the battery voltage falls to this dangerous 10 volts level.
Allowing the battery to discharge down to 10 volts can make the battery flat permanently and reduce its life. . Therefore to eliminate this issue the circuit needed to somehow reduce the hysteresis level. This is done by introducing a couple of diodes at the emitter of the transistor.
We know that normally a 1N4007 diodes would drop around 0.7 volts across it and two if them would make a total of 1.4 volts. By inserting the two diodes in series with the emitter of the transistor, we force the transistor to switch off 1.4 V earlier than its normal specified limit of 10 volts.
Therefore now the lower operating threshold of the circuit becomes 10 + 1.4 = 11.4 volts, which may be considered just OK for the battery and for the automatic restart of the charging process.
Having both the thresholds updated as per the standard charging requirements, we now have an automatic automotive battery charger that’s not only cheap to build but also smart enough to take care of the battery charge conditions very efficiently.
Circuit Diagram
Parts List for the proposed one transistor automatic battery charger circuit
R1 = 4K7
P1 = 10K preset,
T1 = BC547B,
Relay = 12V, 400 Ohms, SPDT,
TR1 = 0 - 14V, current 1/10th of the battery AH
Bridge diodes = Equal to the current rating of the
transformer,
Emitter diodes = 1N4007,
C1 = 100uF/25V
PCB Design
Have Questions? Please Comment below to Solve your Queries! Comments must be Related to the above Topic!!
Hello sir( happy republic day) … My. Question is that sir. I have 13-0-13 4a transformer and i make battery charger from this transformer by using 4 IN 5408 DIDODE, 4700uf/35v and the dc voltage come from that is 18v how to simply adjust 14v 1A from given cricuit diagram with their autocutt function… Please suggest me sir…
Happy Republic Day Apurva. For setting the circuit, apply 14V to the circuit from an external power supply, and then adjust the preset such that the relay just clicks. That’s all, your circuit is all set now.. After this you can connect the discharged battery and switch ON the transformer, once the battery reaches 14V the relay will cut off. Remember you will have to connect the battery first and then switch ON the transformer. And you must NOT use 4 amp for a 7Ah battery. If your battery is a 7Ah, use a 1 amp or 500mA transformer. Or… Read more »
For ur last paragraph where u write.. (alternatively, you can connect a 12V motorcycle side indicator bulb in series with your battery positive for your existing 18V 4 amp output. The lamp will take care of the current).. It means to say i can use this 4amp transformer for charging the 12v 7ah battery before connecting 12v bike bulb for reducing the current… To the dc 18v power coming from the Bridge rectifier..
yes that’s right. The bulb should be 12V 10 watt as shown below. Use only one in series:
After connecting it then how much amp will be reduced.. Sir..
1 amp…use only one bulb!
Are the two capacitors electrolytic or normal capacitors? I’m going to get the components today
They should be electrolytic type.
Thanks and voltage ratings?
any value that may be equal or higher than twice the supply voltage
Hey I added the transformer of greater rating and circuit worked but now I see that my transistor has blown aka Base emitter is open… If I were to place a resistor in series with the Base would it prevent damage to the transitor? also the relay buzzes even with a flywheel diode
Best regards Adrian Sassin
Hi, the transistor can never get damaged because R1 is already present. R1 will restrict the current in case the preset adjustment is shorted accidentally.
The transistor can possibly burn if the flywheel diode is not good. You can try changing the diode with a good quality 1N4007 diode.
To stop buzzing you could try adding a 10uF capacitor across base/emitter of the transistor and and also a 100uF capacitor parallel to the relay coil.
Greetings I used a 15V 1A transformer to charge the battery for the circuit but when I charge the circuit noting I get 0.4 mA on the Dc side and my battery starts to make hissing noise… It cuts off at 13.7 but after and hour or two it goes to 13.4V what is the reason for this hissing and the discharge and what can I do to rectify the problem? I’m using this circuit in conjunction with and emergency light… my battery is a 12V 7ah
Hi Adrian, that looks quite strange, because at 1 amp a 7Ah battery will never hiss. Try adding a current control stage using the following circuit and set the current to 500mA:
https://www.homemade-circuits.com/universal-high-watt-led-current-limiter/
If still you hear the weird sound then the problem could be in your battery.
the one transistor cut off circuit explained above may not be very accurate since the preset adjustment could deviate slightly overtime, giving rise to errors, you can try using fixed resistors instead of the perset to get reasonably consistent cut off response from the circuit
Hey I wanted to ask I’m getting 0.45mA when the battery chargers.. How can I increase current flow to 0.7A to change lead acid 12V battery 7ah… Transformer I’m using is 15V 1A and is quite warm after a few hours
Regards Adrian
Hey Adrian, your transformer may not be actually rated at 1 amp, because most transformers usually print slightly higher rating than its actual capacity. Try some other slightly higher rated transformer, may be upto 2 amps and see the response.
If still the current does not increase then your could be having problems.
Circuito não esta funcionando….tem um erro no R-1 e tambem no C-1 negativo, …. estão sem coneção ( Design do PCB ) só funcionou depois de fazer mudanças
Obrigado por apontar o erro no design do PCB, ele foi projetado por uma pessoa externa, não eu, mas de qualquer maneira, estou feliz que você possa corrigi-lo e fazer com que ele funcione para você
Greetings if I use a 12V 7ah lead acid battery… What will the charging current be that will charge the battery?
Hi Adrian, it should ideally 0.7 amps, but 1 amp will be just fine!
Hi I wanted to ask.. The Base voltage I measured when it starts conducting (when charger cuts off) is 1.45V? why is this voltage so high? Regards Adrian Sassin
Hi, it is because of the combined voltage drops from the two emitter diodes and also the transistor itself
But so far I only connected on emitter diode not two
so it is 0.7V for the diode and 0.7V for the transistor, resulting in 1.4V
sir can I use this circuit to charge my 60ah battery.. if yes. what are the changes required in the circuit
Manish, you can do it just make sure to select a relay with contacts rated at 10 amp
Sir,I built a battry charger using a step down tranfo and this circuit. The challanges am facing is that, If our Main AC is above or below 220v the DC output is varying either above 13v or below 11v and this may damage my battery.So an Idea came to my mind and I changed transfo with a UPS tranfo that has 175v, 195v, 220v and 250v primary and 7v-0-7v secodary and am using outer taps that gave 14v to charge battery.I used four (4) switches, 1st switch From Main to 250v of tranfo, 2nd swtch from main to 220v of… Read more »
Aminu, if your step down trafo is 0-12V rated, and if you use a bridge rectifier with a high value filter capacitor, that will be enough to raise its output 14.5V at 220V.
Or alternatively you can use a 0-24V trafo and use an LM338 regulator to regulate the output at 14V
If you are thinking of employing a voltage stabilizer circuit that will be an overkill..
Sir I did it, and it's working fine.
The transfor is 12v 5A
The bridge rectifier is 10A
The Diode i.e +recitifier to battery+ is 6A 400V
And the Caps is 4700uf 25v.
The output I got is around 14v to 16v.
Now, How can I set the above circuit for autor cut-up?
Thank you very much for your kind help.
Aminu, first adjust the preset such that the slider completely touches the ground side of the preset.
now apply the supply voltage to the circuit….now very carefully and skillfully adjust the preset upwards until the relay just switches ON.
that's all your circuit is set.
…while practically charging the battery make sure the discharged battery is connected first and then power is switched On
the power supply current must be 1/10th of the lead acid battery AH
thanks for the reply sir.
So, do you means, I cannot charge other battery type than lead asid (as you have suggested in last reply) with is circuit?
you can use any form of battery, lead acid is not compulsory, but the charging current can be different for different types of battery, for example if a Li-ion batt is used then the charging current could be as high as its AH value, but for lead acid it is recommended to be 1/10th of its AH
Good Day Sir,
I am done with this battery charging circuit. But the diode you suggested me to use between rectified positive power from transfo to the positive to battery is getting hot. And it is a 6A diode. I tried finding higher current in the market but I didn't get one.
So, can I use MBR20100CT, which is 20A 100v three pin diode in form of transistor?
If yes, I need your help on how to coonect it.
Thanks very much Sir.
Aminu, you can clamp the diode with an aluminum heatsink for cooling it. you can innovate the clamping method by some thinking
Okey, thank you very much sir.
I will try this.
that means MBR20100CT cannot be use, which is very simple to clamp on the heatsink?
Sir, can I connect two or three 6A diodes, in parallel, to double the current?
Aninu, diodes should not be connected in parallel. the heatdink method will be more effective
Sir, KBPC2504 It's a 25A diode brigde.
Aminu, connect a 2200uF/25V filter capacitor and then check the response.
also check whether the bridge is getting warm or not, without load….if it is then your connections could be wrong.
Thank you Sir.
If I got you right, connecting a diode from a 12v tranfor to a positive of 12v battery can charge the battery to 14v?
Aminu, the diode needs to be connected after the bridge/capacitor network, if you use only the diode without the bridge then the output voltage will not reach even 13V
Okey Sir.
Sir, Can I use KBL406 diode bridge and 12v 5A transfo with this circuit to charge a car battery. i.e 12v 50A battery?
Again, I had once connected KBPC2504 to a 14.2v transfo and the voltage at + and – output of the tranfo at diode dropped to around 11v. What mades the voltage dropped to 11v?
Thanks for the help you are offering us Sir.
Hi Aminu, what is KBPC2504, please specify….
Okey Sir.
Sir, could you help me on how to rectify 12v to 14v? And, How/Where do I connect the 60ohm resistor and of what watts?
Thanks for the help.
connect a diode instead, in between with the charger positive and battery positive, this will drop 0.7V and never allow the battery to reach 14.4V or over
charger(+)—–>|——-battery(+)
1n5408 or 6A4
Thank you Sir.
Sir, If am using 12v tranfor, is there any harm to battery if it is fully charged?
So that, no need to use battery charge circuit for battery full cut off.
thanks one again.
12V after rectification/filtration will become almost 14.5V…although it won't harm the battery much, it is not recommended to keep the battery connected with a 14.5V supply after it is fully charged. use a low ohm resistor around 60 ohm in series this will allow you to keep the supply connected permanently.
Sir,
Using the above circuit, can I replace the relay with DTDP, so that, I can configure it to off the charger from main power when the battery is fully charged.
And, If I apply transfo with any volts above 14 (as specify) will it damage the battery? let say the transfro volt is 18 to 20 while the current remain 1/10th Ah of the battery.
Thankd you Sir.
Aminu, you can do it but you may to incorporate an ON/OFF push-switch in parallel with the contacts which is wired with the mains, this push-switch will be used to switch ON the circuit initially.
18V will damage the battery once it's fully charged even if the current is 1/10th…so make sure the voltage is not above 14.5V
Sir, could I use this circuit for 110ah 110ah 110ah 12 volt D.C. battery?
Hi Raja, yes you can connect the mentioned battery by suitably upgrading the relay to a 20amp relay and by replacing the BC547 with a 2N2222
Hi,
Please share the 3 and 6 volt auto cut battery charger circuit.
Regards
DEV
you can try the following design:
https://www.homemade-circuits.com/2013/12/usb-automatic-li-ion-battery-charger.html
no there's no proper place to add LED indicators in this cirucit…
hello, try adding a few more diodes with the existing emitter diodes D5, D6 and see if it makes any difference..