Tricky 12V Battery Charger Circuit

Here is a crude but efficient tricky charger for Lead Acid Battery. It uses a 12 volt car bulb as current regulator and charge status indicator. The brightness of the bulb indicates how much charge is flowing into the battery. When the battery becomes fully charged, lamp turns off. If the lamp is staying on with full brightness for more than 30 minutes, it indicates that the battery is dead and is not accepting charge.

Charging current is obtained from a 15-0-15 volt secondary 2 Ampere step down transformer. Diodes D1 and D2 are rectifiers which can handle 3 ampere current. In order to give “Dirty DC” for charging, a low value filtering capacitor C1 is used. So that the DC voltage will have some ripples which is necessary for better charging of lead acid battery.

Tricky Charger Circuit diagram

The trick of the lamp is interesting. A 12 volt car tail lamp bulb is used in the circuit. It is connected in series with the positive output rail so that current flows through the bulb into the positive terminal of the battery. From the positive terminal, current passes through the battery chemistry into the negative terminal and then returns into the transformer. So the current flowing through the bulb depends on how much charge is using by the battery. When the charger is connected to the battery, the lamp turns on only if the battery requires charging current. OFF state of the bulb indicates that the battery is dead. If the battery holds some charge, bulb will turns on. If the battery is partially discharged and holding 50% charge, bulb will light brightly when the charger turns on. Then the brightness gradually reduces and finally the filament appears as a red hot line. This indicates that the battery is fully charged. The bulb also restricts the flow of current like a resistor.


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  • V.Sambath kumar

    Hi Mark,

    Yes, one can use resistors and diode for limiting the current for charging the batteries. We can also provide high/low voltage cut off for the safety of the batteries. But, as this is a very simple charger (As the name suggests), I have not gone to that extent.

    PS : I AM that good 😉

    V. Sambath kumar

  • Mark

    Well done V.Sambath kumar

    You compensated for too high of a voltage from your transformer and DC supply, and too much current, and also managed to find light bulbs that did what you wanted. You are either very good or very lucky.

    I have tried many light bulbs, in series and in parallel, and combinations, to give me the exact voltages and current I wanted for various stages of charging. I have rarely found bulbs to do exactly what is ideal for use as a visual indicator.

    The main reason my bulb setups haven’t worked like described, and like you described; being dull and bright and off, at the correct times, is because I alter the circuit for the different stages of charging(change bulb sets) AND I have been charging batteries in various conditions.

    Under conditions of a basic single stage charging, of a good condition battery, you can indeed get a bulb setup that goes out at the correct time.

    A way to improve your charger so that you have a better indicator, and control of the charging for various types of batteries, and for batteries in good to poor condition, is to add three things in place of light bulbs.

    Add a variable resistor, to replace the bulbs, so you can make adjustments for all conditions. Add both a volt meter and a current meter as excellent indicators of state of battery, state of charge, and rate of charge, and for doing proper stages of charging that are best for these batteries.

    When I get the parts and am done building my charger, I will be able to: use it to charge virtually any type and size of flooded battery, 6 or 12 volt(made two secondary coils in the transformer I rewound), create 4 charging stages(3 most of the time), and use it to restore batteries in poor condition of sulphate build up. I plan to add overload protection and reverse polarity protection and a fan to keep the transformer and circuity cool.

    Sounds like you have the skills to do this too if you wanted.

  • V.Sambath kumar

    Hi Mohan ,

    This circuit is a pretty good circuit, i have tried the same ckt in 1980 in my work shop but with a small change i used 2 nos 12v bulbs in series.since the xmer is 15v ac it will give 21v dc which will be harmfull to the 12v one no bulb used in this ckt,also since the current limiting is less it will heat up the batteries.when using 2 nos 12v bulbs current limiting will be better.after disconnecting the bat we can short the o/p of the charger ckt and visually observe and get an idea of the intensity of glow of the bulbs during a bat short cond. during normal charge cond both the bulbs will glow dim and when the bat is fully charged both the bulbs will go off.

    Thank you
    V. Sambath kumar.

  • phoe pyae

    I have electri tricycle but I can’t recharging.please help me.

    • Mark

      Insufficient information phoe pyae. If you have a volt test meter, confirm that your charger has the correct voltage. Test the battery to see if it has the correct or some voltage.

  • Emeka

    I doubt that. Because I Ave had the opportunity of measuring the open circuit voltage of most chargers, it’s usually similar to voltage measured when it is hooked up to a battery. To be honest I doubt that. But I will give a try. I don’t want to destroy the battery

    • Mark

      Nigon is correct to check results in circuit, as there will be some voltage drop. You are correct though that it will not be enough.

    • Mark

      Emeka. I also doubt that. All chargers I tested, and have now built, and have repaired, measure or will ideally measure 13.8 to a bit over 14 volt. A couple were 15 volt and were a little more difficult to work with. 21 volts is too much. You can use resistance to drop some of the voltage to get the voltage you want across the battery. Hopefully the resistance will not drop the current so much that it is lower that what you want.

      Sometimes you can get at the secondary coil windings of the transformer and unwind some turns. If the transformer is centre tapped like the tricky charger, you probably can not do this. If you have a transformer with only two leads you may be able to remove perhaps 5 windings to get a decent voltage. If your transformer comes apart or you have an old microwave transformer your could build it/rebuild it.

      The microwave transformer I rewound has two separate secondary windings now. One is about 8 volt AC for a 6 volt charger at about 15 amp capability. The other winding is about 14 volt AC for a 12 volt charge at about 30 amp capability. I will be using a variable resistance along with a volt and current meter to absolutely control the stages of charge.

  • Emeka

    Thanks for all the comments. But I want to be clarified on something. For instance I use a 220/15V 2A transformer to implement the charger, after rectification it can be noticed that the available DC voltage will hover around 22-21VDC.this is not good to charge a 12V battery.I am at the verge of giving up, how do I solve this problem, should I use a resistor to share the 20VDC by connecting it to ground or what,,,i will be expecting a reply

    • Nigon Kouk

      Check the voltage output after you hook it up and start charging with it, the voltage should drop a few volts or so because when amperage is being used the voltage will drop.

  • victorx66

    I have had another dream when i recharged my battery with 120 dc and simple diode to turn on my 12v to 120 dc inverter?”
    Correct, that is a dream, since inverters produce AC 😉

    I wrote a little fast and instead of putting inverter ac i did dc gg, sorry for the fast typing, anyway i think it nothing for a clown like you.

    Since you are a little paranoid keep the bulb, you can give it 2 uses
    1- you can put it in your ass what ever you wants
    2- it can be useful sometimes if you wanna give an low ampere charge.

    As suggested i will tell you that your trickle charge may work if you pull out the cap.

    And since the current flows from negative to positive pole, will be most efficient if you move the bulb to the negative side and leave the positive line free

    Thanks bro for let me wrote some personal experiences in this funny web
    Dont forget present to the community your big academic degrees GGsss. (lol)

  • Mark

    There is another charge stage that can be done now and then called an equalization charge to remove stratification and sulphates.

    When a battery is fully charged and settled and cooled, you do a charge that is an over charge, that will make the battery bubble. Using over 13.6 volts you do not leave the battery this way very long, perhaps 15 minutes.

    If you had your battery on a dumb charger without current limiters, and went past the absorption stage then you already did and equalization charge, likely for too long.

    This is why I am making a charger with just over 14 volts. I can use a bulb or bulbs or resistors to achieve a proper constant current charge for the bulk stage, and limit the voltage at the end of the absorption stage to 13.6, and also do a float and an equalization charge. FYI

  • Mark

    This is what you want to happen when charging, ideally.

  • Victorx66

    Mark says:
    When I apply that to a circuit with 120 volt DC, with a 100 watt(aprox. 144 ohm) bulb in series(suggested by Victorx66), the result of the voltage across the battery varies from less than 1 volt to over 20 volts across the battery. NO CONTROL! Ruined batteries.

    You could put this circuit together and luck out, because the battery happens to be at a state of charge and therefore resistance, that, when you test the voltage across the battery you get less than 14 volt; and if you don’t bother with the math, or further testing, you might be fooled into thinking that this circuit works well, and magically adjusts to whatever battery you try and charge this way; but you have fooled yourself.


    Then it was a dream when i turn on my ham radio after charge my battery applying 120dc and transforming it to 12dc
    I have had another dream when i recharged my battery with 120 dc and simple diode to turn on my 12v to 120 dc inverter?

    Man this is ridiculous.
    The circuit 120DC works in many cases; but it remains to be dangerous, so I proposed the low voltage charger with no more than 14dc-14.5dc

    The bulb is acceptable for example when a battery is bad will consume too much current and the bulb can protect the source in the case of no protection of regulation, also it has a nice appearance but it is all; You can use it if you want but it is not impressible; when loaded with 120dc it is

    In addiction lead-acid Chargers does not bring any bulb or cap, even a simple resistance, because we don´t need to stabilize anything, We need a natural pulse current and not filter in any sense, since more wilder this current can be it will be much better for battery acid

    • Mark

      “I have had another dream when i recharged my battery with 120 dc and simple diode to turn on my 12v to 120 dc inverter?”

      Correct, that is a dream, since inverters produce AC 😉

      “lead-acid Chargers does not bring any bulb or cap, even a simple resistance, because we don´t need to stabilize anything”

      Incorrect. I have another 15 amp charger that has a 15 ohm resistor included, exactly like the tricky charger, because these are necessary at times.

      Of course you can charge a battery with all sorts of circuits, that does not mean you are charging the battery well. Even a car alternator does not charge a battery properly that has been discharged too much.

      The chart I included below is from a very reputable battery charger and inverter manufacturer, and is what nearly every battery manufacturer site I have been studying says is proper or best for the battery. You do whatever you want, I do not care. I am posting corrections here for the benefit of those who want to learn and not blow themselves up, or want to make their batteries last.

      I found a fairly simple circuit that can be added to the tricky charger circuit that takes the place of a bulb or resistor and vastly improves the job of charging a battery well. I may not bother to post it because of you.

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