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# 12V Battery Charger Circuit

This battery charger circuit can be used to charge one or more batteries with the total nominal voltage of 12 V, meaning ten NiCd battery or six 2 V lead acid. The circuit is pretty small and can be built in a housing network adapter. The incorect usage is impossible: connecting the batteries with reverse polarity, shortcircuit of the output terminals or power loss have no impact on the charger or battery.

We can use a transformer with 18 V on the secondary and then using a diode bridge to rectify the 18V ac voltage we get 22V dc on C1.

The completely discharged batteries are charged at the begining with a 6 mA current thru R2-D2 and R4-R6-D1. One the bat. have reached 0.3 – 0.5 V, the base-emitter voltage of T1 is high enough to bring the transistor in conduction.

Green LED D4 is used as an charging indicator and opens T1.
There is a 60 mA current flowing thru R5-R6, this means that the charging of a 500 mAh NiCd battery will take 12 hours.

If the battery is connected with reversed polarity or there is a shortcircuit, the power transistor T1 remains blocked and the charging current can not exceed 6 – 12 mA. The current draw at maximum load is around 80 mA.

## Battery charger circuit schematic

### Battery charger PCB Layout

Components List

R1 = R2 = 10K
R3 = 1K
R4 = 5.6K
R5 = R6 = 12Ω

C1 = 1nF
C2 = 220µF / 35V

D1 = 1N4001
D2 = D3 = 1N4148
D4 = green LED

T1 = BD140
T2 = BC546

##### Related Tutorials

• johnhoon

Hi, just wondering that this charger can charge 12V car battery or not ?

• DANTE

i HAD ONE OF THE Freight Harbor Tools, a Battery Float Charger, IT WORKED GREAT . I reversed the leads and pop goes the Transistor. I took it apart but can only find part of code.

F 136 and IP 41C. Can you help me with a replacement Code.
I know the charger was cheep but I am 76 yrs and don’t have a lot to do. So I took it apart n ow I need a part.
Thanks

• Jim Keith

The part # is TIP41C –very common device.
It is available from DigiKey for \$0.67 each.
Specs for cross reference are: NPN, 6A, 100V, TO-220 package
and there is no need for a 100V rating –60V should work fine.
Good luck!

• Robert

While might be good for charging NiCd batteries (that I don’t Know), this circuit is useless for 12V Lead Acid Batteries(that I do know well). A completely discharged 12v Lead acid Battery has to be charged on 3 stages.1 – Bulk charge (constant current 4A to 10A or more), depending on the battery size,2 – Topping Charge, and 3 – Float charge. To maintain a battery you can use a Trickle 13.2V up to 500mA, a Battery Tender with Hysteresis Threshold 12.6V – 14.5 V up to 1.5A, and a Smart Battery charger up to 20A, with equalizing and battery reconditioning feature.
Buy at Freight Harbor Tools, a Battery Float Charger, Automatic on sale for \$9.99 or (\$6.00 with coupon). The factory 13.2V is only a promise. Open the regulator, replace the 56 Ohm resistor marked as VR1, with a 100 Ohm trim pot, via two tiny wires. Now you have a Voltage regulated battery trickle, from 12.5 V to 14.25 V. Float charging a battery is like dancing on a needle. At 11.4 v the car battery is discharged and at 12.9V is fully charged. Increasing the voltage above 2.26V per element or > 13.5V, the battery start boiling. Tiny current as low as 20mA, for long period of time oxidize the positive elements. I set the regulator at 13.25V on the car and forget. I have more than one car, and while not in drive I trickle the battery to keep at 12.9 V on REST (without charging element). This is only the first AID. Periodically you have to equalize the elements of Battery, This can be done increasing the charging voltage, periodically, to 14.5 V for 2 hours. While the weaker cells are charging the strongest cell are boiling. The goal is to equalize the voltage 2.26V per each cell. Periodically check electrolyte level and top it with distilled water on mark. If decide to maintain the battery, do not buy sealed maintenance free batteries. To be able to equalize you need a Battery tender with Hysteresis Threshold, that start charging at 12.6V and stop charging at 14.5V, giving the battery a chance to rest or relax.

• may

what will be the output voltage after the rectifier diodes and its output voltage in the circuit if the transformer secondary winding is 12V??

• Kuberkoos

To calculate the DC voltage after rectification of a sinewave AC voltage multiply by 1.414

• saideep

can you please tell me the necessity of two transistors t1,t2

• Maikl

T1=BD140 is NPN transistor, not PNP as shown in scheme.

• Popescu Marian

You must have used some strange datasheet if you say that BD140 is NPN… It is PNP and has been like this from its birth, when the silicone met the plastic case for the first time.

• Shibu

Please make the PCD layouts also large as the circuit diagram. That way it is easy to take a print of the whole article

-Shibu

• Mosfetti

If you rectify 18 Vac and smooth it to DC, how do get 22 Vdc? Multiply 18 x 1.414 gives 25.5 Vdc

• Tesla

Does this circuit have over charging protection?

• md