Thursday, November 3, 2011

UPS Battery Charger

The battery charger used in the ups are very slow in nature, means the charging current is low (about 1 - 2 A). So they required a lot of time to charge a high AH batteries. This charger is build around LM338 that can charge the battery with 5a current and fast in nature.

This circuit has Auto cut off facility also. This High current charger can be used for the Fast charging of sealed lead acid batteries. This charger can give 5 Ampere current for quick rejuvenation of the battery. It uses an Adjustable voltage regulator so that 6 volt and 12 volt batteries can be charged. When the battery acquires full charge, charging process will be terminated.

LM338 is a high current variable voltage regulator that can provide 2 to 25v DC with high current output. Its important features are 7 amp peak output current, adjustable output down to 1.2 volts, thermal regulation etc. It is easy to use and require only 2 external resistors to set the output voltage. 
LM338K has only two pins; Vin and Adjust. The body of the device acts as the output pin. On the bottom side, the first pin is adjust and the second pin is Vin. It requires heat sink to dissipate heat since it is draining high current. Two resistors are required to set the output voltage. Resistor R1 (220 Ohms) act as the programme resistor. Usually 1.25v reference voltage (Vref) develops between the output and adjusts pins. The reference voltage is impressed across the programme resistor R1.The voltage across R1 is constant so that constant current flows through the output set resistor VR1.

Input voltage is obtained from a 0-15v, 5amp secondary transformer with the 10 or more amp bridge rectifier. Capacitor C1 makes the DC ripples free and around 16 volt DC enters into the Vin input of the regulator. VR can be used to set the output voltage from 2 volts to 15 volts.

Auto cut off circuit is built around the 5 Ampere Darlington NPN transistor TIP122. When the terminal voltage of the battery rises above 12.5 volts, Zener diode ZD conducts and inhibits the working of the Regulator LM338. When the battery voltage drops below 12 volts, Zener turns off and the regulator again provides charging current. Note for 6v battery use 6v zener instead of 12v.

A fully charged 12 volt battery should have a terminal voltage of 13.5v and that of a 6 volt battery 6.5v. To charge the battery, set the output voltage for charging as 14 volts for 12 volt battery and 8 volts for 6 volt battery.

UPS Battery Charger
Components Required:
T1 .................................. 220v to 15v 5amp
R1-------------------------120 ohm 1 watt
R2(VR)-------------------5k ohm pot
R3, R4--------------------1k ohm 1 watt
D2 (ZD) ------------------12v 1 watt zener
D3-------------------------10 amp Bridge 
C1--------------------------1000uF/50v Electrolyte
C2--------------------------100nF/50v Poly
C3--------------------------100uF/25v Electrolyte


  1. can i use 12-0-12 ct transformer

  2. yes you can, but it should be of 5 amps.

  3. Replies
    1. max amps of lm338 is 5. there is no use of using transformer higher than 5amps.

  4. Can You also introduce a Ammeter (panel) to monitor how many amps

    thanks in advance

  5. Hii nice post. This blog is very informative for all and most uninterrupted power supplies sold for computers 'switch' power, running a small inverter when power is interrupted, then switching back to 'normal' power when it's back on.

    UPS Battery Backup

  6. Hi, I asked a query if an ammeter can be introduced into the circuit, very strange that after waiting for almost a year, yet to get the feed back... thanks

  7. i used this circuit but the is i am getting 19v output when using 15volt transformer and 15v output when i use 12 volt transformer. Variable resistance not decreasing voltage.. Plz help me