Hello there!! I have 2-4.2v Lead acid batteries. I want to make a charger Usng lm317. My question is: Dont I have to change the resistor values if i want to charge a 4.2v Pb acid? And what shall be the output of the power outlet if I want to charge 1 batt at a time. Shouldnt the Led turn off when the batt is fully charged? Suppose, I leave it there for a day after full charge, will it cause any problem? The circuit i am following is given below:

If you think there is a better circuit plz inform me about it. Thanks in advance for your help.

 

It looks like the circuit is a quasi constant current source forcing about 200 mA of current to flow through the battery when it is connected. The LED should light anytime a battery is connected. The 317 forces a1.25V at it's output across the 6.8 Ohm resistor.

 

You need something more like this. Set the output voltage, under no load, for the proper float voltage for your battery. This circuit will provide constant current until the battery is nearly full, then cool down to just enough to keep it topped off.

 

Thanks for answering. what shall be the values of the resistors, transistors, etc if the required V is 5 and the V out of power outlet is 9? Can a LM317 calc. help?

 

The most difficult part is determining the float voltage. It is about YOUR battery. Recent developments in lead-acid batteries have changed the answers. An AGM (Absorbent Glass Mat) battery will need a different voltage than a SLA (sealed lead acid) or a, "add water here" battery.

Read the label.

 

Sorry for not mentioning it- It is a battery extracted from a Flashlight. It has two vents which are covered with rubber caps. The caps looked sucked in after discharge and bulge up when charging. I guess it is a "add water here" battery. nothing is written on the batt except"YG10042".The battery is this:

 

This is how I'd do it.
Adjust the voltage until the battery barely does not bubble when fully charged.

 

Thanks again for answering!! any more suggestions? Will the led turn off after the float charge is complete?

 

No. This is a float charger. It maintains the battery at, "full" for months at a time. It doesn't turn out the light when it's done.

 

Thanks for answering!! doesnt the arrows in your circuit represent earth?(negative)?

 

No. The empty triangles represent, "common", and common is the negative side of the battery in this case. One black triangle represents a power supply, one represents an LED. One arrow represents the emitter of an npn transistor, and one arrow represents the center connector on a potentiometer.

 

Emergency!!

I somehow added too much water to my lead acid cell and it spilled some acid around it when i charged it!! What shall I do now? it has formed white spots in the floor. What about the battery? Should I add acid to it or water. I cant buy H2SO4 because of my paranoid parents. Will giving water to it enough? Or shall I do nothing? Please answer. Sorry for the behavior. I know i have to ask politely. But please answer quickly!!

 

I did not add anything. The batts V is 4.22 90mins after charge. Is that natural?

 

Don't add any acid or H2SO4 to the battery.
Don't add any more water to the battery.
Ask your mum for some baking soda, add some water to the baking soda to make a paste.
Get some paper towels and rub the paste to the spill on the floor.
This will neutralize the acid.
Then clean up with water and wipe until dry.
(You did not say what kind of floor it is, wood, carpet?)

When charging lead acid batteries, place the battery on a rubber mat to collect and contain any spills.

 

Thanks for answering. The floor is made up of marble. After a day, the Batts V is 4.16. Is that normal?