How to make Automatic Charger for a 7Ah Battery Using Atmega

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Automatic Charger for a 7Ah Battery

7Ah Sealed Lead Acid Battery

 7Ah Sealed lead acid battery is a very popular battery which people use in most places like fans, LEDs etc. The reason behind popularity of 7Ah battery is due to its medium size and medium Ah rating. By medium means, its size and ratings lies between the small batteries and large batteries. For medium purposes, the size and rating of battery is ideal.
Automatic Charger for a 7Ah Battery

For example if you want to glow a 20Watt LED then this battery will serves the purpose really well.Before go further, studying the articles related to battery basics, rechargeable batteries and non- rechargeable batteries would be fruitful. Here we will design a 7 Ah charger for this battery.

How to Charge a Battery

You probably already listened that to charge a battery the voltage of charging source should be greater than the battery voltage but the question is how much voltage should be greater and when will I know that you should stop charging now?
There are two questions here. Let’s answer it one by one.
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How much source voltage should be greater than the battery voltage to charge?

It’s not a single value that you can rely on. But an average value at which you can surely charge your battery without hesitation is 20% more voltage than the battery voltage.

To charge a 12 volt battery the charging source should be 14.4 volts. Now the question is that how to attain this volts. I will answer this question later.

When Should you Stop Charging?

During building a charger, the most important question comes in mind that when should I cut off the charging source. This is the very critical point during making automatic chargers because mostly batteries do not bear overcharging and their life drastically decreases due to overcharging. Before I answer this question first understand anot should be charge at C/10 current no matter how big the battery is.

Here,                                    C=Ah

Which means if you have 100Ah lead acid battery then its charging current must not be greater than 100/10=10 A. So the question is how to maintain 10 A current going inwards the battery? The answer is that the internal resistance of ‘12 volts 100Ah lead acid batteries’ is adjusted in such a way that whenever you apply the voltage of 14.4 volts then the battery will automatically draw 10 Amps.
        Now our question was that when should you stop the charging? You don’t have to!! If you are able to apply the constant voltage source of 14.4 volts than the battery will draw current from the source according to its need.

For example in 100Ah battery, if the battery is fully discharged then it will take 10Amps from the 14.4 volts source and if the battery is fully charged then it will not draw any current from the source and at 50% charging it will draw 5amps from source.

       Now I think you are able to understand the procedure of making an automatic charger for sealed lead acid batteries. You can apply this procedure for any sealed lead acid battery but the charging source should be big enough to give the required current to the battery.
Automatic Charger for a 7Ah Battery Schematic

Step by step details to build automatic charger for 7Ah 12 volt seal lead-acid battery through micro-controller

Note: In this article I will give detail description about how to make the charger and how to find out that your charger is working okay or not but if you want to implement any indicators then its your own choice.

By providing constant voltage we will able to make our required charger. We will build a closed loop system in which our charger sense the requirement and act accordingly. We have to provide 7Ah/10 = 0.7 A current to charge the battery. I will use constant voltage technique to apply the required current. ATmega8 micro-controller which is from the family of Atmel AVR micro-controllers is used here to apply 14.4 constant volts.

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