With charging of a battery many questions come to mind like;

  • In order to fully charge a battery how many amps are required?
  • In how many hours does a battery charge completely?
  • What measures can be taken to extend the life of a battery to about 2-3 times more?
  • How can a battery stay full for a long time?
  • Last but not least, why must a lead acid battery be used to power many circuits?

battery lifebattery charger

Let’s carry out a small experiment of creating an emergency light. It’s inevitable to choose a 12V battery as that’s what that is used normally and is quite popular. Its load device like a 12V lamp can also be found easily.

Why must a Lead-acid Battery be used?

Let’s have a comparison between lead-acid batteries and other types of batteries, such as 1.2V Ni-MH battery, 10 batteries.

For basic use, lead acid batteries are easier to use and are much cheaper whilst providing enough electricity so we will choose that here.

It’s a known fact that when we run out of battery power it can simply be recharged many times with a normal service life of about 2 years (according to my experience).

Many people wonder how to charge a battery many times.

Principles of charging a battery properly:

An important thing to quote here is that the principles of car battery charging and motorcycle battery charging are the same. Factors to consider include:

  • Battery capacity Ah
  • Remaining battery capacity before charging. How much is left?

12v lead acid battery

How many amps does a battery need to charge?

First, we must check the size of the capacity of our battery. It has units of Ah. Check out the example is given of the battery that is used. It is a 12V 3Ah size battery that is used in a small motorcycle.

I got this battery for free from my brother who runs a motorcycle repair shop. He said that the owner of the motorcycle will replace it when the battery is about 1 year old but it can still be used for another 2 years and will maintain a constant charge.

I have used this size because it is enough for small electronics for example a solar cell system in the garden which is equal to using an LED light bulb 12V 2W (0.16A).

This light bulb will light up for as long as about 3A/0.16A = 18 h. Isn’t it nice?

To cut the long story short, charge your car and motorcycle batteries properly with a low current and for a long period of time. It will benefit battery life in the long run.

A battery must not be charged with high current in order to make the battery fully charged quickly. This can damage the battery resulting in the overheating, deterioration, or swelling of the battery.

When you but a battery, the manufacturer will recommend a proper charging method.

yuasa electrically filled battery

  • Standard(low current): 0.3A for 5 to 10h
  • Quick (high current): 3A for 0.5h

Hence its clear that to fully charge a battery, I charged with 0.3A of current for 10h to 12h to charge it to full.

Calculation method

Current to be charged = 0.1 x battery capacity (Ah)

For example

  • Motorcycle battery, 12V 5Ah: 0.1 x 5 = 0.5A. So it should be charged with a current of 0.5 A.
  • Car battery, 12V 65Ah: 0.1 x 65 = 6.5A. So it should be charged with a current of 6.5 A.
  • Car battery, 12V 65Ah: 0.1 x 100 = 10A. So it should be charged with a current of 10 A.


Most of my circuits are suitable for small batteries below 10Ah as they charge only as low as 1A. Some people need to charge larger batteries for various purposes. Therefore they require a very high charging current that is much more than 10A. Hence to fulfill their needs, they need a specially designed circuit.

How much voltage can be called a charged voltage?

Usually a 12V car battery and a 12V motorcycle battery have 2 charging modes.

  • Float charge (stand-by use) is charging at 13.6-13.8V.
  • Equal charge (cycle use) is charging at a voltage of 14.2-14.9V.

In conclusion, it should be used with a voltage of no more than 15V and if it is lower than 13V, the charging will not be sufficient enough for your use.

How many hours are required to charge a battery?

Take a look at the graphs on the datasheet of our battery model. You will notice that most of the time when the battery is discharged there will still be about 35% Ah battery capacity left. Therefore, it needs to be charged to an additional of 65% to make the battery full.

Calculation method

Additional Ah amount = 65% x battery size (Ah)

Charge hours = amount of Ah to be supplied ÷ current that should be stored

For example

  • Motorcycle battery 12V 5Ah

Amount of Ah to be charged = 0.65 x 5 Ah = 3.25 Ah

Charge hours = 3.25Ah ÷ 0.5A = 6.5 hrs ≈ 7 hrs.

This calculation refers to the 12V 5Ah motorcycle battery when the battery is dead. We should charge an additional 3.25Ah by charging with 0.5A current for about 7 hours.

  • Motorcycle battery 12V 5Ah

Amount of Ah to be charged = 0.65 x 3 Ah = 1.95 Ah

Charge hours = 1.95Ah ÷ 0.3A = 6.5 hrs ≈ 7 hrs.

This refers to the 12V 5Ah motorcycle battery when the battery is dead. We should charge an additional 3.25Ah by charging with 0.5A current for about 7 hours.

All in all, it should take about 7 hours to charge this battery.

When is the right time to charge a battery?

It’s a basic fact that a battery needs to be charged in order to work when it has the charging capacity left of about 35%.

But how can we know that we have reached that point where the battery is completely charged? Usually, the manufacturer will always guide you about everything related to the battery. Take a look at the battery below.

battery with acid fill

This battery is filled with acid. If we measure the stabilized open circuit or its voltage it comes below 12.4V hence it needs charging.

If you forget and do not charge the battery it will cause the battery to start to deteriorate affecting its lifetime and performance.

How to check that the battery is fully charged?

Normally when the battery is fully charged it has a voltage higher than 12.4V, which is roughly equal to the output voltage of the battery charger. See the picture below:

Battery full charging low current

In normal (old age) battery chargers, there is an Ammeter connected in series between the battery and the battery charger.

When that battery is almost full the current will gradually descend near to zero. The reason is that both voltages are almost the same until the current flow becomes less.

Due to the V = IR rule, if the charging voltage V increases, it will cause the charging current (I) to decrease.

In modern battery charging circuits, the battery voltage is measured. When the voltage indicates that the battery is fully charged, the system automatically cuts off the power which is much more convenient.

Automatic battery charger circuit:

In my experience, if there is an emergency or a need to save the battery, we can use simple circuits. For example:

It charges with low current and the voltage does not exceed 15V. Hence we set the alarm for 7 hours or 10 hours. By the time the alarm goes off, the battery is fully charged normally.


I always try to make Electronics Learning Easy.


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