How to Find the Charging Current of a Battery

Knowing how much electricity (current) to use is important to safely charge a battery, just like needing the right amount of water for a plant.

Most people use a tool called a multimeter to measure this, but not everyone has one.

Here is a cool trick you can try if you do not have a fancy tool:

• You will need a special electrical part called a resistor.
• Connect the resistor to your charging circuit.
• As the battery charges, the resistor will get warm.
• The warmer it gets, the more electricity is flowing.

Circuit Working:

Parts List:

This circuit offers a clever way to determine the charging current without needing a multimeter.

Simply connect a 22 ohm 2 watt resistor in series with the battery and touch the resistor to sense its temperature.

If the resistor becomes very hot it indicates a current of 100mA or more flowing through it.

Although this resistor would normally indicate one watt of energy flow we deliberately use a 2 watt resistor for this purpose to measure the heat as it represents lost energy aiming to minimize losses.

To understand the heat dissipation of a 2 watt resistor you can test it by placing a 560 ohm 2 watt resistor across the terminals of a battery which generates 250mW of heat serving as a reference.

For example, a 1.2 amp hour 12V battery contains 14 watts of energy.

If the charging rate is one watt it will take approximately 16 hours to fully charge the battery.

This circuit is useful for various charging scenarios such as charging from a car solar panel battery charger or pulsed solar charging circuit.

It also acts as a safety mechanism by limiting the current to 100mA.

If the current exceeds 130mA the resistor will become hot and emit a smell.

Note that removing the 22 ohm resistor will increase the current flowing into the battery.

The increase may be as low as 10% for some chargers but can be as high as 100% or more if the battery is connected to the cigarette lighter plug in your car.

Formulas and Calculations:

The above circuit diagram, the formula to calculate a batteries charging current (I) is:

I = C / t

where,

• I is the charging current (amps)
• C is the battery capacity (amp-hours)
• t is the charging time (hours)

According to this formula, the charging current is determined by dividing the intended charging time (hours) by the capacity of the battery (Ah).

Applying the Circuit to the Formula:

See the batteries specs, especially the Amp hour (Ah) capacity.

The batteries datasheet or the battery itself may both carry this information.

Establish the ideal charging duration in hours.

Your batteries capacity and the chargers current rating will determine this.

For example:

Assume you are using a charger circuit capable of delivering a maximum current of 5A to charge a 12V, 50Ah battery.

The charging current may be determined using the following calculation if you want the battery to be fully charged in ten hours:

I = 50Ah / 10h = 5A

Since 5A is within the charger circuits maximum current rating, it is appropriate to set the charging current to that amount in this example.

Important Information

Given its simplicity, this formula might not be totally accurate in practical situations.

Voltage control circuits are frequently included in battery chargers in order to prevent overcharging, which might alter the charging current during the procedure.

For more precise charging details and safety precautions, consult the datasheet of the battery and charger circuit that are relevant to you.

How to Build:

Steps to follow How to Find the Charging Current of a Battery which are mentioned below:

Series Resistor Setup:

• Connect the 22 ohm 2 watt resistor in series with the positive terminal of the battery.
• The negative terminal of the battery should be connected directly to the negative terminal of the power source or charger.

Charging Time Calculation:

• Based on the power rating of your battery and the charging rate one watt in this case, you can calculate the approximate time it will take to fully charge the battery.
• For example, a 1.2A Hr 12V battery contains 14 watts of energy so at a charging rate of one watt it would take approximately 16 hours to fully charge the battery.

Safety Circuit:

• The circuit also acts as a safety mechanism by limiting the current to 100mA.
• If the current exceeds 130mA the resistor will become hot and emit a smell indicating an excessive current flow.

Adjustments:

• Note that removing the 22 ohm resistor will increase the current flowing into the battery.
• The increase may vary depending on the charger used.

Note:

• Please ensure to take necessary precautions while working with electronic circuits especially when dealing with heat and current flow.

Conclusion:

By using a 22 ohm resistor in series with the battery and monitoring its temperature, you can estimate the charging current without the need for a multimeter.

This method offers a practical and educational way to understand the flow of current in a charging circuit making it useful for various charging scenarios

References:

Battery charge current