Simple li-ion Battery Capacity Tester Circuit

Visualize you have a phone battery, but how long does it really last on a charge?

This tester circuit is like a battery fitness checker.

It tells you how much power your Li-ion battery the kind in phones and laptops can hold helping you see if it is getting tired and needs replacing.

This is important because these batteries can lose their power over time making your devices run out of juice faster.

Circuit Working:

Parts List:

This circuit is designed to assess the capacity of a rechargeable cell a critical step given the prevalence of fake cells like the ultrafire

The ultrafire cells weight alone at 30gms raises suspicion it is impossible to achieve a 6 amp hour capacity in a 30gms cell.

Our test showed that our cell had less than 0.5 amp hour capacity.

To use the tester you will need basic components like resistors and diodes which create a load discharging the cell at around 600mA or 2.4 watts.

The resistors and diodes together can handle up to 2.5 watts.

As the cell discharges from 4V to 2V the voltage and current decrease.

By measuring the voltage across the resistors you can calculate the current flow.

Avoid using an ammeter as its voltage drop significantly reduces the current.

Also, steer clear of jumper leads which drop about 350mV.

You can customize the load with different components ensuring they do not overheat.

A buzzer in the circuit signals when the voltage drops to 2V.

Adjust the biasing resistors to match this detection voltage if you use a different transistor as the detection voltage can vary by up to 200mV with different BC547 makes.

Our test cell lasted less than 2 hours confirming its fake nature.

Some fake cells weigh only 28 grams because they contain a much smaller cell inside.

When charging a Li-Ion cell ensure you use a charger that cuts off at around 4.2V.

Charging from a variable power supply without monitoring the terminal voltage can lead to overcharging potentially damaging the cell.

Formulas:

In the above circuit, a lithium-ion batteries capacity is determined using the following formula:

Capacity (mAh) = Discharge Current (mA) x Discharge Time (Hours)

where,

• The batteries capacity (measured in milliampere-hours) is its stored charge.
• The current continually pulled from the battery during testing is called the discharge current (mA).
• The amount of time needed for the battery to fully discharge at a steady current is called the discharge time (hours).

Circuit elements that have an impact on the formula are:

Resistor (3.3k): This resistor sets a circuits maximum discharge current (mA).

You may find the discharge current by measuring the voltage across this resistor and applying ohms law.

Time: The circuit does not directly measure time.

Throughout the discharge process, you must keep an eye on the batteries voltage and note how long it takes for it to reach its discharge endpoint, which is usually about 3V.

Take note:

It is possible that this basic capacity tester circuit is not particularly accurate.

A more complex battery tester circuit is advised for a measurement that is more accurate.

It is important to take safety measures when handling Lithium-Ion batteries.

It is advised to discharge the battery inside a fireproof container and at a safe current rate in accordance with the batteries specifications.

How to Build:

To build a Simple li-ion Battery Capacity Tester Circuit follow the below mentioned steps:

Load Circuit:

• Use resistors to create a load that will discharge the Li-ion battery.
• The load should be adjustable to test the battery at different discharge rates.
• Use transistors to control the load.
• The transistors should be able to handle the current drawn by the load.

Voltage Measurement Circuit:

• Use a voltage divider composed of resistors to scale down the battery voltage to a range that can be measured by the op-amp.
• Use an op amp configured as a voltage follower to buffer the scaled down voltage.
• This buffered voltage will be used as a reference for the comparator.

Display Circuit:

• Use an LCD or LED display to show the battery voltage and the estimated capacity.
• Use switches to control the operation of the circuit, such as starting and stopping the test and selecting different discharge rates.

Assembly:

• Assemble the components on a breadboard or PCB according to the circuit diagram.
• Ensure all connections are secure and follow safety precautions when working with Li-ion batteries.

Testing:

• Connect a Li-ion battery cell 18650 to the tester and adjust the load and reference voltage as needed.
• Start the test and monitor the display to see the battery voltage and estimated capacity.

Adjustment:

• Fine-tune the load and reference voltage to optimize the testing process and obtain accurate capacity measurements.

Completion:

• Once you have finished testing the Li-ion battery disconnect it from the tester and ensure all components are turned off.

Note:

• Building a Li-ion battery capacity tester circuit requires some electronics knowledge and skill.
• If you are not familiar with electronics, consider seeking help from someone who is experienced in building electronic circuits.

Conclusion:

A Li-ion battery capacity tester circuit is a valuable tool for assessing the health and capacity of Li-ion batteries.

By discharging the battery at a controlled rate and monitoring the voltage the circuit can provide valuable information about the battery’s capacity and overall performance.

Building such a circuit requires careful selection and assembly of components but it can help prolong the lifespan and optimize the performance of Li-ion batteries used in portable electronic devices.

References:

Simple 18650 capacity tester circuit