Battery Charger

# Measuring Battery Charge Schematic Circuit Diagram

It is not widely recognized that a standard digital voltmeter can be used to measure the charge level of a battery. This method is applicable to specific types of batteries, such as Lithium-ion batteries. Despite the variety in Li-ion batteries, there are generalizations that can be made. Graphs provided by Panasonic illustrate that the terminal voltage of the cell decreases proportionally with the reduction in charge. Hence, a straightforward voltage measurement is adequate to assess the battery’s charge status. #### Understanding the Graphs and Load Conditions

The provided figure displays three graphs, each corresponding to specific load conditions. To obtain accurate results, it’s crucial to measure the output voltage under the specified load conditions. Additionally, knowing the exact load value is essential. The battery should be under the load for at least a minute for reliable readings.

#### Measuring Voltage with Known and Constant Loads

In cases where the load is known and remains constant, such as in devices like pocket torches, measure the voltage and determine the corresponding charge from the relevant graph. However, if there’s no load, the load is unknown, or it varies, introduce a temporary load using a resistor. For instance, if the resistor’s value is 20 Ω, refer to the upper graph (0.2 C, 180 mA). Note that a single resistor might overheat due to power dissipation, so it’s prudent to use several resistors in parallel, like five 100 Ω resistors.

#### Creating a Precise Battery Graph and Data Analysis

For precise measurements, create a graph specific to your battery. Charge the battery fully, then connect the load, such as the five 100 Ω resistors. Record the output voltage every five minutes and input the data into an Excel sheet to generate a smooth curve. If the time intervals are irregular, enter the actual times and utilize the ‘spread’ option for the curve. Excel can translate the time into charge on the horizontal axis.

#### Calculating Current and Charge

Compute the current in each interval by dividing the mean voltage (average of start voltage and final voltage) by the resistance. Charge is determined by multiplying this current by the elapsed time. The depicted graph pertains to a 900 mAh battery. For example, a current of 0.2 C translates to 0.2 x 900 = 180 mA, where 1 C equals 900 mA, and 2 C equals 1.8 A. It’s important to note that this method is suitable for lead-acid batteries under constant temperature conditions. Keep in mind that the graph for an old lead-acid battery may vary slightly from that of a new one, impacting the accuracy of readings. This method, however, is not applicable to NiCd or NiMH batteries.

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