Maximizing battery life in IoT devices requires careful consideration of how the battery connects to the load. Often, a DC/DC converter is best, offering regulated power for specific applications. This setup enables optimal battery usage and minimal energy waste. The DC/DC converter must be highly efficient for the device’s voltage and current needs.

But how to choose the right DC/DC converter? While datasheets provide an overview, characterizing its efficiency within the system is best. Our tech post demonstrates this process.

System under the test

We investigate the DC/DC efficiency for a system that consists of a 1.5V Alkaline battery, and a DC/DC converter that boosts the voltage to 3.3V to an Arduino Pro Mini, see figure 1. The timer on the Arduino board starts the system at certain intervals. During these power-on intervals we want the system to run as efficiently as possible. We want the system to utilize the high efficiency of the DC/DC converter in the range of 1 – 10 mA (active mode) for the entire battery life cycle, with the decreasing battery voltage over time in mind. For systems where the sleep mode power consumption dominate over the active mode power consumption the efficiency of the DC/DC is not as important.

Battery characteristics

When connecting a load, RL, to a battery with an unloaded voltage, E, the voltage over the load will drop due to the internal resistance of the battery, Ri. The battery voltage, U, will decrease as the current into the load increases due to the voltage drop over the internal resistance. Coin cell batteries and AAA batteries have a high internal resistance so it is not possible to load them with high current as the output voltage would be too low.

If the load is resistive, like with a simple resistor, the current drained from the battery will decrease as the battery discharges and voltage drops. When a DC/DC converter is connected to the battery, presenting a fixed output voltage to the same load, the current drawn from the battery will increase when the battery voltage drops. This will accelerate the discharge, make the voltage drop further which leads to further current increase and so forth.

DC/DC converter under test

In the datasheet (see figure 3) for the DC/DC converter we use here, efficiency is presented as a function of load current at a specific voltage, 1.2V in this case. More information of our system is needed, for example the efficiency for the range of ~0.8V – 1.5V. It would be great to have this curve as a function of the current drawn from the battery with the DC/DC self-consumption and the losses in its components (mainly the inductor) included.

When the voltage starts to drop, the current drawn from the DC/DC converter starts to increase which leads to decreased voltage from the battery (note that we are only checking behavior at room temperature in this study). The efficiency of a step-up DC/DC converter typically decreases with lower input voltage.

Characterization of the DC/DC converter

For the DC/DC characterization, Otii Arc is used as the supply for the system, acting as the battery, but also as a measurement unit. Otii Arc can supply the system while measuring both input voltage and current at the same time as output voltage and current from the DC/DC converter. By dividing the output power with the input power, the efficiency can then be calculated.

To measure the output current from the DC/DC converter a sense resistor is needed, see Figure 4. The value of the resistor must be high enough to allow sufficient resolution for the measurement but low enough to not introduce a too big voltage drop and to not exceed shunt voltage limits.

In this example, a 4.7 ohm sense resistor is used. This will result in a voltage drop of 37mV at 8mA current, which will not affect the performance of the Arduino. Otii’s ADC expansion port voltage range is -81.9175mV to 81.2mV, so 37mV is well within this range.

A simple application is run on the Arduino Mini, flashing an LED, while measuring the DC/DC efficiency for that specific use case.

A Lua measurement script is run in the Otii desktop application to set input voltages from 1.5V (fully charged battery) to 0.9V (almost empty) and calculate the DC/DC converter efficiency over time to reflect a real use case. One .csv file per voltage is exported, containing the measurements and calculations.

Between each measurement, the power supply is disabled to reset the system. By doing this, it is possible to see the entire startup phase and the active phase of the system. The script file and saved .csv files can be found in the download section below.

Results

Figure 5 shows that the output current from the DC/DC converter (ARC ADC Current in the figure) is relatively constant, regardless the input voltage. This can be expected as long as the DC/DC converter works as designed. However, the input current (ARC Main Current, see Figure 5) increases when the input voltage decreases.

The figure 6 shows the DC/DC efficiency for different battery voltages. The average DC/DC converter efficiency varies a lot with battery voltage. At 1.5V the efficiency is more than 87% while at 0.9V it has dropped below 79%. This is measured with an Arduino as load, consuming roughly 5.1mA when the LED is on and 3.6mA when it is off. As expected the data sheet values provide a good indication of this for higher battery voltages, however for lower voltages this data is not provided. Hence, importance of characterization is apparent.

Conclusions

Battery-powered IoT devices mostly stay in sleep mode, using minimal power and periodically waking up for short, power-consuming tasks. Each product type has a unique activity profile and design.

No single power supply design fits all products. To optimize, characterize the battery and load, design power management, and validate the entire system. We demonstrate characterizing a DC/DC converter for an IoT design example in this tech post.

The DC/DC converter datasheet offers performance indications but may be limited. Characterizing its efficiency as part of the whole system is best, which is simple using the Otii tools.