The Tricky Task of Emulating a Battery in an IoT Device

Raise your hand if you’ve ever struggled with emulating a battery in an IoT device. We hear you – creating a setup that is as realistic as that of the finished product with the battery close to the electronics, is nothing short of a challenge. Today we’d like to share some valuable tips on how to get there.

Here’s a TL;DR summary for those of you that are in a hurry:

• The longer the distance, the higher the resistance – remember that the battery in the product is very close to the electronics, while your test setup is more likely to have longer cables that prevent this close connection.
• Beware of high resistance – it can prove fatal, e.g. when a sudden current rush creates a voltage drop in the wires, sending the system into reset.
• Resistance hack #1: Minimize the resistance by choosing short and massive cables.
• Resistance hack #2: Provide a reservoir energy source by connecting one or several capacitors directly to the voltage input of the device.
• Remember to carefully choose the right capacitor (optimal ESR + capacitor size).
• Check your data – resistance hack #2 will affect your measurements.

Feeling powered up and ready to read all about it? Great! Let’s get going.

The longer the distance, the higher the resistance

In real products, the battery is (almost) always connected close to your electronics. This setup is not a coincidence – short wiring leads to a small resistance in the current path between the battery and the load. Sounds good, right?

When we simulate a battery, we often end up with long(er) wires that create unwanted resistance. Big deal? Actually, it is! The current rush creates a voltage drop due to resistance in the cables. If the drop is too big, it might prevent the electronics from working properly.

What causes a current rush?

When a device is cut off from a voltage supply for a while, we face a worst-case scenario. The situation turns fatal because the capacitors on the board are empty and need to be filled with energy again.

During the first nanoseconds of a sudden current rush, all decoupling capacitors – before they are charged up – act as short circuits. Yes, this applies to all capacitors spread over the PCB, from 100 nF to bigger ones. The capacitors cause a huge inrush current that creates a momentary voltage drop in all resistive components that happen to be in the way, i.e. cables, connectors, PCB traces.

Remember to check for resistors on the current path

So, how can you check for resistors? Some devices use a fuel-gauge where the current is measured over a resistor in series with the battery. Li-Po and Li-Io batteries should always come with a battery protection circuit. If your setup includes this circuit, it also possesses a resistance in the current path. Also, don’t forget the current’s return path. All resistance in the path counts, including ground planes.

Two easy Resistance Hacks

To avoid the challenges presented above, we recommend that you take the following safety measures when you emulate a battery in an IoT device:

1. Choose short and massive cables to minimize the resistance in the wires connected from your battery emulator to your device (see Figure 1). Take a look at this handy Wire Gauge Size Chart.
2. Provide a reservoir energy source by connecting one or several capacitors directly to the power input of the device.

Figure 1: Resistance hack #1: Minimize the resistance by choosing short and massive cables. (Image source: Qoitech)

Connecting the capacitor(s) creates a reservoir of charge, supplying the instantaneous charge requirements of the circuits locally. In other words, the charge doesn’t have to travel through the resistance of the power wires.

Let’s look at an example: A mobile phone can easily consume 4 A in a short pulse during power-up. In this case, it’s important to create a big, low-resistance energy reservoir close to the battery connector (see Figure 2).

Figure 2: Remember to create a big, low-resistance energy reservoir close to the battery connector. (Im-age source: Qoitech)

How to select the right capacitor(s)

Before you get started, we’d like to say a few words about selecting the right capacitor(s) for your device. Make sure you’re asking yourself the following questions:

1. What’s the optimal ESR (Equivalent Serial Resistance)?
   The right ESR of the capacitor, together with its number of micro Farads, is crucial. For applications with short, high current bursts, you’ll want a low ESR. If you need to lower the ESR, you can use several parallel-connected capacitors. Take a look at this table with typical values for ESR capacitors.
2. What’s the optimal capacitor size?
   To be honest, this is often a trial and error task. Select the capacitor big enough to make sure your device can power up correctly. But don’t make it too big, because the capacitor will act as a low pass filter, changing the rise-time of your current pulses and impacting your measurements (keep reading to learn more). Also, make sure to choose a capacitor with minimal leakage. The size of the reservoir depends on the needed energy supply (peak current and time) and the acceptable voltage drop, without the system going into reset.

A few words about measurements

A capacitor located immediately at the voltage input of the device will affect your measurements. The reason is that the capacitor needs to fill up with energy, which will take some time. When you use Resistance Hack #2, the rise and fall times of the current pulses will be slower. The results are similar to when a low-pass filter is in series between your battery measurement equipment and your device. Don't worry though – if you use the right capacitors, the small leakage current of the capacitor will be the only measurement error.

Want to dig deeper into the powering of IoT devices?

Take a look at this article about how an efficient power management system allows you to maximize battery performance. Also, if you haven’t done so already, check out Qoitech´s Otii Battery Toolbox! This tool elevates your Otii Standard from a DC power supply to a comprehensive battery profiler and emulator, creating a realistic source for your real-life projects.