Handling large current peaks on CR2032 battery

Question

I’m designing a nRF52832 SOC based device and I came across this issue with energy buffering and battery voltage drop.

I am using a CR2032 battery, nRF52832 SoC and a VCNL3040 proximity sensor.

This sensor has a pulse current up to 200mA for a few milliseconds (attached image below - captured one measurement,) but the battery is rated only for max 0.2 mA continuous load and for peak load the capacity drastically decreases, as it can be seen in this article.

The problem is that these high current pulses are causing a voltage drop from the battery. When the battery is older and cannot deliver enough current, the voltage drops below the operating voltage limit causing a hard reset of the device.


One more important thing is that the device is designed for a long battery life. Most of the time, it is in sleep mode. The sensor measures only once in a few minutes. A BLE data transfer is done once in a few days, therefore the average power consumption is around 5 µA.

As I understand it, the workaround would be to attach a capacitor which will cover the current pulses. That would mean - if I assume full consumption of the proximity sensor 200mA for approximately 5 milliseconds (4 peaks of 1.25 ms) and 100mV voltage drop - I will need a 10 mF capacitor (200 mA)(5 ms)/(100 mV). In this case, I will need a capacitor with high discharge current rating and as low leakage current as possible - I imagine leakage current around 1 uA, but I don’t know how realistic this is.


I found a capacitor of desired capacity and low leakage current (CPH3225A,) but the rated maximum discharge current is only 10µA. According to the datasheet it seems that when increasing the discharge current the capacity decreases.

Is there any way around the limitations?

Answer 1

My car key fob has a CR2032 battery and a small lithium rechargeable battery in order to support a relatively powerful transmitter. So that is another way.

In order to see how much capacitor leakage will affect the battery life, you can calculate the internal self discharge current and look up the capacity of the cell.

The capacitor you linked has an internal resistance of 160Ω (presumably typical) so it's not going to be remotely appropriate for supplying 200mA from a ~3V fully-charged voltage. Short-circuit current would be less than 20mA, so it's hardly better than the button cell itself (in fact, it's aimed at replacing such primary cells).