Improving battery life: BLE device with a 100uF capacitor

Question

I have a BLE beacon and I was wondering if adding a capacitor in parallel with the coin cell battery would help with the battery life by smoothing out the peaks. The scenario shown in the chart is the BLE advertising event (every 1s). So I first added a 10uF then also a 100uF capacitor in parallel and took a trace of the power profile. The two series shown is with a 10uF capacitor (orange line) and the other series with both 10uF+100uF capacitors (blue line). Looking at the charts, I have two questions:

1. For the 100uF series, why is there a large spike at the start of the advertising event ? I expected the larger capacitor to help smooth out the initial peak of the advertising event not make it worse. (there's a bit of CPU work before the main spike which is smoothed out but that should not deplete the capacitor). Instead, it looks like its charging at the same time and I suspect there needs to be some way to limit the draw of the capacitor. I tried adding in various resistors but that just converges to not having a capacitor in the first place.

2. It does not look like adding in the 100uF is helping at all, are there ways to make it help and not hurt or is it better to not have it ?

---

Thanks for the replies. Turns out to be buggy UI software. I reran the tests with different s/w and got results more inline with what you would expect.

Answer 1

I have a BLE beacon and I was wondering if adding a capacitor in parallel with the coin cell battery would help with the battery life by smoothing out the peaks.

YES, definitely.

This is pointed out in this mostly relevant white paper by Texas Instruments (White Paper SWRA349 - Coin cells and peak current draw. Mathias Jensen), which covers exactly your problem. The key section is section 4 (emphasis mine):

4 How to survive a high internal resistance

A common technique to handle high peak currents is to use a capacitor to offload the power source. During high current periods the capacitor will act as the primary power source, while during low current periods the battery will be the primary power source and recharge the capacitor. When dimensioning the capacitor it is important to know the battery’s internal resistance and the load profile. With this information it’s quite simple to dimension a suitable capacitor. For our testing we dimensioned for an IR of 1kohm which resulted in a capacitor of approximately 100uF. In a low cost application, this capacitor size would probably be to large or too expensive, but in a real BLE application the capacitor can be significantly reduced by a factor of 2-5 depending on application, since the load profile is much easier.

Answer 2

One way to make it help would be to lookup the minimum operating voltage of your BLE module. Since you are running off a 3V coin cell it is likely that the BLE will run from 1.8v. If that is the case you can place a high efficiency and low quiescent current buck converter between the coincell-cap-combo and the BLE module which will reduce battery current almost 30%.

Answer 3

Your graph suggests the caps is after a power switch of some kind. It is not doing what you think.

To answer your question you should consider that battery performance toward the end of its life, and at temperature extreme. In both these cases the internal R is much higher. Also at the end of battery life the Vmin is likely to be reached during current spikes: it stops working because voltage dropped out during spikes, not because the batteries energy was fully depleted. C helps all these.

As Dan says you need a low leakage cap. There are specifc low leakage caps, and electros operated near Vrated are high leakage.