Best practice for monitoring coin cell battery voltage given ESR

[David]

I'm using an NRF51 w/ S130 operated directly from a coin cell battery. I use the ADC with a voltage divider to successfully measure analog voltages. One of the challenges of the coin cell is that its ESR is not insignificant and degrades overtime. This means that if I have a substantial current pulse (for example from RF activity) the battery voltage droops in a meaningful way (sometimes ~200 mV). The ESR is not fixed and usually gets worse over time. Suppose my system cannot operate below 2.4V and I want to monitor the battery voltage and [do something] when there's a low battery condition. What's the best way to do this given the ESR? For example with a multimeter I might see "2.6V", but with a healthy current pulse the battery might actually droop to 2.4 momentarily, which could brownout my system. Is there a way to synchronize sampling the ADC with RF activity so I can measure the worst case battery voltage?

[David]

@rct42: I currently use the internal bandgap as the reference voltage.

I have 10 uF bulk capacitance on board and have tried as much as 20 uF with similar results.

That's interesting -- when you use the EVENT_TX are you ensured that the sampling actually takes place during the radio transmitting or is it simply synchronized to the transmission?

Since I'm not trying to do any sort of fuel gauge I think the easiest thing to do is measure the worst case battery V (e.g. during the droop).