You know ... after feedback from many users, I almost want to drop the BT idea, or just leave the header there if people still want that option, but actually offer an RFM69 logger - that can log plenty fast for user analysis (30 times a second with some lost packet tolerance good enough?).
I imagine in some cases the serial output is useful to capture events "of interest", meaning it would be great to see only data that really matters rather than a ton of data that is just linear. Kinda like deep memory with trigger on a scope which can log a very long time window but only give you what you care about. You probably dont care that your device is sleeping at 10uA for 3 days, but you care more about what happens when it wakes up, how often, and what the spikes are. Lots of coding to make all that happen but thats why this device is
programmable by you - the user. The really great suggestions and contributions have a great chance to make it into a future revision.
Another idea for external logging is to have optically isolated outputs - in which case no BT module is needed - that would be nice to have on USB but then that would only work for logging and not charging so a little tricky to make happen, and ... more expensive