I'm nowhere close to having all the electronics integrated, but I took my laptop on a quick drive the other day and recorded this from the kalman filter.

Red line is the filter track. Looks great, right? Well, here's the raw GPS data (in blue):

Yep, same basically the same track. What does this mean? Well, I had a really good GPS signal on this drive, I think I just had great resolution. No major tracking errors. The accuracy meets everything I think I'd need for an actual bike trip, but I'd like to do better and add more bells and whistles, re-enable speed based prediction and include an accelerometer.

In addition, the data ends early for some reason. I was recording this with a CoreFlight app, and whatever I wrote had some error, although it didn't crash anything. I've included more error checking in case this happens again, though an app crash/restart could overwrite the log files, which would be a problem.

I'm seriously getting stuck on how to safely integrate an unprotected 18650 battery into this project though. It looks like I'll have to include separate charging, protection, and 5V regulation ICs to get the Raspberry Pi powered. I'll then send the data lines from the Raspberry Pi straight through to the charging port so that a computer can talk to the Pi while charging the battery.

Next problem: I think I need a high-side battery protection IC now. Most of them are low-side, but the battery will still be grounded through the charging IC.