joewein

The cable I use also wouldn't work if the Garmin was mounted on the stem, but mounted on the handlebar it's fine.

Have you considered a forward mount such as the K-Edge?


For the record, my longest recorded ride distance on my Garmin Edge 500 was 570 km, recorded over 47 hours of elapsed time, using a host mode USB cable and a generic USB battery pack (8,000 mAh). Mine is running firmware 3.20. I first considered recording the ride in multiple chunks, just in case, but then decided to go for it and it worked just fine.

njkayaker

I had no problems charging a 800 while riding using a charger with a rechargeable battery. Two issues are where to put the charger (like in a handlebar bag or a bento box) and rain (when you probably can't recharge).

The 800 has a 1000mA battery.

I can use my charger (it's a modestly sized 5400mA) for my phone too.

Note that you can't get all of the mA out of the charger, which means the useful amount of capacity is less than it's listed capacity.

I believe the ability to use the 800 while charging was added in a firmware update.

joewein

Correct, the inability to charge via a standard USB cable while recording is a problem specific to the Garmin Edge 500, not the 800 with recent firmware.

The listed capacity of Li-ion batteries is always the total number of mAh that can be drawn from the cell in its operating voltage range (from typically 4.2V when full down to a cut-off point of about 3.0V under load).

A USB battery pack has a circuit that steps up the voltage from the raw battery voltage to the USB standard of 5 V. If the current is used to recharge another Li-ion battery rather than being directly consumed by the electronics of the plugged in device then that 5V needs to be stepped down again to a charging voltage of no more than 4.2V. There are losses in this two way conversion, as step-up / step-down circuits are not 100% efficient. On top of that there are losses in the battery itself, which gets warm from internal resistance while the charge/discharge current flows through it.

If you charge one cell from another you'll lose at least 20% of the power I'd guess, based on the specs of conversion chips I've looked at. So to fully recharge a flat 800 mAh battery in a Garmin 500 you would need to fully discharge at least a 1000 mAh USB battery.