If your battery is too flat to flash a new recovery via fastboot, and you're stuck with the old reovery what doesn't charge when the power is off or in bootloader/recovery, you're in a bit of a pickle.
The battery should still charge for the few seconds between when you power on and when it enters the bootloader, so you will just need to keep rebooting it over and over while plugged in until your battery is back up to over 30%.
A few people ahve reported this method to work. Personally, I haven't tried it, but it's worth a shot.
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