Usually use a Planet Bike SF taillight. It's usually in blink mode while riding, but to turn it off you first have to push the button once to switch it to solid on and then again to off. When it's switched to solid and the batteries are weak it dims noticeably within the first second and that's my cue to change out the batteries or recharge them. At that point it'll still run ok for at least an hour in blink mode with decent brightness.