Garmin GPS in Model A

[apbright]

Nearly all modern electronics operate with switched-mode power supplies. Good manufacturers (among which I would count Garmin) will be conservative when specifying the usable supply voltage range. Thus, it does not surprise me that it will operate normally with a 6.4V supply even if the minimum specified voltage is 8V. The Garmin EE's I'm sure designed for under-voltage protection, thus your worst case scenario if your voltage sags much below 6.4V is that it will either switch off, or its battery will stop charging (have been working in engineering in portable consumer electronics firms for 20 years).