Does the Garmin need 5V? How do you know how much capacity the unit will provide at 5V?
How efficient is the boost converter?
If it's a USB connector, it's 5V (plusminus 5%). Li-ion [Cobalt] cells usually output 4.2V full and a safe circuit would cut off somewhere between 3.1V and 3.3V.
Capacity is inversely related to current drawn, so a cell which can deliver 1A for 5 hours is likely to run for less than 1 hour at 5A.
The combination of these effects is why device battery % capacity drops off a cliff below 25%: The voltage is lower when they're this empty so it takes more current at battery voltage to deliver any given current at circuit reference voltage.
@Emyr
How efficient is the boost converter?
If it's a USB connector, it's 5V (plusminus 5%). Li-ion [Cobalt] cells usually output 4.2V full and a safe circuit would cut off somewhere between 3.1V and 3.3V.
Capacity is inversely related to current drawn, so a cell which can deliver 1A for 5 hours is likely to run for less than 1 hour at 5A.
The combination of these effects is why device battery % capacity drops off a cliff below 25%: The voltage is lower when they're this empty so it takes more current at battery voltage to deliver any given current at circuit reference voltage.