Battery Wattage hour requirement for 12v, 3.75 amp device?
Many electronic appliances with motors require several times more current when starting up (for the first few seconds) than they do to run continuously.
To accurately calculate your power requirements, use an amp-hour or watt-hour meter to determine how many watt-hours the machine uses per hour.
Multiply that number by the desired runtime in hours to get the minimum required battery size. Since most batteries don't like being discharged to 0% repeatedly, get a slightly larger one.