How much power or wattage can be produced by a PowerTank? How long will a full charge last at various power levels?
Celestron PowerTank 7 #18774 and PowerTank 17 #18777 are 12-volt DC battery supplies, and behave just like a car battery. Both PowerTanks will deliver as much current (amperage or amps) as the load (whatever is plugged into them) will demand or draw.
A load will draw current depending on its resistance (like a heater or light) or for a motor (how hard the motor is working).
The amount of power in watts delivered by a PowerTank will be the voltage times the amperage.
Celestron mounts will draw a few-tenths amp when tracking and up to 5 amps for a big mount like the CGE Pro when at maximum slew. At 0.3 amps, the power drawn by tracking will be 12 x 0.3 or about 4 watts. The power drawn for the CGE Pro at fast slew rate 9 will be 12 x 5 or 60 watts.
The PowerTank 17 holds 17 amp-hours total charge. To find out how long it will last, divide this rating of 17 by the amps drawn. At 5 amps, it will last only about 3 hours (17 amp-hours divided by 5 amps). If your slew your mount a lot, the PowerTank may only last one evening before needing a recharge. If you only do a few slews and just track, it will last much longer. (For example, 17 divided by 0.3 equals nearly 57 hours.)
The same considerations hold when using other devices like dew shield heaters, laptops, blow dryers and the like. The more the device draws in current, the greater the power output in watts will be from the PowerTank, and the shorter the PowerTank charge will last.
If your dew shield heater is rated at 50 watts, the reverse formula (watts divided by voltage) is used to find the draw in amps. 50 divided by 12 is about 4 amps. So, a PowerTank 17 can power your heater for approximately 4 hours (17/4).
PowerTanks have safety fuses that will limit the amperage to 15 amps or less. You will never be able to receive more than 12 x 15 or 180 watts safely from your PowerTank.