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Originally Posted by bazaud
.. any suggestion on what type and size (Amp/Hr) 12 volt battery I should buy?
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How long do you want to run for? The EQ6Pro draws 11W in sidereal tracking and 22W at maximum slew - average of about 12W. For your laptop, you'll have to figure out the power draw yourself.
Allow for typical 50% depth-of-discharge of the battery to maintain battery life (i.e. multiply capacity by a factor of 2). Many places/people, me included, also recommend a 30% (i.e. a factor of 1.3) safety margin to account for temperature variation, battery age and miscalculation.
So if your laptop draws (guess) 3 amps average at 12V, and your mount is 1 amp average, and you intend to run for 8 hours per evening, then your battery capacity in AH should be:
(3 + 1) x 8 x 2 x 1.3 = 83 AH.
The battery should be deep-cycle as they are designed for low, continuous current and deeper depth of discharge. Car batteries are designed for high instantaneous current and shallow (~5%) depth of discharge.
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Have I forgotten anything else I need to power?
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Dew heaters? But, personally, I wouldn't drive those from the same power source as the electronics - just an opinion.
Edit: A note of caution with regard to advertised capacity ... the battery capacity will change with current - if possible you should view the current/voltage/time curves for the battery before buying. The most indicative capacity for astronomy needs is usually the "20 hour rate", sometimes written as "C/20".