Forgive the information dump ...
Quote:
Originally Posted by guggle
... plus $29 for a charger ... I think I'll settle for the 9amp battery
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Power drawn by the fan will depend on size and design, but assuming it's a typical 80mm fan, it'll be around 3W. The NEQ6 in sidereal tracking draws 11W (I've measured it) and 22W at max slew speed, so that's 14W total for the majority of the time assuming the fan remains on. At 12VDC that's equivalent to 1.17A. I'll assume you're happy to go a bit over 50% depth-of-discharge for a battery you intend to replace maybe in a couple of years, so let's say 60% is the limit. With a 9Ah battery, to not exceed 60% discharge at 1.17A, you'll get around 5 hours usage*, less if you slew a lot.
For an AGM battery you should use a charger with AGM mode (they need different voltages). A 3-stage smart charger is recommended - bulk, absorption and float being the three stages. The charger should be sized for the Ah capacity of the battery to maximise life and not overheat the battery, though the acceptable range is fairly accommodating and opinions vary somewhat. The battery manufacturer should state the maximum initial current and (hopefully) afix it to the battery -
do not exceed this. In the absence of this info, the rule of thumb is: charger rating (maximum current) = 0.3C, so for a 9Ah battery, the charger should have a rating of no more than 3A. However, AGM batteries are usually more tolerant of a higher initial current, and also going a bit smaller doesn't hurt too much.
*
A note on capacity:
Typically, the manufacturer labels the battery with the C/20 capacity - that means when the battery discharges at a current of 1/20th it's capacity (which will take 20 hours). E.g. a "40Ah" battery only has 40Ah capacity when discharged at 2 Amps. Discharge faster and the capacity dwindles.
For a 9Ah battery, C/20 is 0.45A. Your equipment will draw more than twice this current, so the useful capacity will be less than 9Ah, which in turn, will shorten the run time. I don't know the specifics of the battery you're considering, but a rough guess would be a useful capacity reduction of 15-20% and a run-time reduction to 4 hours.