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HI i personally think the battery is your problem sorry to be a downer, i can assume that it is one of those black gel cell ones. firstly once the battery gets to around 10.7 volts it is considered flat. So the theory is 18ah= 1hr at 18 amps or 3 hrs at 6 amps however the capacity is not 18ah because of a few factors, the internal resistance of the battery changes with current draw and thermal losses occur which also draw power (heat) the wire also has losses in it. the screen of the kendric and the electrics inside (heat). and the capacity of the battery is more likely to be around 12ah instead of the 18ah advertised.
you would be amazed at how many amps your scope draws (to track) an object. My HEQ 5 draws 2.3 to run the synscan and track but when i slew the scope it goes to 3.2 amps. are you running the dew heater at 100% all the time or are you changing the duty cycle to the minimum. you only need the scope to be at 1 degree above the dewpoint to prevent dew. also how many heaters are you running?
i am interested in this as i am in the middle of building my own heater and with it turned to 100% duty the main heater element draws 3.5 amps so as you see it does not take long to get to the capacity of a small battery.. try and run two 18ah batteries in parallel this will double your capacity and keep the volts at 12 longer.
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