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Hey Mike
Going way back to my techy days many decades ago, so here goes.
Unless the generator's 12V output was designed for charging a battery, it is unlikely to charge your 100ah battery effectively because it is designed to simply run 12V appliances i.e. it unregulated. This means the output voltage range may vary by up to 3 volts and the DC voltage may be dirty with AC ripples. These conditions may damage your battery or fail to charge it properly.
The charging voltages for batteries vary somewhat with battery types, however typically 14.4V is required for bulk charging deep cycle batteries. Once charged, it requires about 13.2V-13.4V for long term float maintenance. So a dumb 12V outlet won't cut it.
If it generator has a 12V outlet that is designed with a battery charging function, then it would provide charging, however it must also have the smarts to regulate how much charge pours into the battery i.e. it must modify the charge rate to prevent overcharging the battery.
If we assume there is a well controlled battery charging function at the generator end, then it will control both the current and voltage delivered to the battery. Ideally for a 100ah battery the charge current delivered should be about 10amps per hour (10Ah). It may be larger than this, however this figure is quoted as an optimal rate for 100Ah batteries. Hence we can assume the generator would pump about 10Ah into the battery.
On the inverter side, you have to add up the Ah (ampere hours) being taken out of it. So go around and check current drawn on all AC adapters for laptop, camera, scope. keep in mind these are maximum or worst case draw from these devices. Your mount may momentarily draw 5amps or more, but when going sidereal, it may only be tens of milliamps. The laptop will be more constant, however simple image acquisition will draw no where near maximum current, unless you are doing image crunching. The camera will be fairly constant draw and rises towards maximum in hot weather. At a guess, I would assume 0.25Ah for mount, 2Ah for lappy, 2Ah camera, heating 1.5Ah, 0.5Ah lighting etc.
So ball park around 6Ah max is required from the inverter. Keep in mind inverter efficiency is around 87%, the inverter will pull almost 7Ah from the battery to deliver 6Ah. So in very rough terms, 10Ah come in from the generator and about 7Ah go out, you have 3Ah worth of charging occurring whilst generator is running.
I would be very careful ensuring isolation between equipment end and generator chassis to avoid ground loops/feedback.
I have probably stuffed something in my assumptions and calculations and no doubt others will kick in to sort it out. Hope this helps.
Ted
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