Appropriate battery for overnight viewing to drive telescope and GOTO? I have a Plettstone 18" Dobsonian telescope with GOTO and clock drive that runs off a 12 volt cigarette lighter adapter.  What is an appropriate battery to use to run this overnight while viewing all night long and then charge during the day for the following night's use?

In the past I've used deep cycle lead acid batteries, but these are heavy and can fail if not carefully maintained with a float charge.  Also, I don't need cranking power of a car, just sustained energy to drive the scope.  A typical Optima YellowTop deep cycle is rated 75 ampHours.  I don't know what the draw in Amps of my telescope is, but imagine it is less than an amp.

I've also seen these Battery Tender Lithium Iron Phosphate types but wonder if their 10-12 ampHour rating is enough and whether they are meant to be drained over a whole night?  Some say if you get below 8 volts the battery becomes broken, but I doubt the telescope would still run at such a low voltage.

What would a reasonable ampHour rating be?  What kind of battery is a good fit for astronomy equipment overnight?
Before buying anything, get out your Amp-meter and measure what the motors pull in clock drive and GOTO mode, starting from a variety of scope positions.
An 18 incher will have a lot of mass, and Dobsonians are not known for their excellent bearings, so the amount of juice you need for a night of observing could be considerable.
Do you track most of the time, or GOTO...GOTO...GOTO?
How many hours is your typical observing session.
Multiply your average tracking amperage by the approximate number of hours you spend tracking, then do the same for your GOTO work.
Add the two numbers (amp hours) together, and double the result.
That should tell you about how big a battery you need.



The doubling is done to provide leeway, and because taking a standard lead acid below 50% is a sure way to kill it after only a few recharges. If you go Li, you can probably cut the multiplier.