Hello Rene,
Some good advice so far in this thread and by far the most important in my opinion is in Peter's first paragraph which essentially advises on working out your power consumption and convert that to accumulated Amp Hours consumed.
This then determines battery and charger size and type, power and fusing distribution, whilst factoring scale ability and other uses such as camping and blackouts etc.
As also mentioned by others, try not to consume more than 50% in accumulated Amp Hours of a battery's rated sized. So if you consume 40 amp hours between charges, then battery needs to be 80 amp hour minimum.
Unless you are confident, very knowledgeable and have lots of $s for lithium, then battery size is often limited to ability to easily carry and store.
Although I have both 18 and 2x 100 amp hour available for Astro ( one fixed in van ) the 18 usually does the trick for non imaging use and easy to carry. I detest though having to move that 100 Ahr battery around, even if left in ute with extension lead for dark sky sites.
Think big scopes, if they are to big to carry/transport and setup then they are not likely to get used much. Same with batteries.
Getting back to determining power consumption.
There are several cheap and easy to use Wattmeters on the market that make this job very simple providing you have basic DC wiring and soldering/crimping skills or know someone that has. These meters must be connected in series between fused battery and load distribution point with a cumulative count limit of 99 Ahrs and 60 amp through current limit. Larger amp models will have an external shunt.
Providing you are starting off each night with a fully charged battery, then 'Total Amps (Amp hours/Ah) since device initialised' is the primary reading you will need to watch from such meters.
Eg, this one with just fly leads:-
http://www.beachloversleds.com.au/sh...current-meter/
Or this one with Anderson plugs to make for easy connectivity:-
http://www.aussiebatteries.com.au/so...lar-watt-meter
And a Jaycar equivalent meter again with fly leads:-
http://www.jaycar.com.au/Power-Produ...Shunt/p/MS6170
These meters and variations are even cheaper on eBay. They are also great for camping where many people including myself have killed batteries, particularly from portable fridges.
I have built my own multi fused power distribution box which I connect to the battery via the above meter and a fuse link at the battery.
Hope this helps
I use 2 x 18aH starter jump packs I got from Supercheap Auto for $95 each. Have 2 cigarette sockets, powered USB, a light and jumper cables. Been going sold 3 years now.
