Thanks guys! Yea I know about old car batteries I have a few but they don't last long for what I need. They are OK for dew heaters and EK box but not enough for running little laptop.
I know about BatteryWorld and similar websites. I thought maybe someone here have experience with this problem and wouldn't mind to share it here. Anyway, I found on google how to calculate Amp and battery capacity. So that will help me in deciding what to get:
http://overlandresource.com/what-is-...ttery-capacity
Thanks again