Reserve capacity question

Can someone double check my math / logic?

If I wanted to run a BD S2 Sport as a camp light (.9A draw) and my 27F 710 CCA battery has a 165 min reserve capacity:

10% rundown (safe-ish for fairly often use) = 165 minutes * .10 = 16.5 minutes @ 25 amps.
25 amps / .9A = 27.8
27.8 x 16.5 minutes = 458 minutes (7.6 hours)

I just want to make sure I don't run out of starting capability in the middle of the desert, and have a survivorman type deal go down.

 

If it was me I would use a second, deep cycle battery to power anything auxiliary loads while the truck isn't running and leave the starting battery to do its main job (start the truck).

 

I would get a cheap 12v SLA battery from walmart used for running deer feeders. These will have 6-8ah capacity and are designed to be drained and recharged frequently

 

you also might look at their rock light for that. it will depend on how much light you need.

 

I have a 2nd battery planned. I just got a great deal on some lights and wanted to use them before I got the aux batt. set up.