Keeping a battery powered device charged
 

I'll be installing a double din android unit soon and I bought a mobile hotspot for internet connectivity. The device can last approximately 6 hours on a charge and I want to keep it in the car at all times because bringing it inside to charge will be annoying to do every day.

I think leaving it charging on the cars battery for an entire weekend can kill the battery. I thought about buying a 12v timer relay which I can set to kick on for 1-2 hours, every 5-6 hours, every day. This will keep the device charged and reduce the amount of time its pulling from the car battery.

Or, for the fact that the device may not pull power from the car battery once fully charged will it be ok to leave for at most 2 days without running the alternator?

During the week I drive about two hours a day.

Anyone ever do anything like this?

Edit: Before someone suggests this...I can't use my phones Internet connection because I have a unrootable moto x 2014 on Verizon. I'm not paying Verizon for tethering, my mobile hotspot is only $4/month.

Never mind, I have a USB modem on the way.