Hello,

I'm working to get a APRS digipeater setup running on a 50w 2 meter mobile.  

I've got an Astron power supply and the radio currently and could set that up without any trouble.  

But it's my only power supply.  I also don't currently have a working battery setup for when the power goes out.  And I'd like to.

So I thought I could kill two birds with one stone and setup the digipeater to run off a battery on a permanent basis.

The question I don't know the answer to is this.  Is there a RF safe battery tender or smart charger that would work well with a 7amp hour gel cell battery?

You can do it, but if you don't have the battery, the short answer is that a battery + charger is going to cost you as much or more than a power supply.

This is what you seek:
http://www.powerwerx.com/batteries-chargers/super-powergate-backup-power-switching.html