External Base Battery Recommendation
We used deep cycle marine batteries also. Tell cell batteries are smaller and lighter weight but cost more. A quality automatic charger is just as important.
I've used an Interstate "wheelchair" battery I think like D55 or something like that, it powers the radio, recharges great, lasts a couple years and is VERY small with a handle.
Another vote for the marine batteries. Optima yellow or blue tops are great and will go all day. I have also used ATV/motorcycle batteries if I needed to haul them a ways from the survey rig.
Are you operating in the wide open? I would only boost it to 35W for long distances with a direct line of sight.
Depending on how hot the ambient temp is where you are working, running it at 35W can really heat that radio up...I have seen some folks actually use a cooler and ice packs to keep it from overheating. Just something to think about if you are going to be running it all day in the summertime.
All Powers make solar chargers that should keep your base running all day and ready for the next day by attaching to the battery of your choice.
We use a Interstate DCM0075 (75A) battery that runs all day. Put it in a battery box. Heavy, but keeps it from disappearing. The battery charger is a 2A that goes into trickle once charged. The charger is just as important as a good battery.
My issue is the Trimble radio. But that is for another post.