Is it a 12V lead-acid field battery?

If so, just get an aftermarket volt gauge for a car. Connect it to the battery and mount it to the box. I would look at both the open circuit and loaded voltage with the battery freshly charged then continue watch it as the battery gets depleted. Pretty soon you'll have a good idea of what voltage you'll want to recharge at.

The volt meter might also bee a small load which could drain the battery when you aren't at the field. If that's the case, just put a toggle switch in series with one of the wires going to the gauge.

You should be able to find one at most auto parts stores. There are also other options: http://www.pronto.com/user/search.do...90919327&site=

Another source might be RV or boat parts suppliers.

To be honest though, I wouldn't bother with it. The field battery should get you through a full day of flying and you can always recharge it before the next trip to the field. I like to charge everything before every trip so that I don't have any surprises.