What you need is the battery tender Jr. Which you can find at your local automotive store. It will charge any sealed lead acid / gell cell. It has an output of 800mah so it wont fry your battery. Its almost like a peak charger for your field battery. When the battery is full it will turn off and if it sees the voltage dropping it will charge the battery till its full again. YOu could easily modify the connnectors to a banana plug so you can charge the battery through the power pannel. I keep it plugged in 24/7 so its always ready to take to the field with a fresh battery.
Many people destroyed their field batteries by over charging or they left the battery sit and the voltage has dropped below the safe level of 12V. If it gets too low then its extremly hard to bring it back up.