How can I improve the battery charge rate while using navionics?

Connect your iPad to a Mac system and run on the Mac CoconutBattery it will tell you the health of your iPad’s battery. Lets see what you find out.

You also maybe hitting a limit here with this iPad series running this type of program.

Have looked at getting a newer model? The older APU’s pulled more power and given your use the APU is stripping the power faster than the battery (or charger) can offer it. You could try using an external battery pack but I’m not sure that will offer enough juice if your direct power feed is unable to offer enough current either.