Cannot understand why you would need an extra battery as I run my GPS, heated jacket, heated grips etc. I do have a small gizmo (see Gammatronix) that lights up orange if I'm not getting enough OUT of the battery and switch things off accordingly. I do not have a problem with charging at all and regularly run all of these extra 'things' when out and about. When I get to site I usually use the power supply to pump up my airbed and charge my phone and helmet (bluetooth) and if necessary I have an air compressor for tyres. I do run the engine when I use either of the compressors though.
I have not had a problem with the battery flattening but the 'gizmo' does give me a reliable visible indicator of the charging system.

Hope this helps.

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