hello Porsche !
i don't use my rome with my hammer s battery but still keep full charge since a few months.
i have this charger(picture) for some old phone but rome battery is too large...
probably you can find a model more powerful for larger battery.
about rome GPS i have post this question wich probably dissapear in umi super forum.
cordialy.
an excelent charger for an aditional battery of umi rome is a broken umi rome, there is a lot in the world, mine is one of them. its worse screen i have seen