Re: Sounds like a petrol station special
"My relatively old S3 wouldn't even charge while doing satnav duties until I bought a chunky 2A car adapter."
You're doing wrong. I've been using <shameful sell here> Oziexplorer for nearly 15 years over a variety of laptops (including a PIII and an Atom), several WinCE devices, and three different Android phones. Although it offers an "online" map display, I've only ever used offline, and all still get used regularly on battery.
Bottom line is, if your phone gets hot, or you need a charger at all to get any reasonable charge life out of the battery, then you paid too much for your satnav application. If it was free, you especially paid too much for it. I'm sure there are other apps that do a suitable job, just not any of the ones mentioned here it seems.