Welcome, Phil - you'll get a lot of good help on here, and more than a few bad jokes!
While I stick MOSTLY to track power, I have built a couple of battery locos (while retaining full DCC control) to take to other folks' lines which may not have DCC track power available; these threads may be of some interest to you, I hope:
Today, I managed to put in a little time on my Multi-Power Loco project - the idea of which is to take an LGB track-power loco (an OBB 2095 "Whizzy Cranks", as it has a nice boxy body with plenty of room for added internal gubbins) and turn it in to a "touring" loco that I can take, and run...
www.gscalecentral.net
Having had success with my first "Multi-Power" loco project using an LGB 2095 Austrian BoBo Diesel, as described here: https://www.gscalecentral.net/threads/building-the-multi-power-loco.309298/ I've now made a start on the second project, which is putting a similar setup into an LGB Saxon Meyer...
www.gscalecentral.net
Echoing other posts above, I'd recommend not trying to remove batteries from the locos for charging - opening up a typical G scale loco is a lot of faffing around (and usually a lot of screws to remove) and you really don't want to be doing it every running session.
If you can fit a suitable Li-Ion battery pack then it will run a two-motor loco for several hours, and a single-motor one for nearly twice that, so one charge will usually be plenty for a typical running session - it's not like the electric flight and car racing boys who burn through a battery pack (possibly poor choice of words there...) in ten minutes and then need a fast recharge....
On safety, provided you use good quality batteries with in-built protection circuits then you really shouldn't have to worry - most if not all of the horror stories/videos are of li-Po "soft pack" batteries from China, charged and discharged at ludicrous rates for the aforementioned flight and racing purposes; with trains you're not treating/mistreating the batteries to anything like the same degree. Good, protected batteries, a proper matching charger and an onboard fuse, and you should be fine!
Jon.