So my circa 1995 Sega Nomad was having some issues due to an unfortunate match between my basement, it and flood water. Needless to say, there were a few things that didn't work anymore such as the LCD screen or the NiIMH battery pack.
It was a 2 day affair:
On day 1 I took time to disassemble the Nomad, de-solder the old screen, sacrifice a backup camera display screen, pin out the needed video sources and make a new video wiring harness using the new more advanced screen and hot glue the sucker in place. I managed not to let the magic smoke out, so no regrets there. The new screen is much brighter and has higher resolution that the original screen while using less power so that is definitely a win as well.
On day 2 I opened the old NiMH battery pack, removed the old bits, desoldered the original charging socket for later, dremmeled out the old battery supports, figured out the placement for a Lithium ion camcorder battery pack, mapped out the charging circuits in an AC/DC home and car charger for the battery, cut the charger apart preserving the 12V DC portion in such a way that would fit into the remainder of the battery pack shell. I soldered the charging circuits back together and wired the stock DC input socket to the charger inputs. I also used the bi-color charge indicator led and extended the connections to it so it fit in the original charge indicator led hole so it would show charging (red) or charged (green). I made a small wiring harness to tie the battery contact leads, charger and nomad contact leads together and sealed it up with hot glue. I currently get about 4 hours of runtime with the new battery pack.
The Nomad screen upgrade went well though it is about a mm off center, I wish I had centered it more carefully before hot gluing it down, I may fix it later with a heat gun. The battery pack refit might have also benefited from an independent shutoff to disconnect the battery from rest of the harness to prevent slow discharge into the charging circuits. Also, using hot glue to reseal the pack has not proved durable so it will start to pop open occasionally though I don't want to permanently seal it shut either. I'm still trying to work out what to do with that.