I’ve seen a few mods around for converting the Sega Nomad rechargeable battery to a Li-ion battery, the advantages are the higher capacity, low selfdischarge and lower price. Most if not all of the old NiMh that you will get your hands on will be rubbish with a very low charge capacity, if any at all, and being an odd sized cells it would be quite a cost replacing all 6 of them. 2000mah Li-ion batteries and chargers can however be bought on ebay for about $18US. This is the cheap $18 charger and battery that I found on ebay, searching for the F550 battery should get you there. Once it arrives in the mail, you want to take the housing of both the charger and the battery, there are two screws behind the sticker that hold the charger together.
Once apart you can place the circuit board from the charger inside the nomad to see how it fits, I had to file a tiny bit of the bottom slanted edge side of my board with a file. You will also have to remove most of the plastic baffles that once held the NiMh batteries with a sharp hobby knife. You will want to remove the 240 volt spring terminals from the charger, and extend the leads on the dual colour LED, you may also have to unsweat and tilt a capacitor near the top corner of the battery.
So that you can charge the battery with the standard Sega 10 volt plugpack I added a DC to DC regulator, you can get these in kit forms from radioshack or jaycar, and failing that you could rat one from inside the adjustable voltage cigarette lighter power supplies. I had to cut mine down a little to fit. I also took the charge socket from the original charging board and attached it to leads, and fed them into my DC-DC supply. I set the DC regulator to 14 volts which seems to work perfectly.
I removed the socket from the Li-ion charger board and connected wires straight to the output of my DC-DC converter. Once everything fitted in place i connected the battery to the battery pack terminals, then stuck the boards, battery, led and socket down with some neutral cure RTV silicone, let it all dry, and reattached the case.
Finished battery charging happily and working like a charm, the best part is that the standard Sega power supply can charge the battery. Aside from the tiny DC-DC converter which you can rat from somewhere if you’re not game to build it, there is nothing too complex about this mod as it’s more like a game of tetris than anything else.
One thing that I did find with this mod was that the battery would self discharge a little when left for a week or so, this was due to the charging circuit creating a little bit of a drain. I solved this problem by putting a small 12 volt reed relay (as these are tiny and usually rated at around 1.5A) on the positive lead between the battery and the charging board, then connecting the relay coils directly to the plug-pack socket with short lengths of wire. This is dead simple and means that the battery will only connect to the charger when the power lead is plugged in. The 12 volt relay will happily trigger from the 10 volt sega supply, i found my relay to trigger down as low as 7 volts, but if you’re worried you can connect it after the dc-dc converter, i just couldn’t be bothered soldering new wires onto my already mounted PCB, so just piggybacked of the socket. If you are game you could skip the DC to DC converter and instead put a 2 pin figure 8 power socket on the battery pack running straight to the mains pins on the charger, this could however be quite hazardous and would take away from the originality, you would also need to find a dc source on the charging board to drive the isolation relay.