The NiCad battery pack in my ICu2AT wouldn't hold a charge anymore, as NiCads often fail to do... But I had a Nikon camera Li-Ion battery pack that is rated for 7.2V and charges up to 8.4V, at 1500mAH. And the rig seems quite happy running off of a Li-Ion battery, so I removed the NiCads and its charge circuit from the rig's battery case, and stuck the Li-Ion in there instead, with room to spare. Of course, I need to make use of the Li-Ion battery charger that goes with this battery, which has 3 contacts to the battery, so I have to change the battery pack's charger jack. So I can connect the 3 leads to the battery inside the Icom's battery pack. I thought of using a mini stereo jack (3 contacts) but I think the charger and battery would get unhappy with the momentary shorts one gets when plugging in and removing the stereo jack. So I used a 3 pin in parallel jack and plug, the sort used on circuit boards in consumer equipment like VCRs, and soldered inside the Li-Ion charger wire leads from this new connector.

To select an appropriate Li-Ion battery you'd need to match the rig's required DC voltage. And something in the order of magnitude of mAH rating, though this is less important than getting the right voltage. And one that will physically fit in the rig's battery case. That and some soldering, hot melt glue or other such ham radio technique and you should be back on the air.