Joe, for conventional operations the PS-2 both the early 5V with 8.4V battery and the newer 3V system with 2.4V battery need charged batteries to work in conventional.
The first iteration of PS-2 the 5V (2000 to about 2004) are long in the tooth and can be problematic. Must put a new battery in before operating, if you buy one of these and it has been sitting.
The later PS-2 3V (2004 to 2012) are very reliable and won't really be hurt by a weak battery.
Neither has any issues with programming conflicts like PS-1 could have.
Lastly the new PS-3 use super capacitors and have no need for batteries at all.
So look for the newer PS-2 3V and PS-3 engines, or if you find a good deal on a PS-2 5V it can be worth it, but you have to consider you might have to have the board replaced. G