As a clarification, I mean what was it like in a German or Soviet penal battalion. I'm not really sure if US or English penal battalions were raised. From my cursory internet search it seems like they were simply given suicidal missions like serving as "tramplers" in minefields or holding vulnerable positions. It just seems to me that there has to be more to it than that. Why wouldn't the men desert if they knew that they would literally be used to clear mines with their dead bodies and there was no hope of escaping service alive?
If you are interested in the penal battalion system, the 36th Waffen Grenadier Division of the SS is really notorious. Headed by Oskar Dirlewanger, the unit was comprised of serious offenders (Dirlewnger himself was a convicted child rapist). They were used for anti partisan missions on the eastern front, where their lack of discipline and inherent savagery was useful for stamping out popular uprising. I'm not sure how the experience of this unit varied from others in the military penal system, but they were actually given a fair amount of operational freedom. The actions of this unit are seen as some of the gravest war crimes of WW2.