The nominal voltage for most NiMH cell is 1.2v so a 6 cell pack will have a nominal voltage of 7.2v. This will charge up to around 8.35 - 8.4v and should not be discharged below about 6v.
Alkaline type batteries typically have a nominal voltage of 1.5v, fully charged are about 1.65v and are considered depleted around 1.0-1.1v.
Lipo batteries have a nominal voltage of 3.7v, maximum of 4.2v (HV lipos up to 4.35v), and minimum of 3.3v. The lower limit is technically 3.0v, but as the chemistry of the lipo is damaged below 3.0v it's safer to use 3.3v - 3.5v as a lower limit. The other thing is that there is very little capacity stored below 3.6v. The table below is pretty old, but is unlikely to have changed significantly over the years.
Lipos are best kept at a storage voltage of 3.8v .
For lipos you might want to consider a cheap voltage meter. The percentage algorithm is a bit off on mine, but the voltage readings are accurate when tested against a calibrated mulitmeter. It's really useful for comparing individual cell voltages in a multi-cell pack and will even show you the highest and lowest cell voltages and the variance between them. This can be particularly useful in identifying a faulty or dying cell.