There is no industry standard, per se. Manufacturers are all over the place when it comes to battery voltage ratings. For example, I have a Jonsered (Husqvarna) 58V max 2.6Ah battery that is also labelled as 131Wh, which works out to about 50.4V nominal. So, per Jonsered, the nominal voltage is about 87% of the maximum open circuit voltage. This is in contrast to the Greenworks 60V batteries, which have a nominal rating of 54V such that the nominal voltage is 90% of the open circuit voltage. It should be noted that when fully charged, the Greenworks 60V battery open circuit voltage is well over 61V, whereas the Jonsered is below 58V.