All old
mobilerobots.com sites seem to be down but they were archived at
www.archive.org before the business closed, so you can get copies there. Some power outputs are voltage regulated but some are not, particularly 12v direct battery power (and any which have been converted to nominal 5v but without stricter regulation). I don't remember all the details but that would explain the discrepancy especially when connected to a charger. 18v seems a bit high, but it may depend on what kind of charger is connected (and at what charge mode/cycle it is in).