Ego is around 3.3v (less voltage than Kgo) and I only recommend Joye brand Ego. The Ego-T - a lot of people don't care for
tank attys and
tanks vs cartomizers but the Joye Ego-T
batteries have a handy 5-click disable for pocketing them. One interesting new clone with little track record (I bought a battery to fool with) is at Gotvapes - Ego 3-stage instead uses the 5-click to go from 3.2 to 3.7 to 4.2v but I don't know how well that voltage holds up since the voltage of most batteries is best off the charger and then degrades to a plateau that may be flat or tilted a bit down, then drops off the end of the plateau.
The Riva 510 Kits at Liberty-Flights.com only (that I know of) are 3.7v (same as KR808D-1)
They also have a Riva 901 that should be able to use most KR808D-1 cartos since its post is slotted - dunno if those are now the 3.7v model because they sell a lot fewer of those. I also don't know if any of their single batteries are 3.7 other than the Riva 510 SE batteries which are.
I believe the Kgo is 3.7v and may come in both 510 and a KR808D-1 compatible threading (not sure)
Anything other than a real Joye brand Ego may need a short adapter to use Joye Ego mega cartos (big reservoir cartos which are made by Joye - they are tightly designed to fit their Ego)