There are two electronic ignitions on the market that are self powered like a traditional MAGNETO.
One is Emag. EMAG use to make a self powered one (called PMAG) and one that required external battery power (called EMAG). He now only make the self powered Model and they are all called EMAG's I believe. Per the EMAG website you need to still connect ships power to the LEFT and RIGH EMAGS, but in the event you lose electrical power to the plane the EMAGS have their own alternator to generate power.
The other is SureFly. Self powered electronic ignition.
As far as I know there are no other electronic ignitions that are self powered. and they require electrical power. To answer your question about batteries (not EMAG or SUREFLY), only ONE battery, ships battery is needed to power BOTH electronic ignition. They do not use that much power. HOWEVER it is mandatory for redundancy and safety you have a backup battery or electrical system for one of the electronic ignitions, independent of the main electrical. That way you will have at least one EI if the other electrical source fails. You can run on battery power off a small backup typically for 30 to 60 minutes.