It's not exactly the same as carrying a spare can of gasoline but for a battery electric vehicle it's about the closest thing to it and could allay range anxiety concerns for those worried about running out of juice on long trips.
Roadside Electric Car Charging — SparkCharge
GM Wants to Trademark 'Range Anxiety
Slow Rollout of Electric Vehicle Charging Network Could Hinder E.V. Adoption - The New York Times
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VIVOHOME Portable EV Charger, 16A NEMA 6-20/5-15
EV Charging and the Influence of Local Startup Innovation - Cintrifuse
Lectron Electric Vehicle Charging — Lectron EV
On-Demand EV Charging Could Be Coming Soon
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8 Ways To Overcome Electric Vehicle Range Anxiety