I got this little dual USB charger and volt meter from Amazon and installed it on the nightstand next to the bed. The voltage agreed with my multimeter so I guess it is accurate. Now we can charge phones even on the battery all night. The only problem is that it is REALLY BRIGHT! As soon as we turned off the lights last night it lit up the room blue. I'll have to find some panel tape or something. But I do like a built in USB charger that doesn't depend on shore power.