1997 Ford Explorer Battery loosing charge while driving.

Few days ago voltage gauge suddenly dropped to half of normal, at end of day it went into red and lights started dimming. Eventually the battery drained on route and there wasn't enough power for the electronic throttle to get me past 2000rpm. Could not use hazards or headlights else engine would fall below idle. It starts with a boost but barely holds above idle. Alternator?

In idle, batter indicator is on, as soon as it goes past about 1800rpm it fades away but then comes right back when drops back to idle.

battery is fine since I left lamps on all night few weeks ago and it started fine in morning. Thx