losing charge on 1990 pickup v6 SR5

hey guys i'm a newbie to this site but last night i was DD for a bunch of friends and as I pulled into my buddy's driveway I lost a lot of charge and the truck idoled below 300rpm and i just shut it off looking to save some juice for diognostic purposes.

for the last week 2 lights would flicker on and off on my dashboard, the battery light and the brake light the 2 lights that come on for start up so i figured it was just some wires a little messed in the ignition.

also a month back i replace my old altenator with a pretty much brand new second hand one. Is there anything else that could be the cause of this other then the altenator?

thanks for reading.

The BRAKE and CHARGE/ALT coming on together is to tell you the alt is bad (usually the brushes). If it is still happening the used alt could be bad too. If you can get to a parts store, most will check the batt and alt for free. Or even better, pull it and have it bench tested.