I'll try to be thorough and organized. I know just enough about things to be dangerous, at least I'm aware of it.

So this van died on an expressway off-ramp, blocking one of two lanes into a busy city center and creating a sketchy Easter situation with lots of honking and one finger salutes. In order:

• The van was driving normally
• Moderate braking on the off-ramp caused immediate lockup of brakes
• Happened 3 times until we came to a stop
• Car turned off
• Starter would turn and car would start 1 out of 3 attempts
• Car would idle poorly and stall when shifting into drive
• Idle could be maintained with throttle but car would not stay running in gear

Things I checked:

• Main car battery was putting out 10.06v on my cheap multimeter.
• Serpentine belt, oil, coolant, and main battery terminals all appear to be fine.
• What I think is Aux. battery is corroded to hell and the negative lead is broken.
• All accessories power up fine
• Even after a dozen failed attempts to start the car the there is still enough juice to turn the starter

More info about the car:

• It has been neglected severely by my dumbass friend
• It is missing the whole front clip and has been for over a year
• Apparently the main battery is dead and the car has been running on an auxiliary battery for weeks.
• As you can see in the pictures it is a handicap van with hand controls

So, I wanted to ask reddit: Is that little black box the Aux battery? Would replacing that and the main battery potentially fix this issue.

He is my best friend and roommate and I'm holding off on yelling at him for almost killing me until his car is fixed. I also want to make sure it was his fault. This will hopefully serve as a lesson about preventative maintenance. He absolutely needs his car to get around and he treats it like shit.