My first hybrid, strange electrical problem, 2007 Honda Civic Hybrid

Question

Just bought my first hybrid, a 2007 Honda Civic Hybrid. Regrets already.

Need help diagnosing this behavior. I'm about 80% sure the "starting" battery is dead, but hybrid...

Day 1

1. Turn engine, car starts
2. After 5 seconds, all dash lights and interior lights start blinking, powering on/off
3. Rev engine at 2500 RPM for about 2 mins
4. Turn off engine
5. Turn back on, runs good, no problems

Day 2

1. Turn engine, car starts
2. Dash and interior lights powering on/off again (even engine turns off/on)
3. Give RPMs for a bit
4. Turn off engine
5. Try to start, "click-click-click" (dead battery?)
6. Try to start 5 minutes later, car starts (what!?)

Day 3

1. Turn engine, "click-click-click"
2. Jump-start, starts fine
3. All dash and interior lights powering on/off again
4. Turn off
5. Try to restart, "click-click-click"
6. Dome lights work, remote entry works, power locks work, power windows do not work

So either my car battery is really dead or the main "hybrid" battery under the seat is dead. Or possibly something else? Have no voltmeter and limited time, so I wanted to get some answers from SE first before committing.

EDIT I should also note the power steering and brakes fail when the dash is flashing like a Christmas tree. Even while it's being driven.

Answer 1

A new 151R battery fixed the problem.

The nuts that tighten both clamps to the battery posts seem to have been engineered to not tighten enough to hold onto the posts. The positive just barely holds, and the negative slips right off with no possibility of further tightening. Looks like it came from the factory this way.