Sudden Battery Failure?

 Saturday rode for a sweaty 100-ish miles, no problems.

This morning I went to start my Valkyrie.  I turned on the ignition, lights
and everything came on, hit the starter button, then...death.

The starter didn't crank, didn't even click.

Everything went out, lights, radio, idiot lights, all of it.

I turned off the ignition, turned it back on, and the low oil light came on
for a fraction of a second, then nothing.  It does this every time now.

The voltmeter showed 13 volts with ignition off.  When I turn it on, the
voltage goes down to about 1/2 volts across the terminals.  Turn it off, and
the voltage goes up to about 12.5 volts, creeping up toward 13.

I have a load tester, but it's for cars.  It shows "Good" with no load, and
bottoms out with load.

There doesn't seem to be a short or loose/corroded connection.  There's no
smoke, no hot wires, fuses are good, and connections are tight.  This bike
was kept in Southern Utah before I bought it, which seems pretty dry and
arid, and I haven't found any evidence of corrosion.

I'm hesitant to jump it or replace the battery.  If it is a short, I don't
want to do any more damage.

I'm really hoping it's the battery, but I've never seen a battery fail quite
like this.  Usually there are warning signs, or at least dim lights and a
few pathetic clicks from the starter.  This does not appear to be the
original battery, as the hold down strap was last installed by a guy who
thinks a Big Arrow with the word "REAR" inside it should be pointed at the
front.

Any ideas?  Has anybody seen a battery work fine, then give up the ghost
this suddenly and dramatically?

CS

 CS wrote:

Yep.  It's a well known failure mode.  I saw it on a more or less brand
new battery at my neighbor's house a couple of months ago.  From your
description of the symptoms, you have a battery with high internal
resistance.  At low currents it can put out full voltage but when any
significant load is applied, almost all the battery's voltage is
dropped across the internal resistance rather than the load.

Get a new battery, and make sure it is properly installed in its
holder, excessive vibration can cause this failure mode.

 
       Yes.   It happened to me about a year ago.   It was pretty much
the same scenario as your failure.    A helpful nearby
resident let me use her jump starter battery.  The bike ran fine with
the extra battery.   It died the instant I disconnected it.   I tried
disconnecting all the non essentials.  All I wanted was a working
motor.   Even that didn't work.
    I offered the lady a few dollars for her time and such.   Take
your husband out to eat,  have a drink or whatever, I said.   She
wouldn't take a penny.   It's good to find people like that.    She
was a transplanted Californian.  I can't remember how she ended up in
rural Nebraska.