Prius 12V battery exploration

Warmer weather once again means it's time to dig into car innards, and
several questions had come up in the meantime.  Messing with the 12V system
had to wait until after passing state inspection, so that the I/M readiness
monitors wouldn't get unset, but once that was done it wouldn't matter for
the car to completely lose power for a while.  There were two major goals:

    See what happens when the DC/DC converter loses its reference

    Pull the battery and measure its dimensions, to shop for replacements

In addition, cleaning out the battery tray area and checking the condition
of the contacts and lugs and hold-down clamp seemed worthwhile.
Voltage games
Upon returning home from some errands, I left the car powered up, dug down to
the 12V battery area, and connected a voltmeter to the ground lug and main
positive terminal block.  The meter, which I trust reasonably well, read 13.99
volts.  This is in keeping with what we know about the inverter -- it tries
to run the system at 14V, even if that carries slightly higher long-term risk
to the battery.  The dash-panel meter up front generally reads a point or two
less, but probably because of a little voltage drop by the time it gets
through all the connectors and fuse blocks to where that power is taken.

[It is assumed that readers generally know what they're looking at here.  The
battery and reserve-power capacitor box for the braking system are right next
to each other, sometimes leading to confusion.  It should also be noted that
the red (+) cover is a pain in the ass to remove -- I've carved away bits of
the plastic side tabs on mine to make it easier].

The DC/DC converter has a dedicated voltage-sense lead running all the way
up to the inverter so it can *know* what the exact battery voltage is.  The
lead has a 5 amp fuse in line right back at the battery-connector block,
and I've been wondering for a while what would happen if that fuse blew?
The sense lead also has a handy connector, so I pulled it.

Instant beep and triangle-of-doom from up front, of course!  Along with a
"battery" warning icon on the display and the ubiquitous "little car going
through a bandsaw", as someone amusingly put it.  For some reason the 
brake warning came on too [yes, the brake-caps-box plug was still connected].
Didn't bother to pull codes; I figured it would be some variant of P0A09 or
P0A10.  [I wonder if regen braking would have worked? hmm...]  The main thing
to find out was how the system responded.  The positive line went to 14.07
volts -- hardly a radical change.  Plugging the sense lead back in made it go
back to 13.98 or just about what it was before; the "battery" icon self-cleared
off the MFD but all the other warnings stayed.

Next possibility: what happens when the battery itself is disconnected?
Many old charging systems used the battery as a regulator, i.e. as a
nonlinear element that would pull more current as system voltage rose and
thus tend to stabilize it.  There are plenty of old horror stories about
a running alternator losing the battery connection and blowing every lamp
in the vehicle with 17 volts or more.  The Prius DC/DC converter is built
like a modern switching power supply, and I didn't expect anything of the
sort from it, but I had to know.  So with everything still powered up, I
loosened up the (+) connection and slipped it off the battery terminal.
The car was now running completely off its own DC/DC output.  It stayed
rock-solidly at 13.99 volts, no matter if the battery was connected or not.

Final permutation: leave the battery disconnected and pull the sense lead
again.  This would force the DC/DC to fly entirely on an internal reference
without any external influences from the sense lead or a battery helping
to draw off excess current.  The bus voltage went back up to 14.07 and
stayed, and the "battery" warning icon returned.  Same behavior upon
reconnecting the sense lead, battery or no battery.  And no crispy-fried
ECUs -- bonus!

Thus, two long-awaited conclusions:

    Losing the DC/DC sense lead is not a disaster, despite the warnings

    Losing the battery with the car powered up is not a disaster either

With everything disconnected I went up front and powered the system down.
*WHAM*, everything went off much more abruptly than I'd ever seen before,
because of course as soon as the DC/DC shut down everything else lost power
too.  Normally there are a few things that hang on and do cleanup before
powering themselves off; no chance this time.  And at that point of course
I lost reverse-beep and seatbelt-beep settings, the clock, ECM fuel trims,
the power-window auto-up calibration, my [relatively new, bfd] MPG average,
emissions monitors, and numerous other bits of keepalive memory.

But something caused the dash-panel voltmeter to slowly drift back up to
about half a volt, confirmed by the meter still hooked up in the back.
I pulled the connector for the brake capacitor box next to the battery,
and the voltage started slowly drifting down again.  Okay, that small
mystery solved.  Eventually that too lost the remains of its charge.
Next step was to disconnect everything else and pull the battery out for
further study.  The final section of the HV battery ventilation exhaust duct
had to be pulled first; as I did so I noticed that there's an extra hole in
its lower side, sort of aimed into the 12V battery area!

Maybe the exhaust from the big battery's intake of cabin air helps warm up
the 12V as the car gets going in cold weather, as well as drifting to the
outside through the exit vents.  These vents under the rear quarter panels
[both sides] have very loose one-way rubber flaps to help keep stuff from
coming in, but they're such a token gesture that a little sand and crud
manages to get inside the car anyways.  This should probably be cleaned out
during the spring spruce-up since it likely contains a lot of salt.

Finally, the item in question could come out.  [Select the picture for the
big version with all the inch measurements added, suitable for printing and
taking to your favorite battery shop and asking "so, do you have anything
like THIS?"]  The little nylon strap is quite handy for pulling it out of
the somewhat awkwardly-angled body well.

A timely discovery

Well, whaddayaknow, another GS rather than Panasonic.  Sealed and maintenance-
free, according to the labels.  Supposedly it's a spillproof AGM, and during
March 2007 there was a long and heated discussion on Prius_Technical_Stuff
about AGMs versus flooded lead-acid, charging caveats, whether or not to add
water and/or what kind of water, etc.  Well, get a load of THIS:

The pictures are a little subtle, but as I tilted this thing I clearly saw
liquid levels rising and falling against the cell walls!  It's flooded,
regardless of what substance is between the plates.  So if the right rear of
the car gets whacked hard in an accident, you're still going to have sulfuric
acid all over the road.  Maybe that's why it tolerates a full 14V sent to it
all the time -- as a wet cell with the vent tube to the outside, it can
tolerate a little more overcharging.  It may also be why Classic owners are
starting to find their own batteries gassed almost dry at this point, causing
them to erroneously think they're fully-absorbed glass mat.

This one's no spring chicken either, being the original battery from 2004.
It's starting to exhibit a slight bulge, especially on the right side, but
from what I can determine it's still fairly healthy.  Now, at this point
with its dimensions in hand I feel much more confident about looking up
replacement specs on the net and knowing whether what I eventually get will
fit nicely or not.
More measurements
What's even more useful is to know the dimensions of the carrier and
surrounding car body area, and how it might need to be modified to fit a
different type of battery.  Here's what the box could actually hold with
no modification:

... and the vertical distance is bounded by these felt-tipped pegs sticking
down from the hatch floorboard, which at 7"7/8 from the bottom land exactly
on top of this battery and help structural support of the hatch area.

They could be cut down to fit a taller one, of course, and the metal holddown
strap would have to be jacked up on a spacer at the inboard end.  The threaded
rod on the outer side has about an inch left above the nut.  In general there
is quite a bit of extra room in the overall area for those willing to attack
the stock bracket or fab up a new one.

As I reinstalled the battery, I dropped a piece of thin self-sticky foam
pad on top of the bracket to cushion the bottom of the battery a little.
Otherwise it's direct metal contact over a relatively small area since while
the bumps in the bracket meet on the same plane, their area isn't very large.

When my GS starts dying, I suppose that in a pinch I could always plug one
of my modded UPSes into the Anderson connector and boot the car from it, and
drive around with that rig until a real replacement is found.  With only
30 amps really needed to boot up, the workaround possibilities are numerous.
Slowly [?] back to life
As long as everything was pulled apart and powered down, it was time to do
some more tests on idle current.  After cleaning things up and bolting the
battery back in, I could begin feeding current to the car through the
"noid box" instead of just zapping the (+) terminal back on which usually
sparks a bit as various capacitors charge up.  Heck, if the hybrid battery
does a precharge to protect its own contactors and downstream components, why
not do the same for the 12V?  And this is a fun little diag tool.  The high-
impedance setting only brought the system up to 2.7V with the sonalert howling
away.  But interestingly, kicking in the 9 ohm resistor only brought things up
to 5.6V -- there was a serious drain somewhere.  Finally I clipped in a direct
connection, and heard all kinds of complaints from the front of the car --
the system still thought it was on, and there was the screen with all the
warnings and the ToD and the speedo all lit up!  That's 300+ watts of draw it
wanted right away upon receiving 12V, and my alligator clip was getting hot.
I swapped in a bigger one, and went up front and properly powered the car
off [again!].  Now it was happier; various things now had 12V and could go
through their little "off" dances.  Then I could do my quiescent-current test.

But the "door open" light was still on -- fixed by shoving the hatch latch
down two clicks with a screwdriver to fool the body ECU into thinking all the
doors were shut when in fact I needed the hatch itself open.  The noid box
with 9 ohms still howled and only brought the system up to about half voltage.
But then if I held it in that state for over ten seconds or so, the drain
eventually stopped, the noid shut up and system voltage came up to 12.7V.
Upon cutting in an ammeter, I read 11.1 mA quiescent drain which is much
lower than the 35 mA I thought I'd previously seen.  I suppose I'll have to
repeat that particular test later after everything's reinitialized and see
if it's any different.  Maybe I forgot about the "door open" dash light the
last time I did that, and was seeing the extra 20+ mA for the LED...
What's your story?
Hopefully having lots of info about the 12V system collected into one place
is useful for other people.  I will be happy to collect the experiences of
others with aftermarket replacement batteries or just point out to other
webpages about it.  With no aftermarket direct replacement for this battery
yet, the Prius community needs to have access to a broad field of knowledge
about brands, size adaptations, terminal modifications, venting vs. not
bothering, etc.  Send 'em in.

Update: see this followup page for an easy and elegant replacement option and some helpful hints for completing the process.

_H* 070328