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.Dead
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.