A trunk aux-power distribution thingy

Idea

I've got things I want wired into the car for power so they're ready to use without futzing with a plug. They include an inverter and a rechargeable combo flashlight. That light was a great find several years back--from an obscure Hong Kong company that manufactures in China and produces stuff you usually only find when some big chain retailer rebrands it, but the quality is so surprisingly good you start to recognize it whatever the name on the box. Nice components, plated contacts, fully silkscreened PC boards, and so on. Load it up with Nimh C cells and it gives you light for about forever.

Good news dept.

The Prius has easy access to 12 V at the battery in an equipment-friendly environment inside the trunk. In my last vehicle I never got around to wiring the inverter permanently because of the hassle of bringing heavy cable in from the engine compartment. In the Prius it's all right there. And the way the Prius will supply regulated 13.8 V while running the engine only as needed is pretty cool.

Bad news dept.

The Prius has a dinky 12 V battery. Previous vehicle, I just wired the Johnlite's charger circuit into a fused +BAT circuit. It trickled all the time, was always ready to use, and I could go weeks without starting the truck and the big old group 34/78 battery'd show no sign of the extra drain. In the Prius, I don't want to add loads that continue while the car is off. The flashlight will still be kept in good SoC if it charges only when the car is on.

Even this inverter I had on the shelf has the annoying feature of consuming some power when it's "off" just to chase a pattern around the front panel LEDs. So I don't want it hooked directly to the battery either. Has to be a new fused circuit with a relay that knows when the car's on.

And I'd rather not make any new holes, or go dissecting the wire harness to find an IG-switched circuit to tap in the trunk.

Ingredients

My local auto electric shop showed me this little module that holds two fuses and has a 70 amp relay built in. The big M8 stud on the right is the gozinta; the gozoutas are on the left. ("Comzouta"'s not how we say it 'round here.) The upper one is unswitched and the lower one goes through the relay. It takes two JCASE fuses.

I'd rather it had a double-pole relay and two switched circuits, 'cause I won't want unswitched loads, but it's what I could get.

Looking at the bottom of the module, the diagram even shows a built-in resistor across the relay coil to limit shutoff spikes in case you might want to drive the relay with a transistor. And don't those little recessed spaces look just like places a transistor and a couple zener diodes might like to nestle?

Even with the resistor, when I tested the relay coil with a simple switch I caught shutoff spikes below -50 V for a fraction of a millisecond .... That's more than the breakdown voltage of the small transistors in the junk drawer, so I made an extra trip back to the electronics shop for a transistor that can withstand 80 V collector-to-emitter. With that and the two zener diodes (one 4.7 V and an 8.2, total 12.9), the electronics shop gets nearly $3.50 out of this project.

Soldered together, the two zeners, a 10 ohm base resistor, a 1000 ohm pullup, and the transistor make a nice little package. I picked the 10 ohm resistor by theory, but the pullup was trial and error. Much larger and the relay may close below 12.9 V, much smaller and the transistor might not be in saturation at normal READY voltage. 1k ohms and it behaved Just Right.

Turns out in actual operation the shutoff spike barely reaches -5 V and I could have used a junk-drawer transistor after all. Oh well.

With some notches in the plastic for the wires and a blob of silicone to hold it all together, it's a relay module that turns itself on when the voltage is above ~ 13 V (as when the hybrid system is on) and off when it isn't.

This could get ugly if I ever apply a load that sucks the converter output down below the threshold. That would make the relay into a buzzer, until something gives. But I'm not too worried about that because the only load that heavy I might ever apply would be on the inverter, and this inverter goes back to its 'off' state if the input is interrupted. We'll just call it an aggressive form of overload protection....

In the industrial control world, you can get lots of interesting things--terminal blocks, fuseholders, relays, and so on--that are all made to simply snap onto this 35 mm wide, ~|_|~ shaped rail. Bend a piece of rail the right way and it can mount to an existing hole in the trunk structure forward, and one of the protruding taillight studs aft. That's an M5 wingnut for the taillight stud, and a nylon spacer tapped for M6 screws at the forward end. The bend at the taillight end is on an angled line, not straight across.

The rail-mount route wasn't my first idea, I looked around for nice cheap fuse panels for 4 or 6 ATM fuses like the Prius already uses. Easy to find, but I didn't find any that wouldn't be more hassle to wire up, and I had the rail stuff lying around. Anyway, ATM fuses don't come in fractional-amp sizes.

The relay module fits nicely on the rail too--doesn't snap on, but another M6 bolt does the trick. Behind it, a couple of rail-mount fuseholders and a terminal block for ground connections. The gozinta is 8 AWG welding cable.

The first small fuse will be wired to the unswitched side of the power module, and supply the little relay-drive circuit. That's a 200 mA fuse because the circuit shouldn't need any more. Turns out the voltage drop across that fuse is enough to raise the voltage threshold to about 13.1 before the relay closes, which is quite nice really. Almost midway between the normal READY voltage and the lower voltage on the battery with the car off.

The other small fuse will be fed from the switched stud and sized at 1 A for the flashlight. (That's more than it needs for charging, but the light can also be used while plugged in, and then it can draw that much.) The inverter will get a cable directly to the switched stud, and be protected by the 60 A JCASE fuse in the module itself.

Those rail-mount holders are for the 5x20 mm glass fuses, just because I had them around. It turns out there are rail-mount fuseholders for ATM fuses too.

Once it's mounted in the car (sorry for the overexposed nighttime flash pic), the ring terminal of the gozinta can attach to the fuse-mounting bolt in the + battery terminal (disconnected right now to get an angle where it's visible).

Interestingly, the voltage doesn't seem to drop immediately when the car is shut off: there's usually time to get out, open the trunk, and look in at the blinkenlights before the relay actually drops out. But that's fine with me; nothing will be a prolonged drain on the aux battery when the car's off, and that's what I wanted.

-Chap

 

It seems InPower LLC now offers a tidy module, the VCM-06, that does the same thing I cooked up with zeners here, and with a couple nice improvements.

It will switch ON and OFF based on the input voltage going above 13.25 and below 12.8 V. That makes it usable right out of the box to create new circuits in a Prius that will be on when READY and otherwise off, without any need to find a switched circuit to control it. Two connections, battery and ground, are enough.

It can switch 15 amps on its own, with no need for an external relay, and with built-in short and overcurrent protection, so maybe no need for downstream fusing. (If you'll be feeding loads of less than 15 amps and they couldn't handle a 15 A fault current, they'll still need to be fused smaller.) If you have a larger load you need to control, it can be on a relay driven by the VCM-06's output.

The neat thing is (and this is a feature I've been wanting to add to my own for some time), it doesn't have to turn off right away when the car exits READY (voltage drops below 12.8). Instead, it has an adjustable timer you can set so it turns off anywhere from immediately to two hours after leaving READY. This is great because sometimes I've wanted to leave something running on the aux power long enough to refuel the car, grab a bite to eat, etc.

It still watches the battery voltage, and will shut off if that drops below 11.8 V even if the set time hasn't elapsed.

Pretty slick!
-Chap

the first version of that web page the Wayback Machine found was on 27 December 2009, 14 months to the day after I posted this homebrew contraption.

No, wait, they actually scooped me! Their press release appeared in July 2008 issue of Diesel Progress NA. Somehow, I must have missed it.

-Chap

 

I think you might be under a misconception regarding the Prius electrical system: yes, the aux battery is kept charged via DC/DC conversion from the high-voltage system, but only while the car is in READY. When the car is off, the HV battery is completely isolated by the system main relays, and the aux battery is on its own. (And when the car is in READY, it will start the engine as needed to keep the traction battery charged.)

That's why the voltage-sensitive switching described here works just great in a Prius, and does just what it says on the tin.

-Chap

 

Ok, experience report time. My homebrew zener-diodes-and-transistor voltage threshold circuit lasted from October 2008 to now (six and a half years), but just this weekend seemed to get stuck ON so it drained my aux battery.

I haven't pulled it out to determine which component(s) failed or how; I think I'll just buy a VCM-06, which has more features I wanted to add anyway.

The problem was definitely my threshold circuit and not the relay; I discovered the problem by hearing the relay click as soon as I swapped a charged battery in.

-Chap

 

Did put in a VCM-06 around that time and the new features pleased me, but oddly enough this winter something has once again sucked my 12 volt flat a couple of times, which is annoying. Also likely to shorten its life, but I generally swap batteries when that happens and let the drained one recharge on a nice controlled indoor charger, and in my experience they come back reasonably well. I've got a slow discharge tester that'll tell me how many Ah capacity I've lost ... will run that test later. Current limit is fairly low, so on a battery that size it runs for a couple days.

So, I sort of keep unintentionally gathering data on the is-the-12V-battery-the-explanation-of-every-unexamined-trouble-code question ... In my Gen 1, down to about 8 volts during the start attempt, the car does start, at least if I pull the brake pump relays first, and doesn't produce any spurious codes. Around 7 and below, starting is pretty much hopeless, but after getting it started with another source, again I get no mystery codes.

This annoyance has led me to discover something about the VCM-06 that I would never have guessed: it draws about 7 mA in the output-off state. That's definitely more than my original two-zeners-and-a-transistor approach, which was pretty much totally OFF when off. I e-mailed InPower about that and got the response "That measurement is within .1Ma of our tested draw for this product. The power is to allow for the microprocess to remain active in a key off position."

I didn't bother to reply and ask why they need the microcontroller to remain active when off ... it only does interesting things like time delays while it's ON deciding when to turn OFF. When it's OFF, the only thing it needs to do is turn ON when voltage goes over the threshold ... they could use a Schmitt trigger for that.

Makes me want to use the Dr. Strangelove voice and say "if the whole point of the device is to protect the battery by turning OFF to prevent excessive discharge, wouldn't you want it to stop drawing current when it's OFF?" But I don't have a Dr. Strangelove font for e-mail.

Anyway, when I measure the resting current of an unladen 2001 Prius (North American), I get about 63 to 70 mA (I think the 70 mA spikes might be the blinks of the security LED), so the OFF drain of the VCM-06 increases that by around 10%. I guess it probably isn't the thing (by itself) that's sucking my battery flat.

I've seen somewhere an older post where someone measured a Gen 1 at rest at more like 35 mA. I'm not sure what to make of that; maybe I do have something drawing an abnormal amount, but haven't had the time to think of tracking it down yet.

Now, I think when Hobbit measured the resting draw of an unladen gen 2, he got a way lower figure, like 11 mA. That would make the VCM-06 design especially unfortunate ... a gizmo that almost doubles the resting drain of your car, just to be OFF....

-Chap