Start dead Prius after 12v battery drain / prevent inverter fully draining battery

Decent inverters these days have an automatic shutoff feature built in, that will turn the inverter off if the battery voltage falls below a certain threshold, which still leaves you plenty to start the car.

If your inverter doesn't have that, it may be a good time for an upgrade.

Otherwise, you could check whether your inverter has a provision for a remote on/off switch accessory (sometimes connected through what may look like a telephone jack on the inverter). If it does, and you learn the pinout of that connection, you may be able to design a circuit that activates that remote-on when a switched circuit in the car goes high. In that case, you aren't worried about drawing a huge current from the switched car circuit, because it is only used for the remote-on signal, and you aren't fussing with a large relay or the like because the remote on/off feature is built into the inverter.

Failing that, you can obtain a large relay and use it to control the battery power to the inverter, based on a switched circuit from the car. At that point, things are getting sufficiently Rube Goldberg that the idea of upgrading to an inverter that already behaves well becomes even more appealing.

Any of the above ideas that involve tapping some switched circuit in the car can also be done using a Low-Voltage Disconnect like this one, which doesn't require locating any switched car circuit. It can tell when you have made the car READY when it sees the voltage rise above 13.2, and will turn off an hour after you turn the car off, or immediately if voltage falls to 11.8.

There's no need for an especially heavy-duty low-voltage disconnect, as, again, it only needs to be connected to a remote-control signal input on the inverter, or to a relay; the power drawn by the inverter doesn't have to go through it.

The inverter I am using has both its own low-voltage shutoff (somewhere south of 11 volts, still leaving plenty to start the car), and a remote control input, which is driven by my LVD. So it naturally shuts off, thanks to the LVD, an hour after I turn the car off if I forget (or I want it to stay on while I pop into a store or grab a quick bite); or, if the battery falls to 11.8 before an hour, the LVD will shut it off immediately, or if both of those things don't happen for any reason, the inverter's own auto shutoff will trip just below 11 volts.

An old inverter I had came with an auto-shutoff feature, but it also had another "feature" where it would chase LEDs around its front panel to show it was "off". So I had that behind a relay controlled by the LVD. But the better solution was to replace that inverter with something less ridiculous where "off" meant off.

 

+1 to Chap's comments. An inverter that shuts off on low input voltage is the best solution.

Also:

Why not just pop the hood once you manually unlock the car and jump it at the jump points provided for that purpose?

Yes, everyone really ought to have one. This is what I have. I've only ever used it once and that was on a friend's car.

 

There was a lot to unpack in the original post. In just two replies, everything that needed to be said was said. Awesome forum.

 

That will work, but a relay capable of switching the input current to a 2kW inverter will be large and clunky and not necessarily cheap. You'll need a place to put it, and to make up and terminate at least one more heavy-gauge cable to have it in the line.

There's a possibility, that partway through that effort and expense, you'll be thinking "wouldn't it have been nicer to just upgrade the inverter?".

 

What an interesting unit. It's not really strictly a "low-voltage" cut-off because it will cut-off when there is 12.8v after 60 minutes, which is hardly low voltage. It's more like "cut-off the load if the car is off for awhile" switch. Also interesting that its entirely solid state, meaning no electromagnetical relay.

If the "low voltage" parameter, "immediate cut-off" parameter and "delay" parameter were user-programmable, it would be better.

 

They used to sell one called VCM-06, very similar but with a pot to adjust the off delay between a few minutes and an hour. That's the one I have, but as far as I can tell they have discontinued it. I have mine set for about an hour anyway, so if it went poof and I had to replace it with this one, nothing would be very different.

 

Somehow I doubt that. Some I've seen don't trip off until the input voltage gets down to around 10 volts and that won't be good enough to start the car.

The only good solution to his problem is a heavy duty relay and some heavy gauge wiring from battery to relay to inverter, triggered by a switched line in the car.

Note: Given what he said, his present 12 V battery might not last much longer regardless of what he does.
Going completely flat just a few times can take YEARS off it's useful life.

 

I've never observed a Prius to have difficulty starting until the battery voltage hit around 7.

In my various ill-advised tinkerings, I've been down close to 10 more times than I'd like to admit. It doesn't complicate starting the car (nor does it produce bogus trouble codes, as sometimes claimed).

And yes, deep discharges do count against a battery's life; my last one didn't make it past ten years and some of that was surely my fault.

If the OP uses one of InPower's LVDs, the cutoff will be a time delay after voltage drops to 12.8, or immediately at 11.8, whichever comes first, and that sure isn't going to leave anybody stranded.

 

My inverter claims to shut off at 10.8 volts. I don't plan to test that, but anything is possible. I had my 2005 down to about 8 volts before I noticed any symptoms.

Ditto ... almost. As I said above, I was down to 8 volts. Car started fine, but brake pump took a long time to make pressure. That's how I became aware of a dying 12V on my 2005 Prius.

 

Yeah ... "brake pump sounds like dying wind-up toy" is usually the sign that your trip is going to be delayed.

My experience is probably closer to Jerry's; I should have said "in the sevens". By the time it's the low sevens, or seven even, there's probably going to be an intervention needed....