Inverter: How much power? and how?

Isolation modules are available from RV and Offgrid stores. They allow charging and discharge isolation in their design. Some are better than others about reducing charging voltage drop to the secondary battery. A good one ensures the vehicle battery is charged before allowing the secondary battery to charge.

https://www.wirthco.com/battery-doctor/battery-isolators-testers-monitors/battery-isolator-series/150-amp
(Important Note: Wirthco refers to the second 12v battery as the "auxiliary" battery)

 

I actually set up a car in this manner back in the 1960s when I was in college. My friend's Corvette had been stolen and recovered with some missing parts and a hole in the fender to disconnect the battery to disable the alarm. He asked me if I could prevent this from reoccurring. I installed an "auxiliary battery" in the trunk to power the alarm with diode isolation so that the alarm was separately powered.

About 3 weeks later the thieves struck again and cut a hole in the fender to disable the alarm. When the alarm went off, they dropped the car and took off so my friend still had his Corvette. My friend complained that they put a hole in his fender. You can't please all of the people, all of the time.

JeffD

 

Whatever you get, just make sure its a pure sine inverter, I made this mistake and regretted it after spending more money for a Pur Sine Inverter.

Toyota Prius upgraded Power Inverter -

 

As Toyota builds that converter only to supply the equipment they build into the car, plus things you might plug into the max-120-watt "power outlets", they haven't felt any need to commit to any specific output power rating for it.

So you look at other information. There is a 125 amp "DC/DC" fuse at its output, in the engine compartment fuse box. At say 13.8 volts that gives you a maximum of 1725 watts. Fuse selection usually involves adding about a 25% cushion over the expected maximum load (in a house, a 15 amp circuit would not be run for any expected load over 12 amps), so you might divide that 1725 by 1.25 and decide you're looking at 1380 watts tops.

Not every circuit tree in the engine compartment fuse box comes from downstream of that fuse. Some of the ones that don't are heavyish (radiator fans, engine water pump, supplemental heat). So perhaps the converter is a bit beefier. (But if you are planning to locate your inverter in the back near the battery, you are traversing that fuse, because the cable from that fuse box to the battery is downstream of it.) But you shouldn't assume that it's sized to run the car's heaviest loads all simultaneously, because that would be really unusual. And it goes without saying that you should assume it will have to run some of the rest of the car along with your added load.

The next thing you can do is attach a honkin' inverter and start drawing different amounts of current and see what happens. Bob Wilson did that some years ago, albeit with an earlier Prius generation. There are a few things that can happen, in order:

1. The converter has an output signal called "IDH" that is turned on when it is being overtaxed and wants to say "Uncle!". In earlier generations, that was an actual discrete signal coming out on a wire of its own, you could monitor it with an LED. In this generation, it is a message sent over the CAN bus. Edit: nope, rjparker corrected me below; it's still a discrete signal. As the car is built, only one thing listens for the IDH signal: the A/C amplifier, which will disable the supplemental heat element (saving 700 watts) if the converter sends it. I've often recommended that anyone adding large accessory loads to a Prius also consider monitoring that signal (or even building a circuit to shed the added load automatically when signaled, just as the A/C amplifier does). Admittedly that was easier in the earlier generations where you could just tap into a signal wire rather than dealing with the CAN bus.
2. If the converter is overloaded and the IDH signal is ignored, the next thing likely to happen is the output voltage will begin to droop. With many kinds of passive loads, that would naturally lead to a reduction in the power being drawn, and an equilibrium might result. With an added inverter as a load, that won't happen: instead, your inverter will say "hmm, source voltage dropping, I'll have to draw more current to make the same power", leading probably to further droop. In Bob's experiments, voltage droop was seeming to set in around 1000 watts of added load. If the voltage droops far enough, your added inverter will probably sound an alarm and/or turn itself off.
3. If the converter continues to be overloaded further and for long enough, my expectation would be that it isn't very likely to be fried, but more likely to say "I've had enough of this" and turn off. It's possible that turning the car off and giving it some time to cool will bring it back. I'd want to avoid repeating that scenario too many times.

Some charging current measurements were made by AHetaFan six years ago.

Because the battery is a voltage source around 12.8 volts tops, and the converter is a source at least a volt higher, whenever the converter is operating, anything powered on the car's "12 volt" system is being powered by the converter, and the battery is receiving power (being charged), not sourcing it.

That could change if you are drawing so heavy a load that you make the converter output droop and (combined with the voltage drop in cabling) its output ends up below that of the battery at your point of connection. Then you would be drawing some from the battery as well. I wouldn't regard that as a very sustainable arrangement.

There might be something to that energy efficiency argument, but you might also do a little back-of-envelope math to estimate how much practical difference it would make in gasoline burned for your typical usage. The higher-rated inverters will also be physically larger, possibly reducing your options for a tidy installation, weigh more, costing you energy driving them around, and require heavier-gauge wiring and terminations to install (unless you deliberately undersize the connections because you aren't expecting to use its full capacity). They're also likely to cost more, and cost more to replace if you happen to connect a load that damages the inverter (which has happened to me). You'd have to judge for yourself, but sometimes in looking at the whole picture, right-sizing can be more attractive than oversizing.

 

No, sorry, haven't shopped for any inverters in a while. My house already looks like I collect the wretched things.

 

When you start talking about shore power and 2000 watt inverters, I once again suggest a quality RV store to explore their made for purpose systems, battery isolators and deep cycle 12v battery(s).

The system could still run off the Prius dc to dc but isolate itself enough to protect the car. If you occasionally have 120vac shore power ("borrowed" in the wild or purchased at camp sites) then your additional battery is fast charged and your loads are directly served by utility power. You get more than shore power capability with these systems, you get refined and configurable mass market products that also provide reliability and safety. Quality in these parts have purposes that you don't have to discover the hard way. For example the car's 125 amp fusible link is a bear to replace and often is not in stock at the Toyota dealer. So protect your fusible link or it may strand you in the wilderness.

A quality inverter is the Xantrex Freedom XC 2000 817-2080 or similar. An earlier post referenced a decent battery isolator.

By the way, an extra circuit based on the car's IDH signal would be a next level enhancement that could save the fusible link. My gen3 documentation indicates IDH still exists as a discrete wire. It would be low power requiring a high impedance interface such as a mosfet rather than a simple relay. The Xantrex inverter includes a shutdown input.

A Honda inverter generator and a couple of 200w plugin dc to ac modules might be my choice. At least until I joined the RV set. Then I would add about 800 watts of solar to top off the 800ah lithium batteries.

 

Where, where? It's somehow eluded me (and I'm almost positive somebody told me it had morphed into a CAN message).

 

The idea is you have two 12v batteries, ideally of the same chemistry. Look at post 12 for a diagram. Both are 12v batteries, one is isolated from the other. So much can be gained by going down to the RV store and talking to them.

Realize I think the whole idea is not cost effective compared to a generator and low cost plugin modules.

 

Well, glory be, there it is. I stand corrected. Thanks!

 

I'm a bad hack carpenter, can't help you at all there.

On the other hand, I'll tell you that I'd readily agree with the battery sizing advice you've been offered. And I'd underscore the point that a planned 200A draw on 400Ah battery is still near limit.

Now, from personal experience I can tell you that you'd do pretty well with two series pairs of Trojan T-105 batteries in parallel. They would pretty well fill up the rear footwells of the car, and they'd be a very dependable 450Ah 12v battery system for you. I've used them for years for offgrid power.

Note that this is also quite large and heavy. Just to be clear- I'm talking about four lead acid batteries each about the size of a traditional car battery. You'd put two in each rear footwell, and I suppose the inverter could be nested in between. So you're talking about losing all of that space and about 350lbs of payload for this power system vs. a 65lb generator the size of a gym bag.

 

I am not worried about future generations, it is the here and now that concerns me. By then buyers will have 7200 watt inverter systems engineered in from the factory, like the 7.2kw option a Ford F150 truck can have today. Or they could modify a used Rav4 Prime which has a decent sized hv battery to work worth along with adequate load capacity.

5 Fun Facts About the 2021 F-150’s Onboard Generator - PickupTrucks.com News

While the video was a good first day introduction to RV 101 electrics, he is not considering the interface to a $30k Toyota hybrid system and the potential of bricking that system. His main concern is selecting matched products so they "last longer". Proceed with caution.

Have you considered enrolling in a top notch engineering school with a double major in mechanical and electrical engineering? Maybe a minor in business or economics.

 

I'm on my phone and quoting isn't working, read a few posts up a 1500 watt appliance would fry the converter? Explain please because on my 2200 watt inverter I run an air fryer or induction stove... not to max but my kilawatt meter during tests shows max 1600 and change, nothing got fried.

A 2nd battery is next, purchasing oem battery bracket, positive cable, negative cable and vent house to install 2nd on the driver's side cubby opposite of where oem battery is. Original thought was yellow top as main and stock non original battery for my 2nd but the reviews on the yellow top back in 2016-2018 don't make me feel comfortable about using a yellow top. My end goal is to leave the car in ready mode over night and it not kick on as much through out the night.

 

You are installing a 2nd battery in parallel, right? Will the Prius inverter charge both batteries?
I know the voltage is the same, but wouldn't you need more amperage?

 

Yes, using an isolator. Yes when the car is on.

Given my body size and limited space in the car, I have no room for larger battery. I'd love a 100Ah battery by battle born... for now my plan is dual yellow tops or just dual stock like batteries.

I'm really trying to get the car to turn on the least number of times while camping over night. Also would like to know that my fridge/router/electronics can run while car is off and ill always be able to start the car without using a jumper if it goes dead.