Low Voltage in Rear Power Outlet

Anybody done the same check on a different Prime yet?

 

The 2016 prius and later have two modes of inverter operation. I'm not sure if 12.6V indicates constant voltage mode but it's probably worth looking into.

note: the above is quoted from Please explain 12V system to me--how to keep battery charged while using accessories? | PriusChat

 

You're talking about a voltage drop of 2.7 volts at 9 or 10 amps. That would require a total resistance, between the measurement point at the ECU and where you measure at the power outlet, of 0.27 to 0.3 Ω.

I'm not sure of the actual gauge of wire Toyota uses in that circuit. 0.27 Ω would be 42 feet of 18 AWG, 34 feet of 19 AWG, 27 feet of 20 AWG, 17 feet of 22 AWG. I don't know that they've got that much wire bundled up in there (it's funny, they did build a long bundle of coiled-up wire on purpose for the early Gen 3 headlight fix, when they wanted to drop voltage. Let's see, if I'm right about that bundle being 55 cm long and having five loops inside, that's 5.5 m of wire adding up to 0.12 Ω, which would be about right for something between 18 and 19 AWG; I suppose it's a metric size really).

Of course, there are also some connectors on the path, and at least one 15 A fuse. I've just measured a 15 amp ATM fuse at 0.00468 Ω, so it doesn't look like a big piece of the puzzle. It rises to 0.00557 Ω over several minutes at 10 amps, as the fuse heats up.

Where do you suspect the lion's share of your voltage drop is happening?

 

I'm still thinking 0.267 Ω is quite a lot to try to explain away just by guessing at how small the wire is. There can't be too many connectors on that path, maybe one of the connections is lousy. I would at least check; could be an easy fix.

They do use freaky thin insulation ... I don't know what that stuff is, but it sure saves them a lot of wire-harness bulk.

The return path to ground is usually to the nearest harness ground point (and thence through the car body, whose resistance we can take to be really, really low). I don't know where the nearest ground point is to that location, but the wiring diagram shows them all.

 

Common to both outlets makes the wire-length explanation seem weirder to me; I'd assume those lengths are different.

On the other hand, common to both outlets probably reduces the number of possible lousy connections to check.

 

The best connection you can get is a separate circuit fused right off of the battery. The big part of that would be getting through the firewall.

 

I imagine Toyota’s actual wire specifications are proprietary, especially now that they’ve become a subject of litigation, but Sumitomo Wiring Systems Ltd. and Yazaki Corp., two leading wire harness suppliers in Japan, have catalogs of their stock wire and cable products that may be of interest.

Yazaki’s catalog is in Japanese only, but SWS’s catalog is also published in English, so I’ll use it as an example. They have traditional AV (Automotive Vinyl) wire, but also several styles with reduced overall diameters. The conductor resistance doesn’t change much; compare the following, all at 1.25 mm² nominal size:

	Type	Overall Diameter	Insulation Thickness	Maximum Conductor Resistance (20 °C)
1	AV	2.7 mm	0.6 mm	14.3 mΩ/m
2	AVS	2.5 mm	0.5 mm	14.3 mΩ/m
3	AVSS	2.1 mm	0.3 mm	14.9 mΩ/m
4	CAVUS	1.8 mm	0.2 mm	14.3 mΩ/m

 

Hmm ... 1.25 mm² would fall somewhere between AWG 16 and 17, and at 14.3 mΩ/m you'd need just under 18 meters of the stuff to account for a 0.27Ω resistance ... about 62 feet ....

Edit: ok, I see that's the largest area offered in CAVUS in the catalog, so it's possible something thinner has been used. Anything below 0.50 would show insufficient ampacity in their chart even at 20 C, so I imagine that's about as small as it might get. Looks like that would still take about 30 feet.