Toyota's Engineering Choices on Gen IV and Prime

Because of a long-running ordeal with my 2018 Prime, I've discovered that Toyota intentionally designed Gen IV Prii (Primes included) to reduce the output of the DC-DC converter when the car is in drive from 14+v to ~12.5v. We also know that Toyota has been moving to smaller wire in their harnesses to conserve weight. I am not an electrical engineer, so I'm asking somebody with more knowledge to help me understand Toyota's overall strategy with these moves.

My understanding is that lower voltages require higher currents for the same power demand. If the 12v system has a 1,000W load (A/C compressor, headlights, blower, interior lights, 12v accessories, etc.), the DC-DC converter has to put out 71 amps at 14v, or 80 amps at 12.5v. Either way, the DC-DC converter has to draw 1,000W + whatever the conversion losses are from the battery and put out 1,000W.

It is also my understanding that the power carrying capacity of a wire is determined by amperage, not voltage. In other words, a 20 gauge wire can carry 10 amps no matter the voltage: 120 watts at 12v, 140 watts at 14v, and 90 watts at 9v.

If all of that is true, it means that for the same load, decreasing voltage requires larger wire and isn't saving any power (1,000W is 1,000W). If Toyota's strategy is to enable the use of smaller wire in the 12v system, shouldn't they be boosting the voltage of the 12v system rather than decreasing it? I'm not understanding the logic of dropping the voltage when the car is driving. What is to gain when there's everything to lose?

 

Because moving to smaller wire and decreasing the voltage of the 12v system means I can't use the 12v power outlets to power my laptop. When a large part of your job is traveling around with a laptop, that sure puts a major usability ding in the car. Any device that cuts out at 11v is limited to 5 amps in the Gen IV. A device that cuts out at 10v is limited to 8 amps.

 

Ohm's Law is what makes their choice seems bizarre. Decreasing voltage AND wire size at the same time is a double-whammy on the capacity of the system.

 

75W. The outlets cannot put out more than about 65W and stay above 11v. At 75 watts the voltage drops below 11v which triggers the low voltage shutoff on the inverter. I confirmed with all the major manufacturers of sine wave inverters that they do NOT make an inverter that doesn't have a low voltage cutoff.

The reason I didn't start off with a dedicated, hard-wired inverter is because the owner's manual AND the outlets themselves say they'll support 120W at 12v. That specification is false. If you stick strictly to 12v, the outlets are limited to about 25 watts and 65 watts for 11v.

 

Thanks. This is likely the route I'll go to avoid trying to run wire through the firewall myself.

 

I appreciate all the responses trying to help, but I'm really hoping for some insight into Toyota's choices here. Does anyone know what advantage is gained by reducing the voltage of the DC-DC converter? It seems like you'd want a device with a 100 amp limit to operate at the highest voltage possible to maintain efficiency, maximize capacity, and reduce wiring needs.

 

They're gonna have to have something to supply 12v (or 120v AC) to accessories if they want to sell cars. I don't think people will be willing to give up being able to charge and use device in their cars to buy a Tesla.

 

That's what Toyota has now, and that's what they flubbed on Gen IV. Technically, that by itself wasn't a flub, but allowing the combination of smaller wiring and reduced voltage was a flub.