12v battery charging mod for efficiency

Related by not a direct answer to your particular question ---

Past guesstimates have put the total overhead load at about 400 watts. However that includes the inverters and motor/generator controls and everything else on the high voltage side, so it covers more than the just the 12V overhead you are looking for.

 

I think the easiest way to see how much fuel can be saved by not charging the 12V battery is just put a diode in on the battery. So it can start the car but will not be charged.
If Prius charge with 4A, then it’s less then 60W. So not using the headlight will save more - I havent seen any fuel usage drop, even I switch down the headlights.
Just my opinion.

 

What's interesting about this is that amongst the Gen1 Honda Hybrid Insight crowd many of them recommend snipping a specific wire that ensures the 12v battery gets a constant 14.5v charge because the hybrid system that regulates the 12v battery has a history of making for a dead broke down on side of the road reality... And Gen 1 Honda Insight's have no problem getting 68mpg with the right amount of nerdery at the wheel. And then why is it that these same MPG obsessed nerds will argue that a constant charge sent to the 12v doesn't make for any level of concerning mpg loss and makes the flawed Gen1 system more reliable?

And this is a bit personal for me because every time I work on a Gen 1 Insight with battery problems my buddy always tells me that I need to snip that wire and charge that 12v at near constant 14.5 like Toyota does and I always tell him I'm not gonna go needlessly snipping people's wires no matter how convinced he is that it's essential.

So maybe these perspectives can help your perspective?

I think you oughta be playing with build your own Lithium 12v Prius battery instead of playing with a charging mod on existing outdated 12v technology. Ditch lead batteries and then ditch the NiMH battery Pack with the soon to be released Dr. Prius Lithium Hybrid battery pack and then we can talk about mods.