Starting battery and traction battery interface

There is no starting battery on a PRIUS.

The small battery is for other items like entertainment, computers, lights etc.

 

It does get the car to “ready” so it does “start” it.

The traction battery works to maintain the 12 volt fed “stuff”, which also includes charging the 12 volt.

But definitely different than a normal car.

 

True. That's why I mentioned computers - which oversee the "POWER" button.

I was pointing out that there is no "starter motor", nor starting battery as such for the ICE. It always starts in EV mode, even if only for a split nano-second.

 

Certainly the DC to DC converter inside the Inverter uses Hybrid battery power to charge the 12 battery. If you are maintaining the 12 volt battery separately, then the DC to DC converter has less work to do.

 

Welcome to Prius Chat @Jerry P. Keelean from a British septuagenarian. By the time I read your post, you had already got the answers you needed. There are very few automobiles as technologically advanced as the Generation 4 Toyota Prius and it is quite different from the conventional oil burning variety!

 

You are correct your 12V battery charger will not charge the traction battery, even when hooked up for a long period of time. Something else must be causing the 75 mpg indication. To confirm, next time you park the car make a note of what is the displayed capacity of the traction battery, then the next time you drive the car look at the capacity again and it should not have changed.

 

The only way to measure MPG is to divide miles driven by gallons used...the meter is just for your entertainment, i.e., for bragging to friends about your "great" mpg

 

True, if he's using the dashboard meter, it's about 5-7% optimistic, some have reported 10%. It takes various readings from the computer and fuel flow and computes a reasonable idea of the l/100km - but the only way to do it is to do a refill and calculate it. Even one tank isn't a good guide, as the pump might have clicked off early (or late). Only ever fill to the first click - unless it's patently obvious that it hasn't filled.

 

Yes. For instance, loss of mpg is one sign of a weak 12v battery because it needs frequent charging to keep it at normal operating voltage. The electricity for this additional charging of the 12v ultimately comes from gasoline (unless you have a plug-in) via the electric motor to the traction battery to the high to low voltage converter and ultimately to the 12v battery.

So, now that you are trickle-charging the 12v, it no longer needs recharges every time it's used and the energy that would have gone to it now goes to the HV battery where it can be used to propel the car more electrically=better mpg's.

 

I doubt if keeping the 12V battery topped up accounts for the mpg difference reported by the OP, who says he observed 52-62 mpg when not charging the battery, which increased to 75mpg displayed when keeping the battery charged.

Back of the napkin so hopefully my numbers are in the ballpark, but I figure:

12V battery probably has about 0.5 kWh capacity.

I doubt if the OP is draining the 12V battery halfway between drives, but I'll use that anyway as an example. So not counting charging inefficiency, the traction battery will have to use about 0.25 kWh of its capacity to recharge the 12V.

At 133 MPGe the traction battery is using about 25 kWh per 100 miles. Or about 0.25 kWh per mile. So the 0.25 kWh used to recharge the 12V battery could have otherwise been used to propel the car about one mile. I don't think one extra mile per trip would make that much of difference in displayed mpg, except on extremely short trips. And if the 12V is drained less the difference becomes even less noticeable. Unless there really is a huge inefficiency when the traction battery recharges the 12V battery.

Of course the bigger reason for keeping the 12V battery topped up in that scenario is to make it last longer.

 

As others pointed out, is isn't (much) from the 12V battery charger helping out the traction battery, by reducing the energy the traction battery must send to the 12V.

What time of year were you getting 52-62 MPG? Perhaps winter?

Now, during late spring, is the one of the best seasonal times for great MPG in the PNW. My guess is that this 75-ish number comes mostly from the season and any other convergences of best possible conditions.

 

Agreed. I wasn't even focusing on the reported numbers.

Yup, big variances can occur up north. Change in gas formulations also factor in too.