Battery temperature and MPG

I don't know if there was a way for Prius owners to track the temperature of the HV battery before the CAN-View became available, but I believe I'm seeing a connection between the HV battery temperature and the ease with which it goes into electric (so-called "stealth") mode.

We all know some of the other ways temperature affects electric mode. The engine must be hot; the cabin heater will inhibit electric mode, etc. But what I'm seeing is that even when the engine is fully warmed up, if the battery is still cold it is very hard to "feather' it into electric (though the EV switch will work) and if the battery is cold, it takes less accelerator-pedal pressure to kick it out of electric mode.

This all makes sense since a cold battery cannot deliver as much current as a warm one.

Greater battery capacity means better mpg. We know this from the folks who have added extra batteries. HSD is designed to improve mpg by (among other things) buffering the supply and demand for power between the engine and the wheels. It makes sense that additional battery capacity would improve mpg, as demonstrated by the modified Priuses; and it makes sense that a cold battery has less effective capacity than a warm one.

What this means is that a Prius which is driven more miles per day, will have its battery warmer (current flow heats up a battery -- any battery, due to internal resistance) and will go electric more easily, and will also get better mpg.

We have always known that short trips are an mpg-killer, because during the first 5 minutes the car is burning more gas to heat up the exhaust system for pollution control. But now I am suggesting another factor: That there are three characteristic mpg intervals, rather than two:

The first 5 minutes, when the car is burning more gas to heat up the exhaust system.

The next 20 minutes or half an hour (depending on ambient temperature and cold-battery temperature) when the engine does not need to run constantly, but the battery capacity is less.

And then the rest of the driving day, when the battery is hot. -- The middle stage is probably a gradual rise from the first to the third, probably not a discrete "stage."

Conclusion: This could explain why some folks get close to the EPA numbers, or occasionally more, while others are stuck around 45 mpg: I postulate that people who get really high mpg are those who put on more miles per day, and those who suffer the frustration of relatively low mpg are those who drive fewer miles per day. In Fargo I probably put on 8 miles most days (downtown and back from my apartment) and my mpg ranged from 29 in the depth of winter to 45 in summer, while here in Spokane, where I am driving more miles I've been up around 50 mpg, but days when I just drive to the store and back, I take a hit. And this week my mom came to visit so I was driving a lot, and yesterday, with about 150 miles on the tank, it hit 54 mpg, my best ever.

Perhaps some way of keeping the battery warm (an electric battery-blanket?) would be a big mileage booster.

 

Okay Evan. Well, you had the CAN-View long before I did. However, I do see a big drop in battery temp after sitting overnight in my unheated (but not very cold this time of year) garage. It's been around 55 or 60 degrees when I start up, and has gotten up to 80 degrees or so after a long drive. And I see a big difference in the ease with which it goes into electric mode between those two temps.

Maybe an electric blanket to keep it warm? You wouldn't need to apply heat, as much as prevent the heat from leaving it. Of course, the problem is it needs to be cooled while in operation, and a full-time blanket would prevent cooling.