Cold Weather and 12v Effects on Fuel Economy

The last few days of colder weather have really placed a hit on my fuel economy. When we were having days where temperature was up in the 50s and 60s I was looking at a 65mpg tank. Three days makes a difference, when I parked from getting lunch yesterday (2/23) the tank was down to 59.2 mpg.

The cooler weather also affects the small 12 volt battery that Honda has equipped the HCHII and the 2^nd Gen Honda Fit, I do put some load on the 12v system with the engine and IMA system offline and the car won’t take much of that in these temperatures. In the summer I was usually good for a good 10-15 min listening to the radio with the DC/DC converter offline (that’s what I’m calling it, its actually an advanced technique but no matter), but in the colder weather under 40 degrees when I last timed it, the system could only take about 1-3 minutes before I had to start the car.

The reason for having to start the car is so that the smaller battery could take a charge off of the larger IMA battery, which only happens with the car is “running” it will top off the 12v battery during an Idle-Stop (the manual calls it idle stop, Auto Stop as the instrument cluster calls it) event. I find that at 11.3v is the closest I feel comfortable letting the voltage drop, somewhere below that there isn’t enough energy in the battery to turn the auxiliary starter and engine over. Just before that point the car switches from using IMA to start the engine to using the backup starter motor.

Two of my customers have experienced this, one with an HCHII Navi, we were learning the navi system in cold weather the navigation screen started to blink, and I knew from a previous experience that means START THE ENGINE NOW! We didn’t start the car in time and the battery died. The other time was where I learned about the blinking nav screen, it was an earlier cold day, and the vehicle was an Accord four cylinder. The battery was weaker than I anticipated and we did my navigation demonstration. A Civic Hybrid leaps to life when jumped off with a booster box, the Accord barely meandered its way out of its premature slumber on the half dead jump box, and it is an interesting situation to be in.

Moral of the story is that you need to be extra careful with the loads you place on your 12 volt battery during colder weather.

Kacey Green
for the Midlands Hybrid Club
"Promoting the spread of clean personal transportation technologies."

My fuel economy, tracked at every fill-up since purchase, your mileage will vary.

The Midlands Hybrid Club, the home of the hybrid club and hypermiling of the Carolinas, based in Columbia, SC.