Short trips? Here's a trick!

[Rokeby]

Jimbo,

The HSD integrates two power sources the primary being the ICE, the
secondary being the HV battery. Most discussions of maintaining high
FE/MPGs in cold temps invariably include a recommendation to install
an EBH to "preheat" the ICE. All well and good as far as that goes.

But what about warming up the HV battery? I can't remember it being
recommended too. It would seem like a logical next step. And on first
consideration, relatively easily done with some sort of electrical warming
pad, foot warmer, etc.

Or have I missed something... really long warm up time... fire danger... ?

[JimboK]

Originally Posted by Rokeby

But what about warming up the HV battery?

I seem to recall someone suggesting it. If so, I didn't pay much attention to it because "EV deny" from a cold battery has been so rare in my climate. And of course, as opposed to the designed-in capability for an EBH, the car isn't made for it. Many unanswered questions: What would one use for a heater? How to plug it in? How much of a pain to install and use? How to keep it safe? Any unanticipated side effects on battery chemistry or function? Would the fuel savings offset the cost to run it? Would the fuel savings offset the hassle of installing it and using it on the few occasions it's really needed? Etc.

On the other hand, if someone wants to experiment on their own car and then can document safety and effectiveness, I'll consider anything that boosts my fuel economy by 0.002783 MPG.

[Fred_H]

Hi Rokeby, you might find this interesting:
http://www.nrel.gov/vehiclesandfuels...-633_sw_ap.pdf

[Rokeby]

Fred_H,

Thank you very much for the cite.

So, the answer is yes, preheating HEV batteries to room temperatures can
make a significant contribution to efficient charge/discharge at very cold
ambient temps. The most efficient method to preheat is direct application of
high frequency AC to the battery terminals. No way I'm doing that!

But there is the suggestion that by preheating with hot air through the
cooling duct will work, although with major inefficiencies. I guess preheat
could be done with a hair dryer... probably a significant fire hazard.

But there is still the possibility of contact heating with a heat pad, etc.

I've got some reading to do. Here's the result of a preliminary Google
search:

hev battery preheat - Yahoo! Search Results

[Fred_H]

Good morning Rokeby,
I think the most important thing to keep in mind is that it's important to keep the temperatures even within the Battery. Uneven temperatures are bad for the battery.

I would say that we can rule out attaching heat pads to every single battery module, for most do-it-yourselfers. Using a heat pad on the outside of the battery pack would probably result in uneven temperatures unless special measures are taken. For example surrounding the battery completely with heat pads and heating it very slowly for many hours, or circulating air to distribute the heat more evenly.

Which brings us to the air heating method. Of course, the Prius already has a good air circulation system for the battery, so we could just push some hot air through it. But simply hooking up a forced air heater to one end would be very inefficient, because most of the heat would be exhausted into the atmosphere at the other end.
If you could figure out a practical way to recirculate the air without danger of overheating, then I think you could build a halfway efficient battery preheater. You have to make sure that air can not bypass the battery through the heater or recirculate to the battery when the battery needs to be cooled.

I have thought a little bit about this before, but soon realized that on my wife’s daily commute, the benefit would be minimal, because the first part of her route requires no significant battery power, and by the time she gets to the part that does, the battery is sufficiently warm from the cabin heat.

But for people who start their commute with a long city drive in very cold temperatures, I can imagine that a preheated battery could make a significant difference.

If there’s not already an old thread about this somewhere, maybe you could start a new battery preheat thread in the technical discussion section?

For HEVs battery preheat is not really necessary, but for the upcoming PHEVs, it will be important, and for BEVs battery preheat will be crucial in cold weather.

Fred

[Rokeby]

Fred_H.

Thank you for your thoughts. Your input has been very helpful as I think
through this matter.

I now believe that a recirculating warm air system would be most practical.
Most likely, the system would use a 120V AC fan and heating element.
It would require tapping into both the supply and exhaust sides of the HV
battery ducting, and perhap using the cabin as part of the circuit -- some
synergy there; preheating the cabin would tend to cut down on the interior
windshield icing issues some folks experience. All this is not impossible, but
daunting nonetheless.

Originally Posted by Fred_H

If there’s not already an old thread about this somewhere, maybe you
could start a new battery preheat thread in the technical discussion section?

I think you're right. This topic should be given stand-alone visibility. I'm
gathering data from various threads, and hopefuly a picture or two of the HV
battery ducting. Hopefully I'll be able to post the new thread later today.