Ah, so this is what a frozen battery feels like

Obviously Prius owners need to learn how to hibernate.

Tip, learn it from a bear, not from Windows.

 

Tell me about it! Yesterday was my first sub-60 MPG day of normal commuting in a long time. (I probably still would have kept it above 60 if not for an essential 1-mile trip at lunchtime.)

I first noticed this behavior on some of our coldest days last winter. The most noticeable effect for me, in addition to what you've described, was that EV mode (with the switch) was denied. After some dialog with other EV users, it seems that low temperatures and, in particular, low battery temperatures are an "EV-deny" criterion, though one that seems missing from most EV-deny criteria lists.

I didn't pay much attention to battery temperatures last year because last year's severe cold snap was short and, compared to this one, relatively mild. This year I've been watching it, and it appears that 35F is the critical threshold. I'd be interested in hearing from other EV switch users who monitor battery temp.

So we're between a rock and a (frozen) hard place: Using the battery will warm it up, but a cold battery won't allow itself to be used. So what I've found myself doing the last day or two is seeking somewhat different routes early in the commute where the ICE would have to run anyway for propulsion, allowing the shortest warmup time possible. After the ICE hits 157F, normal S3 and S4 behavior occurs, including ICE shutdown. I find myself actually approaching red lights rather quickly to allow time to stop for the S4 idle routine; normally I time them to avoid stopping. Good thing is, by then, I've got SOC levels often at 70% or above, so I can do some pretty good runs of pedal-feathered EV at 34-40 MPH in S3 and all speeds below 40 in S4. With that, the battery seems to warm up pretty quickly, though I still can't use the EV switch until it hits 35F or so.

Today's commute will be interesting. It's 5F right now -- the coldest it's been in these parts in five years.

 

It's -13 F here in the Poconos this am. Think it's the coldest ever since I've had a Prius. Instead of watching the mpg's I watch when it's warm enough for the heater blower fan to come on LOL.

 

Turn the blower on yourself, and you should get the assistance of the electric heaters in the heater core and footwell, if the engine coolant temperature is below 55 celsius (and your car has them - apparently only fitted in 'cold regions'). There are a pair of PTC HTR relays in the centre relay box in the engine compartment, and corresponding 30A fuses in the right-hand-side (as you stand in front of the car) fuse/relay box which control the two heaters in the heater core, and a PS HTR relay next to the PTC HTR relays and a 50A fuse that control the footwell heater.

There's no point freezing to death in the car - turn the heating on and it will warm up the HV battery at the same time it warms you up. Battery chemistry is subject to temperature.

This chart comes from Panasonic EV Energy's webpage:



At a comfortable 20 degrees Celsius, the specific power is more than twice that at freezing temperatures (0 Celsius) and four times that at -10 degrees C (14F).

Much like the catalytic converter pre-heating, it's a trade-off between consuming more fuel immediately after starting up for better economy later on.

 

The heater blower clicks on the first setting after only a few minutes so I always figured it did that by itself when the electric heater started making enough heat??? There's no way the coolant could be warm enough that soon, I have to check it with my new scan guage though

 

Well, I managed 64 MPG on the way in this morning. Light weekend traffic meant fewer red lights, and by the time I got to the first one I was up to 157F, so I could get it into S4. Good thing too, because the battery temp started at 24F and got only as high as 33F; the EV button never worked.

You seem to be making the assumption that those of us concerned with battery temperature are also concerned with body temperature.

Seriously, it is fuel economy fanatics, not drivers trying to warm themselves, that are concerned about battery temperature. Some of us are so obsessed that we keep the cabin heat off in the coldest of weather. On my commute, the car will never make it to S3/S4 temperatures in this weather if the cabin heat runs. Which means, effectively, that the ICE will never shut off unless I'm at a complete stop.

True, cabin heat will help warm the battery to allow EV, but the battery is a large thermal mass and radiant heating (and cooling) is pretty slow. I won't have EV available anyway in the first few minutes of warmup, so I'm concerned more about warming the ICE than warming the battery. I don't care if I can't run on the battery if I can get the ICE to shut off when it's not needed for propulsion.

Meanwhile, I'm not freezing to death. I bundle up pretty thoroughly.

 

The thermostat opening temp is 82°C (nominal).

 

Does the rear O2 sensor report temperature? If so then lagging the cat to keep it's heat in may help reduce the engine running time.

 

A lot of O2 sensors report basic temperature as in "warm enough" or " not warm enough" as they do not work until they are warm.

 

FWIW, you may be able to get "catalyzer" temps on a Scangauge.

These are reported as being untested, and I don't know the difference
between the two. Here are the codes:

Temp1
TXD 07E0013C
RXF 0441053C0000
RXD 2810
MTH 0001000AFFD8
Name Ct1

Temp2
TXD 07E0013E
RXF 0441053E0000
RXD 2810
MTH 0001000AFFD8
Name Ct2

I think I got these over at CleanMPG.com, but now I can't find them there.

 

I have a copy of the 2004 manual that indicates that the fan(s) should turn on at approx 100°C and turn off again at approx 98°C. I don't know (but I doubt) that it is different for later models. That section is missing from my 07 manual.

I have never seen a temp above 93°C on the Scangauge and I have never heard the fans turn on on my car. I don't do any grille blocking.

 

In a recent discussion about battery performance in cold weather in a regeneration thread these graphs were posted. What I gathered in discussion with Ken is that the batteries will have to warm from being charged/discharged. And there won't be much discharge because you can't draw many amps from the cold battery...so that means that charging seems to be serving as the battery heater. It's a pretty large efficiency loss until the battery heats to a reasonable operating temp.

I suppose this explains the characteristic of the ICE merrily charging the battery well into the green even with the grill mostly blocked.

 

Another observation that relates to this thread:

I was out of town on business last week. The car sat for several days in the parking lot before I headed home, so it had a good cold-soak. I hit the road Friday morning, straight onto the highway, with HV battery temperature at 32F. I had a passenger, so I ran the cabin heat. She had a coat on and I had an insulated vest on, so I didn't have to run it too hard to keep us comfortable -- thermostat temps in the low 70s. That's still considerably warmer than I normally keep it. After about 20-30 minutes on the road the HV battery temperature was still 32F.

This demonstrates how ineffective a warm cabin is in warming the battery. I really didn't see any appreciable warming until a little later in the drive when I had the opportunity for regeneration and lower-speed pedal-feathered EV mode.

 

I have two things to say: Block heater and battery heater.

Block heaters make a big difference in warm up time and they are easy to install on the Prius.

Battery heater could be a heating pad under the battery or around it. They make these for lead acid batteries and two of them would cover two sides of a Prius battery. Under would be better but harder to install. Then to keep the heat in, insulation. Just take it out in the warmer months. Foam is best. Light and easy to cut.

Even with the Prius as good as it is there is still lots of room for improvement.

 

I (and others in this thread) have a block heater. It does nothing to warm the battery.

Others have discussed a battery heater of some sort. Between the unanswered questions (see my quote embedded within the OP of that thread) and the rarity (for me) of driving conditions like those described in this thread, it's not worth it. On a more typical winter day the battery behaves fine and there would be no benefit to preheating. On such days the fuel economy hit caused by cold car parts is mostly from a cold ICE.

 

It would be interesting to see some empirical evidence that there is a business case to add a battery heater (additional cost in electricity vs reduced cost in gas).

Note: Depending on your grid power source (hydro, nuclear, coal/petrol), you could decide if you still want to heat the battery even if it costs more in $$$ (to globally save on non renewable energy sources).

I tend to think that the ICE block heater does more than a battery heater would…