HV Battery Pre-Heat... Anybody tried it?

Around here, every cold snap - eg -40 - there are a rash of car fires caused by using the old fashioned interior car warmers.

What's really neat is that the remains of the car are then entombed in solid ice after the fire department gets done putting out the blazing inferno.

I've often wondered how they manage to break the ice free and remove the remains of the car. Then I saw a Bobcat with a chisle attachment that easily pecks away the ice. The guy operating the Bobcat called the attachment "The Pecker"

I'm sure

Hydraulic Breaker Attachment - Bobcat Company

 

Put a hair dryer in the exhaust end of the battery cooling duct and blow warm air through the battery pack backwards, the exhaust will warm the cabin. You will need a non return valve to stop hot battery air going into the cabin space under normal operation.

 

I would like to caution you against putting the heat source on top of the battery. I recommend that you put it under the battery.
Since warm air tends to rise, heating the Battery from the top would probably cause a thermally stratified condition within the battery box, where the top of the battery is much warmer than the bottom. By heating the battery from the bottom, the warm air would rise through the battery by convection from the bottom towards the top, keeping it more evenly warm.
Uneven temperature within the battery can decrease performance, and shorten battery life.

 

JimboK

efusco

Rokeby

Rokeby, excuse me, but I cannot see how you've drawn your conclusion from the two referenced posts. Neither one offers any evidence that heating the battery directly provides any benefit. They didn't try direct heating; how would they know if there was some value? What they are saying is that the charge/discharge cycle in conjunction with the cabin heated after some period of driving have minimal effect on increasing battery temperature, in one case after 20-30 minutes, in the other after three hours. IOW, they have only offered evidence of what does not provide any benefit.

This thread has ended (or has at least become inactive) exactly where it began. There's been a lot of egghead talk, unwarranted conclusions and unsubstantiated opinions, but very little in the way of the OP's appeal for anyone who has tried to warm the battery, or to quantify its effectiveness.

Yet my post drew only a warning of a fire hazard, as though everyone "knew" that three hours of preheating the cabin couldn't possibly offer any benefit to the HV battery. Before anyone assumes that foregone conclusion, I suggest that they actually try an experiment; you know, gather some empirical data, rather than talking the subject to death.

 

A warm HV battery must have a positive impact on FE because it is a fact that a cold battery will not be as efficient at capturing regen energy and cannot discharge as efficiently. This means wasted energy and reduced current capability which means mpg hit. There is no doubt in my mind that an HV battery at optimal temperature will improve the Prius efficiency. It really is only a matter of how much.

I have a graham scanner and I have seen that the max regen current when the battery is <10˚C is about 30amps. It can take over 50 amps when >15˚C. So your brakes are burning off the excess energy when you regen above that 30amps with a cold battery.

I think better insulation and some passive heat would work best.

I'm also working on this issue with my 2003.

 

The core heating of the HV pack with AC is the best method but what they don't explain in the paper is what voltage and amplitude is applied. I'm assuming pack voltage.

From the NREL study liked to earlier:

 

? do not know. It just sounds cool. I'm not that electrically savvy either but very ambitious.

My current approach is a battery blanket on top which is pretty ineffective, with heat rising and all. I want to get something inside the case under the battery at some point but it will have to wait till I have time for all that unbolting. There is a tiny bit of room under the case but the whole pack would have to be lifted a bit and some bolt holes elongated for that to work. Might as well put it under the batteries if possible. Christ from EM has suggested waterbed heater pads as they have a thermostat and are thin.

I'm determined to do something.

 

I'm all about the heat.

There is a lot of gain to be had in this. When I drove down to california, the battery was nice and warm compared to here and I could EV for miles and pull way more amps before the engine kicked in compared to home: >15˚C difference.

All of the electricity including the running draw of 300-700 watts is stored in the battery before being used. The in-out efficiency is so much better when warm, I'm stunned to hear people say it won't make much difference. But then my alternator delete garnered similar responses.

Thanks for the links. It was a warm gesture

 

How about putting a water bed heater under the HV battery?

It's about 300 Watts, but temperature regulated. If you get the more simple ones with a mechanical thermostat, you can just set the thermostat to its lowest setting and have another plug just like for the engine heater.

Interesting thread!

 

Their posts were not deleted. FWD's name is still there. Shawn changed his screen name.

 

When you calculate the energy needed to warm the battery, will the savings in better mileage add up?
Also, as I have mentioned on an earlier thread, I am working (still) on an inexpensive battery charger for the NIMH pack. Starting off with a nearly full pack each day would insure that I could get to the train station in EV mode every day without the ICE running. This would also keep the battery warm, another advantage. I already keep my Prius in an attached garage, but is can still be cool in there, but not as cold as outside.

 

I wouldn't do this. One way that Toyota gets such long life out of the NiMH batteries by operating them in the middle of their charge range - they don't stress them by filling them up near to the top limit of their capacity nor do they discharge them down towards the bottom limit of their capacity.

By topping the batteries off (I presume you mean giving them a full charge) you will greatly reduce the lifetimes of the batteries.

Warming the batteries up with externally supplied heat is one thing (and as has been pointed out, probably isn't energy efficient overall) but heating them up by overcharging (from a reliability point of view) is probably not financially efficient overall.

I'm sure lots of engineering went into designing the battery control system so that the Prius batteries would last well over 150,000 miles.

Good Luck!

 

Since it's cold again, it's no coincidence that I have also been thinking about this again. But it is a coincidence that last week my wife was shopping for a new vacuum cleaner. She had her eye on a model with over 2000 W. I pointed out to her that that is more than three horsepower, enough to power a small motorcycle, and a ridiculous amount of power just to pick up a few grams of dust. So I persuaded her to order a different model with two thirds the power, but which performs just as well, because it is more efficient.

So thinking about the enormous inefficiency of vacuum cleaners, which waste most of their power by heating a large amount of air by a small amount, led me to the idea of using a vacuum cleaner to heat the HV battery. I think that a vacuum cleaner is very close to the ideal heat source to preheat the HV battery:

1. A vacuum cleaner has about the right amount of power. Based on NREL's calculations, I estimate that 2000 W of power, assuming 50% heat transfer efficiency, would heat the HV battery by about 30° C in about half an hour. That is slow enough to ensure fairly even heating throughout the individual cells, but fast enough that it can be effective on fairly short notice.

2. The high airflow ensures even heating throughout the battery pack. One of the worst things for the health of a battery pack is uneven charge between cells, which can be caused by uneven heating.

3. A vacuum cleaner has of course, very conveniently, a built in air filter, which would slow the build up of dust due to the increased airflow in the battery pack.

4. Normally, vacuum cleaners do not operate at such high temperatures as most heating devices, so the fire danger might be lower.

5. The installation, I think, would be relatively simple.


The basic idea was already mentioned earlier. More specifically, the idea is to tap in to the battery ventilation exhaust duct as close as possible to the battery, draw the air from the battery through the vacuum cleaner, whereby it is heated as it cools the fan motor, duct the exhaust air from the vacuum cleaner through a one way valve, and into the battery ventilation intake duct. The heated air is then drawn through the battery by the vacuum sucking at the battery ventilation exhaust duct. So the air is re-circulated in a more or less closed loop. And of course a thermostat or timer should be used to prevent overheating.

If the duct from the vacuum is not routed to and sealed around the battery ventilation duct intake grill on the right rear seatback but tapped in to the duct itself, then a second one way valve, in the battery ventilation intake duct upstream of where the vacuum cleaner exhaust is tapped in, may or may not be necessary, depending on how much air can exit the cabin through other openings. Without the second one way valve in the battery ventilation intake duct, at least a small amount of heated air would escape through the battery ventilation duct intake grill on the right rear seatback. This might be either a loss or an added benefit, depending on your point of view.

Some vacuum cleaners are equipped with a blower port to which a hose can be attached. With other vacuum cleaners, some ducting would have to be attached around the exhaust openings, or the entire vacuum could be encapsulated to capture all the exhaust air.

One thing that I'm not so sure about is how much the intake airflow to the vacuum cleaner would have to be throttled, in order to consume the required amount of power. Electric motors, when not working against a mechanical (or air) resistance, do not consume much power, even at high rpm. Presumably vacuum cleaners are designed to develop max power when working against the air resistance of the floor cleaning attachment while it is resting on the floor. Perhaps using a small diameter duct to the vacuum cleaner or cutting only a small hole when tapping in to the duct would produce the desired effect.

PS Here's a nice picture of the battery and ductwork of the 2nd gen. Prius from the Toyota press site:

 

60 Amps at (rounding) 200 Volts is 12 kW. Well within the capability of the inverter, MG1 and the engine to produce.

The trick here is to use the internal resistance of the batteries themselves to generate heat.

To prevent overcharging, one could inject current into the battery and then pull current out of the battery. Either way, you heat up the battery by putting current through its internal resistance.

If I were to _GUESS_:

1) This is a viable approach to heat up the battery without damaging it.
2) The Prius has powerful enough hardware (the MGs, the engine and the inverter) to accomplish this.
3) It might require a slight hardware change to the inverter (I don't know the topology).
4) It would require significant changes to the software of the hybrid control computer.

This appears to me like it's one of those things that Toyota had to cut / overlook in order to get product "out the door." It might show up on later models as a refinement. On the other hand, Toyota might not bother.

I don't have a dog in this fight. I live in So Cal - for me, 45 F is cold.