Bad for Long Term Health of Hybrid Battery?

Cycledrum – I have the same habit of depleting any excess accumulated Traction Battery charge by EV’ing during that last bit of distance home after I enter the gate of our compound. I do this because the H.V. Battery does a poor job of retaining any healthy charge it may have over night after being shut down. It is essentially “Use It Or Loose It.â€

The HSD battery control software does an excellent job of protecting the life of the H.V. Battery. Over the long haul, anything might fail --- even your bio-system. Just maintain your fuel-sipper well, drive it “Hybrid $mart,†and let the reliability chips fall where they may.

 

Excellent question...because I do the same. I try to use EV as much as possible once I get close to my house (30mph zones). I usually eat up the charge by the time I pull into the garage...right or wrong.

 

A normal hybrid battery does not work like this. Prii have a special version of NiMH chemistry that limits self discharge.

But anyway, it is not going to hurt the battery to do this, but it will likely hurt your gas mileage, rather than help it. The battery is there to (1) start the gas engine (2) capture energy from braking (3) provide power for acceleration. If you run the battery down to low SoC when cruising, the car will at some point turn the gas engine on to recharge the battery, and due to Peukert's, this is less efficient than compared to if the engine had just been used to propel the car in the first place. It is akin to using EV mode for that last mile, then firing up the diesel generator at home to recharge.

 

Seilerts – I know you are a “Battery Curmudgeon,” so I shall not dispute your input on self-discharge, as regards the NiMH chemistry used in Prius batteries.

However, permit me to share two anecdotal data points about the behavior of the Traction Battery in my old GEN II Prius, and now in my new, 2012 Prius “vee.”

The observed behaviors of the H.V. NiMH Battery Array in both my 2G and “vee.” is that some portion of the displayed SOC bars disappears between my pulling in the garage at night, and when my Prius is fired up and rolled out of the garage in the morning. It may not “evaporate” because of “self-discharge,” but is vanishes all the same.

I see you have 2,256+ posts and counting, and are among the “Do Not Use Your Battery, It Will Hurt You Gas Mileage” group. While it is true that when the Traction Battery runs down the HSD will force on the ICE to return it to a “healthy” charge level, and, thereby, consume fuel and incrementally lowering your instantaneous fuel economy, it is also misleading advice because it does not look at the “Big Energy Picture.”

The whole “Hybrid” purpose of lugging around a heavy high voltage battery in a gas-electric hybrid is to help propel it the down the road without using gasoline to give you GREATER DISTANCE TRAVELED DIVIDED BY LESS FUEL BURNED EQUALS MO’ BETTA MPG FUEL ECONOMY.

If you do not use the H.V. Hybrid Battery in a Gasoline-Electric Hybrid Vehicle, the H.V. Hybrid Battery becomes a mere “Boat Anchor” residing in a conventionally-powered car. If one does not want to use their hybrid battery, they should buy a Corolla not a Prius.

I know where you “Do Not Use Your Battery, It Will Hurt You Gas Mileage” advocates are coming from, but it is a narrow view of the overall “wonderfulness” of “Hybrid $mart” use of the Traction Battery in an HSD controlled-operated hybrid. It ignores the “Free Energy” obtained from converting Velocity Energy into Stored Electric Energy via Regenerative Coasting and Braking.

Reuse of Reclaimed Regenerative Energy to improve fuel economy is the fatal flaw of the “Do Not Use Your Battery, It Will Hurt You Gas Mileage” crowd.

How does one squeeze the “Increased MPG” advantage out of battery-stored reclaimed regenerative energy if they work to stay out of the EV mode, and refrain from using battery power?

A better bit of advice to give folks is to learn how to “Become At One With The HSD” and drive “Hybrid $mart,” using the Traction Battery to its best advantage in boosting Per Tank-Full fuel economy. That is how I drive my fuel-sippers.

FWIW, I am currently driving my new, 2012 Prius v (“vee”) at 53.4 MPG, which is 127% of EPA Combined. I could not attain these “$tellar” MPG numbers without “Hybrid $mart,” use of the “vee’s” Traction Battery

 

The battery charge display updates periodically. When you turn the car back on it will update and you will see the change from the final leg of the previous night's journey, which, since it typically involves slower speeds, often means pulling into your driveway in EV mode using up the battery.

I would see something like the same you at the end of my commute since I drive on the flat or uphill and then reverse into the driveway. My wife finishes downhill and drives in forwards so after she's driven it I usually see a full battery.

Seilerts didn't actually write that. He wrote about not making an effort specifically to use EV.

The most efficient way of driving a Prius is to use pulse and glide and only touch the traction battery is when accelerating slowly from a stop up to around 15 mph.

Gliding specifically avoids the use of the brakes. The best hypermilers try to "drive without brakes" and so rarely use regenerative braking.

Using that approach Wayne Gerdes achieved 66mpg in a v.

(Ken@japan has indicated from Gen 3 it's ok to use a very small bit of EV rather than glide but you shouldn't really try to.)

Naturally people can't always use perfect p&g so they end up with regenerated energy, but the car will make use of that energy itself. It's not perfect, so there are limited circumstances in which the driver can improve mileage with EV, but the best general advice is not to use EV deliberately to force the engine to remain off. People who make effective use of EV are generally avoiding inefficient short bursts of engine use and at the end of a trip the engine will be operating efficiently.

 

HV battery is there to capture energy lost, bottom line.
Prius is a ICE car.

Motors are there to propel the vehicle in some extent of low power (EV) or very high power (assist), and to generate (capture).

 

I like to use battery-only when entering my gated community entrance but rarely use 1-2 bars doing it. I find if I deplete down to 2 bars when entering the garage for the day, the next morning, the ICE runs much longer during the first 4 miles or so (3 stop signs/lights), thereby affecting my mpg's. If I have a normal charge, by the time I'm at the gate leaving the community, my ICE shuts down when I stop. Purposely running down the battery to "save" on gas doesn't work - you lose the savings later when you have to recharge. The best mpg's are achieved when both ICE and battery are working together - as designed, supplementing each other as needed.

 

Original question was: Will using EV lead to the early demise of the battery?

IMO, Car (and software) is designed to protect the battery as much as practical.

 

It may be the difference between 11 years and 14 years. Ehh.

seilerts is correct: the battery is there to overcome the deficiencies of the internal combustion engine. Using the battery more than necessary can reduce fuel economy. In this example, draining the battery at the end of the day means that stored energy won't be there the next morning to assist the then-cold engine during launch. But only careful measurement could show how much difference it makes.

 

The short technical answer is yes, using EV for the run home will shorten your battery life.

The longer, practical answer is that you probably won't be able to measure difference in battery life. Every time you cycle a rechargeable battery you use up some of its life. The bigger the SoC swing in a cycle, the more life it consumes.

(Note here that there are many battery chemistries and physical structures. Some actually require cycling to function properly. My discussion is limited to types of batteries currently used in EVs and hybrids.)

So encouraging your Prius to dig deeper into the battery does take a little life from it. The good news, as pointed out by others, is that the Prius is pretty easy on batteries. Toyota's engineers have been very cautious about protecting the battery, so it's very hard to damage it through driver action, other than physically running into something or setting your car on fire.

The bigger question relates to your stated reasons for using this technique. Generally this technique only makes sense if you live on the top of a hill, where each drive will start with a large amount of regeneration. Otherwise you are just hurting your mileage by going through the inefficient gas/mechanical/electric/chemical/electric/mechanical conversion cycle. You don't want to do that unless absolutely necessary, unless just for fun.

Your HV battery is not losing a meaningful amount of charge overnight. The chemistry of Prius batteries makes for a very, very low self discharge rate. What you are seeing is the battery monitor system making a best guess at the SoC each morning. Unlike a gas tank, there is no way to physically measure the SoC of your Prius battery, so the monitoring system relies on a number of semi-related measurements and some voodoo calculations to guess at the SoC. It does a pretty good job during operation, but the results are not as good from a cold start.

Tom

 

Yes.

This is not to say that there is no use for EV mode. The best use would be to avoid a cold start, such as a trip first thing in the morning from the house to a coffee shop that is only a few blocks away. Additionally, in cold weather, if you pull up to a stop light and the ICE is running so as to provide cabin heat, you can engage EV mode to make the ICE shut off. Finally, if you know that you have a steep downgrade coming, you can use EV mode to reduce state of charge before beginning descent, knowing that the battery will be fully charged, if not burning charge off, by the time you get to the bottom.

 

With respect to gen-2, that's what happened both on my commute to work and frequently the return home from errand running. No EV button, but lots of stealth and steep hill climbing afterward. At 118,185 miles when I traded it, all was still fine. My impression was there's plenty of buffer, even when hitting 2 bars on a regular basis.

Factors like extreme heat are likely to have more of an impact.
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i thought that running the battery down on the way home was a good idea because the next morning the engine has to run to warm up and it would recharge the battery anyway?

 

"The bigger question relates to your stated reasons for using this technique. Generally this technique only makes sense if you live on the top of a hill, where each drive will start with a large amount of regeneration."

Did you mean "live at the bottom of a hill?"

I think it was Seilerts who first pointed out that finishing a drive with a full battery is a bad idea because chemical and thermal changes continue after the car is turned off, but the batterie's protective mechanisms are inoperative. So this would be one other reason to draw off charge.

 

To elaborate on what others have mentioned about draining your battery at night just meaning that you have to refill it in the morning - something that you might not notice, especially without a ScanGauge, is that the battery naturally wants to be at 60%. Cruising on the highway, it'll usually be right on the money. When SoC gets above 62%, the car will run the engine at lower RPMs and make up the difference by using battery power. When the SoC gets below 58%, it does the opposite - it runs the engine faster, sending extra power to the battery. Without a ScanGauge, you can't tell exactly where you are; bar #6 on the display covers the range of 55%-66.5% (at least on the Gen II; I don't know if it's identical on the Gen III or just similar).

So you probably won't know it, but the engine will be working harder in the morning to charge the battery. At that point, you lose all the "free" mileage you thought you were getting at night. And even if the engine has to run anyway to warm up, when SoC gets to a certain point, it'll just stop producing power, so it uses a relatively small amount of gas to just idle the engine compared to when it's charging the battery at 4-5 kW as is typical (assuming that you're not burning up the charge just by accelerating, which I often do).