Hybrid Battery Meter

[Andrew888]

After 2 weeks of driving my IS, has anyone had Full Bars on the Battery meter? My wife has a CRZ and from time to time I do see it being full due to regenerative charging. I do know that Honda's IMA is only Battery assistance and Lexus Hybrid Drive works differently i.e. full EV but just curious thats all....

 

 

[Mr Vlad]

Hi. I get full bars regularly but to be honest the bar display could be better as the very top of that part of the screen always looks as tho it has space for another of those blue blocks. I drive on motorways making sure I see the arrow from the engine to the batteries is always on ready for when I switch to Sport and floor it. I love it.

[Thackeray]

My instinct when I first got the car was to want to see the Battery meter full. Instinctively I was probably looking at it like the petrol gauge where "full" is reassuring but "low" is worrying (in case you run out). But actually, as a friend pointed out to me, you need the Battery not to be full if the car is to make the most of regenerative braking. If the Battery is full, any electricity generated from braking is just thrown away. Imagine driving down a long hill with the Battery full - all the electricity generated from using the brakes on the downward slope is lost.

By contrast, when you're accelerating, if the Battery is full, any spare electricity generated from the engine driving the generator (MG1) can be routed to the main motor (MG2), so none of the electricity is lost.

So I realised that the best reading for the Battery meter is probably around half-full or less, giving plenty of space for generated electricity when you're on a long downward slope.

In any case, I've read on a Prius forum that the meter doesn't give an accurate reading of the actual state of the Battery. The Battery is kept between around 40% and 80% charge. So when the Battery meter says empty (I've never seen this) the Battery still has a 40% charge; and when it says full the Battery has an 80% charge. This is done to extend the working life of the Battery as overcharging the Battery can shorten its life.

[Zotto]

Best mpg are obtained keeping the state of charge  around  6th notch on the meter (accordingly on hypermilers ). On my GS I see full charge  (80%) only after a long descent with regenerative braking, and only when it happens I use engine braking with a lower "gear" set manually.

[Thackeray]

Here's a discussion on battery charge levels. If you scroll down to the post by m.wynn, there's a graphic which I think represents the state of charge for each of the bars on the Prius Battery meter. It shows a full range of 40-80% and bar 6 is 55-66%. 

Whether this is the same on the IS300h, I don't know. I don't actually know how many bars there are on the IS300h Battery meter. Does anyone know if it's also eight? My frequent stop-start heavy traffic slow driving usually leaves the car with around three bars when I get back home. In other places, driving at higher speeds often shows a higher reading when the car stops.

[Thackeray]

Reviving an old topic, until now I've never seen eight bars on the hybrid Battery level (ie full) As mentioned in my post above, I usually see two or three bars in the town driving that I often do. But last week, after going down a long winding hill of 2-3 kilometres in France, I saw eight bars for the first time by the time I reached the bottom of the hill. Following discussion in another thread, I only used the regenerative braking going down the hill - the speed limit was 50 kph (30 mph) - rather than "changing down" a gear ratio or two. 

The other interesting thing was that when the Battery was full and I kept on braking, the engine started running. In fact, when I reached the bottom of the hill and stopped at some traffic lights the engine kept running for around five seconds after the car had stopped. Normally, the engine stops immediately when the car stops at lights. Someone in another thread said they were puzzled by the engine apparently running when the Battery was full and the car was slowing down on a motorway.

I assume the engine running while going down the hill with the Battery full will allow the generator to turn more slowly to avoid damage. But does anyone know why the engine keeps running for a short time when the car has just stopped and the Battery is full?

[Herbie]

When going downhill and braking lightly it's the regenerative braking that's working to both slow the car and charge the Battery. When the Battery reaches a fully-charged state then nothing can be done with any electricity that is produced, so the 'normal' hydraulic brakes take over from the regenerative braking system. I would hazard a guess that the engine runs to enable the hydraulic brake servo to do its job and assist 'normal' braking.

The engine probably continues to run for a short while when stopped and the Battery is full to enable cooling of the inverter and hybrid system - just a guess but it sounds plausible to me  

[Vince Donald]

The servo assistance must be electrical as most of the time when slowing down, the engine is usually not running when you get to the mechanical stage of braking, ie from about 5mph if you do it just right. Other items like the AC are also electrical rather than driven by the ICE.

The way the engine / electric motors all connect through the gears, the electric motors can not be disconnected so they will be spinning and electricity will be generated, this has to go somewhere so the engine will spin as a load - it doesn't need to use any fuel, it just needs to be spun by the system to create a load to dissipate the excess energy, the continuing for a few more seconds would just use up a little more energy.

The control of the engine is achieved by using a second electric motor which can spin forwards or backwards to mate the speed of the ICE, the electric motor and the road wheels, its all very clever and actually extremely simple which probably helps with reliability.

 

There are several videos on YouTube explaining how it works, the Weber Auto one actually show the inner workings of an actual gearbox / motor setup from several versions of Prius, which are fundamentally the same system, in one of those videos he mentions the engine spinning as a load for the system when there is excess electricity.

Had this very effect recently on a trip up Scotland on a road know as the Cairn O Mount, Battery fairly depleted at the summit, filled up pretty quickly on descent and then the engine came on about 2/3rds of the way down.

 

Vince

[ColinBarber]

19 minutes ago, Vince Donald said:

The way the engine / electric motors all connect through the gears, the electric motors can not be disconnected so they will be spinning and electricity will be generated, this has to go somewhere so the engine will spin as a load - it doesn't need to use any fuel, it just needs to be spun by the system to create a load to dissipate the excess energy, the continuing for a few more seconds would just use up a little more energy.

This isn't entirely correct. There is no power generated if the circuit is disconnected, which the ECU can do when the Battery is charged or you put the transmission in N for example. The same principle applies to a normal car's alternator - excess electricity isn't generated, only what is required.

The engine is spun to protect the gear mechanism from excess speed, and also to stop the voltage from the motor/generator from exceeding the maximum voltage the system is designed for. To quote the official explanation of the hybrid control system:

Quote

during Deceleration

1. While the vehicle is being driven with the shift lever in D and it decelerates, the engine turns off and the engine motive force output to the wheels will be zero. At this time, the wheels drive motor (MG2), causing motor (MG2) to operate as a generator and charge the HV battery. While motor (MG2) is operating as a generator, it creates a resistance to rotation at the wheels, producing a braking effect.

2. If the vehicle decelerates at a higher speed, the engine (crankshaft) will not stop turning. The engine will maintain a predetermined speed in order to protect the planetary gear unit.

 

[Thackeray]

57 minutes ago, ColinBarber said:

There is no power generated if the circuit is disconnected, which the ECU can do when the battery is charged or you put the transmission in N for example. The same principle applies to a normal car's alternator - excess electricity isn't generated, only what is required.

The engine is spun to protect the gear mechanism from excess speed, and also to stop the voltage from the motor/generator from exceeding the maximum voltage the system is designed for.

Thanks for that explanation. That makes sense for what is happening when you go down a hill with a "full" Battery. (It's probably only at 80% as I understand it but that's the limit set by the system designers.)

But I'm still not sure I understand why the engine would keep running for five seconds or so after the car came to a halt with the Battery on "full charge". It sounded to me as if the engine was firing. The electric motor (MG1) could have kept the engine turning without using fuel to reduce the level of Battery charge. But it sounded more as though the engine was running rather than being spun without using any fuel.

[ColinBarber]

There are several circumstances when the engine will keep running, where to the driver thinks it seems correct for the engine to stop. Typically the system is going through some maintenance/calibration cycle.

I've seen reports of it keeping the engine running after descending a hill with a full Battery for a few seconds - one theory is the system is removing a small amount of charge from the Battery so regen braking works as expected when the brakes are next applied but I've not seen any official documentation to confirm this. It isn't something to worry about, unless the petrol engine fails to shut down for several minutes just let the system do its thing.

[Thackeray]

7 minutes ago, ColinBarber said:

one theory is the system is removing a small amount of charge from the battery so regen braking works as expected when the brakes are next applied

After looking for information on Prius websites, this seems the most likely explanation; just slightly lowering the Battery charge level. That would presumably mean the engine was spinning but not firing, despite my impression that it was running normally.

While looking for an explanation I came across some people who had experienced the engine continuing to run without stopping. This generally seemed to be because something like a plastic bag was blocking the Battery cooling vent! The engine kept running to prevent overheating of the Battery. When they removed the plastic bag blocking the vent, the engine behaviour went back to normal. If only most other problems were as easy to fix!

[Vince Donald]

On 8/27/2018 at 5:30 PM, ColinBarber said:

This isn't entirely correct. There is no power generated if the circuit is disconnected, which the ECU can do when the battery is charged or you put the transmission in N for example. The same principle applies to a normal car's alternator - excess electricity isn't generated, only what is required.

The engine is spun to protect the gear mechanism from excess speed, and also to stop the voltage from the motor/generator from exceeding the maximum voltage the system is designed for. To quote the official explanation of the hybrid control system:

 

In a normal car system, the alternator can be disconnected and then it just spins and provides little resistance, this is correct.

However, in a hybrid system, the energy levels are much higher and crucially the system is connected to the road wheels, as it says in the first part you quoted "(MG2) is operating as a generator, it creates resistance to rotation at the wheels", this is normal regen occuring. 

On 8/27/2018 at 5:30 PM, ColinBarber said:

• While the vehicle is being driven with the shift lever in D and it decelerates, the engine turns off and the engine motive force output to the wheels will be zero. At this time, the wheels drive motor (MG2), causing motor (MG2) to operate as a generator and charge the HV battery. While motor (MG2) is operating as a generator, it creates a resistance to rotation at the wheels, producing a braking effect.

• If the vehicle decelerates at a higher speed, the engine (crankshaft) will not stop turning. The engine will maintain a predetermined speed in order to protect the planetary gear unit.

In the second part this is describing braking at higher speeds, if they switch the ICE off, the MG1 would need to spin extremely fast backwards, http://eahart.com/prius/psd/ has a good demo of this on the Prius, the maximum speed with the ICE off would be about 42mph so when they say "protect the planetary gear unit", they really mean MG1 as they don't want it spinning too fast, this is still normal regen occurring. So from higher speeds, at some point when you are going slow enough, the ICE goes off, if you set the tacho to always on you can see this all the time.

In downhill braking, even at say 20 - 30 mph when the Battery becomes full, this is not the same situation as described in part 2 as at those speeds the ICE would more than likely be already off. But at some point the ICE comes back on, I've witnessed this a few times, descending the Cairn O Mount its easy to watch it all happen as you don't want to come down there at any great speed. So descending at around 30mph in silence apart from the brake pads on the discs, the ICE suddenly come on a short while after the Battery appears to be full, and the ICE runs at something like 2000rpm.

So whats occurring here is a little different, the main braking is being done mechanically, but since the MG2 is connected to the road wheels at a fixed ratio so it will keep generating power, but if you could disconnect the circuit, this would be done using electronics so would have large energy across the electronics, not good.  Also you would remove the load as the load is created by pushing the electricity into the Battery and you don't want a sudden decrease in braking.

So by spinning the MG1 you cause the ICE to spin and this allows the MG2 to spin slower, the energy of this slower spin is used to keep the MG1 spinning which is spinning the ICE, so the ICE is providing the load via MG1 to MG2 and hence the road wheels.

The whole system is beautiful in its simplicity, it however requires some very sophisticated software to keep it all in check.

 

Vince

 

[Thackeray]

10 hours ago, Vince Donald said:

So by spinning the MG1 you cause the ICE to spin and this allows the MG2 to spin slower, the energy of this slower spin is used to keep the MG1 spinning which is spinning the ICE, so the ICE is providing the load via MG1 to MG2 and hence the road wheels.

I think I've now got a better idea of what's happening when the Battery is nearly full and the car is going down a long hill. Have I understood this right?  This is what seems to be happening.

If you look at the power split device graphic on eahart.com/prius/psd/ and adjust MG2 to set the car's speed to around 30mph, MG2 (connected to road wheels) will be spinning forwards and MG1 backwards. Both will be generating electricity which is being fed into the Battery. The engine (ICE) is not turning.

But when the Battery becomes full, something has to be done with the electricity that is still being generated. So the electricity from MG2 is re-routed to MG1 instead of the Battery. MG1 now changes from a generator to a motor and spins forwards rather than backwards. On the graphic, setting the ICE (internal combustion engine) to spin at around 2,000 rpm shows this effect. MG1 is spinning the engine, using the electricity that is being generated by MG2 as the car continues to roll downhill.

 

[Vince Donald]

Correct, though after I posted I realised that I made a small mistake, the MG2 doesn't spin slower, its speed is directly linked to the road wheels, its just that the rest of the system, its just that the MG1 takes the energy  and spins the ICE as I said.

 

Vince