Hybrid Battery charge state

[Gaffer]

Apologies if this has an obvious answer but, after having driven my wife's NX300H for a few weeks, I've realised that I can never get the Battery to full.

It has 8 bars and the max I get is to 6. This is regardless of how I'm driving (e.g. gently and slow, or deliberately slightly faster to get the engine to recharge it, or coasting as much as possible, or 10/15min on the m'way etc etc).

The only thing I can think is that Lexus may be doing what apple do with their iPhones. Overnight charging doesn't just get it to 100%, it actually keeps it at about 80% most of the night and only fully charges it just before it thinks you'll need it (e.g. before alarm goes in the morning).

So keeping the NX at 6/8 is keeping the Battery at 75% - is this deliberate in order to improve Battery longevity? Or is the Battery meant to get to 8/8 full when driving and I'm just not getting it?

[Steve Oliver]

The Battery will generally stay around 60% full or so when driving around normally (obviously it varies up and down depending on how you're driving). But  try rolling down a long hill (mile plus)  somewhere (eg Scottish Highlands) at less than 50mph and you'll see it go right to the top, especially if you're braking on the way down. Once it's fully charged (and it really fills that little gauge on the info display), if you're still descending the hill (potentially still regenerative braking) the ICE engine will restart, and I've no idea what it is doing when it does this, but it seems to run to prevent the over charging. Perhaps someone knows why it does this?

[stepppy]

That is perfectly normal.

I have just come from a 2017 Prius that I had from new, so 7 years old when I sold it.  That would only ever show a 'Full' Battery after a motorway journey, then a long decent down a hill to our home.  So far I have not seem my NX show a full Battery, just one below.  

Thing to remember is that although the Battery gauge may show 'Full', it is actually only around 80% full, the same when it's low, it is not that low in reality.  Toyota/Lexus reserve a portion of the Battery at the top and bottom that is not visible to us, this helps protect the Battery and ensure its longevity.

I hope this puts your mind at ease 🙂

[ColinBarber]

10 hours ago, Steve Oliver said:

Once it's fully charged (and it really fills that little gauge on the info display), if you're still descending the hill (potentially still regenerative braking) the ICE engine will restart, and I've no idea what it is doing when it does this, but it seems to run to prevent the over charging. Perhaps someone knows why it does this?

Depending on your speed the petrol engine will be driven (not actually consuming fuel) to protect the transmission even if the Battery isn't full.

Once the Battery is fully charged then regen braking has to stop (no where for the energy to go) and so it uses the engine as a load to provide a constant 'engine braking' force - without doing so you vehicle would suddenly start accelerating down the hill.

[stepppy]

As @ColinBarber says, when the Battery shows full the engine will spin not using any fuel acting as an air pump to help slow the car and to help spin off some excess energy.

If you put the car in N going down a hill you will really appreciate how much regen slows the car as without it the car just keeps on going. Not advised obviously as you are not charging the HV Battery, but very useful to clean off your brake discs if they are getting a bit rusty.

The times I had a full Battery when I got home, the engine would often come on and I would wait for it to stop and by then it had dropped one bar from the full mark. It had done its job. 

[Thackeray]

11 hours ago, Gaffer said:

Or is the battery meant to get to 8/8 full when driving and I'm just not getting it?

Most people's first instinct is to regard the Battery charge level in the same way as the fuel tank level - Full is good, Empty is worrying.

But this is not the way to look at it. On these hybrid cars, the hybrid Battery is not primarily to drive the car along. Instead, it's a receptacle to store regenerated electricity. If the Battery is full, there's no room for any more regenerated electricity so the power has to be discarded as heat instead. Trying to keep the Battery full would be like going shopping with a full shopping bag so that you have no room to carry your groceries.

After the electricity has been stored, it can then be used to help drive the car along and make use of power that would otherwise have been lost as heat from the brakes.

The computer aims to keep the Battery about half full. This leaves plenty of room for more electricity to be stored, while at the same time having a decent reserve available to power a sudden burst of acceleration.

As others have said, the gauge doesn't represent reality. On other models and probably the NX too, one bar represents around 40% full. The engine will start if it reaches this level to prevent it going any lower. Eight bars is around 80%. Keeping the Battery in this range helps to extend its life.

[RONNIE W HODGEKINSON]

12 hours ago, Gaffer said:

Apologies if this has an obvious answer but, after having driven my wife's NX300H for a few weeks, I've realised that I can never get the battery to full.

It has 8 bars and the max I get is to 6. This is regardless of how I'm driving (e.g. gently and slow, or deliberately slightly faster to get the engine to recharge it, or coasting as much as possible, or 10/15min on the m'way etc etc).

The only thing I can think is that Lexus may be doing what Apple do with their iPhones. Overnight charging doesn't just get it to 100%, it actually keeps it at about 80% most of the night and only fully charges it just before it thinks you'll need it (e.g. before alarm goes in the morning).

So keeping the NX at 6/8 is keeping the battery at 75% - is this deliberate in order to improve battery longevity? Or is the battery meant to get to 8/8 full when driving and I'm just not getting it?

To The best of my Knowledge The system purposely charges only to the optimal charge of 80% as this maintaines the best Longetivity of the Battery itself.. It Never charges up to 100%

[ColinBarber]

On 5/31/2024 at 10:32 AM, Thackeray said:

On other models and probably the NX too, one bar represents around 40% full. The engine will start if it reaches this level to prevent it going any lower. Eight bars is around 80%. Keeping the battery in this range helps to extend its life.

Correct. This is specific NX information showing how the state of charge changes as the vehicle is driven - it lacks scale detail but is essentially the same as all the Toyota/Lexus hybrids with a NiMH traction/hybrid Battery.

[stepppy]

After my earlier post stating that I have not yet got a 'Full' Battery in my NX like I used to in my Prius, yesterday I did.  Unlike my Prius it didn't start the engine just as I drove into the garage to spin off any excess energy. 

Its interesting that the NX seems to keep its Battery pretty much 50-75% full the majority of the time, there is a lot less, so far of dropping down to 2 bars and the engine kicking in to top it back up.  I know the NX has a different Battery chemistry which must be more efficient, plus I didn't realise until I looked at the display, but the rear wheels regenerate too which must help.

[Thackeray]

3 hours ago, ColinBarber said:

Correct. This is specific NX information showing how the state of charge changes as the vehicle is driven - it lacks scale detail but is essentially the same as all the Toyota/Lexus hybrids with a NiMH traction/hybrid battery.

That's helpful to see a graph for the computer's strategy for the state of charge.

My own testing on the IS300h (I had nothing better to do during Covid lockdown!) showed that the engine would start when the Battery charge dropped to 40% (two bars showing) and would stop when the charge had risen to 50% (three bars). Four, five and six bars represented levels of charge from around the low to mid-50s up to around 60%. From memory seven bars would only appear above about 70% and eight bars at 80%. I was reading the percentages from the Hybrid Assistant app.

The videos on the Hybrid Assistant app website seemed to show higher percentages than I was finding, so maybe the fine tuning varies from one model to another.

[Gaffer]

Thanks for all the info everyone 😊. Confirms what I suspected so I'll stop worrying about it (I've found the best way to do that is take the infographic off the split screen completely which CarPlay does perfectly 👍).