I have Toyota Estima 2.4 petrol/electric hybrid 2010 (UK 60 reg) that I bought about 4 months ago. So far I had no problems with anything until a couple of days ago the HV battery stopped charging fully. It would charge much slower than usual up until about 70-75% and that's it.

There is an indicator of charging that goes from 1 to 5 bars. Normally it would show 1-2 bars during coasting and up to 5 bars when braking. As of yesterday, it's at most 1 at coasting and at most 2 during braking, even sharp braking. I can't think of anything that could be a trigger for this change, it just started happening when I started the car in the evening yesterday to go home from work.

I checked ECU for any fault codes - and the are none. What are the most likely reasons for this and how would I diagnose them?

Update: I've monitored it over last night and today and now see a bit more symptoms, but don't know what to attribute them to. Essentially, when the car is cold (i.e. started after a long period of being parked), charging works as I expect it to, with up to 5 bars on braking. After I've driven for about 30 minutes or so, the charging slows down significantly to at most 2 bars. This is regardless of the battery level - whether it's nearly empty or more than half full. This leads me to believe that the issue may be with inverter/converter block or some other part of the charging mechanism.


I suspect your battery chemistry is breaking down. Almost a decade is a very good run, especially considering the technology in 2010 has grown by magnitudes since then.

Those batteries relied on absolute segregation of anionic and cationic states. Over time, everything mellows and it becomes a bit of a mush that can't be restored to 100% design capacity.

(Admittedly, I have no idea what I'm talking about and have no real understanding of the underlying chemistry...)

The decision you might have to wrestle with is whether you want to get a new battery bank (probably hideously expensive) or live with as long as you can the diminished capacity.

There might be some ways to "rejuvenate" your existing battery bank. Deep discharge and recharge cycles can break up the "dendrites" in the chemistry that diminish capacity. Perhaps. Much like cellphone batteries, they thrive on use. Unfortunately, we plug in our cellphones at night and end up not exercising these batteries in a way that maximizes their lifespan.

  • Thanks for your answer. This was my initial thought too, however if this was due to battery deterioration with age, it wouldn't happen overnight - the battery capacity would have been decreasing slowly. Also, additional symptoms I noticed (and added to my question) seem to point to the charging mechanism. – Aleks G Jul 11 '19 at 10:02
  • The Toyota computer is intelligent enough to rejuvenate the battery occasionally. Honda wasn't so precise on the charging controller, and thus, in some use cases (a certain charging pattern), a Honda hybrid battery used to break down. Toyota has not had the problems of Honda. About suboptimal control algorithms of Honda: aa1car.com/library/honda_civic_hybrid_battery.htm – juhist Jul 11 '19 at 16:49

I suspect your battery is working just fine.

My RAV4 hybrid has 8 bars for the battery charge. 99% of the time, it's at most 6 bars. Very rarely do I see it to go to 7 bars. The minimum I have ever seen is 2 bars. I understand that 8 bars is not truly "full" and 0 bars is not truly "empty". Empty would be more likely something like minus 2 bars, and full would be something like 10 bars or so. The car is using only the middle portion of the battery charge/discharge curve.

When starting a cold engine, it warms up the engine completely, charging the battery to provide constant low load for the engine. Thus, a cold start means the charge will go to 6 bars.

When driving at low speeds on a warm engine, the battery depletes to 2 bars, is charged to 3 bars, depletes to 2 bars, is charged to 3 bars, ... So, it will never charge it to 6 bars when driving at low speeds. This is completely normal. Apparently, Toyota has decided that operating at between 2 and 3 bars is the most efficient state of charge.

Have you been driving at low or high speeds when the symptoms appear? If it's at low speeds, that might explain the symptoms. Low speed never charges the battery fully.

Try either of the following:

  • Drive at continuous > 80 km/h speed for 20 kilometers, see if the battery charges to levels you have previously but not recently seen on a warm engine
  • Drive at < 80 km/h speed, but in such a manner that you avoid using the EV mode. If you see the EV lamp light up, press the accelerator more to deactivate the EV mode, then gradually decrease the accelerator to keep within speed limits. Never release the accelerator fully as that immediately activates EV mode.

If you see that either of the two options charges the battery to a high level, your battery is working just fine.

If not, time to go to a Toyota dealership. If you service your car at the dealership, they can do a battery test. Where I live, the battery has 10 year / 350 000 km warranty, which is valid only if the car is serviced at a Toyota dealership where they always do the battery test.

  • Thanks. Unfortunately this didn't help. Driving at 100-110 km/h continuously for about 20 minutes barely charges to 70%. The bars I refer to are the charge speed, not battery level. – Aleks G Jul 11 '19 at 16:56
  • Oh, your hybrid then has a charge speed monitor that my hybrid doesn't. I guess, time to visit a Toyota dealership then! – juhist Jul 11 '19 at 16:57

If you look in the scan tool live data it should be under the ecu called hybrid control or something similar, you can find the state of charge of the hv battery and the individual cell block voltages which can help you identify what the current state of charge is.

Also one of the best parameters to look at would be delta state of charge which would help to tell if your battery has degraded or not. Delta state of charge measures the difference in state of charge between cell block modules in the hv battery and the biggest difference is displayed on the parameter so its a good way to identify if your battery has degraded.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.