The first Generation Prius, model NHW10, went on sale on December 10, 1997.It was available only in Japan, though it has been imported privately to at least the United Kingdom, Australia, and New Zealand. The first generation Prius, at its launch, became the world's first mass-produced gasoline-electric hybrid car. The NHW10 Prius styling originated from California designers, who were selected over competing designs from other Toyota design studios.
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- from KEVIN of -

Hi, I have been away for a few day's and come back to what appears to be some "heavy" posting going on.

Having now caught up and read them all it is clear that there is STILL engrained "presumed" information that is still being worked with here, from old out of date and long ago postings, that is still hindering most of you from understanding even the basics of these NHW10 Hybrids.

I use the term HYBRID to drive home to you that this is what this car is, it is a HYBRID. IT is not an electric car and a petrol engined car bolted together, but is a BLEND of both sides working together in one harmony. The electrical side is not managed by the ECU the same as on a standard electric car would be nor is the petrol engine manged like a normal petrol engine either. If you try and tread each side as if "standard/normal" you will come unstuck, as most of you seem to be doing here.

Toyota developed the NHW10 (Prius 1st gen) and recognized to themselves that the car as a whole was generally "underpowered". They then spent millions of dollars on research and came up with the NHW11, it is only by looking at these changes that they made, that you can see were the NHW10,s weakness may lay, and there by address the needs of an NHW10.

The HV battery pack, yes! it was changed over to Prismatic cells which are clamped together to prevent uneven heat loss, and there by prevent imbalance from easily developing, and also remove the need for equalization charging on such a vast HV battery pack. Basically, in Kilowatt terms the NHW11 HV battery pack is still a similar power output, but the ECU was able to make much better use of it after these changes by not down grading so often.

The biggest change however (to address the under power of the NHW10) was to the ICE or petrol engine. This was up graded from a 43KW output engine on the NHW10 to a 52KW on the NHW11, a massive near 25% increase in power. This was achieved by upgrading the ignition system as a whole (coils and plugs)and reprogramming of the ECU management system on managing the ICE.

This ICE is heavily tuned on an NHW10 to a modified version of the "Atkinson cycle". In layman's terms this means that it prioritizes the economy production over Horsepower production, were as on a standard OTTO cycled engine they prioritizes Horsepower over economy, (the other way around). SO if you neglect the servicing of a standard OTTO cycled engine you loss economy (people often service their standard cars when the MPG goes down), but on the NHW10 it is an Atkinson cycled engine so you should service if the horsepower goes down (in other words, if you get direct or indirect turtle lights coming on).

As the NHW10 was "underpowered" you have to do everything that you can to "wring" out as much horse power out of the ICE as you can. A half blocked air filter or low oil may on their own result in a drop of 5 kw of power on this Atkinson cycled engine, but a full service with modern improved air filters and iridium spark plugs may gain you an extra 5 kw's over original standards, plus other things as well.

The HV battery pack is a weakest link (in how it is managed) in the drive train of the NHW10, but if it was not strained so much with a poor performing ICE (which also drives the charging system) then it would not play up so much.

If all is well then the HV battery pack actually runs within whats is only a 6% usage range, this +/-3% either side of the 60% marker. The 1/2 mark on the MFD is 57% and the 3/4 is a true 63%,(these are actual readings taken),how ever as no energy is wasted this range is extended only during recovery Regen purposes from 20% to 80% then it's back to the 6% range. Each module block must be maintained to within 1 volt at all times(as published by Toyota)even under load, or a "turtle light" is invoked. this could be due to a weak cell or it could be due to poor over all charging due to the under performing ICE. During a turtle light scenario a "marker" is placed at the point of the program where the 1 volt drop a cures during discharge, and this point then becomes the NEW and moved 20% marker, the battery is then charged up (with a time limit) to 80% to get back to minimum imbalance, and an accumulative count is added to each time this happens IV20x1 and IV80x1. So a "turtle light",in the case of the HV battery, is not just a charge boost but a re-calibration as well. In exceptional bad circumstances where a 1.2volt drop happens then an IV10x1 (10% marker) is added and the system stops the car and try's to break down the continuity in a probable bad cell through continuous high rev charging. Once the hybrid warning light comes on altogether (with triangle) then the HV battery pack is retained at a full 4kw down grade on top(this reading is on the hybrid ECU) so the cars management system is adjusted to use the HV battery pack much LESS! and the cars over all economy is permanently reduced.

If you re-con a HV battery pack without resetting the ECU again, then the battery ECU "dumps" the extra charge it can now hold and takes the battery pack back down to the past "bad battery" settings held in it's memory, and you get to drive a kilometer or so on pure HV battery power. In other words the battery ECU can not go back to "good" settings, by itself, no mater what you do.
SO to keep an NHW10 running well in the first place, you have to improve on the original servicing to gain on the original 43kw output of the ICE. This improves not just it's performance but also reduces the reliance on the HV battery pack and leaves more energy left over for the running of the charging system. You also need to understand that wear on the HV battery is dependent on also how the car is used (it was designed as a commuter vehicle) and how the ICE is looked after as this is priority. Secondly you need to keep the HV battery cool (the cells are to be kept down to 24'C) and it is very common find non working air-con systems trying to blow air through blocked cabin filters through to the hv battery pack. Toyota set up this system so that the HV pack would get "room temperature" air flowing in from the cabin, so make sure this all on and works at its best. Lastly use the better 12 volt batteries and reduce your 12 volt usage to a minimum so that the HV battery will not be called up on to prop up the 12 volt system quite so much as this is how Toyota hybrids are set up.

I have customers whom have NEVER had a red triangle come on their NHW10's in 5 or more years and I have never touch their HV battery pack either.
Other's have said "sort it out for me, it will still not run properly even after a service". Only then I find it was last looked at properly 50,000k ago and it has mice living in the cabin filter or something (this has happened!).

Yes I have rebuilt many HV battery packs successfully on those cars that in my opinion have suffered neglect, but what i am trying to get across here is that the HV battery problem is a knock-on affect of a weak performing ICE first and for most. for instance whilst driving up a hill the HV battery pack is only called up on whilst the CV gearbox gears down. There after the car SHOULD drive up normal hills all the way up on pure ICE power alone, if it dose not, then there IS something wrong to call upon the HV battery more than this.

- - @Raza1