Tidbits - Vista's UAC
I have the privilege of owning a Lexus RX450h since December 2011. In that time I have
made an effort to understand the key differences between a hybrid and a car that only has a
One of the key principles is that the RX450h requires 3 batteries to operate. First is the 288
volt, 6.5Ah high voltage pack which operates the 3 x 650 volt electric motors (all wheel
drive). Two are at the front, and one at the rear. Second is the 12v battery that operates the
12v based accessories. Third, is the button battery found in the key fob.
Battery #1 - High Voltage Battery Pack (288 volts, 6.5 Ah)
The high voltage battery pack provides and receives power from the 3 electric motors
(AWD). There are 2 located in the front of the car. One of the motors is a generator which
manages the petrol motor, and is responsible for powering up the petrol engine as
required. Thus there is no longer any linkage between the petrol engine and the 12v
battery. The generator (MG2) charges the 12v battery.
The high voltage battery pack can only be charged by 2 methods. First, is by the petrol
engine. Second, is at a Lexus Service Centre. There are no other methods of charging this
battery. It is imperative that you do not let this battery discharge to the point where MG1 is
unable to start the petrol engine.
To avoid discharging the high voltage battery pack do not:
1) Run out of petrol and continue to drive in electric mode. Once the battery pack is drained,
the car is kaput, and will need towing to a Lexus Service Centre. Lexus warns that the
RX450h is not designed to run without petrol. Which I interpret to mean, Lexus has not
created a low fuel program that manages MG1 and the battery for you. When the low fuel
light comes on, the low fuel program is for you to manage. You need to maximise mileage
prior to running out of petrol, which means, accelerate gently, brake gently (maximise energy
recovery), drive below 60km/h and avoid driving uphill, and get to the petrol station before
you run out of petrol.
2) Not driving the car for a long period of time. Just like your phone or camera batteries, the
battery will discharge over time. Lexus recommends that you drive the RX450h for 30
minutes at least monthly.
Battery #2 - 12 Volt Battery
The 12v battery runs the 12v electrical systems. The system that matters, is the security
system that gives the all clear to the electric motors to start up. The good news is, you have
to run the 12v battery down much more than you would say in the RX350, before you declare
that you have a flat battery. The bad news is, if the battery is fully run down, the 12v battery
will be really flat/fully discharged, and you may need to apply manual procedures to even
open the car.
This can be rectified, by recharging the 12v battery to at least 30% charged. Lexus have kept
the procedure quite similar to jump starting, with one key difference. You can’t jump start
the RX450h. You need to recharge the 12v battery. That’s why Lexus requires that the car
supplying the battery is allowed to run at above idle for 5 minutes. A battery charger will do
the same. If you are using a battery charger, Lexus recommends that the charger not deliver
more than 5A of charge, else it could result in battery damage.
Battery #3 - Button Battery in Key FOB
The button battery in your key fob needs to have enough charge to send the relevant radio
waves to the security system to unlock the car. The good news is, Lexus have built a
backup into the key fob, so that if the high voltage battery pack and 12v battery are good to
go, you can still start the car. Lexus have designed the fob to work without the need for
electricity, where the fob works in a similar manner to the Visa contactless PayWave system,
where your credit card just needs to be in close proximity to the transponder unit. As Lexus
has placed the transponder behind the start button, you need to touch the start button with
the Lexus emblem on your fob. This is no different to the RX350.
Compared to the 2007 RX350 that I previously drove, the pros and cons of the RX450h over
the RX350 are:
+ Fuel Economy
The RX450h shines on a number of fronts here. The comparison is based on suburban
driving (50km/h, 60km/h, 80km/h and some freeway driving) in a hilly suburb:
1) The RX450h uses close to 50% less petrol than the RX350. I use to average 6 km/litre in
the RX350. In the RX450h it's 11 km/litre. Fuel bill feel further reduced by the RX450h
having a smaller fuel tank (by 7 litres) than the RX350. I use to refill the RX350 every
fortnight, the RX450h only needs to be refuelled every 3-4 weeks.
2) The RX450h thrives in slow stop and go traffic. This is not mentioned in the reviews.
The RX450h tips fuel economy on its head. In heavy traffic, fuel economy improves as the
car works the electric motors more at lower speeds, and the car recovers the energy from
braking that stop start entails. So being stuck in a shopping centre car park improves fuel
+ Smoother Torque
The RX450h handles low speed driving (up to 50km/h) really well, due to the car being
managed by its computer systems to run the petrol engine and electric motors in tandem to
optimum power/torque curves, with each motor covering any power or torque gaps that any
motor has, especially the petrol engine.
I have s steep hill on a main road that has a 50km/h speed limit enforced by mobile speed
camera cars. The RX350 engine would labour trying to do 50km/h up the hill, and
encourages you to push the accelerator pedal so that the engine can power the car up the
hill at 60-70km/h, where the engine works more efficiently. The RX450h has a much wider
torque band, and just quietly and smoothly motors up the hill at lower speeds.
+ Air Conditioning
Being able to run the air conditioner for up to 30 minutes on a hot summer's day without
running the engine.
- Torque Steer
The major minus in the RX450h, the focus on efficiency results in the car being principally a
front wheel drive car. Coupled with the ability of the electric motors to immediately apply full
torque, under heavy acceleration, especially where the wheels need to be turned, the car
suffers from torque steer. The RX350 applies power to all 4 wheels, and does not suffer
from torque steer.