Toyota Hybrid (probaby Auris) - starter battery goes flat after a few days of car not being used

[NY]

My parents have a Toyota petrol hybrid car - I think it's an Auris. My dad
is no longer able to drive, and my mum is normally using her own car, so
dad's Toyota isn't being used as often as normal.

My mum mentioned that the Toyota's starter battery (as opposed to the one
used for driving the traction motor during times when the petrol engine is
not going) has a habit of going flat if the car isn't run for a week or so.
This means she has to keep taking the car out for a drive, and to keep the
battery on mains charger in between times.

It could simply be that the battery is getting old and needs changing, but I
wondered whether there is anything inherent to a hybrid which means that the
battery gets drained by some constant current being drawn when the car is
turned off and locked. I would have thought that the only quiescent current
drawn would be things like the clock and the alarm.

[Colin Bignell]

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris. My
dad is no longer able to drive, and my mum is normally using her own
car, so dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the
one used for driving the traction motor during times when the petrol
engine is not going) has a habit of going flat if the car isn't run for
a week or so. This means she has to keep taking the car out for a drive,
and to keep the battery on mains charger in between times.
It could simply be that the battery is getting old and needs changing,
but I wondered whether there is anything inherent to a hybrid which
means that the battery gets drained by some constant current being drawn
when the car is turned off and locked. I would have thought that the
only quiescent current drawn would be things like the clock and the alarm.

How old is the battery and is it wet plate, AGM or some other type?

[NY]

Post by Colin Bignell

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris. My
dad is no longer able to drive, and my mum is normally using her own car,
so dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the one
used for driving the traction motor during times when the petrol engine
is not going) has a habit of going flat if the car isn't run for a week
or so. This means she has to keep taking the car out for a drive, and to
keep the battery on mains charger in between times.
It could simply be that the battery is getting old and needs changing,
but I wondered whether there is anything inherent to a hybrid which means
that the battery gets drained by some constant current being drawn when
the car is turned off and locked. I would have thought that the only
quiescent current drawn would be things like the clock and the alarm.

How old is the battery and is it wet plate, AGM or some other type?

The car is 15 reg so if it's on its original battery that will be no older
than 7 years. The car is a Yaris, not an Auris - my mistake. I wouldn't know
what type of battery it was. Do hybrids need batteries that are suited to
auto-stop/start technology? I imagine they do. When I see my parents at
Christmas I'll look at the manufacturing code on the battery to see what
make/model it is. I'm not sure how much the problem has been investigated by
(or even reported to) their Toyota garage.

There is then the bigger issue of how long they keep that car, since it
looks as if they won't be making any more longer journeys (eg holidays to
their cottage in Yorkshire) and so my mum's smaller car will suffice. I
think that Dad is starting to accept that he won't be able to drive any
more.

[Colin Bignell]

Post by NY

Post by Colin Bignell

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris.
My dad is no longer able to drive, and my mum is normally using her
own car, so dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the
one used for driving the traction motor during times when the petrol
engine is not going) has a habit of going flat if the car isn't run
for a week or so. This means she has to keep taking the car out for a
drive, and to keep the battery on mains charger in between times.
It could simply be that the battery is getting old and needs
changing, but I wondered whether there is anything inherent to a
hybrid which means that the battery gets drained by some constant
current being drawn when the car is turned off and locked. I would
have thought that the only quiescent current drawn would be things
like the clock and the alarm.

How old is the battery and is it wet plate, AGM or some other type?

The car is 15 reg so if it's on its original battery that will be no
older than 7 years.

Probably not wet plate then, but even an AGM battery could need
replacing at that age.


The car is a Yaris, not an Auris - my mistake. I

Post by NY
wouldn't know what type of battery it was. Do hybrids need batteries
that are suited to auto-stop/start technology? I imagine they do. When I
see my parents at Christmas I'll look at the manufacturing code on the
battery to see what make/model it is. I'm not sure how much the problem
has been investigated by (or even reported to) their Toyota garage.
There is then the bigger issue of how long they keep that car, since it
looks as if they won't be making any more longer journeys (eg holidays
to their cottage in Yorkshire) and so my mum's smaller car will suffice.
I think that Dad is starting to accept that he won't be able to drive
any more.

Make sure it is fully charged before selling the car ;-)

[Ken]

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris. My dad
is no longer able to drive, and my mum is normally using her own car, so
dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the one
used for driving the traction motor during times when the petrol engine is
not going) has a habit of going flat if the car isn't run for a week or so.
This means she has to keep taking the car out for a drive, and to keep the
battery on mains charger in between times.
It could simply be that the battery is getting old and needs changing, but I
wondered whether there is anything inherent to a hybrid which means that the
battery gets drained by some constant current being drawn when the car is
turned off and locked. I would have thought that the only quiescent current
drawn would be things like the clock and the alarm.

I've had a Prius since new in 2010. I retired a while back and it
isn't unusual for it not to be driven for long periods of time. It
never gave any trouble until lockdown. I tried to remember to start
the car every couple of weeks and allow it to idle or take it for a
short drive, but often forgot. Then we were setting off for a trip
away. I'd started loading the car while my wife completed her
last-minute whatever women do that takes so long. I'd turned on the
car but not started it, so radio, fans and so on were powered up. By
the time my wife arrived it wouldn't start the engine.

A 3-second connection of jump leads to the terminals under the bonnet
fixed that.

Two weeks later we were in a Tesco car park. Again, I was waiting and
turned on the auxiliary power. It would have none of it when I tried
to start 15 mins later.

So, a few things. It's a good idea to start the car periodically to
allow the 12v battery to be topped up. It's a bad idea to half-start
the car (in my case to press the Start button without a foot on the
brake) and leave it for any time as the 12v battery is quite puny, not
being used to actually crank the engine. It was especially puny In my
case as it was over a decade old.

Finally, if your Toyota hybrid is anything like mine a flat 12v isn't
the end of the world as there are the terminals under the bonnet. You
can't jump start FROM the car, but attaching jump leads to it will
provide enough to start the systems, which is all you need. You can
disconnect very quickly, the traction battery and engine now charging
the 12v.

[NY]

Post by Ken
Finally, if your Toyota hybrid is anything like mine a flat 12v isn't
the end of the world as there are the terminals under the bonnet. You
can't jump start FROM the car, but attaching jump leads to it will
provide enough to start the systems, which is all you need. You can
disconnect very quickly, the traction battery and engine now charging
the 12v.

Ah, does it have terminals under the bonnet? Mum mentioned something
about the battery being in the boot (as in the old Rover 2000) and the
leads on their charger not being long enough to reach so the car has to
be parked with its front end near the door so the back end is near the
rear wall where the mains plug and hence the charger are.

I'll have a look when I go down in a couple of weeks.

If a "flat" 12V battery isn't really flat but can be rejuvenated by a
quick "flash" from another's car's battery, that is a help. I presume a
charger will have the same effect, even if it takes a bit longer. Or are
you saying the the donor battery is still needed to *start* the engine,
as opposed to just for getting the battery into a state where it will
start the engine without the donor?

The car has been a real liability even since they have had it, either
brand new in 2015 or recent second hand a year or so later (I forget
which). They have been locked out of the car (or triggered the alarm) on
numerous occasions because there hasn't been enough battery to operate
the central locking or to disarm the alarm. Initially they didn't know
about the little access "door" in the driver's door handle to allow the
blade of the key to be used to unlock the car the old-fashioned way.
Sometimes a break of a few minutes, stopping at a motorway service
station during a long journey, has been enough to cause the problem - so
the battery will definitely have been charged from the driving they have
done up to that point.

I'd have played merry hell with the garage that sold them the car if it
kept happening: "just f-ing sort it out".

[Theo]

Post by NY
The car has been a real liability even since they have had it, either
brand new in 2015 or recent second hand a year or so later (I forget
which). They have been locked out of the car (or triggered the alarm) on
numerous occasions because there hasn't been enough battery to operate
the central locking or to disarm the alarm. Initially they didn't know
about the little access "door" in the driver's door handle to allow the
blade of the key to be used to unlock the car the old-fashioned way.
Sometimes a break of a few minutes, stopping at a motorway service
station during a long journey, has been enough to cause the problem - so
the battery will definitely have been charged from the driving they have
done up to that point.
I'd have played merry hell with the garage that sold them the car if it
kept happening: "just f-ing sort it out".

I have a Gen2 Prius and a Yaris of the same vintage as yours - the Prius
keeps having battery issues due to lack of use (killed two batteries since
lockdown), while the Yaris 'just works' - despite very similar lack of use.

It sounds like your 12V battery is likely on the way out, because I have
very similar symptoms with the Prius. I've found a trick with that.
Normally the starting procedure is:

<start button once> - dash lights come on, main display boots
<foot on the brake>
<start button once> - car goes into Ready mode

If the battery is on the way out, it can't open the HVDC contactor on the
second press while also running the brake servo pump. The trick is to not
put your foot on the brake:

<start button once> - dash lights come on, main display boots
<start button once> - more warning lights come on, because you didn't put
your foot on the brake
<foot on the brake>
<start button once>

That seems to start reliably if the voltage is ok (> about 11.8v under no
load).

Once you've got it started, leave it turned on for a while to recharge the
12V battery, running the engine as necessary. On the Prius it's only a 4A
charger so it can take a while. Toyota recommend leaving it turned on for
an hour a week to keep the level up (I don't do this, I probably should).
Of course you'll have to leave the key in it, so don't do this where
somebody might steal it.

Post by NY
If a "flat" 12V battery isn't really flat but can be rejuvenated by a
quick "flash" from another's car's battery, that is a help. I presume a
charger will have the same effect, even if it takes a bit longer. Or are
you saying the the donor battery is still needed to *start* the engine, as
opposed to just for getting the battery into a state where it will start
the engine without the donor?

You need the 12V battery to supply about 20A to open the contactor to
connect the HVDC traction battery. If the voltage sags, it can't do that
and there's no traction power. Once the contactor is open, you have the
stored energy in the traction battery (in a Yaris it's only 144V 0.9kWh so not
very big, but enough). If the traction battery is low, it can start
the engine to recharge things.

All you need is enough current to get it over that 20A spike and everything
then works. I have a little lithium jumpstart pack which generally does the
trick if needed.

It is not advisable to run it without a 12V battery, and you should not use
it to jump another car (since the 12V battery is not designed for running a
starter motor).

I've not found the Toyota batteries very good - which is why they tend to
die. They're AGM but folks on PriusChat seem to have a lot of problems with
them. I've replaced the Prius one with a UPS battery - the 12V 55Ah Ritar
from ups-trader.co.uk, as recommended in this thread:
https://priuschat.com/threads/fitting-mobility-12volt-agm-battery.122920/

It doesn't quite have the CCA to do the standard 'two presses' start as
above, but it does work with the 'three presses' start. We'll see how long
it lasts. Since it's designed to be run flat in a UPS, I'm hopeful it's
better than the OEM battery.

I've never had to jump or change the battery in the Yaris, but I'll take a
look tomorrow in daylight and report back what the arrangement is.

Theo

[Theo]

Post by Theo
I have a Gen2 Prius and a Yaris of the same vintage as yours - the Prius
keeps having battery issues due to lack of use (killed two batteries since
lockdown), while the Yaris 'just works' - despite very similar lack of use.
It sounds like your 12V battery is likely on the way out, because I have
very similar symptoms with the Prius. I've found a trick with that.
<start button once> - dash lights come on, main display boots
<foot on the brake>
<start button once> - car goes into Ready mode
If the battery is on the way out, it can't open the HVDC contactor on the
second press while also running the brake servo pump. The trick is to not
<start button once> - dash lights come on, main display boots
<start button once> - more warning lights come on, because you didn't put
your foot on the brake
<foot on the brake>
<start button once>
That seems to start reliably if the voltage is ok (> about 11.8v under no
load).

I've confirmed that process works in the Yaris too. On the first push from
cold a pump runs (brake pressure or inverter coolant, I'm not sure) and I
expect that trying to start while that pump is running would put more
pressure on an ailing battery. The three-press version works, and the pump
isn't running after the second press.

Post by Theo
I've never had to jump or change the battery in the Yaris, but I'll take a
look tomorrow in daylight and report back what the arrangement is.

Here we go.
https://ibb.co/album/F41Qkb

This is the main fusebox under the bonnet:
https://ibb.co/dbmwWsL
The catches on mine are stuck and I didn't want to stress the plastic.
According to this pic:
https://www.ebay.co.uk/itm/362863872817
and comparing with:
https://fuse-box.info/toyota/toyota-yaris-hybrid-echo-hybrid-xp130-2012-2017-fuses
it looks like there aren't jumpstart terminals under there, which surprises
me.

However, getting access to the 12V battery isn't too hard. It, along with
the traction battery, is under the back seat. If you can open the rear door
you can do it fairly simply.

There's three pieces of plastic trim held in by circular trim clips: you
press the button in the middle of the clip to release it, and then it pops
out.

1. Remove 6 trim clips from under the back seat.
2. Remove the middle section of plastic trim
3. Remove the right hand side section
https://ibb.co/99Q2Lsf

4. You can now access the 12V battery:
https://ibb.co/Fbt626D
https://ibb.co/x6xLF1Z
https://ibb.co/NtfjZG0

The terminals are easily available to clip a charger to. You could probably
wedge the trim in place if you're going to need to be charging regularly.

I'm not sure if there is a release mechanism for the seat itself, but I
suspect the tubular frame is bolted down. I think it's possible to unscrew
the battery support and slide it out from the front, without needing to
remove the seat.

(the seat swab above feels loose so there might be a way to remove it from
above - I didn't explore that)

Also underneath the seat on the left side is the 144V traction battery:
https://ibb.co/qDz9ZDp
https://ibb.co/kgctw8G

The orange thing in the middle is the 'service plug grip' which is used to
isolate the HVDC system (technically it splits the battery into two halves
by disconnecting in the middle) which should be done if anyone needs to work
on the high voltage electrical systems (with appropriate HVDC PPE) .

Once the battery is changed, put the trim pieces back and pop in the trim
clips (you have to work them so the button pushes back out first).

I did this process of removing the trim in the time it took to demist the
car, so about 5 minutes.

Theo

[Mark Goodge]

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris. My dad
is no longer able to drive, and my mum is normally using her own car, so
dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the one
used for driving the traction motor during times when the petrol engine is
not going) has a habit of going flat if the car isn't run for a week or so.
This means she has to keep taking the car out for a drive, and to keep the
battery on mains charger in between times.
It could simply be that the battery is getting old and needs changing, but I
wondered whether there is anything inherent to a hybrid which means that the
battery gets drained by some constant current being drawn when the car is
turned off and locked. I would have thought that the only quiescent current
drawn would be things like the clock and the alarm.

No; in a hybrid, any constant current draw (if any, and there are very few
that will actually exist) is taken from the drive battery rather than the
starter battery unless and until the drive battery is completely flat. The
onboard computer will preserve the starter battery at all costs, because
it's the one thing that has to be capable of providing enough power even if
everything else is non-functional. So it does sound more as if the starter
battery is getting old and needs replacing.

Mark

[nib]

Post by Mark Goodge

Post by NY
My parents have a Toyota petrol hybrid car - I think it's an Auris. My
dad is no longer able to drive, and my mum is normally using her own
car, so dad's Toyota isn't being used as often as normal.
My mum mentioned that the Toyota's starter battery (as opposed to the
one used for driving the traction motor during times when the petrol
engine is not going) has a habit of going flat if the car isn't run for
a week or so. This means she has to keep taking the car out for a drive,
and to keep the battery on mains charger in between times.
It could simply be that the battery is getting old and needs changing,
but I wondered whether there is anything inherent to a hybrid which
means that the battery gets drained by some constant current being drawn
when the car is turned off and locked. I would have thought that the
only quiescent current drawn would be things like the clock and the
alarm.

No; in a hybrid, any constant current draw (if any, and there are very
few that will actually exist) is taken from the drive battery rather
than the starter battery unless and until the drive battery is
completely flat. The onboard computer will preserve the starter battery
at all costs, because it's the one thing that has to be capable of
providing enough power even if everything else is non-functional. So it
does sound more as if the starter battery is getting old and needs
replacing.
Mark

Which battery actually starts the engine? In ours, the motor that starts
the engine is a big AC motor/generator driven off the 400V traction
battery. The 12V auxiliary battery only has to have enough juice to power
the computers and operate the contactor that connect the 400V supply to
the inverters. Without that, the traction battery is isolated.

Ours also has the 12V auxiliary battery in the back, but there's a set of
convenient terminals under the bonnet that a 12V booster battery can be
jumped onto to get it started.

The 12V is charged through a DC-DC converter from the traction battery
while it is switched on.

nib

[Theo]

Post by Mark Goodge
No; in a hybrid, any constant current draw (if any, and there are very few
that will actually exist) is taken from the drive battery rather than the
starter battery unless and until the drive battery is completely flat. The
onboard computer will preserve the starter battery at all costs, because
it's the one thing that has to be capable of providing enough power even if
everything else is non-functional. So it does sound more as if the starter
battery is getting old and needs replacing.

I think you have that backwards. There's a 12V battery and a high voltage
DC traction battery. The 12V battery is used to run auxiliaries (lights,
radio etc), to boot the computers, and to open the contactor that protects
the traction battery. The traction battery is what's used to turn the
wheels (+/- engine, depending on the vehicle)

The 12V does not run a starter motor - the HVDC battery runs a motor to turn
the engine over when needed. But if you mean the 'starter' battery is used
to engage the 'drive' battery, then the starter is the 12V and that's the
one used by constant loads, and that's the one that goes flat through lack
of use. If the 12V battery doesn't have enough puff, the contactor doesn't
open and there's no traction power.

Theo

[Mark Goodge]

On 27 Nov 2022 19:49:30 +0000 (GMT), Theo

Post by Theo

Post by Mark Goodge
No; in a hybrid, any constant current draw (if any, and there are very few
that will actually exist) is taken from the drive battery rather than the
starter battery unless and until the drive battery is completely flat. The
onboard computer will preserve the starter battery at all costs, because
it's the one thing that has to be capable of providing enough power even if
everything else is non-functional. So it does sound more as if the starter
battery is getting old and needs replacing.

I think you have that backwards. There's a 12V battery and a high voltage
DC traction battery. The 12V battery is used to run auxiliaries (lights,
radio etc), to boot the computers, and to open the contactor that protects
the traction battery. The traction battery is what's used to turn the
wheels (+/- engine, depending on the vehicle)

If the 12v battery runs the auxiliaries, how come my car's calculated drive
battery range goes down when I turn them on? And how come the 12v battery
doesn't go flat when I'm running the car on drive battery alone for an
extended period, without starting the petrol engine?

It simply wouldn't make sense to run auxiliaries off the 12v battery in a
hybrid. In a pure ICE car, the auxiliaries are run off the alternator when
the engine is running, and only off the 12v battery when the engine isn't
running. The equivalent, in a hybrid, is to run them off the drive battery
when the car is in EV mode and off the generator when the engine is running.
But the difference is that, in a hybrid, the drive battery is still
available to the auxiliaries even when the car is stopped.

Post by Theo
The 12V does not run a starter motor - the HVDC battery runs a motor to turn
the engine over when needed.

But the car will start the petrol engine even if the drive battery is
completely flat. That power can only be coming from the 12v battery.

Mark

[nib]

Post by Mark Goodge
On 27 Nov 2022 19:49:30 +0000 (GMT), Theo

Post by Theo

Post by Mark Goodge
No; in a hybrid, any constant current draw (if any, and there are very
few that will actually exist) is taken from the drive battery rather
than the starter battery unless and until the drive battery is
completely flat. The onboard computer will preserve the starter
battery at all costs, because it's the one thing that has to be
capable of providing enough power even if everything else is
non-functional. So it does sound more as if the starter battery is
getting old and needs replacing.

I think you have that backwards. There's a 12V battery and a high
voltage DC traction battery. The 12V battery is used to run auxiliaries
(lights, radio etc), to boot the computers, and to open the contactor
that protects the traction battery. The traction battery is what's used
to turn the wheels (+/- engine, depending on the vehicle)

If the 12v battery runs the auxiliaries, how come my car's calculated
drive battery range goes down when I turn them on? And how come the 12v
battery doesn't go flat when I'm running the car on drive battery alone
for an extended period, without starting the petrol engine?
It simply wouldn't make sense to run auxiliaries off the 12v battery in
a hybrid. In a pure ICE car, the auxiliaries are run off the alternator
when the engine is running, and only off the 12v battery when the engine
isn't running. The equivalent, in a hybrid, is to run them off the drive
battery when the car is in EV mode and off the generator when the engine
is running.
But the difference is that, in a hybrid, the drive battery is still
available to the auxiliaries even when the car is stopped.

Post by Theo
The 12V does not run a starter motor - the HVDC battery runs a motor to
turn the engine over when needed.

But the car will start the petrol engine even if the drive battery is
completely flat. That power can only be coming from the 12v battery.
Mark

In both our PHEV and our BEV, _everything_ runs off the 12V auxiliary
battery except for the heating/air-con heat pump and the traction motor
(s).

The 12V is charged from the 400V battery while the car is "ON" (and
possibly also while on a charge point). So the accessories (lights,
heating fan, radio, computers) are powered from the 12V battery which is
also being charged, so in net terms effectively from the 400V battery via
the DC-DC converter.

The PHEV doesn't have a 12V alternator on the engine. The 12V is always
charged from the 400V via a DC-DC converter while the car is "ON",
regardless of what the engine is doing.

Of course the engine sometimes drives the big generator, which charges
the 400V battery, which in turn charges the 12V battery.

When the car is "OFF", the 400V traction battery is isolated. In that
state the 12V is not being charged, and so possibly can go flat.

The connection of the 400V battery when the car is switched "ON" requires
a contactor (relay) to operate. The contactor is powered from the 12V
battery. No 12V battery no go.

When the petrol engine is needed, it starts instantly using the AC motor/
generator backwards as a motor, powered from the 400V battery. It can't
do that unless the contactor is closed, which needs the 12V battery!

Your hybrid sounds unusual, if it can start the engine from the 12V with
a completely flat traction battery. For most hybrids it not viable to
start the engine with a 12V battery as it takes so long, the much bigger
AC motor/generators can start the engine and get it up to speed and
connected into the drive in a fraction of a second. In ours for example,
if you kick-down on the accelerator while in EV mode it will start the
engine immediately, it just feels like a slight lag.

nib

[Theo]

Post by nib

Post by Mark Goodge
On 27 Nov 2022 19:49:30 +0000 (GMT), Theo

Post by Theo

Post by Mark Goodge
No; in a hybrid, any constant current draw (if any, and there are very
few that will actually exist) is taken from the drive battery rather
than the starter battery unless and until the drive battery is
completely flat. The onboard computer will preserve the starter
battery at all costs, because it's the one thing that has to be
capable of providing enough power even if everything else is
non-functional. So it does sound more as if the starter battery is
getting old and needs replacing.

I think you have that backwards. There's a 12V battery and a high
voltage DC traction battery. The 12V battery is used to run auxiliaries
(lights, radio etc), to boot the computers, and to open the contactor
that protects the traction battery. The traction battery is what's used
to turn the wheels (+/- engine, depending on the vehicle)

If the 12v battery runs the auxiliaries, how come my car's calculated
drive battery range goes down when I turn them on? And how come the 12v
battery doesn't go flat when I'm running the car on drive battery alone
for an extended period, without starting the petrol engine?

As mentioned below, on the Toyota system the 12V battery is charged from the
traction battery via a DC-DC converter. If the 12V battery is full, there
won't be any current from the DC-DC converter. If the petrol engine isn't
started, loads draining the 12V battery will cause it to be topped up from
the traction battery via the DC-DC, but nothing is topping up the traction
battery so its level decreases.

Post by nib

Post by Mark Goodge
It simply wouldn't make sense to run auxiliaries off the 12v battery in
a hybrid. In a pure ICE car, the auxiliaries are run off the alternator
when the engine is running, and only off the 12v battery when the engine
isn't running. The equivalent, in a hybrid, is to run them off the drive
battery when the car is in EV mode and off the generator when the engine
is running.

The auxiliaries like lights are all 12V loads. The DC traction battery is
anything from 144V (in the OP's Yaris) to 400V (in some PHEVs). You can't
wire 400V to a 12V lightbulb, and doing so would make the car a deathtrap to
work on (the HVDC system is very tightly protected, for good reason). Trace
where the orange wires go - that's the HVDC. Anything else isn't.

Post by nib

Post by Mark Goodge
But the difference is that, in a hybrid, the drive battery is still
available to the auxiliaries even when the car is stopped.

Post by Theo
The 12V does not run a starter motor - the HVDC battery runs a motor to
turn the engine over when needed.

But the car will start the petrol engine even if the drive battery is
completely flat. That power can only be coming from the 12v battery.

The 'drive' (traction) battery can be 'flat' on the display, as in 1 bar,
but it's not flat. On Toyota's NiMH batteries 1 bar equates to about 30%
state of charge. In normal use they'll start the engine long before this.
The range of normal operation is typically about 40-70% SOC. On the plugin
versions and some newer models it's a lithium battery and that allows a
wider range of depth of discharge.

If you park somewhere and leave the car, the HVDC contactor will isolate the
traction battery. That means it's not exposed if the car suffers damage
(something crashes into it, it's in a house fire, etc). It also means the
charge in the traction battery will be preserved. If you come back months
later the alarm may have run the 12V down to zero, but you can charge it
back up, the contactor will open, and the traction battery will be good to
start the car again.

[snip]

Post by nib
The connection of the 400V battery when the car is switched "ON" requires
a contactor (relay) to operate. The contactor is powered from the 12V
battery. No 12V battery no go.
When the petrol engine is needed, it starts instantly using the AC motor/
generator backwards as a motor, powered from the 400V battery. It can't
do that unless the contactor is closed, which needs the 12V battery!

This is an accurate description of the Toyota system.

Mark, what car do you have? It is possible it does something different. In
particular 'mild hybrids' are really just regular cars with a 48V battery
and a starter motor capable of some regen, rather than the Toyota/Lexus
system with Motor-Generators 1 and 2 and a planetary transmission. Can you
drive your car on battery alone?

(I thought the mild hybrids were all-48V systems, although maybe there's a
secondary battery for 12V loads rather than powering them from a DC-DC
converter off the 48V battery?)

Theo