What Does 12V Battery Do ?

[digger11]

Why do you need a 12v "starter" battery in a Lexus Hybrid ?

Can't you just drive off on the Battery Pack and then the petrol engine starts like usual ?

[lemon]

It powers up the computers first, which are needed to regulate the power coming from the traction battery. It also runs all the things that normally use 12 volts in a regular car, like the computers just mentioned, the stereo, the power windows, power locks, power seats etc. etc., since the traction battery is 288 volts and if it was used to run everything, the 288 volts would have to be stepped down to 12 volts.

[jaswood]

In truth you could probably get by with a small AAA battery pak(9) and a capacitor or 2. Once the HV battery SOC is at a decent level and/or the ICE is running, the 12 volt battery isn't really required, a simple 12 volt POWER zener diode and a few capacitors would suffice.

[lemon]

In truth you could probably get by with a small AAA battery pak(9) and a capacitor or 2. Once the HV battery SOC is at a decent level and/or the ICE is running, the 12 volt battery isn't really required, a simple 12 volt POWER zener diode and a few capacitors would suffice.

Except the 12 volt battery is powering the stereo and all the other things that run on 12 volts - headlights too. Would a diode and a few capacitors be able to keep the headlights on?

[digger11]

It powers up the computers first, which are needed to regulate the power coming from the traction battery. It also runs all the things that normally use 12 volts in a regular car, like the computers just mentioned, the stereo, the power windows, power locks, power seats etc. etc., since the traction battery is 288 volts and if it was used to run everything, the 288 volts would have to be stepped down to 12 volts.

ok thanks.

[jaswood]

In truth you could probably get by with a small AAA battery pak(9) and a capacitor or 2. Once the HV battery SOC is at a decent level and/or the ICE is running, the 12 volt battery isn't really required, a simple 12 volt POWER zener diode and a few capacitors would suffice.

Except the 12 volt battery is powering the stereo and all the other things that run on 12 volts - headlights too.

Would a diode and a few capacitors be able to keep the headlights on?

The ideal design for the 12 volt battery and 12 volt battery charging system will be that NO POWER is drawn from the 12 volt battery during operation of the HSD system. The design would be such that once the 12 volt battery SOC is depleted during the starting process it will be fully recharged ASAP and that full charge will be maintained throughout the full period of HSD operation.

As you know, should the HV battery SOC become too low due to the "bleed" into the 12 volt supply load then the system will simply restart the ICE in order to restore the HV battery SOC.

There might be a slight voltage "top-off", different voltage charge maintainance level, 13.2 volts vs 13.5 volts, when the ICE isn't operating vs not. But never enough difference in SOC to take a chance on not having enough charge for the next "start" even after a decently long period of storage.

This is unlike a more standard battery/alternator charge system wherein due to engine RPM and extreme load levels the alternator may not have enough "juice" for periods of time. Even so, by system design, the 12 volt battery will NOT be, CAN NOT be, on the average, the source of power supplied to the 12 volt system loads.

[lemon]

I'm pretty sure I read in my service manual that the 12 volt battery is used to power all the 12 volt accessories in the vehicle. I can check when I get home. It is recharged as necessary via the traction battery (and some sort of inverter/converter I think - to manage the 288 volts of the traction battery). Perhaps this is why the battery is so prone to failure - the amount of discharge/charge cycles?

[jaswood]

I'm pretty sure I read in my service manual that the 12 volt battery is used to power all the 12 volt accessories in the vehicle. I can check when I get home. It is recharged as necessary via the traction battery (and some sort of inverter/converter I think - to manage the 288 volts of the traction battery). Perhaps this is why the battery is so prone to failure - the amount of discharge/charge cycles?

NO, the 12 volt battery SYSTEM is used to power the 12 volt accessories. Once you "energize" the HSD system the HV battery will be used to provide ALL loads requiring a 12 volt source.

The only real reason, cause, for the 12 volt battery is to sustain all of the ECU "learned" parameters, the clock, and the radio settings during periods of non-HSD operation.

Absent the above volatile R/W memory "keep-alive" voltage requirement, months long requirement, there would be no need for a 12 volt battery at all.

[digger11]

So why doesn't it just "start" via the inverted down 288V pack ? This never goes flat.

[jaswood]

So why doesn't it just "start" via the inverted down 288V pack ? This never goes flat.

But it would go "flat" were you to set and listen to the radio with the HSD not "started". This way no one cares if you fully deplete the 12 volt battery.

[digger11]

So why doesn't it just "start" via the inverted down 288V pack ? This never goes flat.

But it would go "flat" were you to set and listen to the radio with the HSD not "started". This way no one cares if you fully deplete the 12 volt battery.

Would it though ? It is a very big battery pack. 288v converted down to 12v will only ever draw nominal current.

[lemon]

If the computers are needed to regulate and control the THS (Toyota Hybrid System) and the traction battery, wouldn't it be dangerous to power those same computers using the traction battery. What if there were some sort of problem with the battery pack? If the computers were powered by it, and not the 12 volt battery, one could be looking at serious trouble if there were some sort of problem with the traction battery (for example, if it was putting too much power out, it could damage the very computers that are supposed to be controlling it). With these computers powered by the 12 volt battery, they can power up first, check the status of the THS and the traction battery etc. etc. before activating the hybrid system.

[jaswood]

So why doesn't it just "start" via the inverted down 288V pack ? This never goes flat.

But it would go "flat" were you to set and listen to the radio with the HSD not "started". This way no one cares if you fully deplete the 12 volt battery.

Would it though ? It is a very big battery pack. 288v converted down to 12v will only ever draw nominal current.

I think I can state with great confidence that without the HSD system "started" the 12 volt battery will be isolated from the HV battery. Sit and play the radio all you wish all you will discharge will be the 12 volt battery.

[jaswood]

If the computers are needed to regulate and control the THS (Toyota Hybrid System) and the traction battery, wouldn't it be dangerous to power those same computers using the traction battery. What if there were some sort of problem with the battery pack? If the computers were powered by it, and not the 12 volt battery, one could be looking at serious trouble if there were some sort of problem with the traction battery (for example, if it was putting too much power out, it could damage the very computers that are supposed to be controlling it). With these computers powered by the 12 volt battery, they can power up first, check the status of the THS and the traction battery etc. etc. before activating the hybrid system.

The reality of the matter is that once the HSD system is "started" the HV battery charging and brake regeneration system is the primary source of 12 volt current flow. Just as the alternator is in a regular car.

The DC-DC down converter, HV down to 12 volts, provides the isolation to protect the various components that require only 12 volts.

If that failed somehow.......

[jaswood]

Why do you need a 12v "starter" battery in a Lexus Hybrid ?

Can't you just drive off on the Battery Pack and then the petrol engine starts like usual ?

Other than providing power for the door locks (unlock) various lighting aspects, power for the hazard flashers in case the HSD system fails, the only reason I can think of is listening to the radio while watching the submarine races.

[lemon]

Why do you need a 12v "starter" battery in a Lexus Hybrid ?

Can't you just drive off on the Battery Pack and then the petrol engine starts like usual ?

Other than providing power for the door locks (unlock) various lighting aspects, power for the hazard flashers in case the HSD system fails, the only reason I can think of is listening to the radio while watching the submarine races.

And, it also powers up all the computers prior to starting the hybrid system? Pretty sure I've read that in my service manual somewhere.

I also wanted to post something I found in the manual for your opinion Jaswood, but I'm not at home. Could just be bad wording, but I read that all the 12 volt accessories were powered by the battery, which was in turn recharged via the hybrid system. The wording makes it seem that the battery is powering everything and is being kept topped up by the traction battery. I'll post the exact phrase when I get home.

[jaswood]

Why do you need a 12v "starter" battery in a Lexus Hybrid ?

Can't you just drive off on the Battery Pack and then the petrol engine starts like usual ?

Other than providing power for the door locks (unlock) various lighting aspects, power for the hazard flashers in case the HSD system fails, the only reason I can think of is listening to the radio while watching the submarine races.

And, it also powers up all the computers prior to starting the hybrid system? Pretty sure I've read that in my service manual somewhere.

I also wanted to post something I found in the manual for your opinion Jaswood, but I'm not at home. Could just be bad wording, but I read that all the 12 volt accessories were powered by the battery, which was in turn recharged via the hybrid system. The wording makes it seem that the battery is powering everything and is being kept topped up by the traction battery. I'll post the exact phrase when I get home.

There is a SOLID heavy guage wire connected between the positive battery post, the DC-DC down-converter output terminal, and the power distribution fuse "box". If the 12 volt current flow requirement is greater than the down-converter can supply at any given moment then the battery SOC is drawn down.

If, on the AVERAGE, the down-converter cannot supply an output current slightly above the level required then the 12 volt battery will end up with a low SOC.

Think or a water pump, a pressure tank accummulater, and an open faucet, all connect in parallel with a large pipe. The water pump is sized such that it cannot supply enough flow if the faucet is fully open. But since the faucet is almost NEVER fully open the pump has times when the accummulator can be recharged.

[lemon]

OK..here is the section I mentioned from the service manual.

DC/DC converter. The power source for auxiliary equipment of the vehicle such as the lights, audio system and the air conditioning system (except A/C compressor), as well as the ECUs, is based on a DC 12 V system. Because the THS-II generator outputs at a nominal voltage of DC 288 V, the converter is used to transform the voltage from DC 288 V to DC 12 V in order to recharge the auxiliary battery. The converter is installed on the underside of the inverter.

So the wording confuses me....does the converter transfrom 288v to 12v and everything 12 volt based runs off the converter, or does everything run off the 12 volt battery, and the converter is used to recharge the 12 volt battery? From reading that section of the manual, I'm thinking everything runs off the 12 volt battery and the converter is used to charge the battery.

When the THS system is off - READY light not on - I can run the stereo, the lights, the power door locks, windows, sunroof, door, the A/C (inside portion, not compressor) - all off the 12 volt battery. When the THS system is on - READY light on - am I still running all that stuff off the 12 volt battery or does it somehow switch to power from the converter?

[jaswood]

Since the output of the DC-DC downconverter is connected directly to the battery positive post that is no real way to tell from moment to moment. Quite obviously if, at ANY TIME, the 12 volt current load is greater than the downconverter output capability then the battery will be used to make up the difference.

But again the downconverter MUST be the primary overall average source of 12 volt power else the SOC of the 12 volt battery would deplete.

[jaswood]

OK..here is the section I mentioned from the service manual.

DC/DC converter. The power source for auxiliary equipment of the vehicle such as the lights, audio system and the air conditioning system (except A/C compressor), as well as the ECUs, is based on a DC 12 V system. Because the THS-II generator outputs at a nominal voltage of DC 288 V, the converter is used to transform the voltage from DC 288 V to DC 12 V in order to recharge the auxiliary battery. The converter is installed on the underside of the inverter.

So the wording confuses me....does the converter transfrom 288v to 12v and everything 12 volt based runs off the converter, or does everything run off the 12 volt battery, and the converter is used to recharge the 12 volt battery? From reading that section of the manual, I'm thinking everything runs off the 12 volt battery and the converter is used to charge the battery.

When the THS system is off - READY light not on - I can run the stereo, the lights, the power door locks, windows, sunroof, door, the A/C (inside portion, not compressor) - all off the 12 volt battery. When the THS system is on - READY light on - am I still running all that stuff off the 12 volt battery or does it somehow switch to power from the converter?

The design should be such that you CAN NOT deplete the SOC of the HV with the HSD switched off, I would be extremely surprised if it were found to be otherwise. So no, the downconverter MUST be disabled with the HSD system switched off.

On the other hand the downconverter MUST be enabled whenever the HSD system is "live".

[Bernie Legacy Prius owner]

In a Toyota Hybrid the 12 volt battery keeps all the electrical
system running while your car is parked. This includes the security
system, the key fob sensors, the clock, and the memory in many of the
computer systems. This is a fairly small drain but it builds up 24
hours a day 7 days a week. It also has to have enough power to start
up the computer and everything else that was left on before the high
voltage battery system is turn on. Once the high voltage battery
system is turned on a high voltage to 12 volt converter
turns on that produces about 14 volts. At this voltage the sealed 12
volt battery charges and will not over charge but it is a slower
charge rate than you might be used to.

The reason the high voltage battery is not used for this load when
the car is park is that it could easily drain the high voltage
battery enough to shorten its life. So if you leave you car parked to
long the 12 volt battery gets sacrificed and it is fair cheaper to
replace.

Since we are talking about replacement, lead acid batteries come
with a chemistry configured to produce lots of cranking amps (not
needed in a hybrid because the high voltage battery pack cranks the
engine) and deep discharge batteries that produce fewer amps but can
be draw down many more times before they fail. The deep discharge
type is the better choice in a hybrid because that is all it does. It
gets slowly discharged when the car is parked.

optima-smilling-short.bmp

[pj8708]

In a Toyota Hybrid the 12 volt battery keeps all the electrical

system running while your car is parked. This includes the security

system, the key fob sensors, the clock, and the memory in many of the

computer systems. This is a fairly small drain but it builds up 24

hours a day 7 days a week. It also has to have enough power to start

up the computer and everything else that was left on before the high

voltage battery system is turn on. Once the high voltage battery

system is turned on a high voltage to 12 volt converter

turns on that produces about 14 volts. At this voltage the sealed 12

volt battery charges and will not over charge but it is a slower

charge rate than you might be used to.

The reason the high voltage battery is not used for this load when

the car is park is that it could easily drain the high voltage

battery enough to shorten its life. So if you leave you car parked to

long the 12 volt battery gets sacrificed and it is fair cheaper to

replace.

Since we are talking about replacement, lead acid batteries come

with a chemistry configured to produce lots of cranking amps (not

needed in a hybrid because the high voltage battery pack cranks the

engine) and deep discharge batteries that produce fewer amps but can

be draw down many more times before they fail. The deep discharge

type is the better choice in a hybrid because that is all it does. It

gets slowly discharged when the car is parked.

Bernard, Thanks for the excellent information. However, the LOC guidlines do not allow any promotion of sales for products, links to an outside site or source, or mention/advertisment of an unautorized product/service. Accordingly I will delete the last portion of you thread. Thanks again for posting the information.