how does the Prius drain the traction battery in park?

It can charge in park (in particular when you first power it up).

In park, you can rev the engine - it could charge but the computer usually just revs the engine freely. This is partly due to software that can't (yet) be changed.

In drive, the engine and MG1 and MG2 are connected and no mechanical locks are in place anywhere in the HSD. If you put your foot on the brake and accelerate, all the engine goes 'to the wheels', but they don't move, so the only place it goes is into the MG1 and MG2, charging the battery (warning - it can charge very fast and may not be good at all for your battery doing this too much).

Regarding the current draw based on engine speed, I am not completely sure. However, what I have found is that, if doing pulse and glide, when 'gliding' with no current draw to the motors, I use about 1HP from the battery, but if I put it into neutral, I use about 0.3HP. So, the most efficient pulse and glide is to go into neutral during the coasting section (probably not the most legal, but anyway).

I think part of it is that, when in drive, the car uses more energy to have the engine 'ready' to be used. I don't know, but maybe it pumps the coolant faster, or powers up more parts of the engine computer or something. I neutral while rolling it always uses less power.

When revving the engine in park, I assume that, the faster the engine spins, the more electrical work all the electrical systems have to do (including water pump if nothing else), so they consume more power.

 

I think things like water pumps, the inverter, ac or heat still run when the car is in park.

 

Yes but.......having the engine run (and rev up) when there really is no obvious need for it to, can some times be a sign of your HV battery going bad.

 

Depending on the model year all 4 of those are electric (on the PRIME). The Gen 4 still uses ICE heat.

 

There's no easy way to tell by the sound whether the engine is spinning MG1 or MG1 is spinning the engine ... as shown by the number of threads where somebody whose engine isn't starting will post "my engine starts and runs fine for n seconds and then stalls".

Not to mention the sort of in-between times where the engine is just idling, MG1 is freewheeling with no appreciable power being claimed or supplied, and the battery is just discharging at the rate of power being consumed by other things.

 

Correct me if I am wrong , but doesn't the ICE spin as a air pump when SOC is too high. Seem to remember reading somewhere that by cutting fuel and spark, MG1 could turn over the ICE to burn off excess energy from the battery. Scangauge would show engine RPM revving but without fuel and spark it would drain the battery instead of charging it.

 

That happens in Brake mode.

 

The HV battery could be going bad.......in effect, self discharging, making the system think it needs ADDITIONAL charge and thus reving the engine to accomplish that.
Many, many owners have reported similar behavior as their HV battery gets weak.

AND.....anytime something "unusual" starts happening, it is a good idea to check the 12 V battery and it's connections too.

 

There's no need to overthink or worry about a problem with the car ...

If you tell the engine to rev in P, it will rev, because you're telling it to.

If the battery needs charging, it will direct some current into the battery. If it doesn't, it won't. If it's above the charge level target, it will use up charge.

All in a day's work if you're a Prius.

 

OR that something is failing (HV battery) and causing an unusual result and symptom.......that wasn't really a part of the "engineering".

 

"Increase the rate of discharge" suggests the car was already discharging rather than charging, probably because it judged discharging to be the thing to do at that moment. You asking it to spin the engine faster gave it a convenient excuse to increase the rate of discharge.

The car does also charge, when charging is called for, right? If that never happened, it would have no charge at all by now, no?

If you wait for a moment when it is idling in P and charging, and you push the go pedal, does the rate of charging increase? (At least until the car decides no more charging is needed?)

 

Maybe "Drain the Battery" is the wrong terminology?
I believe HSD, will drain the battery ( use battery power ) or charge the battery, based on a lot of factors that it recognizes as it is being used. Therefore, I think it correct to say, being in Park, will place demand on the Hybrid Battery and/or Charge the Battery, as the system see's fit. But the system is active in Park.

My understanding, and correct me Prius Chat collective, if I am wrong, is the only time HSD is incapable or doesn't charge the battery is when the vehicle is in neutral.
Which is why you have the occasional story of a Prius going through a ride thru car wash, and getting the low battery warning.