12 Volt: How much is used to start?

I don't believe the 12 volt battery is used at all. The ICE is started using one of the Motors which, of course, run from the HV battery. There is no separate "starter".

 

There's some experts on here that will hopefully come along and help you... The reading/measurement you are looking for to find out if your battery is faulty is how often the 12volt battery is being charged vs. how often the 12volt battery is not being charged.

If your battery is constantly being charged it can take a pretty good sized bite out of your MPG. What kind of MPG have you been getting lately?

 

Correct it just throws a relay and the big battery takes over. Now I think the small things like radio, interior lights ect. run off the 12v battery, and yes if 12v battery is getting low it will effect MPG.

 

You are asking the wrong question and the measurement is irrelevant. If you want to know how much is used, it's about 0.01-0.015 AHr. Insignificant as the battery is about 50 AHr when new. The measurement you are making is probably based on voltage, which is a totally inaccurate method to rate a battery because at the pulse currents the Prius uses, even a small resistive connection (dirty terminals etc) will cause enough voltage drop to swamp any indication the instrument may need to determine the state of the battery.

Connect your instrument -directly- to the battery terminals, not through the existing connection.

The car will draw a few amps due to various electronic systems when running, but that is supplied by the inverter once you are in "Ready", and when you go to "Ready" it draws about 50 Amps for about a second to get there. Hence my estimate of 0.01-0.015 AHr.

If you are wondering if the battery is good, use a 50 Amp "load tester" or a "pulse battery tester" and for both you need to connect directly to the battery on clean terminals.

If you are getting voltage drop it may be the battery terminals are oxidized and need cleaning or perhaps the ground lead is corroded at the body connection.

 

I made a math error in my post (now corrected). Forgot to also divide by a further 60, so my original post showed A-min not A-Hrs.
What I'm getting to is you want your measuring instrument to be isolated from the current path of the load on the battery. So for example, if the load (car cable) is on one side of the battery terminal put your instrument terminal on the other side. Just to ensure you don't measure the voltage drops in the current path.
But your battery is now about 3 years old and is probably short on water. If there is no way to add water (and there isn't with the new batteries as far as I know) then you probably need a new battery.

Further, a little tip. You can charge the battery from the "boost terminal" under the hood. No need to open the hatch. Pearl S is resting in the garage, hood open, Battery Tender connected by clips to the boost terminal and chassis ground. I recommend if doing this to rest the fuse box cover over the connection so you don't forget to put it back on (which is what I did a few months ago - luckily there isn't enough room under the hood for it to wander off!).

 

This is a misconception !!!I mean the "pretty good sized bite out of your MPG" ..

The battery is charged by less then 5A at around 14.6 V that is equivalent of just one low-beam light bulb. Of course anything using electricity comes from the fuel eventually.

 

Here is the clue !!!! This is really not good for your battery. So if you regularly run down your 12V battery it will "age" a lot faster and it quite possible having issues just after a year. Depending on temperature mostly the Prius should be OK for up to 2 weeks starting out with a fully charged battery.

If you can, you should probably use a battery tender or drop-charger or whatever it is called down there OR just leave the car on in P (if you can safely). That will impact the MPG a little ....

I drive the car every day (almost) and in cold winter days' morning it take over 1,5 hour to re-replenish the battery!! If you have as ScanGauge or other scanning tool like Tourqeu, you can tell if the car's computer thinks it is full by turning off all lights, fan, heating and A/C and the Voltage should go down to 13.6V which sometime referred as the floating voltage. When enough load is on it should be around 14.5-7V.

The manual also describe how to check for SOC by measuring voltage after a few hours of parked state. Just do not open driver side door. Precharging the break will interfere !!!

 

I agree with your logic in theory, but in practice there are many threads on here regarding people seeing a huge improvement in MPG when they replace their bad 12volt battery. So I suspect it's more about how the overall system responds when the battery never reaches a full charge and that 5a load at 14.6 V throws something else off in the overall functioning of the system that causes the engine-inverter system to operate at a less than optimum level. Not sure what it is but I bet one of the super experienced electronic geeks on here understands it...

 

Love to hear it !!! I have read this many times without actual explanation. Yes higher voltage do mean higher wasted energy but again ... the voltage is the same when you turn on even DRL or heating fan ... regardless of the 12V battery status. I have tested and observed this many times.

Now I think what others reporting as a MPG loss is that in ideal conditions spring and late summer you may see mpg loss!! When you do not need light for most of your driving and no need to heat/cool the car will run about 13.6V almost all the way of driving ...

But anyone with plausible explanation for the "good sized bite out of MPG" part let's hear it.

 

I don't know if this makes any sense at all but, if the 12v bat. was bad and only reading say 12 volts and would not take a charge. Then would not the electric system keep throwing max charge at the battery in an attempt to charge it. Causing a greater loss. Instead of the charge going into the big battery as additional EV miles , it keeps trying to charge a bad battery. No? Causing the ICE to run more of the time.

 

I recently did such an experiment for Prius Technical Stuff for a different reason. We were determining the load on the 12v system during different states while in READY. However, there were capture periods with the car off and then booting into READY which will answer your question. Here are my results:

I just did a startup sequence with the Prius and captured some oscope shots. I saw no bad behaviour on the 12v rail when I jammed the accelerator pedal to the ground (in park) and took 80A out of the HV battery in a spike as measured on the CAN bus. I jammed the accelerator to the floor, then let off, then to the floor again. My HV battery was pretty well charged however at was resting at 220VDC with 70%-ish SOC.

The full time capture:


This is the full time capture map:



Region A is when the car was off and I just had my probe hooked up. The battery is very aged and dying (6-7 years old) and reads 11.8v when idle. Region B is after pressing the brake pedal (Line 1) and the brake pump primes. The dip with the long RC time recovery is when my HID headlights kicked on and the ballast ramps down the power needed to keep the salts ignited. Region C is when the HV relays flicked in sequence. Then there is a long delay where the car is READY and the inverter is not supplying power yet which starts at Line 2. This dropped my 12v rail to 10.8v average for a period of almost 2 seconds. I believe this may be how Prii get "stuck" in quasi states or experience issues on startup and refusal to power down. My aux battery is clearly on the edge of functionality and way past the safe limits of use. Region D is when the car is READY and the inverter has kicked on (Line 3). The 12v rail spikes to 15.5v momentarily on the startup ramp, then settles and stays at 14v almost even.

A zoom in on the "interesting" parts:


Here is a zoom in on one of the many brake pump spikes:


Here is a zoom in on the big spike in C:


Here is a zoom in on the startup ramp of the inverter and the resulting overshoot:

 

I stuck a cheap analogue induction ammeter on SWMBO's Gen 2 12V battery (6 years old) and it spiked to around 20A on initial switch on then settled down to almost zero (large scale, 0-30A, difficult to read miniscule amounts) before the ICE started. It then went up to 10A when the ICE started but dropped back to around 5A after about a minute.

 

Thanks this is really cool information ... and absolutely explain some of what users experienced.

 

Great conversation... Great details... But what in those oscope pics explains why people get a 5mpg boost when they replace an almost dead battery? Is it possible to oscope a brand new battery to show where all the energy loss is occurring?

 

When a battery dies or is depleted, the internal resistance of the cell increases waaaaay beyond tolerance. It goes from milliohms to multiple ohms. Now when a cell is completely dead, you won't be able to put the sulpher from the lead plate back into the acid ("charging the battery") and it is wasted. It is also a big voltage drop. So 14v in and then 2v-3v drop immediately. So now your I2R losses are through the roof. You can be burning hundreds of watts of power doing nothing other than heating up the battery.

33KW/gal of gas, 25% efficiency, 8KW/gal usable, average 165W/mi. Now if you are burning 80W doing nothing all the time heating the battery up, you are at 245W/mi or a 245/165 or 148% increase. That would mean getting 50mpg before would lead to 33-34mpg with a very bad battery.

Yay physics, chemistry, and math...

 

Awesome... Thanks so much for helping myself and "bad battery means good sized bite out of MPG" denier named szgabor | PriusChat

 

The above math is a gross oversimplification, but it works about the same. It does not just drop over night though. The increase in internal resistance happens over time.