12 Volt: How much is used to start?

Agreed... But until it was explained in detail it was hard to have the basis to make the argument that people who replace a really old battery are happy with the suprising MPG boost.

 

Interesting but something is wrong with the math and mpg drop ... if 80W extra would mean 50 mpg to 33-34 mpg then turning on just the low beam (totaling 110W) would also drop your mpg the same rate ...

Many of us can tell that running the lights has very very little effect certainly not dropping from 50 to 33 mpg. Not even full A/C drops that much which is certainly more then 80W.

It just doesn't happen ... also if you would put 80w pure heat loss into the battery (no chemical reaction anymore) that heat int that small place would almost certainly blow up the battery ...

So I do agree with the chemistry part extra load etc. And I am NOT denying that dying battery is not a load on the system (therefore some mpg hit). What I am saying is that it is NO way it would take it from 50 to 33.

Here is what my commute today was 1:15 and the car was unable to fully charge the battery (did not lower voltage) so it was charging it by the nominal 5Ax14.6V approx 73W. I can assure you my mpg was not 33-34 range car reported 52.1 for the 22.2 miles (I know from my record that about 5% error rate btw real and MFD which in this range means about 2.5-2.7 mpg less ) calculated mpg would be close to 50.

 

You made a fundamental error here... You used units of power, or watts - you should be using units of energy, or watt-hours. Specifically, a gallon of gas contains 33 kWh of energy, or 165 Wh/mile. To follow along with your example, assume that you're traveling at 25 mph on average; so in 1 hour, you use 165 Wh/mile * 25 miles = 4.125 kWh. Add to that one hour of 80W wasted in the battery (thus, 80 Wh), and you have 4.205 kWh, a difference of under 2 percent - and that's for fairly slow driving. Charging the battery is not going to waste enough energy to affect your fuel economy. The high beams are 60W apiece - if 80W was enough to drop your fuel economy that much, think how awful it would be when you were burning 120W to see on a dark country road!

My assumption on how the "bad battery kills mpg" theory works has always been that it was a computer problem; the low voltage causes trouble in some computer (like perhaps it loses or corrupts some data), which then manifests as lower fuel economy. That's not to say I buy in to the theory, however - I'm not convinced so far.

 

Thanks for the correction this seems a lot more plausible !!! I knew something was up with 2k1Toaster's math and what we could see ...

Tried the same argument many times that low-beam (nominal 55W) and two of them would beat the 80W and would have the same effect and that is clearly not the case. And anything more than 80W heat dissipation in the battery is just again not possible (even 80W for extended period of time within that small battery without cooling would be suspect).

Again I have no doubt that extra 80W load will have some effect but not likely anything like he is saying. 2% ??? not sure my long time average (over 3 years) was 51Mpg so 2% loss would be about 1Mpg ... that is hard to notice ... a week of rainy commute over a tank makes more of a difference than that.

But really like to have some plausible explanation for this myth. Your theory makes no sense (sorry) if the car boots up all and every computer is powered from traction battery. And even if you completely remove the battery (same as bad) car only losses a few things like radio settings and so on not mileage and stuff. Not sure what happens with the tripA/B data I think it is saved.

I am sure people who reporting mpg loss due to bad battery doing so from calculated numbers (hope). Tanks after tanks.

 

Completely correct.

I shouldn't do math before I'm about to pass out lol.

 

There are so many variables affecting mileage I don't see how anyone could blame a poor 12V battery.
But I'm still waiting for a good explanation.
The bad battery affecting some computer doesn't wash. When in "ready" the inverter is supplying and "regulating" the 12V power. The battery is just a giant filter on the system. A poor one at that, being how far it is from the rest of the computers. So if it's "bad" it's just a little poorer than normal as a filter.

 

I suspect after reading everyone's comments that a bad battery hurts MPG in the same way running your heater or A/C hurts MPG... And while I don't think the actual charge that a really old 12volt battery draws from the car is where the main loss of MPG is coming from, I do think that the sensor that monitors 12volt battery level is telling the car to run the engine/inverter system more often in the same way it does when you have the heater or A/C on. Or at least that's my best sense of what's happening.

 

I did not do this but used stuff like this before ... so

each vertical line on the scope on this image is 2s

each horizontal line the same way is 2V

And from the text it start around 12V at the very left side so you can see the changes I hope then on his other picture you can figure this from the top row where the scope shows the scale...

hope this helps

 

The bad 12V battery can impact the fuel efficiency when it is shortened. The 12V system is capable of producing 100A, which is more than 1 kw. If a 12V battery is shorted, the system will feed it with high current. The battery will still remain cold, since the current just passes it by.

 

As for a first post here, this is total nonsense !!!! Sorry this doesn't even pass K12 physics class.

 

Thanks for greetings. Please explain why you think this cannot happen.

 

Sorry it was harsh but where to begin Ohm's law ??? Yes the inverter (DC/DC) can provide 100A current.

But if the system pushes out 1KW something must consume that same amount (heat somewhere) if not the battery then what ?? You have to have both end of the equation !!! The First Law of Thermodynamics.

 

If you attach a very thick wire (low resistance) to a regular 12v transformer, where will be the heat?

 

The transformer will heat up and burn out. The energy cannot be created or destroyed. It has to go somewhere.

The problem with your idea is batteries don't short out as a perfect short. Instead, if they had an internal "short" they will absorb energy and heat up - a lot! To the point they will blow the release caps if the current is high enough.

The -second- problem is -most- Prius batteries fail due to loss of water (you can't add water to top them up). This results in what is basically an -open- circuit.

It is possible a bad battery somehow affects one or more computers in the Prius, causing poor mileage. I'm just waiting for an explanation of how this works.

 

GEEZ...

Your posts are getting better and better ....

In both the transformer and the wire will heat up no questions. I2R to be precise and if you are lucky something will melt down. And everything feeding the transformer ...

 

David is correct, the heat will be in transformer not the circuit. It may not burn out, if the power feed is limited to safe parameters. Same might happen in Prius, most of the heat will be produced in 12V transformer and absorbed by cooling system.

Unfortunately, Prius will start even if your battery is dead not allowing its owner detect that it needs to be replaced. This is exactly what the thread starter is experiencing - dead battery but the car still starts.

My battery had one cell shorted, 1L of water missing but still starting the car at -30C. Well, I would also say at -35C it wasn't doing it from the first time. And yes, you can open Prius battery, just remove the sticker, transparent plastic and there is a normal battery with cups.

I am not keen discussing how -most- of the prius batteries die. But I am sure some people may experience fuel consumption issues with shorted batteries. 50 -> 45 MPG is only half a liter difference that can can easily happen due to additional power consumption, at the same time very noticeable for a Prius owner. Even regular car owners with smaller engines admit the fact that bad battery leads to additional power consumption, especially during idling.

 

the SKS system is always on although it would go a "deeper" sleep after given time (do not remember 3 days?). With Gen2 you could actually turn it off ... not anymore.

However if the key(s) are too close it keeps that alive and that alone will drain your battery in about 2 weeks (less if the battery is already "old" or not fully charged. Not sure what is your situation we have one of the fob on the house key (little stupid) so if we just walk by the car it will notice and keeps it up. And if the car parked in a public garage my guess all other similar FOB will keep the car's security system alive.

I believe someone measured that security system about 10-15mA drain on the battery. Which is pretty low. That alone will not explain observed dead at airport.

I guess you can measure the drain at the fuse box, one by one but that is pretty cumbersome.

Let us know if you find something.

 

I'm on my third 12v battery in 6 years, and have never seen variance in my fuel economy. Those that do? Simple. The placebo effect. They *think* they are going to get better economy, and since they are thinking about it, they drive a bit more cautiously, so they *do* get better economy, but it has nothing to do with the 12v battery.

To answer the original question, the amount of power it takes to startup is almost nothing. I've been able to start up just fine with a battery that can't hold up voltage above 10v ( dead ) just running the car in accessory mode for 30 seconds ( without headlights or anything ).

There is a simple way to check the battery: power the car on in accessory mode ( no break pedal ), and put it into service mode by holding down the info button and flicking the headlights 3 times. There's a voltage readout in service mode. A healthy battery should be able to run the headlights for a few minutes and still maintain at least 11-11.5v and climb back to 12v when you turn the headlights off. My first battery change I noticed early with this and almost certainly saved myself a roadside assist since I was about to go on a road trip where the temperatures were lower.

I wasn't so lucky the last time, and it turns out, you can't even pop the trunk to get at the battery when it's totally dead, nor manually disengage the parking break to push the car out of the garage so you can get jumper cables on it. Quite annoying. I ended up having to climb over the rear seat and remove the panels and manually actuate the latch, then replace the battery.