What is the logic between 12v battery and lower MPG?

A weak 12v battery constantly pulls excess charging current from the 12v bus. This energy has to be supplied by the engine via the HV battery/inverter and therefore reduces your MPG. This is a small factor unless the 12v battery has a shorted cell and then it can be quite noticeable.

JeffD

 

It is not so much a case of low voltage when the car is in ready mode and charging the 12 volt battery. The problems mainly show themselves at initial turn on.
First when you open the car door to get in this can activate the brake accumulator hydraulic pump for about 10 seconds. This draws about 10 amps and rises steadily as pressure builds, and the motor load goes up. This current draw can lower the voltage of a week battery.
On getting in to the car you put your foot on the brake turning on the brake lights "about a 3 amp load" then press the start button starting the many computers in there boot up process. It is at this time the problems can appear especially if you have left some things running when you turned the car off "such as the heater fan". The MFD and various computers draw around 20 amps and check all the systems mostly by voltage comparisons.
If signal voltages are returned low "out of specification" fault codes can be issued. All these checks are done before the computers pull in the HV DC relays to to enable ready mode and the battery charging.

John (Britprius)

 

That is 27 miles/US gallon which is really bad, unless your driving pattern is to drive 2 miles at a time, in sub-freezing weather.

11.5V in ACC-ON mode is not very good. Put your battery on a charger overnight now.

However I doubt that is the cause of such poor fuel economy.

 

Yes provided that any false codes issued do not prohibit the running of the systems. These false codes can be generated because of lower than expected return signals to the ECU's and possibly because the ECU's are running at a voltage below there operational thresholds.

John (Britprius)

 

One of the most common low 12 volt battery symptoms is a message on the MFD saying there is a problem with the parking pawl and to drive to level ground. "Words to that effect".
I suspect this is because the signal that shows the parking pawl is in it's correct position stopping the car from moving is to low to register. The car then thinks it is not in park so refuses to go to ready and start the engine. If the car was not in park it would move as the engine started. This can be felt if the parking brake is not on and the engine starts in park. A slight forward movement can be felt as the engine starts.

John (Britrius)

 

a bad 12 volt battery is just one, of a myriad of problems that can lead to lower mpg's. that and low tyre pressure are simply low hanging fruit that are easy to check before moving on to more difficult things like throttle bodies and etc. that's why we have the 'why am i getting low mpg?' sticky.

 

The "12V system" is a name for the electrical system of most automobiles which maintains a voltage up to 14.4V in most cases. When the car is running (ready-mode in the Prius) a voltage regulator maintains the voltage at around 14.2-14.4. When the car is not running, the battery takes over to run accessories, but over time, as all batteries do, the voltage gets lower and lower over time. When the car is running again, it charges the battery back up to it's optimal voltage of 14.2-14.4. Correct me if I have the exact numbers wrong, but that's the general idea.

It's similar to the idea of toilet tank tank. The water supply keeps the tank topped up with a float-valve, and when it's used, the level drops until the water supply brings it back up to the proper level again.

Perhaps a better analogy for trickle-charging is an air-tank on a truck. If there is a slow leak, causing the air pressure to drop over time, hooking up an air-supply line with a small "trickle" amount of air to fill the tank may seem like a solution, but if you leave it too long, the tank will become over-pressured and may be damaged. Much better to feed it a regulated supply, which tests the pressure and only tops it up as needed.

A battery-maintainer or an automatic charger are a regulated supply of electricity.

 

it's never been documented. anecdotal evidence says that some people have seen large gains after replacing their battery. it could be because those particular batteries were defective in some way, and causing a large drain on the system, which the hybrid battery has to replenish, and the engine has to run more to top up the hybrid battery. but it's all conjecture, and i know zero about batteries.

 

do you have a feel for average mpg's this time of year in the past? have you owned the car since new?

 

If your 12 volt battery has a shorted cell, and the rest of the cells are ok the charging current can easily be over 50 amps. At 14 volts "with a shorted cell the charging voltage would be higher than this" 50 amps charging current would equate to 700 watts. That is enough to drive in pure EV mode for 2.5 miles.
However to replace that 700 watts by using the engine to drive the generator. You have the losses when you burn fuel in the engine, the transmission losses, the losses in the generator, losses in converting the AC from the generator to DC, losses in charging the HV battery, losses converting the HV battery voltage to 14 volts to charge the 12 volt battery, and losses charging the 12 volt battery. It all adds up meaning you have to use a lot more than 2.5 miles of fuel to replace the lost 700 watts.

John (Britprius)

 

Typically if the battery measures around 11 volts after being charged then stood for at leased 12 hours. Fully charge battery cells are around 2.1 to 2.2 volts depending on the type of battery. So 6 cell battery should be 12.6 to 13.2 volts. If you have a shorted cell you only have 5 cells so the voltage will be down by 2.1 to 2.2 volts.
There are other reasons why the voltage could be down such as all the cells being sulphated badly so that the battery has very little capacity.
It is shorted cells that cause large charging currents and thus large losses though. This is usually manifested in the battery getting hot while on charge and excessive gassing. Both these can easily be missed in the Prius because the battery vents under the car, and is tucked away where it cannot easily be felt for temperature. These effects only occur at high charging currents such as when the car is in ready mode. Not when your using a battery charger with a low output current.

John (Britprius)

 

Sounds like the alignment could be part of the problem. Do you have a printout of the alignment? Maybe some numbers are at the very limits of the specification range.

 

[/QUOTE]
Thanks John for the reply. So basically the computer does its checks on the low battery only (no help from the HV battery) and saves the results for the duration of the trip?[/QUOTE]

There's very little checks done on the 12 volt battery. What he said was all the various ECu's check all the systems they control. The system does not complain much if it has a bad 12 battery and it presents a higher than Normal load on the inverter dc-dc .
In fact the system gets quite confused if the battery gets a little low and throws a few nags like park on a level surface.

One nag that is battery related that it does throw is the red Master Caution Triangle only momentarily upon boot up. It will flash for one second at somewhere around 11.2.

Bottom line is if its the original battery or it has seen 11.5 its a bad battery and a charge will only mask a down the road failure and expose your car to the dreaded jump start. That's why the 12 volt in this car is a big deal. It can cost you thousands of dollars.

At night before u make ready the car turn on the headlights and then ready the car. If the headlight dim even slightly the 12 volt battery is done.

 

Or take the battery to a battery seller and get them to load test the battery. If I live in the North and I have even a doubt, I test and replace if even marginal. One 'stopped in an awkward place' costs far more in cash and compromised safety than a new battery of the best pedigree.