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Can a car battery affect mpg?

  • 15-12-2010 4:13pm
    #1
    kc66
    Registered Users Posts: 2,267 ✭✭✭


    Over the last 4/5 months I noticed the mpg in my car dropped from around 41/42 to 36/37. Theres been no change in driving habits or roads. At the same time the battery was always a bit dodgy. I replaced the battery a couple of weeks ago and the average mpg is back to around 41. Any way these are linked?


Comments

  • #2
    Seweryn
    Registered Users Posts: 4,487 ✭✭✭Seweryn


    kc66 wrote: »
    Over the last 4/5 months I noticed the mpg in my car dropped from around 41/42 to 36/37. Theres been no change in driving habits or roads. At the same time the battery was always a bit dodgy. I replaced the battery a couple of weeks ago and the average mpg is back to around 41. Any way these are linked?
    No... The mpg drop is due to colder weather - engine warms up longer than during milder weather conditions.


  • #3
    mickdw
    Registered Users Posts: 23,256 ✭✭✭✭mickdw


    I guess if the battery was so bad that it wasnt taking proper charge, the alternator would be working overtime. If the alternator was continually pulling power from the engine, it would effect MPG


  • #4
    martyc5674
    Registered Users Posts: 2,520 ✭✭✭martyc5674


    If you swapped out your batt you may have reset your ECU. Depending on how you drove after the reset can effect MPG/ performance/ power
    If you want it economical disconnect it for a few hours. Reconnect and drive it nice and handy

    Marty


  • #5
    martyc5674
    Registered Users Posts: 2,520 ✭✭✭martyc5674


    Mick is probably on the money tho!!


  • #6
    Pkiernan
    Registered Users Posts: 4,315 ✭✭✭Pkiernan


    A low battery being charged by an alternator would not lead to a 12% decrease in fuel economy!!

    It might be a symptom of other issues though.


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  • #7
    si_guru
    Registered Users Posts: 2,347 ✭✭✭si_guru


    What kind of car is it.... if it is a Land Rover ANTHING could be possible.


  • #8
    bmw535d
    Registered Users Posts: 1,160 ✭✭✭bmw535d


    surely an alternator going full tilt all the time would increase fuel consumption?op did you change anything else? air filter perhaps?


  • #9
    Pkiernan
    Registered Users Posts: 4,315 ✭✭✭Pkiernan


    bmw535d wrote: »
    surely an alternator going full tilt all the time would increase fuel consumption?

    Yes, but it would not cause such a massive decrease in fuel efficiency.


  • #10
    Nissan doctor
    Closed Accounts Posts: 8,223 ✭✭✭Nissan doctor


    The rotational mass of the alternator doesn't change regardless of the electrical drain being put on it. So it cannot effect fuel economy on its own. If the battery/charging system is in poor condition then the reduction in power supply to the likes of the coil/coil packs could cause a reduction in economy due to a weaker spark.

    However, I'm willing the bet the lower mpg is down to the cold weather and associated slower driving rather then some fault with the car.
    Cold weather also reduces tyre pressure, which effects fuel consumption.


  • #11
    bmw535d
    Registered Users Posts: 1,160 ✭✭✭bmw535d


    when you put a heavy electrical load on a car you can hear the revs decrease as the extra load in turn goes on the engine even with the idle control valve, there for an alternator on full load will use more fuel than an alternator with little load on it. why do you think modern cars charge the battery while braking?so as to not waste fuel with accelerating.


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  • #12
    Seweryn
    Registered Users Posts: 4,487 ✭✭✭Seweryn


    Nissan doctor wrote: »
    The rotational mass of the alternator doesn't change regardless of the electrical drain being put on it. So it cannot effect fuel economy on its own. If the battery/charging system is in poor condition then the reduction in power supply to the likes of the coil/coil packs could cause a reduction in economy due to a weaker spark.
    It actually would affect the mpg. I didn't read up the original post correctly. If the battery is really bad - as bad that you would have to jump start the car every time and if the battery consumes a lot of current to get charged, then the engine will burn more fuel to produce the extra energy.


  • #13
    kc66
    Registered Users Posts: 2,267 ✭✭✭kc66


    The car is a 1.8 2001 Volvo S40. I don't think it can be weather related as it started happening around August or September.
    The battery was not so bad it had to be jump started every time but if lights or radio were left on for 10 minutes while the engine was off it would probably have to be jump started. When the cold weather came a couple of weeks ago, if it hadn't been driven for a day it would need a jump start.


  • #14
    Flyer1
    Registered Users Posts: 664 ✭✭✭Flyer1


    Simple answer: No


  • #15
    bbam
    Closed Accounts Posts: 7,410 ✭✭✭bbam


    Nissan doctor wrote: »
    The rotational mass of the alternator doesn't change regardless of the electrical drain being put on it. So it cannot effect fuel economy on its own. If the battery/charging system is in poor condition then the reduction in power supply to the likes of the coil/coil packs could cause a reduction in economy due to a weaker spark.

    However, I'm willing the bet the lower mpg is down to the cold weather and associated slower driving rather then some fault with the car.
    Cold weather also reduces tyre pressure, which effects fuel consumption.

    I appreciate what you are saying about the rotational mass being constant as the alternator is contstantly being turned by the fan belt..
    However when the alternator is actively charging the EMF of the power being generated is what increases the resistance thus slightly loading the engine, I use the word slightly as it would be essentially neglegable to the overall fuel consumption.. And I wouldn't expect it to cause the drop in mpg seen by OP.


  • #16
    martyc5674
    Registered Users Posts: 2,520 ✭✭✭martyc5674


    Nissan doctor wrote: »
    The rotational mass of the alternator doesn't change regardless of the electrical drain being put on it. So it cannot effect fuel economy on its own. If the battery/charging system is in poor condition then the reduction in power supply to the likes of the coil/coil packs could cause a reduction in economy due to a weaker spark.

    However, I'm willing the bet the lower mpg is down to the cold weather and associated slower driving rather then some fault with the car.
    Cold weather also reduces tyre pressure, which effects fuel consumption.

    I disagree...the greater the electrical load on the altenator the more energy required to keep it turning, whether it would increase the fuel consumption by 12% is another question.

    Its the equivalent of saying that because the 4 wheels of the car are already rotating that the energy required to push it up a hill is the same as is required to push it along the flat.
    Marty.


  • #17
    mickdw
    Registered Users Posts: 23,256 ✭✭✭✭mickdw


    Nissan doctor wrote: »
    The rotational mass of the alternator doesn't change regardless of the electrical drain being put on it. So it cannot effect fuel economy on its own.

    The energy required to turn the alternator when its working to capacity compared to that required when its not under any load is much greater so it most certainly does have an effect on engine fuel efficiency. I wouldnt have thought it would have caused a 10% drop in MPG however given the details in this case, it very much looks like the battery condition did indeed result in additional work being put on alternator and therefore resulting mpg drop.
    If a battery could be charged in car without any fuel penalty, we would be into free energy territory


  • #18
    SouperComputer
    Registered Users Posts: 6,949 ✭✭✭SouperComputer


    The electronics on modern cars can behave in strange ways. Its not beyond the realms of possibility that a suboptimal battery would cause a loss of MPG. At a basic level, if for some reason the spark is not as strong as it needs to be, less of the charge drawn into the cylinders is burned and therefore the car makes less power for a given throttle opening. Cold weather for sure will have an impact MPG as well as a dud coolant temp sensor.

    There's no doubt that at higher electrical loads the energy used to produce the current demanded from the alternator has an effect on the load on the engine however modern engines are so quick to adjust idle its barely noticable. There's no change in rotational mass, however as the electical load increases so does the "magnetic load" on the alternator. For a good part, the mechanical resistance is greater than the "magnetic" but when electrial demand increase the "magnetic" load becomes resultant and affects the engine for sure.


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