kc66 Registered User
#1

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?

Seweryn Registered User
#2

kc66 said:
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.

mickdw Registered User
#3

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

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martyc5674 Registered User
#4

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

martyc5674 Registered User
#5

Mick is probably on the money tho!!

Pkiernan Registered User
#6

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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si_guru Registered User
#7

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

bmw535d Registered User
#8

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

Pkiernan Registered User
#9

bmw535d said:
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

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.

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bmw535d Registered User
#11

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.

Seweryn Registered User
#12

Nissan doctor said:
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.

kc66 Registered User
#13

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.

Flyer1 Registered User
#14

Simple answer: No

#15

Nissan doctor said:
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.

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