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post #1 of 11 (permalink) Old 12th November 2009 Thread Starter
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How does the Battery affect the MPG?

Hi all,

I had a limp mode incident a few weeks back (no warning lights) , where I also seemed to lose fuel efficiency... fearing the worst...(cracked inlet manifold / clogged egr ) I took it in for diagnostics....

turned out the battery had failed and was running the wrong voltage or something. Dealer thought this was the case (battery is 5years old) .. 233 quid later for the battery and diagnostics, all is good, and amazingly My MPG has definately been restored to pre- limp mode incident. (it dropped to at best 29mph, but now I can get back to 35mpg etc on the motorway).

Question - can battery really affect MPG or is it just a computer failing or something - seems very strange, but I am just gratefull it wasn't anything more serious!!
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post #2 of 11 (permalink) Old 12th November 2009
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Hi there.

I wouldn't have thought that the battery itself should impact on the general running or efficiency of the engine itself, as the engine should produce a surplus of charge to restore what has been taken away by the starting process, and whatever ancillaries are being operated.

The limp home mode, basically places an electronic ceiling over the amount of power the engine is capable of producing, just to prevent further damage - strange no warning lights or anything was found by the diagnostics though? Possibly a sensor picked up the battery's internal failing, and this triggered the ecu 's response?

5 years for a modern battery is pretty normal, and at this time of year, they do have a harder life - which usually sees off those which are borderline.

I'd keep an eye out for any re-occurence of the limp home scenario, but at least the battery should have a 3 year guarantee.



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post #3 of 11 (permalink) Old 12th November 2009
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Maybe in reality the mpg hasn't really altered at all.
It could be the onboard computer threw a wobbler due to a poor voltage supply. Plus a car battery tends to smooth out any voltage irregularities but if duff won't be able to do so.
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post #4 of 11 (permalink) Old 13th November 2009 Thread Starter
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yeah it seems most likely that it was just the computer playing up.... it did behave very strangely after the incident - but the bottom line it just wouldn't register anything over 29mpg - even if I started it while coasting on the motorway - now it does and had it showing 45mpg + again, so I feel much more reassured, as planning a long drive over xmas...

Interesting though, as I was convinced it was the inlet manifold - there seems to be threads with similar symptoms pointing to that, but it turned out to be just the battery! Thanks!
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post #5 of 11 (permalink) Old 13th November 2009
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Yup, it sounds like low voltage has upset the computer. In 'limp' mode, the ECU is not really making its best efforts to run the engine efficiently. It's just trying to keep it going without damaging it.

Theoretically, a weak battery could increase fuel consumption by increasing the load on the alternator. The alternator senses the voltage of the battery and varies its output accordingly. With a fully charged battery, it will practically free-wheel, but with one that regularly drains, the alternator will take a lot more effort to turn, which will sap engine power and use more fuel.

A 120 amp alternator running at maximum output will draw 1.44kW of mechanical energy from the engine. If Wikipedia is correct (and bear in mind that it occasionally isn't) then internal combustion engines are still roughly only 20% efficient in converting fuel energy into mechanical energy, so that's 7.2Kw of fuel energy going in to produce 1.44kW of electricity coming out. A litre of diesel contains 9.7kW of energy.

The upshot of this? A bad battery (or any other heavy electrical drain) could, in theory, cost you nearly a litre of extra fuel for every hour of driving. Though in reality, if you were pushing that much energy into a battery that wasn't storing it, I'd expect something to either melt or explode.
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