jrkitching said:

The rate of self-discharge increases as the battery ages, so with an old battery, the alternator has to work that much harder each morning to put back what the battery has lost overnight.

Any extra load on the alternator has to be paid for in fuel.

Click to expand...

That's obviously true, but I would be amazed if a battery made a 14 mpg difference in consumption - over 20% - because it needs a bit more work from the alternator first thing every morning.
If that were so, then working the alternator at max on a cold, damp night with lighting/aircon/heating on would crucify consumption on any car.
I have periods where I drive nights, and I have not seen huge differences in consumption from normal daytime averages.

Be interested to hear what others have found?

It just occurred to me that if SS won't work at 12.5 volts, I gathered this because SS does work after a few miles run and I have yet to decide at what voltage SS stops working or starts working because to be honest SS isn't important to me. Then maybe the onboard computer gets dizzy and doesn't do it's arithmetic.

There is no other explanation for this change in fuel-ecomomy read-out than the effect of a new battery.

Sweetsixteen said:

That's obviously true, but I would be amazed if a battery made a 14 mpg difference in consumption - over 20% - because it needs a bit more work from the alternator first thing every morning.
If that were so, then working the alternator at max on a cold, damp night with lighting/aircon/heating on would crucify consumption on any car.
I have periods where I drive nights, and I have not seen huge differences in consumption from normal daytime averages.

Be interested to hear what others have found?

Click to expand...

The difference might be more than you'd think, especially if the battery voltage never gets high enough to reach the charging cutoff. The extra energy used basically gets dissipated by the battery as heat - in extreme cases, very old batteries have been known to boil or even explode.

It's also why many car manufacturers disconnect the alternator when doing official fuel consumption testing.

I'll work out the math when I've got a minute or two.

Thank you.

It's particularly interesting because so few people in the trade understand and just to re-iterate it was batteries beyond their best that burn out my motorhome control unit called EBL99. This unit has an intelligent charger that deals with the engine and habitation batteries together with solar and wind collecting/sharing. The control board burnt out because the batteries were beyond their best and overloading the charger when connected to 240 volts. Presumably the same/similar was happening when the engine was running and trying to keep 300 AH's happy.

Back to the 500 multijet, it's worth undoing the boot-let where the electrics go through for the rear hatch door. On mine a 12v+ was partially severed, then take the rear panel of and pull the plug out and back in that supplies the number plate lights. I sprayed mine with contact spray though in hindsight maybe taping the plug/socket together would have been a good idea. The plug and socket disconnect and it's classic with history across the 500 range.

A small car alternator with a 55A rating could be producing 770W, if you factor in that an alt is <80% efficient, belt/ pulley losses, heat build up etc, it could be using about 2hp.

At a steady 35mph, a small car uses less than 8-10hp to cruise. Having to use 10-12hp to cruise at the same speed would have a pretty marked influence on MPG.

As for diesels, just got a $10,000 bill for my VW TDi at 30,000 miles, but out of warranty.

UFI said:

As for diesels, just got a $10,000 bill for my VW TDi at 30,000 miles, but out of warranty.

Click to expand...

O.M.G.

Don't tell me, the cam belt broke ?

No - that was for a new long-life low-energy battery.
Never mind, the lower fuel bills will eventually absorb the cost!

It's a gear driven cam (one of the reasons I liked it), but instead it's leaky injectors/ cracked head/ cam replacement as a precaution.

The new 2.0's have a 210,000km cambelt change interval so we'll see how that pans out...my guess is a lot of destroyed valve trains at around 120,000.

We have the new 2 litre VW in a Beetle, such a smooth powerful unit. I hope they don't let us down. So far we had five and never spent a penny other than routine servicing.

A pal had a lot of trouble with his turbo, something to do with corrosion and the resulting expansion.

I think I'll buy an old Land Rover, quite a few people round these parts have them.