Dumb question

[OLD_ian]

How much power does a cars alternator consume? If its trying to charge a dead ish battery?

I've had to replace the cars battery as it wasn't charging though alternator is working fine. Wouldnt start this am so had rac out to boost. said battery was shot after testing it. said alternator was good. Driving it afterwards it felt much quicker... Or is that just down to me not driving it for a month? Seriously it feels loads stronger. Starts better than it has since I've owned it as well.

 

[Elliot]

sure it wasnt a connection issue with the leads to the battery that the RAC chap rectified.

 

[Paul 290T]

I read somewhere the alternator is 1.2kW, so that's about 2bhp, allowing for mechanical losses.

 

[OLD_ian]

Good question. I don't think so. No corrosion at all on the terminals or connectors. Old battery was cheapest of the cheap, doubtless put in by the dealer I bought the car from, and at least three years old.

ORIGINAL: Elliot

sure it wasnt a connection issue with the leads to the battery that the RAC chap rectified.

 

[OLD_ian]

A bit of googling has suggested a range of numbers between two and five HP. So likely placebo effect combined with not driving it for a month combined with driving it on warm clean dry roads for the first time in five months. Much more traction than its had for a long time.

Edited to correct shockingly bad spelling!

ORIGINAL: Paul 290T

I read somewhere the alternator is 1.2kW, so that's about 2bhp, allowing for mechanical losses.

 

[Lowtimer]

Yes, the higher rated alternator, the 115 amp one, if it's doing that at 14v, is putting out a max of 1.6KW, so even if it were only 50% efficient (and I expect it's around 60 to 65%) than it would be consuming 3.2KW from the engine, which is less than 5 bhp.

 

[OLD_ian]

actually it does look like there may be something more to it... got this back from an engineer friend.

Remember good old V=IR and P=VI? We can deduce that P = RI^2 or indeed that P = V^2/R and that if the voltage is fixed (it is because you have a regulator) and the resistance tends to zero (which we can because chances are your battery is a rattling box of metal sloshing around in an acid metal mixture) then the power required to charge a knackered battery tends to the infinite.

So yes a nice new shiny battery with a nice high resistance would make your car work better . First thing to swop on a motorbike that handles fine but feels sluggish on the throttle is the battery.

That load to try charge the battery is going to be constant from almost zero revs to the redline. So while the difference wouldn't be noticeable at high revs, pulling away at low revs it would be. Or it could be my imagination

ORIGINAL: _ian

A bit of googling has suggested a range of numbers between two and five HP. So likely placebo effect combined with not driving it for a month combined with driving it on warm clean dry roads for the first time in five months. Much more traction than its had for a long time.

Edited to correct shockingly bad spelling!

ORIGINAL: Paul 290T

I read somewhere the alternator is 1.2kW, so that's about 2bhp, allowing for mechanical losses.