Electrickery - electrical power when engine running

[alicrozier]

Maybe Robin can explain... When the engine is running how does the battery affect the power available for ancillaries?

I inherited a tiny carbon tat battery with the Exige. It weighs virtually nothing but can go flat in a week if left in the cold. On a motorway trip Walshy and I noticed the lights dimmed a bit when the electric windows were operated (lights, heater fan, radio, TomTom also all on). Obviously bored (as you do) we experimented with various things…found that with everything on and both leccy windows held against the up position was worst case and the lights virtually off!

Would a stronger battery help? (or just take some measures to stop welshmen ersing about with my car)

[Rich H]

As I understand it (Robin will correct me later) the alternator puts out 13.8 volts to run the electrics and charge the battery.
As it gets loaded up with electrical stuff the current demand increases.
The alternator does it's best to keep the voltage up but as the load increases further the voltage starts to drop away to maintain the current.
The lamp brightness is voltage dependant so as the voltage drops form a nominal 13.8 the bulbs go dim.
That also explains why the lights get brighter when you start the car, the voltagfe goes from 12V (Battery) to 13.8 (Alternator)

My Porker has a factory voltmeter so you can watch it happening.... the indicators are by far the worst and the headlights go dim-bright-dim-bright in sympathy...

Not sure a bigger battery will help unless it's getting below 12V.

Check your earths are good that can make a lot of difference.

[2F45T4U]

If you dropped the voltage across this small battery by powering all the accessories, the voltage could drop below the point where there is not enough for the excitation of the alternator, so it effectively switches off and powers directly from the battery. The battery being less of a man than the standard one would produce the same results as you described.
But i would have thought the battery charging light would have come on at this point depending on where the threshold is.

It would be interesting to repeat this experiment at a higher RPM if you have not done so already. most alternators struggle to regulate properly below the 1000 or 950rpm point especially with a large load.

[alicrozier]

To clarify, the experiment was at (ahem) 4-5K rpm...we were on the M6 toll Road.

[Corranga]

To clarify, the experiment was at (ahem) 4-5K rpm...we were on the M6 toll Road.

Driving on the motorways in first gear again?

[alicrozier]

To clarify, the experiment was at (ahem) 4-5K rpm...we were on the M6 toll Road.

Driving on the motorways in first gear again?

Well 4th anyway...

[robin]

There are two effects here.

The first is cabling impedance, the second is alternator voltage/current capability at a given RPM.

Given that your RPM was high enough not to worry about it, you can assume your alternator is capable of pumping out 70A at standard charge voltage ~14v.

The ECU, fuel pump, injectors and ignition coils will use ~20A.

The battery will use between 50A and 5A depending on state of charge and condition. Most of the time it will use <10A, even if you started completely flat - that's because the voltage rises very rapidly when the battery is flat or dead. When the voltage reaches the target charge voltage, the battery essentially uses 0A.

The battery will supplement alternator output in extreme circumstances (according to Kirchoff's law).

You will never reach the point at which you cannot excite the field in the alternator - the ECU/injectors/coils will have stopped working before you reach that voltage.

If the effect you were seeing was a function of the battery being too weedy, the condition would vary over time - when you first overload the alternator, the battery would top up the current to what's required and you wouldn't notice; then after a minute or so, the battery would begin to suffer and the effect would become observable.

I suspect what you're seeing is cabling impedance. If 10 circuits share a common cable (live or earth or both) then as you turn more and more of them on, the impedance of the common sections drops more and more voltage and the lamps dim/motors slow, etc.

Chances are you have a duff earth connection or a burnt or rusty spade terminal that is common to more than one circuit, or the wiring design is sh*t ...

Cheers,
Robin

[alicrozier]

Thanks Robin.

First of all I'll check if it still does it...not noticed it recently (sans welshman) hence I was more interested in why.
Then worth going around checking the earths, the gearbox one has certainly been off a few times.

Then again we can't rule out your 2nd conclusion...

[mwmackenzie]

On the back of that do you think this would work at all??

http://www.gmcmotorsport.co.uk/index.ph ... cts_id=194

[Rich H]

Yes... it'll switch off the alternator reducing drag on the engine and release a little bit of power. But only a tiny bit!
If your battery is charged and your electrics are mostly off then there is little to be gained.