charging car battery

forgotmyname

Hazard lights probably use about 8 amps. Nothing to an alternator putting out 60 - 100+ amps.

Sidelights also. A tiny draw in the alternator compared to what it can out out.

Neglible load on the alternator.

Rover_Driver

forgotmyname wrote: »

Hazard lights probably use about 8 amps. Nothing to an alternator putting out 60 - 100+ amps.

The output of an alternator is relative to its speed, it will only be producing its maximum output at high speed, when the engine is idling the output is low, which is why the general advice advice is to take the car for a run once it has started.

All that leaving lights etc. on when idling does is to reduce the available power to charge the battery.


Eg. If the alternator output at low speed is only about 10 amps, and the hazard lights are on using 8 amps, that only leaves only about 2 amps available to charge the battery. Which is why lights, heaters etc. should be switched off.

forgotmyname

A modern alternator will put out a good 80 - 90% of its rating at tickover speeds.

My Mondeo has heated front screens pulling almosty 60 amps. + rear screen at 10+ amps. The revs barely move when i switch those on yet the voltage never drops below 14.5 volts.

If it needed more revs i would expect the voltage to decrease as it struggles to power the front screen and rear screens.

Less than 50rpm increase and it ticks over at 750rpm normally. So wtill under 800rpm.

Rover_Driver

forgotmyname wrote: »

My Mondeo has heated front screens pulling almosty 60 amps. + rear screen at 10+ amps. The revs barely move when i switch those on yet the voltage never drops below 14.5 volts.

The voltage shouldn't alter, it's the current (amps) output that varies with the alternator speed.
At idle, that sort of loading could take all of the alternator output, and any shortfall from the battery, so no charge would be going in to the battery.

forgotmyname

If you draw more amps than it is putting out though the voltage will drop.

Yes its possible all the load is going to the screens but still providing enough power to keep the battery charge light off.

Not even close to putting the battery light on despite the low tickover speed.

Rover_Driver

The voltage may not drop, depending on the state of the battery and the warning light will be off if there is an output from the alternator, it is not output dependant.

forgotmyname

If the battery was in a discharge state. More power being drawn than the alternator can supply the battery light would be on.

But even with 60 - 70 amps at under 800rpm that doesnt happen.

Too cold to mess with test meters. If it warms a bit i will see how much power gets to the battery with the heated screens on.

Rover_Driver

If the battery was in a discharge state. More power being drawn than the alternator can supply the battery light would be on.



Unlike a dynamo, an alternator warning light is not affected by the rate of output.
It is either 'on' if the alternator is not working, or 'off' if it is.


The revs don't drop because they are controlled by the engine management system, and the slight increase is because it senses the extra load on the engine from alternator.

nobbysn*ts

forgotmyname wrote: »

If the battery was in a discharge state. More power being drawn than the alternator can supply the battery light would be on.

But even with 60 - 70 amps at under 800rpm that doesnt happen.

Too cold to mess with test meters. If it warms a bit i will see how much power gets to the battery with the heated screens on.

You can let us know how well the electronics cope with the load dump when you whip the battery lead off to put the meter in with the engines running. Should be a good little experiment for you.

Strider590

Get a multimeter on the battery terminals, if you read more than 12.8v with the engine running, then it is being charged by the alternator...... I've never known a car where the voltage did not rise with the engine running, so yes it does charge at idle.