hi all, ive recently fitted a new alternator and im having a few issues when the engine is runningat idle the multimeter at the battery reads about 13 volts when i bring it up to fast idle then it starts reading about 14 -15 volts , not sure if this is supposed to be like this , my next problem is that when the engine is idlin and i switch on the lights (side and headlights) then the battery starts to slowly run flat and the voltage starts to drop gradually ,would this indicate that there is a shortage of some kind somewhere along the way ? thanks in advance for any help

 

Don't mind me if I ask some questions before offering any hopefully worthwhile answers.
Could you tell us why you needed to replace the alternator? Was it actually a new or rebuilt one?
When the engine is running at a fast idle (1000+ RPM) it should maintain a voltage of 14 -15 volts at the battery terminals with all accessories off.(avg rdg -14.2 volts)
When running down the road the alternator is supposed to be able to supply all the needed current to operate the vehicle. If when you have a load on the system like,ie,
headlights , blower motor, wipers etc the alternator should be able to maintain at least 12 volts at the battery at avg roadspeed. If not, there are a few reasons for it not being able to do this. Among them are, poor connections, faulty alternator, PCM problem if alternator is controlled by the PCM,and defective battery(s) and sometimes a loose drive belt.
Could you please clarify just what you mean by "battery runs flat"?
That's the first time I have ever heard that expression so please help me with that.

 

I had the same problem on my old Cougar, and the explanation is pretty straightforward: any alternator has two windings: the excitation ("field") winding - deployed on the rotor and powered through a couple of slip rings, and the generating winding, deployed on the stator. The field winding supply is controlled by an electronic device ("charging controller") that makes the alternator output exactly what is needed to overcome the loads and keep the battery charged without overcharging. At any given rpm, the alternator's field winding is powered such that the generator puts out at least 12V ... however, this is not possible unless the generator spins faster than certain, unit-specific, rotational speed ("start speed"). You go below that and the alternator has no way to output enough to charge the battery.

Back to your problem: if the new alternator has a start speed higher than your original's (which happened to me), it won't be charging the battery at idle unless you put a smaller pulley on it to make it spin faster. You can't put too-small-a-pulley though, because if it is too small, then you'll be spinning the alternator too fast when you rev up the engine (and risk damaging the alternator). That's about it ...