HI car is a 2011 focus 1.6tdci with stop/start,car wouldn't start due to flat battery, looks like the original so we got it changed at Halfords they then performed a test which showed the alternator to be faulty,when doing a search there are 2 options, a 150a and 120a.

Does anyone know the correct one or is there a way for me to check ?

Cheers Rich

150a with aux heater.  

120a without aux heater.

I'm fairly sure all the Mk3 1.6 TDCIs have an aux heater so would expect it to be 150a.

32 minutes ago, TomsFocus said:

150a with aux heater.  

120a without aux heater.

I'm fairly sure all the Mk3 1.6 TDCIs have an aux heater so would expect it to be 150a.

 Thank you.

Is it the case that a faulty alternator wouldn't light the battery warning? I'm aware that this car has a smart/variable system so is new to me.

10 minutes ago, ossett2k2 said:

 Thank you.

Is it the case that a faulty alternator wouldn't light the battery warning? I'm aware that this car has a smart/variable system so is new to me.

I would expect to see the battery light if it wasn't charging at all.  Though maybe not if it was still providing some charge.

Did Halfords reset the BMS after replacing the battery?

They said they had reset the bms,I wasn't there,so before I order the alternator I'll do a reset to be sure.

 Any tips on brand to buy or avoid please? So many to choose from.

This is the report on new battery.

I'd stick to Lucas, Bosch or Valeo.

That test does appear to show little to no charge being produced.  It is variable, but should be a lot more than 12.3v average, even on a brand new battery.

If you've got a multimeter, it would be worth testing again now it's been driven.  Literally just a basic voltage test across the battery terminals with the engine running. 

Nice one,appreciate the advice,I'll test in the morning, found a lucas at a reasonable price so I'll go with that.

For test purposes ONLY.

Just unplug the small connector on the alternator (Smart charge connector). With the engine running at a fast idle you should be measuring between 13.6 and 13.8V on the battery terminals.

Repeat the test with the plug reconnected and on a good fully charged battery the voltage on the battery terminals will be around 12.5V to 12.7V

Thanks @unofix if it still reads 12.3v with the plug on and 13.8v with it off is it still a faulty alternator? Or bad smart charge?

Before thinking about replacing the alternator check that the chassis earth lead to the engine block is in good condition.

43 minutes ago, unofix said:

Before thinking about replacing the alternator check that the chassis earth lead to the engine block is in good condition.

Where is this  located please? 

They are generally located towards the back of the engine, but there are so many variations depending on the exact engine and the model year.

Google "focus chassis to engine earth strap location" and it will bring up a dozen or more photos of the locations.

I have fit a new alternator and now charging around 13.7v when running and when I switch lights, heater on which I believe is OK.

 The battery light came on,on checking the receipt it says it's a 12v 150a but on the website when I ordered it says 14v 150a.

  Is it safe to run the car like this until I get an exchange?

 Ps what a nightmare of a job and definitely not looking forward to the repeat!

Just to update,the alternator is the correct one so unsure why the battery light is on? I'll do some digging around tomorrow,see if I can find the fault. 

46 minutes ago, ossett2k2 said:

Just to update,the alternator is the correct one so unsure why the battery light is on? I'll do some digging around tomorrow,see if I can find the fault. 

Have you taken it for a drive yet?  May take a few minutes for the BMS to settle and the light to go out.

4 minutes ago, TomsFocus said:

Have you taken it for a drive yet?  May take a few minutes for the BMS to settle and the light to go out.

Just a quick 5 minute drive,I'll have a longer drive tomorrow, I've also got forscan so should be able to access more info.

Update on this problem. 

 Car is sorted now,turns out I didn't push the little plug on the alternator all the way home,was about 2mm out.

Car runs great now,it's been running a bit rough for ages,thought it was dpf,runs smooth and has more punch.

 Thanks to you guys for the valuable help,looks like a great forum.

Cheers Rich