Hello is there any common faults with the smart alternators? Iv got a new battery fitted and it's constantly showing 12.1 volts. As I drive the voltage briefly goes up to 13, and sometimes 14 volts only for 5 seconds or so and goes back down to 12.1. if lights are on it will adapt and give a little more volts but it never seems to charge the battery at 14volts like id expect. Surely when the car first starts and it sees it's low on 12.1v it should put out 14 or 15 volts straight away ? But it doesn't?

Does it ever fail to start ?

How are you testing it.

It hasn't failed to start, it sounds a bit weak sometimes when it starts.

I used a voltage gauge that I plugged into the 12v socket inside. It's exactly accurate iv checked with a multi meter on the battery and it's always the same as what the gauge says.

It's as if the alternator doesn't know the battery is low.

2 hours ago, jmmy18860 said:

Iv got a new battery fitted

Was the new battery coded to the car?

Thanks. AG Falco

Thanks for the reply, no it wasn't coded !!! I can't believe you can't just fit a battery to a car nowadays. I used obd 11 and coded it to the car. It seems to of sorted it out now. The alternator likes to stay at 13.2 volts most of the time and regularly fluctuates up to 14.7 every minute or so. and then back down to 13.

What was ever wrong with the old alternators that put out 14.7 volts permanently? They can't be saving that much energy using smart alternators because the engine is definitely less efficient then diesel were 10-15 years ago.. it's only doing about 40-45 mpg. Cleaner emissions maybe but not more efficient.

One of the main reasons for the smart alternator control is to allow headspace in battery capacity to allow for energy recuperation while braking or in overrun.

Together with automatic Stop-Start, this allowed the vehicle to be classed as a micro-hybrid.

It doesn't work the way you expect because it's a smart alternator🫣

I have watched the behaviour of my alternator when solo and when towing

When towing it sits at a steady 14.4v as you might expect.

When solo it only charges and goes up to 14.4v on overrun, never when accelerating. It's a smart alternator designed to minimise the load on the engine and thus increase MPG.

Find a long hill where you can drive down in gear with no throttle using engine braking. Check the voltage and you will find it's where you expect it to be.

On 30/03/2025 at 16:56, jmmy18860 said:

Thanks for the reply, no it wasn't coded !!! I can't believe you can't just fit a battery to a car nowadays. I used obd 11 and coded it to the car. It seems to of sorted it out now. The alternator likes to stay at 13.2 volts most of the time and regularly fluctuates up to 14.7 every minute or so. and then back down to 13.

What was ever wrong with the old alternators that put out 14.7 volts permanently? They can't be saving that much energy using smart alternators because the engine is definitely less efficient then diesel were 10-15 years ago.. it's only doing about 40-45 mpg. Cleaner emissions maybe but not more efficient.

I can easily attain 40-45 mpg, with my 1.4TSI Octavia - over a wide mix of driving - you should be able to get significantly better with a diesel!

Fuel usage is highly dependant on driving style.

9 hours ago, Warrior193 said:

I can easily attain 40-45 mpg, with my 1.4TSI Octavia - over a wide mix of driving - you should be able to get significantly better with a diesel!

Fuel usage is highly dependant on driving style.

Yesterday I managed 62.5 mpg over a 25mile trip. That was normal driving sticking to the speed limits, 40-50mph, no large hills unlike at home.