Stop/Start and Battery Condition.

[Locknut]

I'm just wondering if it is possible to judge battery condition by how the stop/start performs. (I had a quiet day today) I was in traffic today and the stop/start turned off the engine, then within one or two minutes the system restarted the engine. Obviously there was a drain on the battery and the system detected when the battery was down to its minimum reserve. It was daylight at the time and the load on the battery consisted of heater fan, DRLs, radio and maybe the brake lights.

I don't think that is a very big load for the battery, especially because the car is a diesel with a very big battery. Is it normal for a battery to last just on or two minutes before restarting or should it last longer? Can the length of the "off" period indicate the health of the battery? How long do your cars stay off?

BTW: my battery is just over five years old.

[Elliot2000]

The fact that your stop start works at all is a good sign that your battery is ok- with a weak battery it will never operate the stop start.

The length of time it is off for can’t be used as any sort of indicator as it’s monitoring so many things like the interior heater - if it drops too much from what u have it set too it will then restart the engine the boost the heat, same for air con in hot weather. Turning steering wheel slightly will also make the engine start, and how much vacuum is left in the brake servo - none of which has any connection with battery health

[Darlo74]

I was told there’s a separate battery that operates the stop/start function to stop it being a drain on the main battery

[Elliot2000]

Darlo74 said:

I was told there’s a separate battery that operates the stop/start function to stop it being a drain on the main battery

U have been told wrong- just one battery but they are heavy duty agm type

[Locknut]

Darlo74 said:

I was told there’s a separate battery that operates the stop/start function to stop it being a drain on the main battery

You are probably thinking of hybrid cars such as the Prius which have a 12v battery seperate from the main motor batteries. The 12v battery operates things like the lights, heater fan, radio etc.

[Locknut]

Elliot2000 said:

... Turning steering wheel slightly will also make the engine start,..

I tried that today and it works! I had not realised that, now I have a handy way of re-starting the engine.

[Triumph Man]

Elliot2000 said:

The fact that your stop start works at all is a good sign that your battery is ok- with a weak battery it will never operate the stop start.

This - my Girlfriends Countryman never (well since she bought it) used start-stop. Then random dashboard lights started coming on, diagnosed as a failing battery. Lots of money later, all's well, and Stop-Start kicks in as it should.

[Mr Tidy]

Locknut said:

Elliot2000 said:

... Turning steering wheel slightly will also make the engine start,..

I tried that today and it works! I had not realised that, now I have a handy way of re-starting the engine.

My recommendation would be to turn it off every time you use the car - it's just there to "cheat" emissions tests!

I bought a pre-registered 09/07 123d with stop/start in 01/08.

It needed a new AGM battery before it was 5 years old at 55K miles - cost over £250 at a BMW Indy because it had to be coded (Sytner quoted over £300).

Then it needed a new starter motor before it was 7 years old at 77K miles (that cost just over £600).

By contrast all these cars don't have stop/start, so no AGM Battery either!

My 56 plate Z4 Coupe needed a new battery in 12/16 at 80K that cost about £120!

Mrs Tidy has a 12/06 Mini that is still on it's original battery after 55K miles.

My 03/06 325i needed a new battery at 137K miles - it cost £125.

[Locknut]

Mr Tidy said:

My recommendation would be to turn it off every time you use the car

I accept that fully. However BMW seem to have deliberately put the switch in an awkward place obscured by the steering wheel. I find that I have to lean over to see around the wheel when ever I use that switch.