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Discussion Starter · #1 ·
Happy New Year!

Quick question. So like many others I disconnected the battery sensor (see picture circled red) to disable the start/stop function because its both annoying and I don't believe in the logic that restarting your little 1.0 for the sake of the environment is worth the extra strain on the engine.
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BUT I've just seen somewhere online that these sensors do more than simply telling the car when to engage start/stop? So I'm asking those with more knowledge (and maybe an inclination/means to go and measure) whether I'm actually being an idiot and should actually reconnect it to make sure my alternator is sending power to the battery etc?

Thanks :)

P.S. I don't have a voltmeter so cant go and test it myself.
 

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Your alternator will stop and start according to measured voltage. This seems better than having it spinning all the time, electrical load or not, as it wears and saps power. If you disconnect the voltage reader, the alternator will still charge the battery like in older cars, if that's the worry. You'd know in a few miles if it wasn't charging!
 
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If you really hate S-S, which seems odd to me, and don't want to press the cancel button every time you start a journey, I expect there is a way to disable it via OBD codes... Also, restarting the engine puts no real strain on it at all... You may argue it wears out the starter faster, but they are pretty robust and rarely go wrong.
 

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Discussion Starter · #4 ·
Your alternator will stop and start according to measured voltage. This seems better than having it spinning all the time, electrical load or not, as it wears and saps power. If you disconnect the voltage reader, the alternator will still charge the battery like in older cars, if that's the worry. You'd know in a few miles if it wasn't charging!
Thanks for the reply,

Yeah it's been fine so far, but just checking I'm not missing something?
So with the sensor disconnected, will the alternator be spinning the whole time or is there another voltage sensor somewhere? Am I actually unwittingly putting more strain on the engine 🤔?

Thanks again,

Tom
 

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Discussion Starter · #5 ·
Thanks for the reply,

Yeah it's been fine so far, but just checking I'm not missing something?
So with the sensor disconnected, will the alternator be spinning the whole time or is there another voltage sensor somewhere? Am I actually unwittingly putting more strain on the engine 🤔?

Thanks again,

Tom

I can answer my own question - There's an alternator "voltage regulator". Well there ya go, every day's a school day.

Thanks again.

T
 

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Yeah, but it's not a sensor - the voltage regulator is to prevent the voltage off the alternator getting to too high. There are a bunch of diodes to convert AC off the alternator to DC plus a chip to stabilize the DC voltage to get the right charging voltage - in principle, a bit like a mains-to-USB charger that you use on your phone.
 
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The battery voltage sensor (BVS) is only there for the stop/start function. The alternator regulator is completely separate. You can change the voltage limit in VCDS which will effectively turn stop/start off but it's much easier to just unplug the BVS.
 

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A smart alternator must have a means of knowing the battery voltage in order to know whether to provide electrical power or not to top up the battery or run extra electrical gear*. The battery is normally kept a little below max capacity so that the alternator can be usefully turned on when slowing down (on the overrun) or when braking. If the battery goes below a certain voltage, the alternator is turned on anyway, even if you don't slow down. All this needs the BVS to be operational, but if the BVS is removed, nothing bad happens apart from wasted efficiency.

Contrary to what I said above,I think the alternator used on Up!s spins all the time, while some vehicles actually decouple drive in the same way the A/C compressor does, to reduce rotational/frictional losses.

*you can hear it operation - drive a while in daylight, no headlights or heated screen on, then turn the fan onto max. Then let go of the throttle - you will likely hear the fan speed up. This is because the alternator, having read the battery voltage, decides to top it up a bit, so the voltage rises slightly and the fan spins a bit faster.

Smart alternators are a great idea, unless you are towing a caravan which needs fridges to run, and leisure batteries to charge. The voltage splitters won't work, so you have a use a DC-DC system to not get a warm fridge and flat leisure batteries after a long drive! I found out the hard way on a run to Marbella.
 
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Only ups with start/stop has BVS so i believe it is there to monitor battery voltage only for the start/stop system to enable it to disengage it when voltage is low one of the reasons the why disconnecting it will stop the system as it does not know the voltage of the battery and assumes it is low.
 
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