Hi all,
Since replacing the battery on my 2011 TDV8 (https://www.fullfatrr.com/forum/topic66148.html) the car has been having a few issues. This mainly involves the battery getting charge from the alternator (14.3 to 14.8v) on a run, but discharging from 12.9v when returning from a trip down to 12.3v in just a few seconds. The car always starts first time in the morning, but the voltage can be anywhere between 11.5v and 12.1v.

I have checked for a battery drain on all the fuses in the passenger area and found no drains. I haven't yet checked the engine bay fuses.

I am wondering if the issue may lie with the BMS.

To test this I was wondering if I unplugged the BMS connector from the negative pole. Would there be any issue running the car without the BMS? I would like to see if the battery revived a full charge (and retains it over night) without the BMS.

What do you all think?

Phil

I'm not convinced there is anything actually wrong here and I certainly wouldn't disconnect the battery monitoring system as it's essential to regulate the charging on a 'smart' system esp with an AGM battery that can be degraded with overcharging. 2012 TDV8 Vogue (L322)

I think it is fairly common to see an apparent quick discharge if you check the battery immediately after a run or charging as it settles down to its actual state of charge. 2012 tdv8 Westminster arrived Jan 19 gone Dec 23

200 TDi discovery bobtail gone
RR classic v8 bobtail gone
300 tdi Discovery 3 dr gone

Yep, you should wait at least 30 mins before checking voltage on a charging or discharging battery, lots of stray voltage around...

As JayGee says, it sounds normal, the BMS does it's thing, fully charging the battery like the old setup is not conducive to battery longevity....  Pete

__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
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Sounds normal for the intelligent BMS. I'd leave it alone. Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
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It all depends upon how you are measuring the 11.5-12.1VDC, firstly, is it at the battery? was the car locked overnight and you measured after unlocking (in which case, there will be some current drain, which can be up to 30A even before you turn the key) or are you using some other method?
The comments above about charge voltage & surface charge dissipation are all valid. 11.5VDC can be low or normal, depending upon the test conditions, temperature etc.

If you were to disconnect the BMS sensor, you'll actually be putting less charge into the starter battery as the alternator control will go into a battery protection mode.

Hi all,
Thanks for the replies. I certainly won't be disconnecting the BMS given the above advice. I am still somewhat surprised that the voltage on a new battery is so low after a drive (11.9 to 12.3v) yet the car still starts. I am currently checking the voltage using the 753 hidden screen but only after opening the car and after turning on all the systems (but before starting the car).

I will will try I'll monitor the battery voltage using a multimeter over the next few weeks, by disabling the bonnet switch and leaving the bonnet ajar overnight (so not activating all the electronics systems. I'll monitor the battery voltages for a couple of weeks and report.

Phil

That's the best way to do it - as you're probably now aware, the '753' screen displays only the voltage at the module, not the battery voltage, and by having the ignition on, you're drawing a not insignificant current which in turn will cause the voltage to drop slightly. It would also pay to record the ambient (or ideally, the battery) temperature and plot this with the voltage as it will have an effect upon the terminal voltage and help you understand any fluctuations better.