XC70: Electrical Explanation of battery monitor please?

[Ian21401]

XC70, 2010, D5.
When engine off but ignition on. After just a couple of minutes “low battery” warning appears. When checked the battery voltage shows 12.6 to 12.7 volts across terminals, which in my experience is not low. I understand that there is a battery monitoring device connected to the positive battery terminal. Could someone explain, in relatively simple terms please, whether it is possible that this device needs to be re-calibrated, if indeed it can be, and if so, how ?

[Model flyer]

Its set that way to warn you that you may flatten the battery if listening to the radio etc. If you want to listen to the radio leave ignition off and press the radio switch . It should then run until battery does get low before shutting off and saving all the other current draw .
My Jaguar does this as did my V70 even with new fully charged battery fitted . Jaguar and Volvo were both owned by Ford its all part of the Ford effect .

[Ian21401]

So, basically, it’s a warning that you are using the battery and may, in the fullness of time, have a low battery, but necessarily now.

[apersson850]

Even if you use the radio alone, it will turn off after a while (maybe 20 minutes or so).
However, if the battery actually is low, it will give a warning about that, and terminate the radio session prematurely.

So no, I disagree. It's not just a warning that your battery may be low next minute, hour or fortnite. But the BMS system doesn't just measure voltage, but also current. It keeps track of consumption and charging, to see if you have too much consumption in relation to charge level. So in a way, maybe it can give you a warning before the voltage is too low, but it's not a general "you're listening to the radio, so you'll eventually have a problem" warning.

[SwissXC90]

Originally Posted by Ian21401

So, basically, it’s a warning that you are using the battery and may, in the fullness of time, have a low battery, but necessarily now.

No.
It is a warning that the battery has gone under the LOW BATTERY threshold.
A lot of people think the threshold is set rather high, but you cannot change it. Only Volvo can.

Ignore the message at your peril.

[green van man]

Howe are you reading the battery voltage, with a digital multi meter?

While the meter may be telling you 12.7 volts what's the capasity. I have known digital meters to read 239 volts on a mains supply but the light did not work, useing a test lamps which puts a load on the system established it was zero volts not the 239 the meters, checked with 2 said indeed under load conditions the digital meters agreed zero volts, open circuit they still insisted 239.

When changing a battery it initially accepts a high current which drops as the internal back emf biulds, at about 75% charged a meter will give a reading of 12+volts, however continued charging will biuld the capasity of the battery.

What sort of running does the car do, take it out and clock 50 miles, see if you get the message then, if so, a new battery is required. Just starting the engine will take 20 minuits of running to replace, D5 where it's tall geared and it's only doing 1500 rpm at 60 mph so if the car does short runs with frequent stop starts your battery is working hard all the time and never fully charged, it needs some miles to keep it healthy. Having 2 cars the volvo rarly starts unless it does 20 miles before being switched off and mostly it's 90, even taking into account the fact thathat it may not start for 3 weeks the running it does get is enough to keep the battery OK. We're I running it every day for 5 miles or so and switching off I would be looking at external charging to keep the battery charged were it not getting a good run once a month.

Paul.

[apersson850]

Yes, the digital multimeter has a high input impedance (normally 10 MOhm), but there are consumers in a car that always load the battery somewhat. So it's not the same thing as measuring the voltage in an open circuit.

Although the voltage of a battery that's being charged does rise, there's no emf involved. ElectroMotorical Force occurs in moving things, like motors (as can be heard from the wording).

[green van man]

Originally Posted by apersson850

Yes, the digital multimeter has a high input impedance (normally 10 MOhm), but there are consumers in a car that always load the battery somewhat. So it's not the same thing as measuring the voltage in an open circuit.

Although the voltage of a battery that's being charged does rise, there's no emf involved. ElectroMotorical Force occurs in moving things, like motors (as can be heard from the wording).

During my apprenticeship 50 years ago emf was electro motive force, a back emf was one working in opposition to the normal perceived polarity of the battery, as in regenerating brakeing . The internal resistance of the battery raises during charging and opposes the flow of the charging current, hence a back emf. When fully charged the forces acting against the charging current are almost equal to the charging current, hence it takes an inordinate amount of time to reach saturation point where the battery physically cannot store any more charge. Of course once the charger is disconnected the battery starts to self discharge due to that internal resistance and circulatory currents within the cells.

Any advise I offer here is based upon my personal knowlage and experiance, I try to explain it clearly so that non technical folk will understand. It's up to them wether or not they follow it. I learned by asking and try to help them that ask where I can. I belived that was the function of this forum seeing as we cannot discuss esoterical subjective subjects such as religion and politics for fear of pi55ing each other off.

Paul.

[Ian21401]

The vehicle in question is my daughter’s car. She is a self employed “pop in pet sitter/minder”. The nature of the job means that on some days she is doing short trips from one client to the next with the associated engine starts, so that part is very relevant. There are other days when she has a 30 minute drive to a client and may be there for a considerable length of time.
I now have a better understanding of what the BMS is monitoring and why.
Thank you all for your information and please let us not have an argument over EMF and back EMF.

[apersson850]

No, we'll not have any argument, since I've now done some research and found that in English nomenclature, emf is used not only for Electro motion force (which comes from the motion of a motor or generator), but the same expression is also used for the voltage generated by a battery. In spite of no motion being involved in the battery.

I apologize for my obstruction. We define these things slightly differently in Swedish.