240 General: New (to me) 1980 Volvo 244

[Othen]

Originally Posted by Laird Scooby

At idle with no load, it should be between 14.4-14.0V Alan so 13.3V is a bit low. I'd suggest you check the charging voltage with your multimeter directly on the battery at tickover to confirm if the new gadget is just reading low as they can also suffer a poor connection in the actual lighter socket.

Once you know if there is a discrepancy the gasget will be more useful as you will have a baseline to work from.

... Yes, as expected Dave: I get 14.35v at tick-over and a reading at the device of 13.3v, - about a volt of potential drop to the accessory socket would be about right (and there would have been a few low current devices like the interior light providing quite an easy path to the -ve return). I'm happy that the new meter is giving me some useful information about the health of the battery and charging system as any deviation from what it shows now would be a good warning.

[Laird Scooby]

Originally Posted by Othen

... Yes, as expected Dave: I get 14.35v at tick-over and a reading at the device of 13.3v, - about a volt of potential drop to the accessory socket would be about right (and there would have been a few low current devices like the interior light providing quite an easy path to the -ve return). I'm happy that the new meter is giving me some useful information about the health of the battery and charging system as any deviation from what it shows now would be a good warning.

A useful test then Alan!

Now you can add ~1V onto the displayed voltage and know whether it's charging or not - over 13.8V it's charging, below it's discharging which would be expected at idle with a high load like HRW, headlights, heater fan and wipers but should jump above 13.8V once the engine speed is above 1500rpm.

[Othen]

Originally Posted by Laird Scooby

A useful test then Alan!

Now you can add ~1V onto the displayed voltage and know whether it's charging or not - over 13.8V it's charging, below it's discharging which would be expected at idle with a high load like HRW, headlights, heater fan and wipers but should jump above 13.8V once the engine speed is above 1500rpm.

Agreed about the 1v potential loss between the battery and accessory circuit (or maybe it is a systematic inaccuracy of the device, it doesn't matter as long as it is constant).

Surely the system will charge a 12v battery (so when fully charged 12.6v) at anything above 12.6v because there will be a potential gradient from the alternator to the +ve terminal. The electrons will only go the down the potential gradient (well, actually they go up because they have a -ve charge, but you know what I mean). What would be critical about 13.8v? Any potential greater than 12.6v will give a potential gradient towards the battery - surely, that is just the physics?

[Laird Scooby]

I'll explain properly later Alan but for now i'm on a fast turn-around after shopping and needing to take Sasha out.

[Othen]

Originally Posted by Laird Scooby

I'll explain properly later Alan but for now i'm on a fast turn-around after shopping and needing to take Sasha out.

No need to go to any trouble Dave, I'm just curious and no more. In practice it makes no difference (to the RB) as I've achieved the aim with a £3 part.

Alan

PS. I just googled the problem, this article says 12.9v is needed to charge a 12.6v battery, which sounds entirely plausible, but at that potential difference charging will be slow:

https://www.powerstream.com/car-batt...lly%20charged.

... this isn't in any way important; I'm happy I can just keep an eye on the RB's charging system and as a result of your excellent suggestion I have calibrated the £3 device.

[Laird Scooby]

The (nominally) 12V lead-acid car battery is made up from six 2.2V cells, totalling 13.2V at theoretical full charge.

However, in common with it's smaller cousin, the electrolytic capcitor, it's impossible to achieve theoretical 100% charge. For practical purposes therefore, it is accepted that 12.6V is fully charged as being a point where self-discharge is at a minimum and maximum power is held within the battery that it can consistently hold.

Because it is in fact a 13.2V battery, to maintain that it needs a float charge voltage of 13.8V, 0.1V per cell above the cells voltage. Anything under 13.8V and it will slowly be discharging because it is <0.1V per cell above that voltage needed to maintain the voltage.

To initiate charging, the voltage needs to be a further 0.1V per cell above the level to maintain it, hence 6 x 0.1V + 13.8V = 14.4V, exactly the value most voltage regulators have when cold. This drops to 14.0V once warm and the battery has meanwhile taken charge.
However alternators are designed such that if the battery voltage drops below 14.0V, it will increase the charge rate to restore the status quo.

When you move on to "Calcium Technology/Silver-Calcium" batteries, they need an additional 0.1V per cell to start them charging so need ~15V to start them charging and a minimum of 14.7V to keep them from starting to discharge. They will charge (but very slowly) on 14.4V but will rapidly sulphate because they are not being charged fast enough to separate the opposite charged molecules from the plates.

Hence 13.8V is the "magic figure" for the crossover point, below that a normal lead-acid battery will discharge, above that it will charge.

[john.wigley]

You'd have made an excellent physics teacher, 'L.S.', far better, I fancy, than the one that we had. Explained like that, even I might even have absorbed enough to scrape a pass in a subject that I failed abysmally at 'O' level Things were quite different in the 1960s; yes, we were taught, but the learning was made neither engaging nor fun - perhaps through intent.

Regards, John.

[Othen]

Originally Posted by Laird Scooby

The (nominally) 12V lead-acid car battery is made up from six 2.2V cells, totalling 13.2V at theoretical full charge.

However, in common with it's smaller cousin, the electrolytic capcitor, it's impossible to achieve theoretical 100% charge. For practical purposes therefore, it is accepted that 12.6V is fully charged as being a point where self-discharge is at a minimum and maximum power is held within the battery that it can consistently hold.

Because it is in fact a 13.2V battery, to maintain that it needs a float charge voltage of 13.8V, 0.1V per cell above the cells voltage. Anything under 13.8V and it will slowly be discharging because it is <0.1V per cell above that voltage needed to maintain the voltage.

To initiate charging, the voltage needs to be a further 0.1V per cell above the level to maintain it, hence 6 x 0.1V + 13.8V = 14.4V, exactly the value most voltage regulators have when cold. This drops to 14.0V once warm and the battery has meanwhile taken charge.
However alternators are designed such that if the battery voltage drops below 14.0V, it will increase the charge rate to restore the status quo.

When you move on to "Calcium Technology/Silver-Calcium" batteries, they need an additional 0.1V per cell to start them charging so need ~15V to start them charging and a minimum of 14.7V to keep them from starting to discharge. They will charge (but very slowly) on 14.4V but will rapidly sulphate because they are not being charged fast enough to separate the opposite charged molecules from the plates.

Hence 13.8V is the "magic figure" for the crossover point, below that a normal lead-acid battery will discharge, above that it will charge.

Well Dave, you are certainly right that a 12v lead-acid battery is made up on 6 cells, but when I took my physics degree the maximum open circuit potential difference each one was of 2.1v, not 2.2v.

:-)

Alan BSc(hons) Bristol