240 General: New (to me) 1980 Volvo 244

[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.

[Laird Scooby]

Originally Posted by 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.

Thanks John!

I think in the 60s it was "those are the facts, learn them"? In the 80s when i was at school/tech college etc, it was more a case of these are the facts, why and how they relate to each other - not always at the same time and sometimes it was just a case of learning things for what seemed like no reason. Then the next bit came and everything suddenly made sense.

Not fun as such but at least we had become accustomed to being taught things for a reason, if it seemed dry and boring there was usually better to come.

[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

[Laird Scooby]

Originally Posted by Othen

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

Have a read of this then Alan :

https://www.silvertel.com/images/tec..._batteries.pdf

https://itacanet.org/eng/elec/battery/battery.pdf

It also appears that some time after 1988 when it was still considered as being 2.2V per cell, things have changed to 2.1V per cell. However note that i did mention this and the reasons for it, maybe i was taught that way and they changed it afterwards but SLA batteries were always 2.2V/cell until that point.

Maybe the teachers got lazy and couldn't be bothered explaining the difference and decided for all practical purposes to change it to 2.1V - while finding those links i found a reference to a book, dated 2000, that specified they were 2.1V.

Has new research been done in between whiles? Have they decided to use the prqactical figure rather than the theoretical figure? I don't have the answers to those.

What does matter is that when your car is cold, the alternetor outputs 14.4V to the battery, dropping it to 14V once warm.

[Othen]

Originally Posted by Othen

:-)

Alan BSc(hons) Bristol

I was being a bit of a pompous arse when I wrote this last night Dave, I'm sorry about that and I apologise. My physics degree was nearly 40 years ago and has no more bearing on this than would O level chemistry.

Originally Posted by Laird Scooby

Have a read of this then Alan :

https://www.silvertel.com/images/tec..._batteries.pdf

https://itacanet.org/eng/elec/battery/battery.pdf

It also appears that some time after 1988 when it was still considered as being 2.2V per cell, things have changed to 2.1V per cell. However note that i did mention this and the reasons for it, maybe i was taught that way and they changed it afterwards but SLA batteries were always 2.2V/cell until that point.

Maybe the teachers got lazy and couldn't be bothered explaining the difference and decided for all practical purposes to change it to 2.1V - while finding those links i found a reference to a book, dated 2000, that specified they were 2.1V.

Has new research been done in between whiles? Have they decided to use the prqactical figure rather than the theoretical figure? I don't have the answers to those.

What does matter is that when your car is cold, the alternetor outputs 14.4V to the battery, dropping it to 14V once warm.

The physics behind the maximum open circuit potential difference that a lead-acid battery can maintain is actually limited by the chemistry: a less exact science (as Ernest Rutherford said: when it comes to sciences there is physics, and the rest are stamp collecting (or words to that effect)). The voltage is determined by the molarity of the acid - and so is a bit hit and miss.

As far as I know nominal 2v cells have always actually been 2.1v since they were invented. A normal lead-acid battery (like the one on the RB) should be charged at 14.4 volts (the chemistry dictates this) and straight after charging (within seconds) will register just over 13v, but this will fall to 12.6v within an hour or so (the extra half a volt or thereabouts is due to the capacitance between the plates - which as you will know will soon leak away). It will then stay at 12.6v for weeks, maybe months. It doesn't matter how long it is charged for, it will still never maintain more than 12.6v - just limited by the molarity of the acid.

12v lead-acid batteries should be charged at 14.4v (due to the chemical process), but once charged won't maintain more than 12.6v. If fully charged they won't discharge if the voltage at the +ve terminal is above 12.6v (because the electrons will follow the potential gradient from the alternator and regulator to the car's wiring) - but if not fully charged (as it won't be after powering that great big starter motor) the chemical charging process won't get underway until the alternator and regulator are providing something approaching 14.4v (actually 12.9v, so your figure of 13.7v is about right - but the chemistry will depend on the molarity of the acid at the start and (very significantly) the temperature). So, the bit between 12.6v and about 13.7v is a grey area where the battery (if fitted in a car) won't be discharging, but the chemical process to increase the molarity of the acid may not have started - so you have a good point. Simple really, isn't it: 12v batteries are really 12.6v, but they won't start charging until the difference in potential across the posts is about 12.9v (ish) - hence float charging to maintain the battery is generally set about 13.7v.

Anyway, I'm really sorry I was a pompous arse above.

:-)

[john.wigley]

Alan, I salute you. That is a perfect example of what makes an already good forum great. Would that such behaviour was at times more prevalent in the lounge ... J.