Something I don't understand about batteries

[cereal tiller]

a bit off topic,has anyone used a sterling digital alternator charge controller?

i have aquired an unused one,and wondered if it would be of benefit for charging my batteries more efficiently.

my engine has a 90 amp alternator with adjustable output voltage regulator currently set at 14.4 volts

also a standard vehicle 60 amp alternator which peaks at 13.9 volts,it is the 60 amp alt. that i am considering installind the digital to.

good idea,or not?

[Gibbo]

a bit off topic,has anyone used a sterling digital alternator charge controller?

i have aquired an unused one,and wondered if it would be of benefit for charging my batteries more efficiently.

my engine has a 90 amp alternator with adjustable output voltage regulator currently set at 14.4 volts

also a standard vehicle 60 amp alternator which peaks at 13.9 volts,it is the 60 amp alt. that i am considering installind the digital to.

good idea,or not?

 

It will make hardly any difference to the 90 amper. It will make a big difference fitted to the 60 amper. Your thinking is correct.

[cereal tiller]

thank you,one more question,the battery temp.sensor is missing as well as the alternator temp. sensor.

the info,with it says it it is ok to run without these items,would they be at all necessary?

my batts are 7x95 A/H numax that are now 6 months old.

from 12 volts at rest the voltage reaches 14.4 within 20 minutes at fast engine idle,and the smaller alternator tapers off after 10 minutes approx.

[Gibbo]

I think I'd be wanting the alternator temp sensor. The battery ones I wouldn't actually worry about in the UK.

[ditchcrawler]

thank you,one more question,the battery temp.sensor is missing as well as the alternator temp. sensor.

the info,with it says it it is ok to run without these items,would they be at all necessary?

my batts are 7x95 A/H numax that are now 6 months old.

from 12 volts at rest the voltage reaches 14.4 within 20 minutes at fast engine idle,and the smaller alternator tapers off after 10 minutes approx.

Are you charging with the two alternators into one battery bank?

[cereal tiller]

yes,2 alternators previously described charge the boat's domestic bank,and a 3rd 35 amp alt. looks after the engine battery and the midships battery which powers the water pump and forward galley 12v lights.

this system is used to reduce any voltage drop,as the boat is 70feet in length.

[by'eck]

Oh my word....

 

The internal resistance of a battery is absolutely, 100%, definitely not higher when it fully charged. Absolutely not.

 

The highest internal resistance of a battery is when it is flat. There is absolutely no question whatsoever about this.

 

 

 

Correct.

 

This isn't even open to debate!

 

Apologies my original post on this thread was misleading. I was referring to resistance perceived by the charging device when connected to battery(s) & providing charging current.

Edited May 23, 2011 by richardhula

[by'eck]

I think I can see what you are meaning...

 

 

... but I think that the value of the current flowing is as a result of, and dependent on, the (magnitude of the) voltage difference of that of the charger and that being produced by the state of the chemistry in the battery...

 

Nick

 

Pardon me if I'm wrong, I but I think you may be jumping on Gibbo's explanation when he says:

 

The reason the charge current falls is two-fold:-

 

Firstly, at a fixed charge voltage, as the battery approaches full charge, the "at rest" voltage of the battery rises sharply. Therefore there is less voltage difference between the charge voltage and the battery voltage. Less voltage difference obviously means less charge current.

 

Secondly, as the battery approaches full charge state, the chemical reactions within the battery change. Less energy goes into recharging the battery (i.e. converting lead sulphate back into sulphuric acid) and more goes into producing gasses. This is why batteries begin to gas vigorously once they approach or reach a fully charged state. In fact this is one of the old methods technicians used to use to detect when a battery was fully charged.

 

This sudden burst of gasses within the electrolyte increases the bulk resistance of the electrolyte which causes a further fall in charge current.

 

His first explanation is IMHO inadequate. How can you talk of separate charge and battery voltages and their reducing difference when the two are electrically connected. There is only a common voltage then which if constant, allows charge current to be determined by the battery.

 

His second reason(s) is/are totally understandable and I'm tempted to wonder if they are the only relevant ones.

 

To sum up, it is clear that a battery particularly as it reaches a full charge, exhibits substantially different resistive properties compared with after charge source has been disconnected and it has settled down.

Edited June 10, 2011 by richardhula

[RobinJ]

During February and March, voltage readings after charging to 100% were in the region 25.4v (12.7v per battery) and the drop after three weeks or so with the SmartGauge reporting around 70% was usually around 25.2v (12.6v per battery) - I am not sure how relevant this is. The charging voltage varies between 28.6v and 29.6v and the 100% figures quoted in each case is a reading taken after the voltage has stabilised about an hour after charging had ceased.

 

Comparative voltage figures for the engine start batteries are 25.5v after charging and 25.4v after three weeks or more.

Those figures don't look that bad?

I understood that 12.6V was roughly equivalent to 80-90% SOC?

 

I have noticed that in cold weather batteries seem more reluctant to accept charge, what effect it has on discharge I'm not too sure (they always appear to go flat quicker?).

[Nickhlx]

Pardon me if I'm wrong, I but I think you may be jumping on Gibbo's explanation when he says:

 

His first explanation is IMHO inadequate.

His second reason(s) is/are totally understandable and I'm tempted to wonder if they are the only relevant ones.

 

To sum up, it is clear that a battery particularly as it reaches a full charge, exhibits substantially different resistive properties compared with after charge source has been disconnected and it has settled down.

 

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 

"How can you talk of separate charge and battery voltages and their reducing difference when the two are electrically connected. There is only a common voltage then which if constant, allows charge current to be determined by the battery."

 

Because there is a continuous voltage gradient between the semiconductor junctions in the charger, (because all conductors have resistance) through the wiring within, the connectors and the cables before it reaches the plates immersed in the acid, so the voltage at the start of the path is absolutely NOT the same as at the end of that path - The charger is trying to maintain a pre-set voltage and is dragging the voltage at the plates towards that voltage - in so doing, current is flowing ( but diminishing as the voltages become closer as the chemistry of the plates changes). It is DEFINITELY NOT the case that the batteries internal resistance is increasing with increased state of charge of the battery, as Gibbo confirms.

 

It is so wrong, I either think you must genuinely not understand / appreciate what is going on, or I wonder if you are having a laugh !

( I certainly am)

 

We'll have to agree to differ, then...

 

Nick

Edited June 10, 2011 by Nickhlx

[by'eck]

We'll have to agree to differ, then...

 

Nick

 

I think you jumbled up the quotes but...

 

Because there is a continuous resistance gradient between the semiconductor junctions in the charger, through the wiring within, the connectors and the cables before it reaches the plates immersed in the acid, so the voltage at the start of the path is absolutely NOT the same as at the end - The charger is trying to maintain a pre-set voltage and is dragging the voltage at the plates towards that voltage - in so doing, current is flowing ( but diminishing as the voltages become closer as the chemistry of the plates changes). It is DEFINITELY NOT the case that the batteries internal resistance is increasing with increased state of charge of the battery, as Gibbo confirms.

 

During the absorption cycle when a charger is holding a steady voltage to the battery (generally trying & succeeding with healthy battery), charge current slowly drops. I don't think we are in disagreement with this. However this can only happen if the resistance it sees (call it what you will) increases. The charger doesn't know its connected to a battery, it can surely only respond to the dynamic load conditions within its programmed cycle.

 

I believe Gibbo generally agrees with this as why else would he offer an explanation.

 

Really this is becoming a question of semantics. Can't it just be accepted that a healthy battery being charged says no more thank you when it reaches a full charge.

Edited June 10, 2011 by richardhula

[Nickhlx]

I think you jumbled up the quotes but...

 

 

 

During the absorption cycle when a charger is holding a steady voltage to the battery (generally trying & succeeding with healthy battery), charge current slowly drops. I don't think we are in disagreement with this. However this can only happen if the resistance it sees (call it what you will) increases. The charger doesn't know its connected to a battery, it can surely only respond to the dynamic load conditions within its programmed cycle.

 

I believe Gibbo generally agrees with this as why else would he offer an explanation.

 

Really this is becoming a question of semantics. Can't it just be accepted that a battery being charged says no more thank you as it becomes fully charged.

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I would say one or two of my chargers ( in particular) definitely do know when they are connected to a battery, and for several reasons :

 

i) one has a light that illuminates when it is connected and goes out when you disconnect the crock clip.

ii) one has LEDs that change to show what phase it is in, which can not possibly be changing due to a timed program, as this

happens relatively quickly if the battery is nearly fully charged - it is therefore not blindly following a "programmed cycle" but

reacting to feedback from the "load" at the other end, so must "know" that it is connected.

 

The point being discussed, and not reaching agreement over, is that you are trying to maintain that the resistance of the battery increases with increasing state of charge.

That I can never accept or agree to as it is plainly wrong.

 

I will let others comment / confirm / deny as I don't know how else to argue that.

 

Nick

[by'eck]

From Gibbo:

 

This sudden burst of gasses within the electrolyte increases the bulk resistance of the electrolyte which causes a further fall in charge current.

[Nickhlx]

From Gibbo:

 

 

 

 

So.... You are saying that in its transient gassing stage the internal resistance is higher - well, of course, as Gibbo says.... gas has a higher resistance than electrolyte and is crap at conducting electricity...

 

Now, forgive me, but I (and Gibbo, judging by his reply....) which I quote.....

 

"Oh my word.... The internal resistance of a battery is absolutely, 100%, definitely not higher when it fully charged. Absolutely not. "

 

am quite clear that your point was that a charged battery has a higher resistance than a flat one.... and, I say again, IT DOES NOT.

 

 

We were not discussing whether a gassy electrolyte has a higher bulk resistance than one whose electrolyte is 100% liquid, (i.e. when in its rested state), but the internal resistance of a flat battery vs. a fully charged one.

 

Also, as Gibbo says, ( which again confirms the point being made)

 

"The highest internal resistance of a battery is when it is flat. There is absolutely no question whatsoever about this" so he read it that way too.

 

 

Good "attempt" at a recovery though....

 

Nick

[weeble]

I think you jumbled up the quotes but...

 

 

 

During the absorption cycle when a charger is holding a steady voltage to the battery (generally trying & succeeding with healthy battery), charge current slowly drops. I don't think we are in disagreement with this. However this can only happen if the resistance it sees (call it what you will) increases. The charger doesn't know its connected to a battery, it can surely only respond to the dynamic load conditions within its programmed cycle.

 

I believe Gibbo generally agrees with this as why else would he offer an explanation.

 

Really this is becoming a question of semantics. Can't it just be accepted that a healthy battery being charged says no more thank you when it reaches a full charge.

 

Ohm's law is for wimps! Translate the battery and charger sources to their Thevenin or Norton eqvivalent circuits and you might better understand what really goes on during the dynamic charging cycle.

[RLWP]

I had to google them

 

it is often possible to replace an uninteresting subset of queues by a single queue with an appropriately chosen service rate

 

Wise words indeed!

 

Richard

[Robin2]

Pardon me if I'm wrong, I but I think you may be jumping on Gibbo's explanation when he says:

 

 

 

His first explanation is IMHO inadequate. How can you talk of separate charge and battery voltages and their reducing difference when the two are electrically connected. There is only a common voltage then which if constant, allows charge current to be determined by the battery.

 

His second reason(s) is/are totally understandable and I'm tempted to wonder if they are the only relevant ones.

 

To sum up, it is clear that a battery particularly as it reaches a full charge, exhibits substantially different resistive properties compared with after charge source has been disconnected and it has settled down.

 

 

I think the problem here is the use of the word resistance to describe what goes on inside a battery coupled with the fact that there is a concept of the internal resistance of a battery when it is discharging.

 

A battery is an electricity generator whose voltage rises slightly as it charges up. At any given state of charge the flow of current from a charger to a battery depends on the difference between the voltage that the battery chemistry generates and the voltage that the charger generates. This difference declines as the battery charges up and consequently the charge current declines. I can't conceive of explaining this using the word 'resistance' in the electrical sense of that word.

 

Now imagine a battery is discharging and delivering a certain current at, say, 12.5v. If the demand for current increases significantly the voltage will drop. This is due to the fact that the chemical reactions within the battery cannot act quickly enough to maintain the original voltage at the higher current. This feature (declining voltage with greater current) is commonly referred to as the internal resistance (or impedance) of the battery. Zinc-carbon batteries have a much higher internal resistance than NiCd or lead acid batteries and that's why they are not used in situations where a high current is required - such as to power a model airplane. In this context a battery's internal resistance will always be lowest when it is fully charged - but this has nothing to do with charging the battery. And as far as I know this notion of internal resistance is just a convenient shorthand and does not really mean that Ohms law applies inside batteries.

 

I would be very surprised if the gassing of a battery reduces the flow of current - though I will defer to the experience of those who have measured it. It seems to me that every bubble that is released needs a certain amount of electricity and more bubbles would need more current. I presume the amount of gassing will reduce if you reduce the charge voltage and thereby reduce the current.

Edited June 10, 2011 by Robin2

[Gibbo]

I would be very surprised if the gassing of a battery reduces the flow of current - though I will defer to the experience of those who have measured it. It seems to me that every bubble that is released needs a certain amount of electricity and more bubbles would need more current. I presume the amount of gassing will reduce if you reduce the charge voltage and thereby reduce the current.

 

Try it. Get a battery at 90% SoC. Get a really big power supply and bang 20 volts across it (you need a big power supply otherwise it can't get past the gassing voltage - it draws maximum current just below this). You'll get one hell of a lot of gassing, but not much current. Reduce the voltage to 14 volts and watch the current increase. Honest.

[NB Alnwick]

I assume that the SmartGauge software makes allowances for the gradual decline in the condition of a battery bank as it 'ages' - this may explain some of the changes that I have seen - our batteries were second-hand when they were fitted two and a half years ago, so it is likely that they have lost some of their capacity. At present, with the invertor on and driving a TV, Sky Box, Computer and several small 240v adaptors, the battery bank (according to SmartGauge) drops from around 90% down to 40% in an evening (18:00 to 23:00) - two years ago it would have dropped to 65% or thereabouts and I could have gone for another day or two before the Smartgauge recorded anything as low as 40% - by comparison, I now need to charge the batteries on alternative evenings rather than every three days.

 

Is there a simple, non-technical, way of assessing the best time to replace batteries? Or have I just established (above) that we have reached that point already?

Edited June 10, 2011 by NB Alnwick

[nb Innisfree]

I assume that the SmartGauge software makes allowances for the gradual decline in the condition of a battery bank as it 'ages' - this may explain some of the changes that I have seen - our batteries were second-hand when they were fitted two and a half years ago, so it is likely that they have lost some of their capacity. At present, with the invertor on and driving a TV, Sky Box, Computer and several small 240v adaptors, the battery bank (according to SmartGauge) drops from around 90% down to 40% in an evening (18:00 to 23:00) - two years ago it would have dropped to 65% or thereabouts and I could have gone for another day or two before the Smartgauge recorded anything as low as 40% - by comparison, I now need to charge the batteries on alternative evenings rather than every three days.

 

Is there a simple, non-technical, way of assessing the best time to replace batteries? Or have I just established (above) that we have reached that point already?

 

We can still manage with a two hour charge per day, our yardstick for replacement will be when we need more than 3 hrs and therefore two charges per day.

[Justme]

Work out how much extra fuel you use having to charge every two days instead of every 3.

 

Then compare to the cost of batteries.

[Robin2]

Try it. Get a battery at 90% SoC. Get a really big power supply and bang 20 volts across it (you need a big power supply otherwise it can't get past the gassing voltage - it draws maximum current just below this). You'll get one hell of a lot of gassing, but not much current. Reduce the voltage to 14 volts and watch the current increase. Honest.

 

I did say I had no experience of this. But my comment was made in the context of normal battery charging voltages - not laboratory experiments at 20v. Just out of idle curiosity, is my comment/question correct at 14.9v - i.e. that there is no reduction in current due to the gassing when the gassing starts?

[paulcatchpole]

I assume that the SmartGauge software makes allowances for the gradual decline in the condition of a battery bank as it 'ages'

 

100% is 100% of what the capacity of the battery is now, so long as it hasn't 'shrunk' to less than 50% of what it started with.

 

I've got a set of Vince's finest, which I've killed a bit (not Vince's fault) and the Smartgauge doesn't like them very much these days. I've replaced them since, but am keeping them in reserve, charged, to see if I can eliminate one dead one from the set (he says, with wild and hopeless optimism).

 

Yours sound to be in a better state than that, if your smartgauge is behaving with them so far.

 

PC

[Gibbo]

I did say I had no experience of this. But my comment was made in the context of normal battery charging voltages - not laboratory experiments at 20v. Just out of idle curiosity, is my comment/question correct at 14.9v - i.e. that there is no reduction in current due to the gassing when the gassing starts?

 

Afraid not. But sort of

 

Maximum charging current occurs just below the gassing voltage. If the voltage is increased above this, then less current goes into charging (ie the batteries would actually charge faster by reducing the voltage). Sometimes the current into the batteries (ie what you can directly measure) will also actually decrease. Though usually it increases slightly or stays about the same, the extra current going into electrolysing the water and heating the battery up.

 

I assume that the SmartGauge software makes allowances for the gradual decline in the condition of a battery bank as it 'ages'

 

It does indeed.

 

this may explain some of the changes that I have seen - our batteries were second-hand when they were fitted two and a half years ago, so it is likely that they have lost some of their capacity. At present, with the invertor on and driving a TV, Sky Box, Computer and several small 240v adaptors, the battery bank (according to SmartGauge) drops from around 90% down to 40% in an evening (18:00 to 23:00) - two years ago it would have dropped to 65% or thereabouts and I could have gone for another day or two before the Smartgauge recorded anything as low as 40% - by comparison, I now need to charge the batteries on alternative evenings rather than every three days.

 

That certainly does sound like reduced battery capacity.

 

Is there a simple, non-technical, way of assessing the best time to replace batteries? Or have I just established (above) that we have reached that point already?

 

The simplisitc way is to replace them when they will no longer support your normal useage pattern in relation to how you'd normally charge (or want to charge) them. Alternatively a financial decision can be based on Justme's suggestion.

[nicknorman]

I think the problem here is the use of the word resistance to describe what goes on inside a battery coupled with the fact that there is a concept of the internal resistance of a battery when it is discharging.

 

Yes I think this is at the heart of the problem, to model a battery as a voltage source in series with a resistor is far too simplistic. I am sure that a fully charged battery has a very low discharge apparent internal resistance but quite a high charging apparent internal resistance since the process for current flow is entirely different in each direction (chemical reaction with the plates/acid for discharge, vs gassing of the water for charge)

 

Nick