A new alternator controller?

[smileypete]

Yes. Once the batteries are nearly full. It does nothing until that point.

Hi,

 

If the batteries are charged from 50% to 14.2V, very roughly what state of charge would they likely be at?

 

Edit: Maybe it would be useful for Chris to measure this.

 

cheers,

Pete.

Edited October 8, 2007 by smileypete

[blackrose]

Yes. Once the batteries are nearly full. It does nothing until that point.

 

But as you have already told us on at least one occasion, it is very important that the batteries are completely charged before being discharged again, and it is this final stage of charging that is often neglected because it is so difficult to get that last bit of charge into the batteries. Surely then, a device which increases the charge efficiency at this (or any) stage has got to be good?

 

I'm an electrical amateur and I'm only going by my own & Chris' measurements, and just as importantly by what you have said yourself. This argument has been going around almost as long as I have been on the forum, but when I pointed out this apparent contradiction last time my question was left unanswered.

Edited October 8, 2007 by blackrose

[Gibbo]

Hi,

 

If the batteries are charged from 50% to 14.2V, very roughly what state of charge would they likely be at?

 

Edit: Maybe it would be useful for Chris to measure this.

 

cheers,

Pete.

 

Impossibe to say.

 

Think about it......

 

A 200 amp charger on a 100Ahr battery will take it straight up to 14.2 volts, therefore in that case 50%

 

A 1 amp charger on a 100Ahr battery will not get to 14.2 volts til it's about 99%

 

Gibbo

[Gibbo]

But as you have already told us on at least one occasion, it is very important that the batteries are completely charged before being discharged again, and it is this final stage of charging that is often neglected because it is so difficult to get that last bit of charge into the batteries. Surely then, a device which increases the charge efficiency at this (or any) stage has got to be good?

 

I'm an electrical amateur and I'm only going by my own & Chris' measurements, and just as importantly by what you have said yourself. This argument has been going around almost as long as I have been on the forum, but when I pointed out this apparent contradiction last time my question was left unanswered.

 

I've never seen this "apparent contradiction" before. Ever.

 

You're confusing "charging the batteries" with "conditioning the batteries". They are two completely different things. Unfortunately there is some crossover which confuses the issue.

 

Getting the last few percent into a battery takes a LONG time. There is nothing that can be done about this. No amount of clever devices are ever going the change the chemistry of lead acid batteries.

 

From 50% to 100% takes an absolute minimum of 6 hours and in most cases a lot longer. Makes no difference what voltage you try to force at them. Any charger that apparently does this in 2 or 3 hours simply hasn't done it. It's just created sufficient illusion of having done it by surface charging the batteries. Come back to them the following day and you'll find they're nowhere near 100%

 

It is true that if one charges at too low a voltage then this time will be increased. But anything above 14.2 volts will do it and increasing the charge voltage higher than this will make next to no difference.

 

Going above the gassing voltage does not recharge the battery. Well it does, but scarily slowly. Nowhere near as fast as charging AT the gassing voltage. The benefit of going above the gassing voltage only starts once the batteries are nearly fully charged. It then starts to break up the stubborn crystalised sulphate.

 

The subject is *incredibly* complicated. The IEEE are still publishing papers on lead acid battery charging because despite well over 100 years of use there are still chemical reactions going on in there that people are only just discovering. It was only last year that someone finally worked out the full chemical reactions! Turns out that the "half reactions" that everyone has known about for years were only half the story.

 

Gibbo

[Guest]

I've never seen this "apparent contradiction" before. Ever.

 

You're confusing "charging the batteries" with "conditioning the batteries". They are two completely different things. Unfortunately there is some crossover which confuses the issue.

 

Getting the last few percent into a battery takes a LONG time. There is nothing that can be done about this. No amount of clever devices are ever going the change the chemistry of lead acid batteries.

 

From 50% to 100% takes an absolute minimum of 6 hours and in most cases a lot longer. Makes no difference what voltage you try to force at them. Any charger that apparently does this in 2 or 3 hours simply hasn't done it. It's just created sufficient illusion of having done it by surface charging the batteries. Come back to them the following day and you'll find they're nowhere near 100%

It is true that if one charges at too low a voltage then this time will be increased. But anything above 14.2 volts will do it and increasing the charge voltage higher than this will make next to no difference.

 

Going above the gassing voltage does not recharge the battery. Well it does, but scarily slowly. Nowhere near as fast as charging AT the gassing voltage. The benefit of going above the gassing voltage only starts once the batteries are nearly fully charged. It then starts to break up the stubborn crystalised sulphate.

 

The subject is *incredibly* complicated. The IEEE are still publishing papers on lead acid battery charging because despite well over 100 years of use there are still chemical reactions going on in there that people are only just discovering. It was only last year that someone finally worked out the full chemical reactions! Turns out that the "half reactions" that everyone has known about for years were only half the story.

 

Gibbo

Which is why most people without shore power accept that the batteries will only ever get a bulk charge, and will be cream crackered after about three years. As two Vetus dealers said to me " a battery boiler may help a little, but don't expect miracles," or words to that effect anyway. Bear in mind that they could have promised the earth and taken me for a few bob, as they both sold the devices. I can't speak for other engines.

[Gibbo]

From....

 

http://batteryuniversity.com/partone-13.htm

 

The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.

 

It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge.

 

There is no getting away from this fact. However much people want to.

 

Correct settings of the voltage limits are critical and range from 2.30V to 2.45V. Setting the voltage limit is a compromise. On one end, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate. A continually over-saturated condition at the other end, however, would cause grid corrosion on the positive plate. It also promotes gassing, which results in venting and loss of electrolyte.

 

Note the voltage limits. 2.45 per cell = 14.7 on a 12 volt battery. That's for calcium batteries not deep cycle or hybrids.

 

2.3 volts per cell = 13.8. That's for pure antimony which no one makes anymore because they sulphate up like mad because you can't charge them at very high voltages because they gas like mad.

 

A multi-stage charger first applies a constant current charge, raising the cell voltage to a preset voltage (Stage 1 in Figure 1). Stage 1 takes about 5 hours and the battery is charged to 70%. During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated. The topping charge takes another 5 hours and is essential for the well being of the battery.

 

Note he doesn't say "Oh if you wack a dead high voltage on it you can halve these times". The reason he doesn't say that, is because you can't.

 

The battery cannot remain at the peak voltage for too long; the maximum allowable time is 48 hours.

 

Like I said. You don't need a float function on an engine driven charger. 48 hours at full acceptance voltage is fine.

 

There is alot of info on that site that backs up what I say. Believe it or believe it not. Makes no difference to reality.

 

Gibbo

 

Which is why most people without shore power accept that the batteries will only ever get a bulk charge, and will be cream crackered after about three years. As two Vetus dealers said to me " a battery boiler may help a little, but don't expect miracles," or words to that effect anyway. Bear in mind that they could have promised the earth and taken me for a few bob, as they both sold the devices. I can't speak for other engines.

 

Absolutely spot on.

 

Gibbo

Edited October 8, 2007 by Gibbo

[Theo]

From....

 

http://batteryuniversity.com/partone-13.htm

There is no getting away from this fact. However much people want to.

Note the voltage limits. 2.45 per cell = 14.7 on a 12 volt battery. That's for calcium batteries not deep cycle or hybrids.

 

2.3 volts per cell = 13.8. That's for pure antimony which no one makes anymore because they sulphate up like mad because you can't charge them at very high voltages because they gas like mad.

Note he doesn't say "Oh if you wack a dead high voltage on it you can halve these times". The reason he doesn't say that, is because you can't.

Like I said. You don't need a float function on an engine driven charger. 48 hours at full acceptance voltage is fine.

 

There is alot of info on that site that backs up what I say. Believe it or believe it not. Makes no difference to reality.

 

Gibbo

Absolutely spot on.

 

Gibbo

 

I am now convinced and ready to die for my beliefs, so which alternator controller comes closest to the ideal.

 

Nick

[blackrose]

I've never seen this "apparent contradiction" before. Ever.

 

You're confusing "charging the batteries" with "conditioning the batteries". They are two completely different things. Unfortunately there is some crossover which confuses the issue.

 

Getting the last few percent into a battery takes a LONG time. There is nothing that can be done about this. No amount of clever devices are ever going the change the chemistry of lead acid batteries.

 

From 50% to 100% takes an absolute minimum of 6 hours and in most cases a lot longer. Makes no difference what voltage you try to force at them. Any charger that apparently does this in 2 or 3 hours simply hasn't done it. It's just created sufficient illusion of having done it by surface charging the batteries. Come back to them the following day and you'll find they're nowhere near 100%

 

It is true that if one charges at too low a voltage then this time will be increased. But anything above 14.2 volts will do it and increasing the charge voltage higher than this will make next to no difference.

 

Going above the gassing voltage does not recharge the battery. Well it does, but scarily slowly. Nowhere near as fast as charging AT the gassing voltage. The benefit of going above the gassing voltage only starts once the batteries are nearly fully charged. It then starts to break up the stubborn crystalised sulphate.

 

The subject is *incredibly* complicated. The IEEE are still publishing papers on lead acid battery charging because despite well over 100 years of use there are still chemical reactions going on in there that people are only just discovering. It was only last year that someone finally worked out the full chemical reactions! Turns out that the "half reactions" that everyone has known about for years were only half the story.

 

Gibbo

 

Ok, I'm completely out of my depth, but I thought we were talking about the amount of amps going into the batteries that Chris W mentioned had doubled with his battery boiler. You then said that this would only occur when the batteries were almost full. My point was about this last bit.

[Gibbo]

Ok, I'm completely out of my depth, but I thought we were talking about the amount of amps going into the batteries that Chris W mentioned had doubled with his battery boiler. You then said that this would only occur when the batteries were almost full. My point was about this last bit.

 

This is where it starts to get complicated.

 

The increase in charge current will be seen for a short time between the batteries reaching the regulation point of the internal regulator and them reaching about (it depends on the size of the alternator) somewhere between 75 and 85% state of charge.

 

Below the internal regulator voltage the external reg makes no difference. Above the state of charge shown above the charge will actually be less than without the external reg. The measured current may be more. But that current is going into producing gas and heat in the battery as opposed to charging the battery.

 

Only for a short period of time in between these will an increase in charge voltage (ie what the external controller does) actually achieve anything.

 

Think about it. If it was really that simple everyone would just charge their batteries at 20 volts and there wouldn't be all these discussions and complicated bits of kit to try to get round the problem. But why not 20 volts? Becasue the battery gasses like mad and all the bubbles in the electrolyte increase the resistance which REDUCES the charge current, not increases it.

 

The perfect charger is one I've been trying to develop for years but can't come up with a reliable, reproducible, method for detecting battery gassing that is simple to fit. Other than that I'd have done it.

 

It's simple really. Hold the batteries EXACTLY at the gassing voltage (not above it, not below) and that equates to the fastest charge you will ever get. Going either side of the gassing voltage REDUCES the charge going into the batteries.

 

It's simple enough to prove. Get a half flat wet cell battery and put it on a power supply. Adjust the voltage between 14.0 and 14.8 volts and you will find a peak current JUST below the gassing point. The problem is the gassing has a delay on it which you have to wait for. The gassing point and delay also changes like mad depending upon the state of charge of the battery and the temperature of the battery.

 

Gibbo

[blackrose]

This is where it starts to get complicated.

 

And I thought it was complicated enough already!

[Gibbo]

And I thought it was complicated enough already!

 

Trust me it's horrible.

 

I've been studying lead acid batteries for 5 years now on and off.

 

A colleague of mine did his Master's thesis on them and only scraped the surface.

 

For someone to think "it's some lead and some acid, just wack the voltage up, that will charge them quicker" shows a sort of quaint naivety.

 

Gibbo

[smileypete]

Impossibe to say.

 

Think about it......

 

A 200 amp charger on a 100Ahr battery will take it straight up to 14.2 volts, therefore in that case 50%

 

A 1 amp charger on a 100Ahr battery will not get to 14.2 volts til it's about 99%

OK, how about a couple of tests as follows:

 

1) Charge from 50% to 14.2v, then for another hour using the internal regulator. Measure the state of charge/SG the next day.

 

2) Charge from 50% using the same time period in 1) using the external regulator. Measure the state of charge/SG the next day.

 

Would this give some idea of the extra charging done by the external regulator over the internal regulator, also the final state of charge in either case?

 

cheers,

Pete.

[Guest]

This is where it starts to get complicated.

 

The increase in charge current will be seen for a short time between the batteries reaching the regulation point of the internal regulator and them reaching about (it depends on the size of the alternator) somewhere between 75 and 85% state of charge.

 

Below the internal regulator voltage the external reg makes no difference. Above the state of charge shown above the charge will actually be less than without the external reg. The measured current may be more. But that current is going into producing gas and heat in the battery as opposed to charging the battery.

 

Only for a short period of time in between these will an increase in charge voltage (ie what the external controller does) actually achieve anything.

 

Think about it. If it was really that simple everyone would just charge their batteries at 20 volts and there wouldn't be all these discussions and complicated bits of kit to try to get round the problem. But why not 20 volts? Becasue the battery gasses like mad and all the bubbles in the electrolyte increase the resistance which REDUCES the charge current, not increases it.

 

The perfect charger is one I've been trying to develop for years but can't come up with a reliable, reproducible, method for detecting battery gassing that is simple to fit. Other than that I'd have done it.

 

It's simple really. Hold the batteries EXACTLY at the gassing voltage (not above it, not below) and that equates to the fastest charge you will ever get. Going either side of the gassing voltage REDUCES the charge going into the batteries.

 

It's simple enough to prove. Get a half flat wet cell battery and put it on a power supply. Adjust the voltage between 14.0 and 14.8 volts and you will find a peak current JUST below the gassing point. The problem is the gassing has a delay on it which you have to wait for. The gassing point and delay also changes like mad depending upon the state of charge of the battery and the temperature of the battery.

 

Gibbo

Which is why they call 'em battery boilers.

Edited October 8, 2007 by Guest

[blackrose]

Which is why they call 'em battery boilers.

 

It's why you call them battery boilers! (I do it in jest). I think you've misunderstood. Since an external regulator will only charge up to the gassing point of the batteries it can't overcharge or boil them.

Edited October 8, 2007 by blackrose

[Guest]

It's why you call them battery boilers! (I do it in jest). I think you've misunderstood. Since an external regulator will only charge up to the gassing point of the batteries it can't overcharge or boil them.

I haven't misunderstood and disagree. Suggest you re-read Gibbo's post, he puts it much better than I ever could.

[blackrose]

I haven't misunderstood and disagree. Suggest you re-read Gibbo's post, he puts it much better than I ever could.

 

I read it perfectly well the first time. It is slightly ambiguous and can be read either way, but at 14.8v an external regulator is not overcharging a battery. I think you already know that. Anyway, if Gibbo can charge his batteries at 16v then nobody with an external controller is boiling a battery.

Edited October 9, 2007 by blackrose

[chris w]

I haven't misunderstood and disagree. Suggest you re-read Gibbo's post, he puts it much better than I ever could.

 

My alternator controller has battery temperature monitoring on it and the temperature only goes up a couple of degrees over the entire charge cycle and remember that they are in the engine compartment so they are going to get warmer anyway due to heat from the engine.

 

The implication given above is that the batteries are at some really elevated temperature through the use of an alt controller - well it just isn't so in practice in my case at least.

 

I guess the ultimate test will be to install one of Gibbo's Smartgauges (which I intend to do) and so accurately read the state of charge vs time vs current.

 

Chris

[dor]

Well, to add to all this, here is a bit of real life experience.

 

I used to be a great believer in external regulators, probably prompted by having a weedy little alternator that would be hard pushed to recharge a torch battery.

 

I now have a Vetus 4.17 with a 95 amp alternator fitted, with no external regulator. Now I've got my instrumentation sorted, I can report that the charge voltage peaks at 14.5 volts. At this point the little green charge indicator floats in the batteries say that the batteries are fully charged.

 

Charging current when the batteries are down a bit stays up around the 65 amp mark for quite a while, then will gradually start reducing as the voltage increases.

 

There are 4 110AHr service batteries plus a 95 AHr starter battery.

 

I really can't see any reason to fit an external alternator as I think I will gain next to nothing. Have I got this right?

[chris w]

Well, to add to all this, here is a bit of real life experience.

 

I used to be a great believer in external regulators, probably prompted by having a weedy little alternator that would be hard pushed to recharge a torch battery.

 

I now have a Vetus 4.17 with a 95 amp alternator fitted, with no external regulator. Now I've got my instrumentation sorted, I can report that the charge voltage peaks at 14.5 volts. At this point the little green charge indicator floats in the batteries say that the batteries are fully charged.

 

Charging current when the batteries are down a bit stays up around the 65 amp mark for quite a while, then will gradually start reducing as the voltage increases.

 

There are 4 110AHr service batteries plus a 95 AHr starter battery.

 

I really can't see any reason to fit an external alternator as I think I will gain next to nothing. Have I got this right?

 

The difference Dor will be less pronounced in your case were you to fit an external controller because you happen to have an internal regulator with a high voltage (14.5v). The increase, were you to fit an ext controller, would be of the order of another 15 amps. That's not 15 amps on top of your 65A because, when your alternator is pumping out that kind of current, the battery voltage will be below the internal regulator voltage and thus in the period before an ext controller would even kick in. It's an additional 15A later in the charge cycle.

 

An ext controller would still have five benefits:

 

1. It would raise the battery voltage to 14.8v which would assist desulphation

 

2. It would produce significantly more current (probably double as compared to without an ext controller) in the latter stages of charging

 

3. It would produce this additional current over a long time period

 

4. It would produce a much higher current (in the later stages) even at low engine revs

 

5. It would monitor both battery and alternator temperature and would cut off and sound an alarm if any abnormal temperature rise should occur. One forum member wrote about his alternator's being blocked by his dog's hair and burning out through overheating (ie: with no ext controller fitted)

 

 

The internal regulator on an alternator is not a multi-stage charger and is the equivalent of charging your battery on a high current "Halfords-type" car charger. There is simply one continuous constant voltage stage with decreasing current as the battery voltage rises.

 

If this were OK, why do we bother with multistage mains chargers and instead go buy a cheap car battery charger? (rhetorical question)

 

Chris

[Theo]

The difference Dor will be less pronounced in your case were you to fit an external controller because you happen to have an internal regulator with a high voltage (14.5v). The increase, were you to fit an ext controller, would be of the order of another 15 amps. That's not 15 amps on top of your 65A because, when your alternator is pumping out that kind of current, the battery voltage will be below the internal regulator voltage and thus in the period before an ext controller would even kick in. It's an additional 15A later in the charge cycle.

 

An ext controller would still have five benefits:

 

1. It would raise the battery voltage to 14.8v which would assist desulphation

 

2. It would produce significantly more current (probably double as compared to without an ext controller) in the latter stages of charging

 

3. It would produce this additional current over a long time period

 

4. It would produce a much higher current (in the later stages) even at low engine revs

 

5. It would monitor both battery and alternator temperature and would cut off and sound an alarm if any abnormal temperature rise should occur. One forum member wrote about his alternator's being blocked by his dog's hair and burning out through overheating (ie: with no ext controller fitted)

The internal regulator on an alternator is not a multi-stage charger and is the equivalent of charging your battery on a high current "Halfords-type" car charger. There is simply one continuous constant voltage stage with decreasing current as the battery voltage rises.

 

If this were OK, why do we bother with multistage mains chargers and instead go buy a cheap car battery charger? (rhetorical question)

 

Chris

 

Do any of those on the market fit these criteria, Chris?

 

Nick

[chris w]

Do any of those on the market fit these criteria, Chris?

 

Nick

 

 

Well mine does; that's how I generated the list.

 

Chris

[Theo]

Well mine does; that's how I generated the list.

 

Chris

 

Which one is it then? Please PM me if you don't want to be seen advertising.

 

Nick

[smileypete]

Which is why most people without shore power accept that the batteries will only ever get a bulk charge, and will be cream crackered after about three years.

Maybe because that's the most economical (in the short term) and convenient way to go.

 

If a solution involved a bit more spending on batteries and connections, and a little more hassle, would they be interested?

 

I really can't see any reason to fit an external alternator as I think I will gain next to nothing. Have I got this right?

Certainly sounds like it, if you're happy with the charge time and battery life.

 

cheers,

Pete.

Edited October 9, 2007 by smileypete

[blackrose]

5. It would monitor both battery and alternator temperature and would cut off and sound an alarm if any abnormal temperature rise should occur.

 

Chris, as you know, my external regulator doesn't have the remote control panel that performs these functions. It's got a battery temp sensor so I assume it would cut out (but not warn me with an alarm) if there was an abnormal temperature rise? My battery monitor will give me an overvoltage alarm.

 

On the subject of an abnormally high alternator temperature, would my external regulator shut down?

Edited October 9, 2007 by blackrose

[Gibbo]

Well, to add to all this, here is a bit of real life experience.I used to be a great believer in external regulators, probably prompted by having a weedy little alternator that would be hard pushed to recharge a torch battery.I now have a Vetus 4.17 with a 95 amp alternator fitted, with no external regulator. Now I've got my instrumentation sorted, I can report that the charge voltage peaks at 14.5 volts. At this point the little green charge indicator floats in the batteries say that the batteries are fully charged.Charging current when the batteries are down a bit stays up around the 65 amp mark for quite a while, then will gradually start reducing as the voltage increases.There are 4 110AHr service batteries plus a 95 AHr starter battery.I really can't see any reason to fit an external alternator as I think I will gain next to nothing. Have I got this right?

Yes. It would make hardly any difference at all.A very mild desulphation and that's about it.Your green floats in the batteries show that they have reached their maximum SG so increasing the charge voltage will do piss all other than waste some water and corrode the positive plate.They're obviously not getting sulphated up otherwise the SG wouldn't get high enough to show the green floats.The external controller would only come into effect AFTER the batteries reached 14.5 volts. And at that point you've already shown that they are fully charged with the green floats.Gibbo

My alternator controller has battery temperature monitoring on it and the temperature only goes up a couple of degrees over the entire charge cycle and remember that they are in the engine compartment so they are going to get warmer anyway due to heat from the engine.

So really you're saying that the battery temperature sensor isn't needed because they don't get too warm anyway.

The implication given above is that the batteries are at some really elevated temperature through the use of an alt controller - well it just isn't so in practice in my case at least.

No. What I said was that extra voltage just generates gas and heat. I didn't say it produced LOTS of heat.And your batteries aren't getting warm so the temp sensor is another waste of money :)So all you really needed to do was increase the charge voltage. Handful of cheap diodes will do that. As I've been saying for ages.Gibbo

The difference Dor will be less pronounced in your case were you to fit an external controller because you happen to have an internal regulator with a high voltage (14.5v). The increase, were you to fit an ext controller, would be of the order of another 15 amps. That's not 15 amps on top of your 65A because, when your alternator is pumping out that kind of current, the battery voltage will be below the internal regulator voltage and thus in the period before an ext controller would even kick in. It's an additional 15A later in the charge cycle.An ext controller would still have five benefits:1. It would raise the battery voltage to 14.8v which would assist desulphation

Agreed in principle. However his batteries are reaching their full SG so 14.5 volts he currently gets is achieving this. So in practice it won't make any difference in this regard.

2. It would produce significantly more current (probably double as compared to without an ext controller) in the latter stages of charging

The VERY latter stages. Maybe knock 5% off the total recharge time.

3. It would produce this additional current over a long time period

It wouldn't actually. It would produce an increased current for a VERY short period of time.His batteries are already reaching 100% as shown by the SG reading. This seems to coincide with his alternator peaking at 14.5 volts. INcreasing the voltage AFTER this is not going to reduce the recharge time because they are already full.

4. It would produce a much higher current (in the later stages) even at low engine revs

You'tre trying to make that sound like something it isn't. An external controller cannot make any difference if the current is limited by the RPM.

5. It would monitor both battery and alternator temperature and would cut off and sound an alarm if any abnormal temperature rise should occur. One forum member wrote about his alternator's being blocked by his dog's hair and burning out through overheating (ie: with no ext controller fitted)

Internal regs already have this feature. They reduce the output if the alternator gets too hot. You can't have it both ways. One day you say this is bad as it reduces the charge voltage, the next day you say it is needed because the alternator burns out without it.

The internal regulator on an alternator is not a multi-stage charger and is the equivalent of charging your battery on a high current "Halfords-type" car charger. There is simply one continuous constant voltage stage with decreasing current as the battery voltage rises.If this were OK, why do we bother with multistage mains chargers and instead go buy a cheap car battery charger? (rhetorical question)

Because a mains charger is on for days or weeks at a time. An alternator is not and therefore doesn't need a float cycle. And as Sterling recommend leaving the internal reg connected, it cannot go into float anyway because the internal reg stops it doing so!Gibbo

I read it perfectly well the first time. It is slightly ambiguous and can be read either way, but at 14.8v an external regulator is not overcharging a battery. I think you already know that. Anyway, if Gibbo can charge his batteries at 16v then nobody with an external controller is boiling a battery.

14.8 volts is WAY above the gassing voltage. Reducing the voltage to the gassing voltage will increase the charge rate.You can charge batteries at 16 volts. But you have to be bloody careful and keep your eye on things. Unfortunately there's no easy way for a charger to monitor what it needs to in order for this to work. And the same thing can be said about charging at ANY voltage above the gassing voltage.Gibbo

 

Edited to say: And the carriage return fairy can sod off!

Edited October 9, 2007 by Gibbo