voltage drop, batteries under load.

[MtB]

I just use 1 little digital voltmeter I bought off ebay for £1.50 something. They come with very nice, either orange or blue digits. This little chap is in my sitting room on the boat. Marvelous. I won't have any more battery gauges at all which all club together, helping to flatten the batteries. Some folk have huge arrays of gauges with monitors monitoring the gauges and more monitors monitoring the monitors, and so on and so on and so on. As for something called Smart, that word worries me.

 

Yes it's a word beloved of middle-ranking salesmen and managers isn't it, when for example they exhort their underlings to "Work smarter, not harder".

 

Pass me a bucket please, I think I'm gonna HURL...

[bizzard]

I've tried to explain the issues to you, patiently and repetitively, but you are intransigent and now somewhat rude. I'm out.

I'm out too, he called me Buzzard!! after I gave the low down in what I thought was a very clear, concise and in depth way by my own analysis of voltage drop under load.

[Dave Clinton]

Loddon, Naughty Cal,

 

Do either of you know how much fuel your engine will use on half power for eight hours? I bet you do, and if you don't it'll take you less then ten minutes online to find out what your engine should burn in that time.

 

My question is the same, but applied to batteries.

 

This is stuff we all should know.

 

DaveGood

 

This, more than anything, reveals the flaw in your reasoning. I can't tell you how much fuel your engine will use on half power (revs, load, rated BHP? - first catch your rabbit) in 8 hours (reasonably easy to define). Even if given the basic parameters, it would depend on many variables I have no access to. Wind, water depth, boat shape, weight, weediness, age of engine, transmission efficiency to name but a few.

 

It is also true that the 'solution' to your postulated problem is a region in a multi-dimensional array that contains very little extra information on which we can make any decision (charge the batteries, use the hairdryer, switch the TV off) than most people can find out with a voltmeter and a bit of experience or, if too lazy or electrically illiterate, with a smartgauge.

 

Getting stroppy with people who have given you considered and intelligent responses won't get you any closer to a 'truth'

 

Allan Keeping Up has given you the most accurate answer you will get from this discussion.

[dmr]

Wasn't someone asking a similar question just a few weeks ago? And did I not post a little graph of on-load voltage?

A forum search should find this.

I am happy to provide more detailed information if required I have the required equipment (batteries voltmeter and ammeter) but it might be much more rewarding for the op to do these measurements himself, especially in view of what I charge for engineering measurements!.

 

............Dave

[DaveGood]

Loddon

 

I read what you wrote,

 

" roughly

 

With no load and least two hours after charging..."

 

Nicknorman, you made a very eloquent series of statements, which while useful, did not contribute anything I wasn't already aware of from my own reading, but utterly failed to answer the simplest of questions, if a battery bank of given amp hours is placed under a given amp load how much will the voltage drop?

 

We all know batteries drop under load because we've all seen it.

 

That must follow some basic physical law.

 

But no, apparently I'll need access to a supercomputer to work it out while all smartgauge needs to work out SOC,a much more complex equation, is one number, voltage over time.

 

You don't know now much a battery drops under load and you want me to believe it's fantastically difficult to work out, and would have no significance when calculating SOC if I did.

 

 

 

DaveGood

Edited March 30, 2015 by DaveGood

[DaveGood]

Dave Clinton

 

If I place a fully charged battery bank of 440 amp hours under a ten amp load, what voltage drop will I see?

 

If you know, tell us, if you don't, can you find out and tell us?

 

If you can't do either,any contribution you make to this thread should have a higher entertainment quotient.

 

DaveGood

 

PS if the mods don't ban me soon, Betty Boo will as soon as she sees this thread.

[DaveGood]

Dmr,

 

started the search, a shed load of possible threads. I'll work my way through every last one,

 

And in advance,

 

Thank you,

 

DaveGood

Edited March 30, 2015 by DaveGood

[ROBDEN]

Dave Clinton

 

If I place a fully charged battery bank of 440 amp hours under a ten amp load, what voltage drop will I see?

 

If you know, tell us, if you don't, can you find out and tell us?

 

If you can't do either,any contribution you make to this thread should have a higher entertainment quotient.

 

DaveGood

 

PS if the mods don't ban me soon, Betty Boo will as soon as she sees this thread.

 

Can't you do this yourself? Charge up your batteries, put a known load on it and watch for the voltage drop.

This would then be a good base line for any future readings on YOUR boat under YOUR conditions.

 

Rob....

[by'eck]

By,eck

 

The smartgauge gives a state of charge reading under dynamic load, so for that matter does your volt meter, your rev counter, and your temperature gauge.

 

Which ones do you not trust when under "dynamic load", all, some, none?

 

DaveGood

 

I'm sure it does although I understand extraordinary situations cause it temporary inaccuracies. In answer to your OP that made no mention of Smartguage, I was explaining about the dynamics of surface charge/discharge effects not dynamic loads.

[John Williamson 1955]

The voltage drop of any battery, no matter what the chemistry of that battery, under load will vary depending on the temperature, the condition of the battery and the state of charge at the time. The voltage drop at the load will also vary according to the resistance of the connections between the battery and the load, including the wire used.

 

To get an indication of what happens with any one system, you need to use an accurate voltmeter and an ammeter that senses the magnetic field round the cabling to avoid any voltage drop in the meter shunt. Then, starting with full batteries, vary the current in steps, noting the voltages. Then while the battery voltages drop as the charge gets less, repeat to taste. You will then have the figures you need. With lead acid batteries, there's normally a quite accurate but non-linear relationship between the state of charge and the specific gravity of the electrolyte within any particular battery.

 

With correctly rated wiring and batteries, the voltage variation with load will be much less than the variation with connected with the state of charge, though. Hundredths of a volt, rather than tenths.

[dmr]

The voltage drop of any battery, no matter what the chemistry of that battery, under load will vary depending on the temperature, the condition of the battery and the state of charge at the time. The voltage drop at the load will also vary according to the resistance of the connections between the battery and the load, including the wire used.

 

To get an indication of what happens with any one system, you need to use an accurate voltmeter and an ammeter that senses the magnetic field round the cabling to avoid any voltage drop in the meter shunt. Then, starting with full batteries, vary the current in steps, noting the voltages. Then while the battery voltages drop as the charge gets less, repeat to taste. You will then have the figures you need. With lead acid batteries, there's normally a quite accurate but non-linear relationship between the state of charge and the specific gravity of the electrolyte within any particular battery.

 

With correctly rated wiring and batteries, the voltage variation with load will be much less than the variation with connected with the state of charge, though. Hundredths of a volt, rather than tenths.

 

No!

 

A shunt based system is fine for measuring current.

Also all this stuff about thickness of cables and quality of connection is nothing but a red herring. The OP is interested in BATTERY volt drop so if he has any sense he will be measuring voltage at the battery terminals, just like Smartgage does, so cabling is irrelevant. A volt drop of tenths of a volt will still be measured and this volt drop is within the electro-chemistry of the battery itself.

 

..............Dave

[DaveGood]

Rob,

 

I live aboard, off grid. No shorepower, on a boat where it is a complete b*****d to get at the batteries to measure SG and confirm the state of charge that way. My boat is my home, it's either under charge or providing power.

 

I have to rip out the back cabin to get at the batteries.

 

To begin with I assumed that the voltage drop of X amps on Y amphours of lead acid batteries, within reasonable limits, would be easily found info. After all, it's technology over a hundred years old.

 

Try finding it!

 

So far I've tried four major battery manufacturers, two won't answer, one thanked me for my interest, and the fourth told me it was "proprietary information".

 

Next I went to a number of websites that concern themselves with (among other things) battery usage.

 

Look back through this entire thread, there's something like sixty responses, not one of them an answer.

 

A lot of them deploy carefully worded explanations as to why a simple, straightforward physcal\chemical process is far too complicated for me to worry my pretty little head about.

 

Oh, and the people saying this don't have the slightest clue what they are talking about.

 

So they'll start referencing stuff they've never measured but heard mentioned, "magnetic fields" or, "non-linear relationship" is a dead giveaway.

 

 

If you wanted to know, say, the equivalent question in an equally old technology.

 

Such as....

 

" How much fuel does a diesel engine, (something built from scores of moving parts) burn per hour to produce 1 HP?"

 

You'd get a correct answer delivered in one short sentence from dozens of people in a few minutes.

 

DaveGood

Edited March 30, 2015 by DaveGood

[Sir Nibble]

The volt drop on the batteries is the product of the current and the internal resistance of the battery. The internal resistance depends on the state of charge so it would be possible to create a graph but you would need to be accurate to millivolts and milliamps at least to make it worthwhile.

[Sir Nibble]

The answers you have had are pretty good. You may think there MUST be an answer but there isn't. It will alter with the age of your batteries, between makes and even between individual units. The battery voltage under load certainly is an analogue of the state of charge but need lab conditions not a boat to be accurate. It's a good, perfectly logical idea but not really practical under real world conditions in my opinion.

[DaveGood]

dmr,

 

I'm keeping an (amused) eye on you.

 

This is a sixty plus thread, most of it we!l meaning waffle, some of it outright bull####, and the rest of it faux outrage that I dare question thier wisdom and knowledge.

 

It's sixty plus waffle, BS, and grandstanding, zero on info and answers so far.

 

I forget who it was who used the term "Non-Linear Relationship", a few posts back, (like he works at CERN ) in some kind of effort to demonstrate he knew what he was talking about, but I bet if whoever that was, if out of their sight for more then five days....

 

.....his mother, his wife, and his children will have forgotten him too.

 

DaveGood

Edited March 31, 2015 by DaveGood

[DaveGood]

Just so you all know,

 

After sixty plus responses and no useful information ( I'm investigating dmr's hints as to another thread where the holy grail may be found ).

 

I can state that the majority of you have been well meaning, some of you have promoted yourself as bearers of knowledge you don't poessess, none of you have been useful ( with the possible, still to be determined status of dmr ).

 

I consider this thread closed and will not look at it again.

 

It has been of no use at all.

 

DaveGood.

Edited March 31, 2015 by DaveGood

[dccruiser]

Just so you all know,

After sixty plus responses and no useful information ( I'm investigating dmr's hints as to another thread where the holy grail may be found ).

I can state that the majority of you have been well meaning, some of you have promoted yourself as bearers of knowledge you don't poessess, none of you have been useful ( with the possible, still to be determined status of dmr ).

I consider this thread closed and will not look at it again.

It has been of no use at all.

DaveGood.

 

 

What an utterly rude and unnecessary post .... a lot of what has been said to you has been very valid but you have chose to ignore it.

Had i posted sooner i would have told you i had changed my batteries rece tly from 12 volt cells to very heavy 6 volt units from a scissor lift, in both cases they are Open Lead Acid but the new batteries fall in voltage for the same fixed load over the same time period as the old ones even though the new ones have a 120amp lower combined capacity, so although you dont want to accept it, it does very much vary with the state and type of battery , the readings i have taken have been with a multimeter and more recently a smartguage and victron bm702.

[dccruiser]

I would just like to add, after 30 years of owning boats, i have found every battery and/or bank has behaved differently even when renewed within a few weeks or months at the most they have become very unique depending on usage and charge patterns, if you choose to dismiss this out of hand then it is you who has less knowledge than you think.

If you want the most accurate chart write one that pertains to your batteries on your boat , just remember to update it every 3 months to keep it accurate and also remember you need a summer and winter one.

 

Rick

[ROBDEN]

Dave.

 

I know you’re not reading this (bet you are really) but I understand your frustration.

 

Like you, I’m an off grid live-a-board with hard to get at batteries and have posted questions

on here and haven’t got the answer I wanted or expected.

But sometimes, there is no answer…..yet.

 

Happy hunting. If you do find the answer, please let us know.

 

Rob….

[GUMPY]

Is it just my reading of this thread or does DG have echoes of CP in his tones?

[Tony Brooks]

 

No!

 

A shunt based system is fine for measuring current.

Also all this stuff about thickness of cables and quality of connection is nothing but a red herring. The OP is interested in BATTERY volt drop so if he has any sense he will be measuring voltage at the battery terminals, just like Smartgage does, so cabling is irrelevant. A volt drop of tenths of a volt will still be measured and this volt drop is within the electro-chemistry of the battery itself.

 

..............Dave

 

 

That is not what he said in his original post - that is unless you think he actually has a single 1440 Ah battery. I am not sure measuring the voltdrop across one battery in a bank will give anything but a rough estimate of the state of the other batteries.

[nicknorman]

Is it just my reading of this thread or does DG have echoes of CP in his tones?

No, CP was quite intelligent. Anyway we know who DG is.

[Tony Brooks]

 

SNIP

 

So they'll start referencing stuff they've never measured but heard mentioned, "magnetic fields" or, "non-linear relationship" is a dead giveaway.

 

 

If you wanted to know, say, the equivalent question in an equally old technology.

 

Such as....

 

" How much fuel does a diesel engine, (something built from scores of moving parts) burn per hour to produce 1 HP?"

 

You'd get a correct answer delivered in one short sentence from dozens of people in a few minutes.

 

DaveGood

 

 

Well that just about sums up the OP's grasp, he only has to look at the many fuel consumption threads on here to find out.

 

No you would not, or if you did it would not be correct EXCEPT under one very specific test condition. There fuel consumption would vary according to speed and load and would only be correct if all the conditions of the test were quoted. Come to think of it I think batteries are much like that because of Mr P's work. Their actual capacity will vary with the speed of discharge.

[bottle]

Rob,

 

I live aboard, off grid. No shorepower, on a boat where it is a complete b*****d to get at the batteries to measure SG and confirm the state of charge that way. My boat is my home, it's either under charge or providing power.

 

Exactly the same as me

 

I have to rip out the back cabin to get at the batteries.

 

 

Exactly the same as me

 

To begin with I assumed that the voltage drop of X amps on Y amphours of lead acid batteries, within reasonable limits, would be easily found info. After all, it's technology over a hundred years old.

 

Try finding it!

 

So after 100 years the data does not exist, I wonder why?

 

So far I've tried four major battery manufacturers, two won't answer, one thanked me for my interest, and the fourth told me it was "proprietary information".

 

Probably a nice way of saying they do not know after 100 years of experience

 

Next I went to a number of websites that concern themselves with (among other things) battery usage.

 

Well that is probably because they have not been going for a hundred years.

 

Look back through this entire thread, there's something like sixty responses, not one of them an answer.

 

A lot of them deploy carefully worded explanations as to why a simple, straightforward physcal\chemical process is far too complicated for me to worry my pretty little head about.

 

The explanations explain why the answer is not known, for the last 100 years.

 

Oh, and the people saying this don't have the slightest clue what they are talking about.

 

But they do, there have been many explanations by those that have been in the industry for many years, not quite a 100myears for a lot of them though.

 

So they'll start referencing stuff they've never measured but heard mentioned, "magnetic fields" or, "non-linear relationship" is a dead giveaway.

 

Is it?

 

 

If you wanted to know, say, the equivalent question in an equally old technology.

 

Such as....

 

" How much fuel does a diesel engine, (something built from scores of moving parts) burn per hour to produce 1 HP?"

 

You'd get a correct answer delivered in one short sentence from dozens of people in a few minutes.

 

No you would not, you would get an approximation as to what it would be in ideal situation, with out any outside influences.

 

As before, see the get out clause in the car adverts.

 

DaveGood

 

 

I wish you well in your quest, when you find the answer, the 'Holy Grail' will be in the next room.

[dmr]

dmr,

 

I'm keeping an (amused) eye on you.

 

This is a sixty plus thread, most of it we!l meaning waffle, some of it outright bull####, and the rest of it faux outrage that I dare question thier wisdom and knowledge.

 

It's sixty plus waffle, BS, and grandstanding, zero on info and answers so far.

 

I forget who it was who used the term "Non-Linear Relationship", a few posts back, (like he works at CERN ) in some kind of effort to demonstrate he knew what he was talking about, but I bet if whoever that was, if out of their sight for more then five days....

 

.....his mother, his wife, and his children will have forgotten him too.

 

DaveGood

 

I am genuinely pleased that I am amusing you, the forum should be an entertaining as well as informative place.

 

Smartgage is expensive so I understand your desire not to get one, I held out for several years, but now I have one I know its worth every penny.

 

You effectively want to do your own smartgage on the cheap, so you must already have a voltmeter. Get a low cost clamp ammeter then you can make your own measurements of volt drop which will be much more useful than anything the forum supplies (or doesn't), and by the look of this thread a lot less aggravation.

 

There are a few people here who understand electricity, but many more who don't but still keep repeating incorrect facts that they have previously read on the forum or heard from their mates, so even if someone answers your question how will you know if what they say is true?.

 

Is there a facility to search ones own posts??? if so I will try to find my volt drop graph????

 

..............Dave