voltage drop, batteries under load.

[DaveGood]

Does anyone have some accurate figures for this?

 

The only thing I've seen is a voltage drop of 0.32 volts on a battery bank of 1440 Ah when placed under a 20 amp load.

 

If we could cobble together some idea of what voltage drop to expect from a battery bank of known capacity when placed under a known load, us CC\liveboards, could have a rule of thumb for estimating state of charge.

 

DaveGood

[nicknorman]

Does anyone have some accurate figures for this?

The only thing I've seen is a voltage drop of 0.32 volts on a battery bank of 1440 Ah when placed under a 20 amp load.

If we could cobble together some idea of what voltage drop to expect from a battery bank of known capacity when placed under a known load, us CC\liveboards, could have a rule of thumb for estimating state of charge.

DaveGood

It will vary tremendously according to such things as temperature, battery condition and state of charge, whether taking current from the "surface charge" bit or from deeper in the batteries etc. Why not just get a Smartgauge, which works by a very complicated combination of voltage drop and rate of change of voltage that is far too difficult for someone with a voltmeter to emulate?

[Tony Brooks]

Sorry Dave but without a conductor (interlink) cross sectional area and length of cable that figure has absolutely no value or even validity.

 

You already have a rule of thumb for estimating the state of charge from the bank's voltage reading and as voltmeters have a very high resistance the current flow is so low you can consider the voltdrop in the measuring circuit to be zero.

 

Generally accepted rested voltage figures:

 

12.7 to 12.8 = Fully charged

12.5 CONSIDER half charged (actually 3/4 charged)

12.2 to 12.4 CONSIDER fully discharged (actually 1/2 charged).

 

Work within those figures and you will maximise your battery life by minimising the depth of discharge.

[DaveGood]

I'd love a smartgauge,

 

But I can put together a 200 watt solar system for less money, Bimble solar sell 120 watt kits for less money.

 

The smartgauge takes the battery voltage, does a bunch of clever maths and comes up with a figure.

 

Well I know my battery voltage, I know my capacity, I know my draw in amps.

 

All I need is reliable numbers on what voltage drop to expect from a given amp load to form a reasonably accurate guesstimate on battery SOC.

 

DaveGood.

[Arthur Brown]

Rule of thumb for use of a voltmeter, If your rested battery voltage is 12.4 or below then charge it. Full charge is when the battery has been constant voltage charged to 14.4v for an hour then float charged at 13.5 for an hour or more.

 

Best available method is to buy and fit a Smartgauge (perhaps with accessories) and charge battery when it says 50% capacity remaining.

[Naughty Cal]

Does anyone have some accurate figures for this?

 

The only thing I've seen is a voltage drop of 0.32 volts on a battery bank of 1440 Ah when placed under a 20 amp load.

 

If we could cobble together some idea of what voltage drop to expect from a battery bank of known capacity when placed under a known load, us CC\liveboards, could have a rule of thumb for estimating state of charge.

 

DaveGood

What the hell are you powering with all those batteries?

[Phil Ambrose]

What the hell are you powering with all those batteries?

A big old bad ass widebeam liveaboard.

Phil

[Paul C]

I agree with post #2, and would add that because this varies with a number of things, it means that voltage reading during a known discharge is an unreliable way to determine state of charge of batteries in general. It might be that you could "map" a table of voltages and loads and correlate it to state of charge, but that would be unique for your battery bank including its temperature at the time its done (so you could do a 3 dimensional table with temperature too......) Of course, the only way to do this would be to accurately measure state of charge somehow....

 

This is why the large amount of info out there which correlates voltage to state of charge is only valid for at-rest condition, including a resting period and removing surface charge from the batteries to obtain a valid/accurate reading.

 

Short-term, the only thing that loads care of is having enough voltage to run, to that end so long as they get their required voltage, they'll work. Long term, the only realistic way to measure state of charge is to either take an at-rest voltage, or use a battery monitor of varying complexity/accuracy eg smartgauge, amp-hour counter, some combination thereof, etc.

 

Is it really such a challenge to achieve an "at rest" condition, eg switch everything off for a bit? Fridge and its contents will cope for a little while being off, can't think of anything else which is needed 24/7/365.

[Richard10002]

Rule of thumb for use of a voltmeter, If your rested battery voltage is 12.4 or below then charge it. Full charge is when the battery has been constant voltage charged to 14.4v for an hour then float charged at 13.5 for an hour or more.

 

???

 

I thought rule of thumb "full charge" was when the batteries are being charged at 14.4V and the Amps drawn are less than about 2% of the bank capacity.

 

I dont think this is something you can put a time on, unless I have misunderstood something?

[nicknorman]

???

I thought rule of thumb "full charge" was when the batteries are being charged at 14.4V and the Amps drawn are less than about 2% of the bank capacity.

I dont think this is something you can put a time on, unless I have misunderstood something?

You are correct, although personally I think 2% is on the high side. If the battery is in good condition, really fully charged the current will be more like 0.5% but then it depends on what you mean by "fully charged".

[DaveGood]

What does the smartgauge itself measure?

 

Far as I can tell it measures probably three, maybe four parameters.

 

Voltage, for certain.

 

Time, for certain.

 

Battery temperature, probably.

 

And amphours in and out ( maybe, website isn't clear)

 

It can't measure other factors such as battery inter connect length and dimensions.

 

Well I know my battery voltage, I know the temperature, I've got a watch and know how to read it, and I have a good idea how many amps are being fed in and out of the battery at any one time.

 

I know more then the smartgauge can sense.

 

I don't know more about batteries then the guy who designed the smartgauge and embedded his knowledge within it as an algorithm. But if he achieved an accuracy of 1 per cent as regards to state of charge, I'm sure iI can get within plus or minus four to five hundred per cent of that number, and that's within five per cent.

 

All I want to know is, does anyone have reliable numbers for how far a battery bank of a given size drops it's voltage under a given amp load?

 

DaveGood

[bizzard]

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.

[nicknorman]

What does the smartgauge itself measure?

Far as I can tell it measures probably three, maybe four parameters.

Voltage, for certain.

Time, for certain.

Battery temperature, probably.

And amphours in and out ( maybe, website isn't clear)

It can't measure other factors such as battery inter connect length and dimensions.

 

It only measure voltage and time, it doesn't measure temperature (of the batteries) or current. However it does measure voltage and thus rate of change of voltage, to very high resolution, and 24/7. It notices periods when there is no discharge (dv/dt=0) and uses that measure of no-load voltage to refine its iterative calculations. All that is beyond you, unless perhaps you were to spend 24/7 staring at a precision voltmeter and writing down and processing everything you see. But if you are going to do that, I suggest your time would be better spent getting a minimum wage job in Tescos/McDonalds etc and after a couple of weeks, spending your spoils on a Smartgauge. And then relax...

 

What the Smartgauge certainly doesn't do is try to work out the SoC based on momentarily measuring the voltage with some known load on, since that strategy will never be satisfactory.

[DaveGood]

Nicknorman,

 

I understand what you are saying, but consider this, as far as "high resolution" measurement of voltage goes, the owner\ founder of "smart gauge" on his website says his device uses a voltmeter accurate to no more then 1 per cent. I've got voltmeters that accurate, so do most of us, it' the industry standard and they cost pennies.

 

How does measuring volts over time differ significantly from measuring amps over time as a measure of battery usage and state of charge? It' amps we use, not voltage, though voltage responds quicker and therefore gives higher resolution.

 

If it not possible (as the majority of posters here state) to get a volt reading that can be used to gauge SOC, unless the batteries have been disconnected for hours....

 

How does Smartgauge work? It either disconnects the users from all power in and out of the batteries for hours at a time, frequently, to get an accurate reading, or it doesn't need to, or it's a fraud.

 

I'm betting its not a fraud.

 

I'm also betting that since I know, to the same degree of accuracy as Smartgauge, what my battery voltage is at any given moment....

 

That I know what amps is being supplied to, or from the batteries at any given moment ( data smartgauge doesn't have.)

 

That I know what temp the batteries are currently at, and a whole host of other factors smartgauge doesn't know and can't measure.....

 

I can, using rule of thumb, off the top of my head calculation, work out that the current state of charge of my live board, in use day to day, battery bank to within five per cent soc.

 

If only I had some accurate numbers on how far down a given amp load drags the voltage of a given ah battery bank.

 

Which is information no one seems to have.

 

 

DaveGood

Edited March 30, 2015 by DaveGood

[nicknorman]

Nicknorman,

 

I understand what you are saying, but consider this, as far as "high resolution" measurement of voltage goes, the owner\ founder of "smart gauge" on his website says his device uses a voltmeter accurate to no more then 1 per cent. I've got voltmeters that accurate, so do most of us, it' the industry standard and they cost pennies.

 

How does measuring volts over time differ significantly from measuring amps over time as a measure of battery usage and state of charge? It' amps we use, not voltage, though voltage responds quicker and therefore gives higher resolution.

 

If it not possible (as the majority of posters here state) to get a volt reading that can be used to gauge SOC, unless the batteries have been disconnected for hours....

 

How does Smartgauge work? It either disconnects the users from all power in and out of the batteries for hours at a time, frequently, to get an accurate reading, or it doesn't need to, or it's a fraud.

 

I'm betting its not a fraud.

 

I'm also betting that since I know, to the same degree of accuracy as Smartgauge, what my battery voltage is at any given moment....

 

That I know what amps is being supplied to, or from the batteries at any given moment ( data smartgauge doesn't have.)

 

That I know what temp the batteries are currently at, and a whole host of other factors smartgauge doesn't know and can't measure.....

 

I can, using rule of thumb, off the top of my head calculation, work out that the current state of charge of my live board, in use day to day, battery bank to within five per cent soc.

 

If only I had some accurate numbers on how far down a given amp load drags the voltage of a given ah battery bank.

 

Which is information no one seems to have.

 

 

DaveGood

I was careful with my words. "High resolution" is not at all the same as "high accuracy". The smartgauge is not very accurate with its voltage measurement, but what it can do is to detect very small changes of voltage and hence accurate rate of change (because it has high resolution measurement of voltage, that makes the dv/dt accurate), and it is this dv/dt that is a major factor in it being able to work out the SoC under load.

 

As I said, it has a battery model based on a number of parameters, many set by the designer after years of testing, but also some set by its own programme - ie "learned values". It works only because its internal battery model is constantly being fed by the only variable it can measure (voltage, well and time as well I suppose).

 

If you just connect it and turn it on, it will read rubbish until it has watched the battery, taking perhaps 5 measurements per second, for hours or days (certainly at least one reasonable discharge/charge cycle).

 

In summary, it works in a completely different way from the method you are proposing. The NASA battery monitor I believe works in a similar way to your proposal (ie takes into account voltage and current) but again, it has the advantage over you of being on the case 24/7 without stopping for so much as a cup of tea!

[DaveGood]

Nicknorman,

 

Lead acid battery technology is over 140 years. In essence it's a couple of conducting plates separated by an acid electrolyte.

 

A two year old toddler who jams an iron nail and a lead pencil into a lemon has made a battery.

 

There are no moving parts.

 

Yet, it appears, the workings of one are so difficult and so complex we plebs are discouraged from seeking even the most basic information on how they function.

 

To ask how big a voltage drop I can expect if I load x amps onto a battery of known ah is the equivalent of me asking what is the mpg of the latest ford focus. Instead of this basic info being widely available, I'm being told (It is being implied) that that information is irrelevant, I wouldn't understand it if I knew it, and it would mislead me if I did know it.

 

And I'm being told this by people who they themselves, do not know.

 

This technology is more then 140 years old, why is this info not widely known?

 

DaveGood

Edited March 30, 2015 by DaveGood

[Keeping Up]

The technology behind a piece of string is hundreds if not thousands of years old, but it isn't any use expecting anyone here to tell you how long it would be.

[dor]

Does anyone have some accurate figures for this?

 

The only thing I've seen is a voltage drop of 0.32 volts on a battery bank of 1440 Ah when placed under a 20 amp load.

 

If we could cobble together some idea of what voltage drop to expect from a battery bank of known capacity when placed under a known load, us CC\liveboards, could have a rule of thumb for estimating state of charge.

 

DaveGood

 

I know what you are getting at, and agree it would be useful to look at a total ammeter and see how the current drops the volts. However, as pointed out, there are a lot of variables so it might be difficult to correlate one boat with another. However, I think you can get a good idea of what is going on through experience of your own system. Sad character that I am, I have a voltage display in my saloon (under the TV) so can easily see what the battery voltage is. I get a good idea of how well my batteries are holding up by noting what the voltage drops by when the fridge kicks in. If the batteries are well charged it can show very little drop, other times it might drop the voltage by 0.3 or 0.4 volts. It's all part of being familiar with your own system.

[DaveGood]

Perhaps so Keeping up.

 

A piece of string can be any length, but an amp, a volt, an ohm, a watt cannot be changed to suit anyone's whim, or win an argument.

 

DaveGood

[nicknorman]

Nicknorman,

 

Lead acid battery technology is over 140 years. In essence it's a couple of conducting plates separated by an acid electrolyte.

 

A two year old toddler who jams an iron nail and a lead pencil into a lemon has made a battery.

 

There are no moving parts.

 

Yet, it appears, the workings of one are so difficult and so complex we plebs are discouraged from seeking even the most basic information on how they function.

 

To ask how big a voltage drop I can expect if I load x amps onto a battery of known ah is the equivalent of me asking what is the mpg of the latest ford focus. Instead of this basic info being widely available, I'm being told (It is being implied) that that information is irrelevant, I wouldn't understand it if I knew it, and it would mislead me if I did know it.

 

And I'm being told this by people who they themselves, do not know.

 

This technology is more then 140 years old, why is this info not widely known?

 

DaveGood

Hmm well the planet/weather is a lot older than that, people have been studying it for centuries, but can you tell me whether its going to rain on the 10th May? I like your analogy of the mpg of a Ford Focus. Well yes you have some manufacturer's figures produced under ideal and un-natural conditions, but as to the real-work value well it depends on temperature, how heavy your right foot is (on both accelerator and brake), when you change gear, how fast you go, how much traffic, whether your tires are inflated correctly, what the road surface contamination is like, how many people in the car, when you last had it serviced. Do I need to go on?

 

No-on is saying that the information you seek is irrelevant, that you wouldn't understand it or that you might be mislead by it. We are simply saying that the information doesn't exist, just like no-one knows whether it will rain on a day a month ahead.

A battery might seem simple but when you start to look at how the chemical reaction actually plays out with all the physical consideration, it is far from easy to model.

 

As you say, the people whom you are asking the question of don't know the answer. Perhaps it is because we are all stupid or perhaps it is because there is no answer. But either way, when you find out the answer for yourself do tell us - you will be a hero!

[Naughty Cal]

Even if you know the voltage drop for a given load, what use will it be to you?

[Paul C]

Even if you know the voltage drop for a given load, what use will it be to you?

 

Known variables: voltage reading of battery, load currently applied could be used to predict state of charge.

[bottle]

ps.

 

to put Nicks first paragraph into simple language, read the small print on the adverts for cars.

 

It says the same

 

and so it is with batteries.

 

Sorry to say but the answer you want can only be got by you monitoring your batteries, on your boat, with your usage or buy a Smatgauge and get a very accurate idea on the SOC of your batteries.

 

(Only at rest or whilst being discharged, even Smatgauge manual says it is up to 10% inaccurate whilst batteries are being charged)

[by'eck]

Does anyone have some accurate figures for this?

 

The only thing I've seen is a voltage drop of 0.32 volts on a battery bank of 1440 Ah when placed under a 20 amp load.

 

If we could cobble together some idea of what voltage drop to expect from a battery bank of known capacity when placed under a known load, us CC\liveboards, could have a rule of thumb for estimating state of charge.

 

DaveGood

 

It would be impossible to provide this as the major variable is surface charge state which cannot easily be measured since its dynamic. My first thoughts were that your 0.32 volt drop was a little high for a 20 amp load on such a large bank, but if the batteries had been charged hard immediately before there would be a greater volt drop than otherwise, some caused by the load but probably more due to the surface charge being absorbed into the plate thickness. Clearly thick plate deep cycle batteries will display this phenomena to a greater degree than say a starter battery.

 

Similar issues when a high discharge has been applied to the bank. Here you would have surface discharge to contend with. If a light load was applied after such, the distribution of charge through the plates thickness could still leave you with a voltage rise.

 

Hopefully you see the futility of trying to predict such.

[Paul C]

Slightly at a tangent, has anyone fitted a Merlin DataCell II? This is effectively a Smartgauge Mk2, as well as using custom algorithms on voltage and voltage change over time, it also uses a shunt ammeter to properly measure current too.