Tail Current - Setting

[Joshua]

I am trying to fine-tune the remote monitor and controls of my charging system.

 

In particular the Victron Battery monitor.

 

One setting is called ‘tail current’, I understand its function and how to set it but it depends on another parameter called ‘Battery Capacity Ah’.

 

Ordinarily I would simply read this as the stated capacity of my domestic bank i.e. 6 x 110 ah batteries = 660, but my little manual describes ‘Battery Capacity’ as:

 

“The battery capacity for a 20 h discharge rate at 20˚C”.

 

What exactly does that mean?

 

What is my battery capacity in this respect given that I have 6 x 110am batteries (new)?

 

 

 

 

Technical Detail:

 

The ‘tail current’ setting is very important as it dictates when my monitor thinks my battery bank is fully charged and on that setting my generator relies to tell it when to turn itself on.

 

The ‘tail current’ is set as a percentage of the ‘battery capacity’ and must be greater than the ‘charge current’, which the generator ‘floats’ at.

 

The allowable range is .5 to 10 and the default (based on a battery capacity of 200) is 4.

 

I have had mine currently set to the minimum .5 but now think this is too low as after 9 hours charging today, the monitor still does not register a fully charged bank.

 

I accept that it probably isn’t truly fully charged (the input amps are still reducing but at a painfully slow rate, finishing at around 3.5 amp when I gave up).

 

At that point I reset the tail current to the default 4% and the battery monitor immediately read a 100 SOC (It would because 4% of 660 is about 26, way more than the 4 amp my generator was putting into my batteries), this now sounds too high.

 

I obviously don’t want to have a 100% SOC reading when in fact it is only say 90%, as then when I set my generator to come on at 50% it will in reality be at only 40%.

 

Any advice on the best way to set this parameter gratefully received.

 

 

 

 

 

 

 

Thanks.

 

 

 

 

Joshua

 

 

[Theo]

 

 

"The battery capacity for a 20 h discharge rate at 20˚C".

 

What exactly does that mean?

 

What is my battery capacity in this respect given that I have 6 x 110am batteries (new)?

 

 

 

Your batteries will have different capacities depending upon how rapidly you discharge them. You can get more out of a battery if you discharge it slowly. I think that you will find that if there is only one capacity listed on the battery casing then it will be the 20A rate.

 

I have not met the term "20h discharge rate" before. Are you sure that you have typed this correctly?

 

I imagine that you meant to type 6 x 110Ah for the battery bank? I rather suspect that if you use 660Ah as the bank capacity you will be fine, but there will be someone along in a bit to correct any mistakes that I have made.

 

You could always get back to Victron to clarify

 

HTH

 

Nick

 

Nick

 

 

[Gibbo]

Where to start?

 

When you recharge a battery, you never get to the fully charged state you were previously at. It is impossible because there will be a tiny amount of sulfate left on the plates which represents chemicals that that can no longer partake. This is a normal part of battery aging. The batteries are now as fully charged as they can ever be (so they really are at 100%) but they have slightly less available energy than the last time round.

 

So now we've cleared that up we can deal with issue at hand.

 

When you recharge, the batteries are not really fully charged until the charge current drops to about 0.1% of the battery capacity/hours. Say you have a 100Ahr battery. They are fully charged when the charge current has dropped to 0.1amp. Two problems with that...

 

1. It will take a ridiculous amount of time. Not a problem if you're on shorepower, but if you're relying on engine/generator charging it means you're running the engine for hours and hours just to get the last 1% or so into the batteries and to allow the monitor to resync (so it doesn't start telling lies). It doesn't make sense to do that.

 

2. As the batteries age, sooner or later (a matter of weeks in reality) you will find that the charge current never drops below that level because the internal self drain and other losses swallow it up.

 

So you have to make a compromise between getting your batteries properly fully charged and getting them sufficiently charged so that you're at least somewhere in the region of fully charged and the battery monitor will resync.

 

2% is about right for shorepower charging, 4% is about right for engine/generator charging. That's really about as close as you can expect to get without analysing the entire system and your expectations for a week

 

I use 0.5% or 0.25% for running tests where the final figures are critical.

 

As with most things electrical, it's all compromises.

 

“The battery capacity for a 20 h discharge rate at 20˚C”.

 

It's the normal rating for battery capacity. If you discharge faster, you get less total energy out of it (Peukert's effect). Some batteries are rated at the 5 hour rate, or the 10 hour rate. But the most common is the 20 hour rate.

[by'eck]

The tail current percentage will not vary with battery capacity.

 

Either accept the default 4% or maybe take it down to 2%, otherwise the generator will never turn off, assuming its triggered to do so at 100% indicated SOC.

 

A lot depends on the float current of your fully charged bank which as pointed out, clearly needs to be less than the current calculated from tail current percentage of bank capacity.

 

If you are concerned that the generator won't be commanded on until below 50%, raise this threshold to 55% or 60%. TBH if I had an automated system such as this I would have the generator powering up at a much higher SOC, say 70% to improve battery life. I appreciate a balance has to be found though to prevent excessive cycling on/off.

[Chris Pink]

 

 

“The battery capacity for a 20 h discharge rate at 20˚C”.

 

 

 

it means if you multiple the current discharge to flatten the battery in 20 hours you get 110Ah (the capacity)

 

which is probably not as much as 5.5A because it's not flat flat, 80% if i remember right.

 

and if you discharge it twice as fast you'll get less than 110Ah (10 hour rate). 20 hour is reasonably kind to the manufacturers figure but is not a bad 'real world' median.

[Gibbo]

which is probably not as much as 5.5A because it's not flat flat, 80% if i remember right.

 

It is actually flat flat. 10.5 volts is usual.

[Joshua]

Where to start?

 

When you recharge a battery, you never get to the fully charged state you were previously at. It is impossible because there will be a tiny amount of sulfate left on the plates which represents chemicals that that can no longer partake. This is a normal part of battery aging. The batteries are now as fully charged as they can ever be (so they really are at 100%) but they have slightly less available energy than the last time round.

 

So now we've cleared that up we can deal with issue at hand.

 

When you recharge, the batteries are not really fully charged until the charge current drops to about 0.1% of the battery capacity/hours. Say you have a 100Ahr battery. They are fully charged when the charge current has dropped to 0.1amp. Two problems with that...

 

1. It will take a ridiculous amount of time. Not a problem if you're on shorepower, but if you're relying on engine/generator charging it means you're running the engine for hours and hours just to get the last 1% or so into the batteries and to allow the monitor to resync (so it doesn't start telling lies). It doesn't make sense to do that.

 

2. As the batteries age, sooner or later (a matter of weeks in reality) you will find that the charge current never drops below that level because the internal self drain and other losses swallow it up.

 

So you have to make a compromise between getting your batteries properly fully charged and getting them sufficiently charged so that you're at least somewhere in the region of fully charged and the battery monitor will resync.

 

2% is about right for shorepower charging, 4% is about right for engine/generator charging. That's really about as close as you can expect to get without analysing the entire system and your expectations for a week

 

I use 0.5% or 0.25% for running tests where the final figures are critical.

 

As with most things electrical, it's all compromises.

 

 

 

It's the normal rating for battery capacity. If you discharge faster, you get less total energy out of it (Peukert's effect). Some batteries are rated at the 5 hour rate, or the 10 hour rate. But the most common is the 20 hour rate.

 

Thank you very much.

 

 

 

 

On that basis, would it make more sense for me to programme my generator to stop and start charging by reference to charge rates and battery voltage respectfully, rather than SOC readings?

 

 

 

 

4% of my 660 bank would give me 26 and I can relatively quickly charge to rates well below that.

 

If I understand you correctly, would it make sense for me to note the point at which charging rates more or less grind to a halt and use that as my datum?

 

For example, If I note the point when the SOC seems to hit a brick wall (irrespective of what that figure is) and note at that stage that my charging current is say 8 amp, would it be sensible to set a tail current of 1.5 (that being about 10% of 660)?

 

 

 

 

How do I then calculate a sensible SOC for the generator to automatically start charging again?

 

I have to say that I am not very confident of the SOC readings, after extensive charging today to deliberately get the bank as charged as reasonably possible my SOC was still only reading 89%

 

Would it be too crude to assume that charging to a more sensible regime, I am only ever going to get about 90% charge and so set 60% SOC as my bottom line (that being in reality 50%)?

 

 

 

 

OR, would it be more sensible to use a given minimum voltage as the trigger to start charging? Say 12 volts and forget SOC altogether?

 

 

 

 

Joshua

 

 

[Gibbo]

For example, If I note the point when the SOC seems to hit a brick wall (irrespective of what that figure is) and note at that stage that my charging current is say 8 amp, would it be sensible to set a tail current of 1.5 (that being about 10% of 660)?

 

I think this might be a clue here. Am I right in guessing that you charge away, the current starts to taper off, but the actual indicated SoC doesn't seem to ever get to the top?

 

I think your meter hasn't got the right charge efficiency in it. If it calculataes that itself, it might need more cycles to get there (most [including my own] can take tens of cycles to get the right figure - there's a good technical reason for that). If you have to enter it yourself, you might have entered a figure that's too low.

 

What's the exact model? Is it really a Victron or is it a badged TBS?

[Joshua]

I think this might be a clue here. Am I right in guessing that you charge away, the current starts to taper off, but the actual indicated SoC doesn't seem to ever get to the top?

 

I think your meter hasn't got the right charge efficiency in it. If it calculataes that itself, it might need more cycles to get there (most [including my own] can take tens of cycles to get the right figure - there's a good technical reason for that). If you have to enter it yourself, you might have entered a figure that's too low.

 

What's the exact model? Is it really a Victron or is it a badged TBS?

 

It's a Victron BMV 6025.

 

 

 

 

We can manually tell it that the bank is 100% SOC but haven't done this.

 

 

 

 

After installation of the whole system, it remained on a land line for a couple of months until we got to test it off line.

 

We then had all sorts of problems in that despite the SOC suggesting we were at for example 60%, the voltage was so low at that stage that the bank couldn't even start the generator. The bank initially seemed to work fine then suddenly collapse.

 

2 or 3 weeks ago we fitted a brand new bank of batteries and they have only ever been charged either by our alternator or generator. Initially, the SOC readings seemed fine, they dropped overnight and returned to 100% as we cruised. For the first 10 days we cruised 8 to 10 hours a day so the generator was never used and the alternator never had much to do.

 

It's only really the last week when we have stopped and lived off the batteries for 2 or 3 days at a time and then charged back up. We simply are not getting back to a 100% reading, seems to slow up at 84% and practically stop at 90%.

 

I have just checked the Charge Efficiency Factor and this is set to the default of 95%

 

 

Joshua

Edited August 19, 2012 by Joshua

[by'eck]

SOC is a meaningless figure on the BMV600 unless you have it calibrated perfectly. I have generally left mine to defaults, changing just the Cb (battery capacity) and CEF to 93%.

 

Using it to monitor charge voltage and current drawn gives a far more accurate indication of battery SOC, the reading of which I use as a rough guide only.

 

To gain confidence try matching off charge/off load battery voltages with SOC according to this table. Clearly at the moment its severely out of kelter.

 

If you can get the BMV600 to trigger on battery volts remember that any load on the bank will temporarily pull the volts down so triggering on SOC may be better if it can accurately maintained.

 

Surely if in doubt having it trigger generator on a little early would be much better than too late. Your mention of it starting at 50% SOC or 12 volts both seem way too low.

[Joshua]

SOC is a meaningless figure on the BMV600 unless you have it calibrated perfectly. I have generally left mine to defaults, changing just the Cb (battery capacity) and CEF to 93%.

 

Using it to monitor charge voltage and current drawn gives a far more accurate indication of battery SOC, the reading of which I use as a rough guide only.

 

To gain confidence try matching off charge/off load battery voltages with SOC according to this table. Clearly at the moment its severely out of kelter.

 

If you can get the BMV600 to trigger on battery volts remember that any load on the bank will temporarily pull the volts down so triggering on SOC may be better if it can accurately maintained.

 

Surely if in doubt having it trigger generator on a little early would be much better than too late. Your mention of it starting at 50% SOC or 12 volts both seem way too low.

 

Many thanks for that, I did reply (twice) to your previous post but bizzarly my replies simply wouldn’t register on the forum, a reply to Gibbo at the same time, did!

 

 

 

 

Thanks also for the link to the volt/SOC relationship. I have been introduced to this factor and have looked at other similar charts. The problem is, as the article says, the difficulty of getting a truly at rest battery without the inconvenience of turning off everything on the boat (including the fridge) and then waiting for it to settle. Nevertheless I am going to pay much more attention to those readings and maybe set a low reading (say 12.2 Volts) to trigger the generator, irrespective of whether it is under load or not.

 

 

 

 

In the meantime, I am still a bit lost as to how to go about tuning my monitor to achieve a more accurate/stable SOC reading. I understand its limitations but unless I can get it stable and more accurate, it is no use as a basis for my charging regime.

 

 

 

 

There are many parameters I can set but not sure which to tweak or how to calculate the rate.

 

 

 

 

At one time, the SOC / Time To Go relationship was out of sync and I discovered that setting the right peukerts value for my batteries, even though this was only .05 different from the default, made all the difference, consequently I am wondering if there are similar small tweaks that I can try that might stabilise my SOC or is it just a case as Gibbo suggested of recycling the batteries enough times to let the monitor find its feet, so to speak?

 

 

 

 

Joshua

 

 

[by'eck]

After fully charging the batteries (make absolutely sure they are) just turn cycle the unit off/on or perform reset. SOC will by default be back at 100%.

 

The only other tweak I would suggest is the Peukert's percentage value so that indicated SOC stays in line with charge state as indicated by off charge voltage. A small indicated current draw (say less than 5 amps) will make little difference. Don't adjust anything from default values apart from bank capacity though until you have seen how the unit performs over several days and charge/discharge cycles.

[smileypete]

Why not fit a Smartgauge, should be well within budget.

 

Use the Victron monitor as a glorified voltmeter/ammeter, and control the genny manually.

 

cheers, Pete.

~smpt~

[Richard10002]

I think you are over complicating this.

 

At some point you feel that you are no longer getting "value for money" out of your generator. If this is 10A, then set the tail current value so the generator stops at 10A.

 

At some point, you believe your batteries are emptied enough to want to start the generator. This will be around 50% to 60% SOC, do set the generator to start when the monitor shows this figure.

 

The Victron SOC will never be absolutely accurate, as it is working on vary comPlicated equation. But it will be good enough for what you need.

 

If you find the generator is starting too early, change the setting, if too late, change the setting.

 

If you are cycling between 50-75% SOC and "full", you will be doing OK. When I say "full", I mean the point at which the tail current setting stops the generator.

 

Most of us do this manually

[nb Innisfree]

In the meantime, I am still a bit lost as to how to go about tuning my monitor to achieve a more accurate/stable SOC reading. I understand its limitations but unless I can get it stable and more accurate, it is no use as a basis for my charging regime.

 

There are many parameters I can set but not sure which to tweak or how to calculate the rate.

 

Joshua

 

As SoC relies on an accurate battery capacity setting and calculating that ever decreasing capacity relies on doing a regular destructive full discharge then it really is an impossible task to have a reliable SoC reading from a solely shunt based monitor. It's like chasing around in ever decreasing circles.

 

What's needed is some sort of monitor where the designer has done lots of exhaustive testing that enables it to make the best possible guess as to battery SoC.

 

I wonder if such a thing exists....

[Chris Pink]

 

 

What's needed is some sort of monitor where the designer has done lots of exhaustive testing that enables it to make the best possible guess as to battery SoC.

 

I wonder if such a thing exists....

 

Yes, it does, it's called a boater.

[grahame r]

As SoC relies on an accurate battery capacity setting and calculating that ever decreasing capacity relies on doing a regular destructive full discharge then it really is an impossible task to have a reliable SoC reading from a solely shunt based monitor. It's like chasing around in ever decreasing circles.

 

 

 

The Victron will give an accurate amp hour count if discharged from fully charged and an accurate voltage reading at rest if you turn everything else off. From the voltage reading you can estimate the actual SOC, you then know how many amp hours it took to get to that state of charge therefore you know the total capacity of the battery bank. No need to destroy anything.

This will not be totally accurate as Peukert's factor has not been taken into account however if discharged at a reasonable rate it will give a meaningful figure that may well surprise most people. My guess is that the actual battery bank capacity if far smaller than most people think. Taking out say 300 amp hours from a supposed 600 amp hour battery bank leaves it at a reasonable 50% but if the bank turns out to be only 400amp hours then it's at a suicidally low 25% so trying to establish the true battery capacity is important and should be done regularly as it will decrease with age.

[Joshua]

The Victron will give an accurate amp hour count if discharged from fully charged and an accurate voltage reading at rest if you turn everything else off. From the voltage reading you can estimate the actual SOC, you then know how many amp hours it took to get to that state of charge therefore you know the total capacity of the battery bank. No need to destroy anything.

This will not be totally accurate as Peukert's factor has not been taken into account however if discharged at a reasonable rate it will give a meaningful figure that may well surprise most people. My guess is that the actual battery bank capacity if far smaller than most people think. Taking out say 300 amp hours from a supposed 600 amp hour battery bank leaves it at a reasonable 50% but if the bank turns out to be only 400amp hours then it's at a suicidally low 25% so trying to establish the true battery capacity is important and should be done regularly as it will decrease with age.

 

Thanks for that Grahame, sounds like a simple and very useful little test.

 

My standpoint is, that there is no point having a high tech monitoring system that delivers a host of potentially useful information unless (a) the data is accurate and The user understands the significance of the data.

 

Chris Pinks cryptic comment is right, but unless you make an effort to deal with the above 2 factors its hard to see how you can become a well informed 'boater'.

 

 

 

 

Joshua

 

Edited to remove a phantom yellow disk

Edited August 20, 2012 by Joshua

[WotEver]

Chris Pinks cryptic comment...

... was intended to avoid saying "SmartGauge"

 

Tony

[Joshua]

... was intended to avoid saying "SmartGauge"

 

Tony

 

Think you have your wires crossed?

 

 

 

 

nb innisfree did obviously mean this, Chris obviously did not, on the contrary.

 

 

 

 

Joshua

 

 

[WotEver]

nb innisfree did obviously mean this, Chris obviously did not, on the contrary.

Which was precisely my point.

Edited August 20, 2012 by WotEver

[Joshua]

Which was precisely my point.

 

Oh I see, personal thing, I thought you were contributing, pity; this had been a useful thread up until now.

 

 

 

 

Joshua

 

 

[WotEver]

Oh I see, personal thing, I thought you were contributing, pity; this had been a useful thread up until now.

 

 

 

 

Joshua

Yes indeed, I agree. It's a shame Chris made it personal.

[liaoning]

The tail current percentage will not vary with battery Dell Latitude D520 capacity.

 

Either accept the default 4% or maybe take it down to 2%, otherwise the generator will never turn off, assuming its triggered to do so at 100% indicated SOC.

 

A lot depends on the float current of your fully charged bank which as pointed out, clearly needs to be less than the current calculated from tail current percentage of bank capacity.battery ASUS N71JQ

 

If you are concerned that the generator won't be commanded on until below 50%, raise this threshold to 55% or 60%. TBH if I had an automated system such as this I would have the generator powering up at a much higher SOC, say 70% to improve battery life. I appreciate a balance has to be found though to prevent excessive cycling on/off.

[Gibbo]

Spam alert?