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Merlin Smartgauge specifications and accuracy


nicknorman

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It has been repeatedly claimed on here that Merlin do not publish accuracy specifications for their Smartgauge product. Typing "Merlin Smartgauge specifications" into Google gives this link as the second hit:

 

http://www.merlinequipment.com/page/document.asp?id=159

 

Which contains the accuracy figures of within 5%SoC during discharge, within 10%SoC during charge, and voltage within 0.5%.

 

Hopefully this categorical information will put a stop to the spurious claims that they don't publish any accuracy figures.

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I don't have a smart gauge though I have considered one. Are those tolerances not a little wide? Ian

That's a bit of a non-question because it depends on what you think it should be! Oh and I'm assuming you mean the SoC accuracy rather than the voltage.

 

But perhaps it's appropriate to consider a few things:

 

1/ What do you want it for?

 

2/ What exactly is SoC?

 

3/ Is there an alternative gauge that is better?

 

 

For 1/, most people want it to get a general idea of how the batteries are doing and in particular when to recharge. Does it really matter whether, when the SoC is actually 60%, it reads 55% or 65%? And don't forget those accuracy figures are worst case, normally it will be better than that.

 

2/. It is very hard to define SoC. It depends on what the future discharge rate will be, future battery temperature etc which cannot be known, and what you consider "flat" to be. When a battery is nominally at 0% SoC there is still some extractable energy. So unlike a voltmeter or ammeter which is measuring a well-defined thing, the Smartgauge isn't.

 

3/ this is the key point. If you have an AH counting gauge AND know the battery capacity precisely ie it's real value, which changes as the battery ages and with temperature (which you can't know) AND always fully recharge to 100% SoC, then probably such a gauge is more accurate.

 

However in reality you don't knew the battery capacity and may well not always recharge to 100% SoC, in which case such a gauge has an error WITHOUT LIMIT, and an error that gets worse with time. Unlike the Smartgauge which gets better with time. This is why the manufacturers of AH-counting gauges such as the BMV and the Mastervolt equivalent, don't publish any accuracy figures for SoC.

 

So if you want a precise SoC reading, the only accurate way is by specific gravity readings. But if you live in the real world and don't like acid holes in your clothes, and want a device to give you SoC in 1 second rather than a few minutes of grovelling around in the engine bay, then the Smartgauge is the best option. So you may consider the tolerances a bit wide, but there is nothing better in practice.

 

Just as a real world example of the alternatives, friends have a BMV AH-counting gauge. They phoned me up to say that the lights were very dim and the water pump wasn't working, even though the gauge said 80% SoC. It transpired that the batteries were goosed, only having 20% of their rated capacity, and were totally flat, but the gauge was oblivious to that. If they had had a Smartgauge it would have been showing near zero SoC. So the SoC was about 80% out. Now that IS a big error!

Edited by nicknorman
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(snip)

So if you want a precise SoC reading, the only accurate way is by specific gravity readings. But if you live in the real world and don't like acid holes in your clothes, and want a device to give you SoC in 1 second rather than a few minutes of grovelling around in the engine bay, then the Smartgauge is the best option. So you may consider the tolerances a bit wide, but there is nothing better in practice.

 

(snip)

 

How acurate is this in practice, unless readings have been taken from new, and the battery's history is known? For example, if a battery is tipped and acid spilled, it is likely to be topped up with water, which will reduce the S.G. reading.

 

Also, the S.O.C. obtained by S.G. will be the percentage of the "as new" capacity**, in contrast to voltage or Smartgauge readings.

 

 

 

** This may be a more useful figure!

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... if a battery is tipped and acid spilled, it is likely to be topped up with water, which will reduce the S.G. reading.

 

 

Yes, which is why that should never be done.

Edited by WotEver
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How acurate is this in practice, unless readings have been taken from new, and the battery's history is known? For example, if a battery is tipped and acid spilled, it is likely to be topped up with water, which will reduce the S.G. reading.

 

Also, the S.O.C. obtained by S.G. will be the percentage of the "as new" capacity**, in contrast to voltage or Smartgauge readings.

 

** This may be a more useful figure!

I'll certainly admit to not being an expert on measuring SoC using specific gravity, but regarding your points my best guess is that:

 

Yes if you spilled acid and topped up with water, you would inevitably have a lower s.g. But realistically how often does that happen with canal boat batteries?

 

Regarding an aging battery with reduced capacity I don't think it's that simple. If the capacity is down due to sulphation, the s.g. will never get to its 100% SoC value (for a normal charge). So when the battery is fully charged you might get an s.g- measured SoC of say 60% for a badly sulphated battery. When the s.g. indicates zero SoC the battery will indeed be flat. Which aligns with what you are saying.

 

However if the capacity is down due to lost plate material then a fully charged battery will have its original s.g and thus show 100% SoC even though it's only at (say) 60% of its original capacity. The battery will become flat long before the s.g. indicates it would be.

 

Realistically an aging battery is likely to have both these effects in some combination so I think I agree with you that s.g. is not actually a reliable way to measure SoC other than for newish batteries. I was just repeating what I have heard others say, which is always a bad idea!

 

So what are we left with as the "gold standard" for measuring SoC? Is it just rested battery voltage, or is there something wrong with that too?

 

By the way, why do you think that knowing the SoC as it would be if the battery had the original capacity, is more useful than knowing the actual SoC?

Edited by nicknorman
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The whole concept of state of charge is a bit uncertain so its not worth worrying about Smartgage having an uncertainty of a few %.

Over a period of time the effective (rather than potential) capacity will probably slowly reduce due to sulphation, then hopefully an equalisation charge will restore most of the lost capacity.

There is also some evidence that the state of charge as indicated by specific gravity lags behind the amp-hours put in during charging, then increases disproportionately towards the end of the charge cycle. Then there is Peukert who suggests that the capacity is reduced if discharging at a high current, and people on this forum (including myself I think) who believe that this lost charge comes back if we wait a while.

Worrying about Smartgage missing by a few percent is the least of our troubles.

 

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

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smile.png It has taken you long enough to find them and I have searched their site and emailed them for them to no avail.

 

 

I think the first thing is that there is no perfect piece of kit for measuring SoC of a lead acid battery they all have their problems. The only guaranteed way I know with flood lead acids is to use a hydrometer, that used with a bit of care will give a good answer And does not need acid on trousers.

 

 

Looking at the amp reading devices they have to my view certain advantages, with a bit of care it is possible to work out the actual capacity of a battery using it in conjunct with a hydrometer. Additionally using its ability to read the amps going in or out of the battery when charging it is easily possible to measure the tail current which with a bit of care can tell you when the battery is fully charged. As for measuring SoC set-up properly some of them are good, almost as good as a hydrometer.

 

 

With regard to the smartgauge I have some major concerns, I have heard of/seen gauges that do not meet any acceptable level of accuracy. Now the excuse given is it came out of the factory out of spec. That worries me that a box that is protecting batteries costing upwards of £150 should be sold in such a state. It could lead someone to replace their batteries prematurely, it could lead to batteries being damaged if the misread tells customers the battery has more in it than it has.

 

 

At no point does it tell you the capacity nor do I know of a way of using its readings and say a hydrometer that it will.

 

 

Recently a member of the forum was persuaded to buy a smartgauge and interestingly he dumped a set of batteries I believe because according to the smartygauge they had no capacity left, the new smartgauge was defective, now were the batteries finished or not we will never know. Then from the forum he was persuaded to buy expensive batteries and the cycle repeated. However perhaps a copy of some notes I sent the owner after visiting him might help. To the to me it was another defective gauge. You will note in they I say I have not seen a spec for the gauge well that was true up to today. BTW when it says it can be 10% out charging that means it can get to a 90% charged battery and show 100% but that is where it stops showing 100% it does not go on rising until the battery is fully charged, albeit 10% out.

 

 

 

 

 

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The previous owners of our boat gazed at me in confusion when I asked him about batteries. They said they'd never considered batteries, charging or monitoring EVER, during their 4 years of ownership! Kept in a marina, used on holidays and weekends.

 

How different we can be!

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The previous owners of our boat gazed at me in confusion when I asked him about batteries. They said they'd never considered batteries, charging or monitoring EVER, during their 4 years of ownership! Kept in a marina, used on holidays and weekends.

 

How different we can be!

 

As a full time liveaboard I treat batteries as a consumable much like diesel. My Nasa monitor tells me what is being thrown in from whatever is being used to charge at that particular time. It tells me voltage. My life experience tells me plus how low the amps being thrown into the batteries when my batts are charged. If needed during the evening I simply check remaining voltage as equipment onboard is being used. I never ever even look at the silly percentage charged bit as they are never right and not needed. As the batteries age daily charging simply gets slightly longer day by day and morning voltage gets slightly lower day by day.

I only ever buy cheapos and simply change them when I feel the extra charging needs addressing. I never even check water and these batts are my first sealed ones. If I get three winters I am well impressed but usualy get 2 winters per set which is great in my opinion.

Life is too short to worry about batteries.

 

Tim

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Graham - I'm on the iPad so adding coloured bits is too difficult

 

Your second para. Yes I agree, there is no perfect piece of kit. However having thought about it I also maintain that measuring SoC by specific gravity is only reliable for fairly new batteries that still retain their full capacity. Perhaps you could explain how you think measuring SoC can be done accurately using specific gravity for a sulphated and plate-shed battery such as one finds on many canal boats?

 

Your third para. Measuring tail current is a good way to decide when to stop charging, but you don't need a fancy AH gauge for that, a Clampmeter will do. I should perhaps explain that the Smargauge's error on charge, whilst being up to 10%, is repeatable under the same circumstances, it is not a random error. So for example one might find that when it first reads 100% SoC the tail current (measured by Clampmeter) is still too high, and after another hour the tail current is satisfactory. Thereafter one just charges to 100% and for a further hour. Again I come back to the fact that 100% SoC is ill defined. One person might consider it 4% of capacity, another 2%, another 1%, whilst the reality is that 100% SoC is only asymptotically approached and thus never reached. After a day on charge it might be 0.3%, but who wants to run their genny for 24 hours?

 

Your 4th para. Yes we have found that out of the very many gauges forumites have, one is way out of calibration, a couple of others have been marginal. This is a manufacturing issue, not a design issue. Fortunately it is easy to check the calibration since if the voltage reading is correct, the SoC reading will be within tolerance. There is only one adjustment, that is the voltage calibration. Of course I quite agree with you that a customer should not have to make that check. However I think it is unreasonable to damn a piece of equipment due to a manufacturing quality control issue that is easily detected, especially when in every other respect the device is the best option for most people.

 

Your 5th para: since the effects of battery deterioration on s.g. is unknown I agree, but then you can't to what you are suggesting with an AH gauge for the same reasons.

 

Your 7th para: yes it has been very unfortunate that Tommy's gauge was defective. However I'd say he was unlucky, these things can and do happen. Diagnosing by remote control is always difficult and since many of us know that the Smartgauge normally works very well, you can rightly accuse us of placing too much reliance on it in this case. Lesson learnt, but when you are dealing with electrical questions from someone who doesn't even have a multimeter, it is difficult. Regarding your last bit as I have already mentioned 100% SoC is so ill defined. Under charge the Smartgauge will carry on counting up until it reaches 100%SoC. At this point the tail current might be 4%, 2%, 1% or possibly less. At what point should it read 100%?

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Funnily enough a mate has just popped round for a brew and we were in the middle of a conversation about Smartgauge for his boat and what they do. I know bugger all about serious battery monitoring, my feet are very firmly in the Mrsmelly camp, but i said i know somewhere that does and this this thread is 4th down on recent topics.

 

He asks...I have 4 leisure batterys 2 x 110 & 2 x115 of indeterminable age if I fit a SG will it tell me what the capacity of my batterys are now and to wire in is it just a case of poss and neg one to each end of the bank.

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(snip) (1)So what are we left with as the "gold standard" for measuring SoC? Is it just rested battery voltage, or is there something wrong with that too?

 

(2) By the way, why do you think that knowing the SoC as it would be if the battery had the original capacity, is more useful than knowing the actual SoC?

 

1) I use Smartgauge cheers.gif On Kelpie,there is also an amp hour counter, which, combined with Smartgauge, gives a consistent measure of battery capacity (which is currently about 100 AH, for a bank of four nominaly 110AH batteriessad.png )

 

2) A percentage of a known quantity might be more useful than of an unknown one. In Kelpie's case, 60% does not mean a discharge of 44AH will result in 50%! I'd expect doing it by S.G. would result in a starting figure of around 13%. Must try it before I replace the batteries!!!!!!!

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1) I use Smartgauge :cheers: On Kelpie,there is also an amp hour counter, which, combined with Smartgauge, gives a consistent measure of battery capacity (which is currently about 100 AH, for a bank of four nominaly 110AH batteries:( )

 

2) A percentage of a known quantity might be more useful than of an unknown one. In Kelpie's case, 60% does not mean a discharge of 44AH will result in 50%! I'd expect doing it by S.G. would result in a starting figure of around 13%. Must try it before I replace the batteries!!!!!!!

1) Yup, I do the same. I'm lucky though, our 2 year old bank still has its rated capacity!

 

2) I suppose it's a matter of opinion. Personally I think knowing a percentage of a thing you have, rather than a percentage of a thing you don't have, is the more useful!

He asks...I have 4 leisure batterys 2 x 110 & 2 x115 of indeterminable age if I fit a SG will it tell me what the capacity of my batterys are now and to wire in is it just a case of poss and neg one to each end of the bank.

No, the Smartgauge doesn't tell you what the actual capacity is, it only tells you the state of charge as a % of what you actually have.

 

Yes, you just wire a + and - diagonally across the bank.

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No, the Smartgauge doesn't tell you what the actual capacity is, it only tells you the state of charge as a % of what you actually have.

 

Yes, you just wire a + and - diagonally across the bank.

 

 

Thanks

 

 

 

Edited by Rod a mod from Tod
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(snip) 2) I suppose it's a matter of opinion. Personally I think knowing a percentage of a thing you have, rather than a percentage of a thing you don't have, is the more useful!

 

 

 

I've got to agree, once you know what you have clapping.gif

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Measuring tail current is a good way to decide when to stop charging, but you don't need a fancy AH gauge for that, a Clampmeter will do.

A lot of mains chargers have indication of how much charge they are putting in at the time as well, be it a multi-stage charger than tells you what stage its in, or inclining a form of current output display.

 

If I had a Smart Meter I expect I would augment it with an shunt-based Ammeter, which I guess you might not need with a AH based battery gauge, but the cost of a ammeter is now where need that of eather type of meter to having it as well is quite feasible.

 

 

Daniel

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I think if an idea of remaining actual total capacity is wanted there would be a need to measure amp hours used.

 

To me the ability to measure the batteries total capacity is important as it tells me how the battery life is going. I cannot see how that can be done with a simple shunt ammeter.

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I think if an idea of remaining actual total capacity is wanted there would be a need to measure amp hours used.

To me the ability to measure the batteries total capacity is important as it tells me how the battery life is going. I cannot see how that can be done with a simple shunt ammeter.

I'd agree with you if you wanted a convenient way of doing it, but if you are prepared to switch on a known load (measured by the Clampmeter) and leave it on for X hours with no other loads so the current remains steady, you can calculate the AH used and compare with the (correctly calibrated!) Smartgauge SoC and thus work out the capacity.

 

Of course fitting an AH counter as well is a much more convenient way of keeping track of actual capacity, but at a price.

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If I had a Smart Meter I expect I would augment it with an shunt-based Ammeter...

I completely agree with you Dan. Shunt ammeters are pocket money these days and can tell you a lot in addition to being one of the best ways to know when to stop charging.

 

Knowing what your bank's capacity actually is is for many people a completely irrelevant number. What they need to know (apart from that their charging regime is good) is how much 'fuel' they have left in the batteries. If that 'fuel' is running out too fast then they either need to reduce usage or improve their bank. It doesn't matter if the capacity gives a usable 250Ah or 120Ah; they don't care. They simply need to know that they have sufficient for their needs.

 

Everyone is different. To some, it's important to know everything about their batteries - Ah used, Ah replaced, current capacity and more. SmartGauge wasn't designed for them, it was designed for those folk who just want a battery fuel gauge, and a simple ammeter is probably the best companion for one.

 

Tony

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