Smart Gauge and Batteries again!

[lewisericeric]

I'm still having difficulties understanding batteries

 

I'd like to give you a scenario and see if you guys know more about it than me (which you probably will!)

 

I came home from work last night from work and had my engine on for hot water until about 7PM. This topped the batteries to 100% as well as the solar panels had put them up to 96% during the day.

 

Ok, I have a shoreline fridge which I normally turn off around 900PM. Last night I turned it off around 930.

 

Here is what I had on from 8.00PM until 1230AM:-

 

iPad was charging - 2 amps and hour approx

Laptop charging - 2amps approx

25w lamp on - 2 amps an hour

Wifi Internet dongle on - 1amp an hour

Inverter on - say 3 amps an hour worst case scenario

 

So....

2+2+2+1+3 = 10 amps an hour

 

10amps x5 hours = 50 amps from 8.00pm until 1230am ( let's say 1AM juSt to round it up.)

 

The fridge was only on for an hour and a half so let's say that the fridge uses 5 Amps an hour and that is over estimating as it is a 12v shoreline. So let's say another 8 amps was used over the evening for the fridge.

 

Total ampage = 58 amps

 

My battery bank is 330ah , so am i correct in thinking that I could work out what % my batts are at using the following calculation:-

 

330-58 =272

 

272/330x100 = 82%

 

However when I went to switch off and check the batteries they were at 67%!!!!

 

I checked the smart gauge and the voltage with the inverter still on and a light or two still on was flipping from 12.4-12.45v which, looking at battery tables online suggests around 80-85% charge.

 

Is the % screen on smart gauge wrong and the voltage screen more accurate? What should I be looking at ? Any one else have these problems or issues. I just find it hard to believe that my batts went from 100 to 67% in 4 and a half hours with a lamp on and a few small items on charge.

 

What do you guys think? Advise from the tech experts would be much appreciated!

Edited September 21, 2012 by lewisericeric

[matty40s]

My technical advice is to get a LED lamp toreplace the 25W one.

 

It does seem rather a large usage, however, there may be other variables

- did you not run a tap at some point??

- or how old are your batteries, they may not be 330AH capacity any more...

[lewisericeric]

Yes pumps and other cabin lights were being used periodically but only for short bursts of time.

 

The batteries were new in march of this year

[dmr]

Im going to risk a telling off from Gibbo here........

 

The Smartguage is not always quite as accurate as its spec (and many forum members) suggests, and in particular is not good during charging.

It can show a 100% charge when the actual charge is less than 100%, and depending on how you use the batteries, it can take a few hours before the smartguage sorts itself out. This could partly explain your rapid fall from 100% to 67%.

(these errors are not really a Smartguage fault, its just the physics of batteries, and the smartguage instructions do talk about the charging errors).

Also, as Matty said, your batteries might not be 330Ah. They might be old and/or they might be a bit sulphated.

Smartguage will say sulphated batteries are at 100% when they are full in their current condition. It might be that a really long charge (or an equalisation) could increase this capacity.

If you really want to get to the bottom of this then you would need a smartguage and an amp-hour meter, or maybe a hydrometer.

Give the batteries a really good charge and see if that helps.

 

Despite these comments I personally feel that the Smartguage is a truly wonderful device and that every boat should have one.

I am looking forward to the new version!

 

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

[smileypete]

What do you guys think? Advise from the tech experts would be much appreciated!

Maybe do a discharge test on the batts; give them a FULL charge then discharge at 16.5 amps for 10 hours. (16.5 x 10 = 165Ah = 50% of 330Ah)

 

If the batt voltage collapses below 11v before the 10 hours is up, the bank has lost >50% of capacity and the SmartGauge won't read SoC accurately, so I understand.

 

cheers, Pete.

~smpt~

[lewisericeric]

Despite being at 67% though the voltage was 12.4-12.45 . This suggests around an 80% charge but I'm guessing that's not accurate either as that was still with a load on it? Eg lights on and invert was still on at that point. When should I take voltage readings to get the most accurate reading?????i think I read somewhere that everything should be off for around half an hour and then the voltage levels out?

 

I agree dave, I really don't think it's accurate either the smart gauge, more of a guesstimate! However would you say the voltage screen is more accurate than the % screen.?

Edited September 21, 2012 by lewisericeric

[dmr]

Although the smartguage can get the 100% reading wrong I find it is really pretty good when it gets to the 50% region.

I suspect that if anything it might under-read slightly at the 50% point, but then I am not sure just how accurate my specific gravity readings are.

 

I am very surprised about the 67% reading at 12.45 volts.

I think Gibbo needs to answer this one, but I suspect either:....

...There is a solar panel connected??????

...The smartguage is not set to the correct battery type

 

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

[lewisericeric]

Battery type is set to 01 which is wet battery which ours are.

 

 

There is solar panels connected....how could that interfere with the smart gauge readings???

[David Schweizer]

lewisericeric,

 

I have a problem with electronic devices telling me how things are performing, and I have plenty of anecdotal evidence based on the electronics in my car to demonsrate that they cannot be relied upon.

 

From my perspective I would ask, was everything still working satisfactorily in the morning? were you able to re-charge you batteries during the day with a combionation of your solar panekls an the alternator? if so are you actually bothered or just curious?

 

I have never been convinced of the need for devices like the Smartguage, clever though it may be. If everything works fine and contuinues to do so, why do you need a box of electroniocs to tell you what you already know?

Edited September 21, 2012 by David Schweizer

[lewisericeric]

Curious and bothered David.... Otherwise if I keep dropping the batts to below 50% charge status I will knaacker them very quickly and I don't want to do that seeing as I got them only in march of this year

[Gibbo]

This one is easy...

 

I checked the smart gauge and the voltage with the inverter still on and a light or two still on was flipping from 12.4-12.45v which, looking at battery tables online suggests around 80-85% charge.

 

That's wrong. Where did you get that from?

 

12.40 volts is 70% not 80%

[matty40s]

Eg lights on and invert was still on at that point.

 

I think this bit is key to your reading anomaly.

If you are drawing load, it will not give you a true figure. The reading must be taken (minimum) 1 hour, a few hours, or, as I seem to remember Gibbo suggested a few days ago, 24 hours to give your batteries a time to rest and give a true state of charge.

Edited September 21, 2012 by matty40s

[Paul C]

I have never been convinced of the need for devices like the Smartguage, clever though it may be. If everything works fine and contuinues to do so, why do you need a box of electroniocs to tell you what you already know?

 

One way of managing something (generalisation) is to have it slightly over-specified that it will rarely/never eg run out of power. Another way is to monitor salient information and use this information wisely, to pre-empt small issues BEFORE they become large issues (like, water stops flowing because the pump doesn't work; the fridge stops working and food goes off; or the TV doesn't work; or lighting doesn't work).

 

I personally like the Smartgauge and because it works using different technology than measuring current and/or voltage, offers an alternative (and better) way to determine the state of charge of a bank of batteries. The only reason I don't have one is I've heard a rumour than an improved one is out in November.

[Gibbo]

The Smartguage is not always quite as accurate as its spec (and many forum members) suggests, and in particular is not good during charging.

 

It actually is. You have to compare it against the alternatives. I've seen amp hour counters 100% out.

[Keeping Up]

Lewisericeric,

 

You say you find it difficult to understand, but you have managed to get the basic approach exactly right. You may find it easier if you spend some time thinking about the terms Amp and Amp-hour, and getting them straight in your mind. Then whenever you do the maths (or myths) make sure you use the correct terminology throughout for not only will it make things clearer, it will also highlight any points where you have made any errors.

 

The rate at which anything consumes current, is measured in amps. Thus in your example your iPod takes 2 amps (not 2 amps an hour or anything else) and your 25 watt lamp also takes 2 amps.

 

If you multiply the current in amps by the time in hours you get the total amount of energy that has been taken, in amp-hours. So drawing current at a rate of 2 amps for a time of 5 hours means you have taken 2 amps x 5 hours = 10 amp-hours. That is also why battery capacities are quoted in amp-hours, which indicates how much energy you can take rather than how much current you can take.

 

Think about the units again, until you have them clearly defined, and you will definitely find it easier. Not that the people at Shoreline have not been able to understand this, and they are completely wrong when they quote the current taken by their fridges in "amps per hour" - that is a complete nonsense, they just mean "amps".

 

You have worked out your numbers correctly, as far as I can see at first glance. My suspicions are that your batteries were not holding 330 amp-hours when you thought they were full. This could be because they are not brand-new, and so their capacity isn't what it was; or it could be because they weren't actually at 100% (the Smartgauge site does say it won't be completely accurate when the batteries are charging), or more likely it is a combination of the two. Unfortunately a voltage reading, when the battery is either charging or discharging (or has been doing so within the previous few hours) is likely to be meaningless.

[lewisericeric]

Gibbo I typed in battery voltage charts in google and came up with a few, And most were saying 12.42v was 80% and 12.5 was 90??

 

Sorry if I was wrong!

Edited September 21, 2012 by lewisericeric

[Gibbo]

Gibbo I typed in battery voltage charts in google and came up with a few, And most were saying 12.42v was 80% and 12.5 was 90??

 

Show me them. They're wrong. So in actual fact your problem is that you either have more loads on than you realise or your batteries have lost capacity.

 

 

 

 

http://www.mpoweruk.com/soc.htm

[lewisericeric]

I was looking at the first 3 or so on the google image page to get an idea seeing as I'm no expert on batteries. I was mostly looking at the 3rd one and there's a few variations of it around too

 

http://www.google.co.uk/search?q=battery+voltage+charts&hl=en&client=safari&tbo=u&tbm=isch&source=univ&sa=X&ei=h2pcUNTRDsiI0AX4o4G4BQ&ved=0CDIQsAQ&biw=1024&bih=644#biv=i|6;d|fuHdExb9_gpKBM:

 

But thanks for giving me some useful and more detailed ones

Edited September 21, 2012 by lewisericeric

[Gibbo]

I was looking at the first 3 or so on the google image page to get an idea seeing as I'm no expert on batteries. I was mostly looking at the 3rd one and there's a few variations of it around too

 

http://www.google.co.uk/search?q=battery+voltage+charts&hl=en&client=safari&tbo=u&tbm=isch&source=univ&sa=X&ei=h2pcUNTRDsiI0AX4o4G4BQ&ved=0CDIQsAQ&biw=1024&bih=644#biv=i|6;d|fuHdExb9_gpKBM:

 

But thanks for giving me some useful and more detailed ones

 

Take this one.

 

That's the one you appear to be using. I'd suspect that the others have been copied from it because they have the exact same numbers (to 2 decimal places!) and that would simply never happen. Not even on the same battery during two different tests. Clearly some plagiarism going on there. Unfortunate that it comes up as the first hit.

 

I think someone in a marketing department made that up whilst eating his dinner. It's supposed to be an off load voltage chart. Off load, 10.5 volts does not mean 0% Soc. Off load, 10.5 volts means "I've broken my battery".

 

Note that the ones I posted all disagree wildly with that chart, but roughly agree with each other. Note also that they not exactly the same. Thus it would appear different people have actually run the tests instead of just copying what someone else made up.

 

Of the (literally) thousands of cycle tests I've run I can categorically state that the chart is complete and utter rubbish. The graphs are indeed correct.

[Grace and Favour]

Gibbo I typed in battery voltage charts in google and came up with a few, And most were saying 12.42v was 80% and 12.5 was 90??

 

Sorry if I was wrong!

 

I suspect (and I'm learning about batteries too!) that the deciding clue in reading battery charts is that the battery must have been resting for a number of hours before a specific reading can be taken:

 

Certainly if one's inverter is/has been on recently that will skew the figures, (though whether it skews them to be optimistic or pessimistic I don't know)

[lewisericeric]

Thanks gibbo, still not sure where I stand tho as my voltage was flashing between 12.4-12.45 and if that's not accurate until around 12-24 hours later then surely when all the loads were taken off my voltage would improve and I actually wouldn't have been at 67% ( kind of echoing what grace and favour said there?)

[Gibbo]

Thanks gibbo, still not sure where I stand tho as my voltage was flashing between 12.4-12.45 and if that's not accurate until around 12-24 hours later then surely when all the loads were taken off my voltage would improve and I actually wouldn't have been at 67% ( kind of echoing what grace and favour said there?)

 

Your loads are small enough to make almost no difference. On a 330Ahr battery bank, a 10 amp load will be dropping the at rest voltage by around 0.02 volts. Not worth worrying about.

[nicknorman]

 

Certainly if one's inverter is/has been on recently that will skew the figures, (though whether it skews them to be optimistic or pessimistic I don't know)

 

It will pull the voltage down a bit, making the SoC seem lower if you are trying to determine it just with a voltmeter. Similarly, having solar panels connected might raise the voltage so making the SoC seem higher. With the sun coming out and going in, I am sure it could result in really confusing voltage readings!

[Grace and Favour]

It will pull the voltage down a bit, making the SoC seem lower if you are trying to determine it just with a voltmeter. Similarly, having solar panels connected might raise the voltage so making the SoC seem higher. With the sun coming out and going in, I am sure it could result in really confusing voltage readings!

 

Ah! I see, thanks - - - -

 

(And thanks to Gibbo for those graphs - - - - I'll print them out for reference!)

[lewisericeric]

So r my batteries knackered or summat?! Only had em since march. Kept water topped up n never really let em drop below 70 only in rare cAses like last night.

 

Do extension leads have an effect on how many amps an item uses , just out of interest?