calculating leisure battery capacity ? Am I on right lines?

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Sailingsaves

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I looked at this website:
http://www.practicalcaravan.com/forum/equipment-accessories/39293-leisure-battery-testing-how

The chap concluded that a leisure battery that was fully charged used 18.4 Amps in 4 hours and left his battery at 75% discharged.

so 75% discharge took 18.4Amps

18.4Ah/75 = 0.245 (to obtain what 1% discharge represents)
(Or 18.4 divided by 75/100 = 18.4 x 100/75 = 0.245)

i.e. 1% of charged battery = 0.245A
so 0.245 x 100 (to get 100% charged capacity) = 24.5Ah.

NOW:
My second hand leisure AGM battery (rated 100Ah) used 1 Amp for 1 hour.
At rest the voltage was 12.85V
After 1 hour draw it rested at 12.81V

IFF (sic) I assume 12.88 is fully charged (instead of 12.7 Volt), then is the following correct?
12.85V to 12.81 Volt = 5% discharge.

So 5% discharge took 1Amp.


1 divided by 5/100 = 1 x 100/5 = 20
i.e. 1% of charged battery = 20A
so 20 x 100 (to get 100% charged capacity) = 200Ah CLEARLY WRONG !!!!!

or, at should it be
1 divided by 5 = 0.2
0.2 x 100=20Ah total capacity ?!

Anyone help?

IN SUMMARY, I took a battery rested at 12.85V for 24 hours. Drew 1 Amp for 1 hours and after 2 hours of rest, battery returned to 12.81V - what capacity does this 10year old battery have now?

Thanks if anyone an cure my headache.
Cheers in advance.

For info, one website I have lost location of stated:
12.7V = 100% charged
12.64V = 95% charged
12.58V = 90% charged
etc
 
Sailingsaves said:
IN SUMMARY, I took a battery rested at 12.85V for 24 hours. Drew 1 Amp for 1 hours and after 2 hours of rest, battery returned to 12.81V - what capacity does this 10year old battery have now?

Thanks if anyone an cure my headache.
Cheers in advance.

For info, one website I have lost location of stated:
12.7V = 100% charged
12.64V = 95% charged
12.58V = 90% charged
etc
Click to expand...

First there is the problem of surface charge. Even then those voltage numbers can in no way be taken as a scientific calibration scale and the scale is not necessarily linear in any event.

They are only rules of thumb !
 
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nigelmercier said:
Nope. Battery capacity is specified over a defined discharge time.
Click to expand...

Quite: old Peukert’s law rears its head again :rolleyes:

For the OP: I think 20 Hrs is the standard rating for discharge scaling down to a usable voltage of around 10.5. Drain faster than that and less amp hours will be available, whereas slower discharge rates will produce somewhat more.
 
I think there is some confusion over units. Amps/ahr/watts. Capacity of a battery is measured in amp hours at nominal voltage of 12v.
Fully charged is often taken as 12.6v, fully discharged 11.6v although completely discharged is obviously 0v.
Anything other than that is confusing to my small brain.
Not quite sure what the OP is trying to find out.
 
Firstly the chap with the caravan got his arithmetic about right to begin with ... then someone challenged it and he changed his mind


My rule of thumb is
Very roughly
12.7V (or more) rested is fully charged.
11.7 rested is to all intents and purposes completely discharged.
That is conveniently a 1 volt difference . So every 0.1 volt below 12 .7 represents a 10% discharge making 12.45 about 75% charged and 12.2 volts about 50%

But its a rough rule of thumb and batteries vary



So his 12.45 volts rested was about 75% charged and his initial calculation of capacity of around 73 Ah was about right

Except that capacities are quoted at the "20hr" rate so a somewhat lower test current would have been more appropriate.


Your 1 amp for 1 hour test is far too small a discharge to make any sort of sensible estimate of your battery capacity. Increase it to 5 amps, for a 100Ah battery, and discharge for a good few hours ( 10 perhaps), Allow the battery to rest and check its volts.


But really you are over egging all of this
 
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Sailingsaves said:
what capacity does this 10year old battery have now?
Click to expand...

10 years old? Realistically the answer is almost certainly "very little".
As it's an AGM battery, its charged open circuit voltage is likely to be more like 12.9 than 12.7 as AGM batteries normally have higher specific gravity acid which in turn produces a higher voltage.
Taking 1 Ah out of a 100Ah battery should not produce a significant change in the rested open cct voltage so your readings support my initial conclusion above.
I would stick a headlamp bulb taking about 5A on it, which in theory - as it's a 100Ah battery, almost certainly on a 20 hour rating - should last 20 hours to about 10.5V on load to be nominally fully discharged.(yes, use on load voltage - rested open cct volts on an old battery will not give you a realistic guide to its actual capacity.)
I suspect you will find it will drop off a lot quicker.
 
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I have always found that batteries discharge twice as fast as any book, guide or website says. Working on that principle I have never been disappointed. Same with the solar panels, I assume ½ the output the book says.
 
Plevier said:
10 years old? Realistically the answer is almost certainly "very little".
As it's an AGM battery, its charged open circuit voltage is likely to be more like 12.9 than 12.7 as AGM batteries normally have higher specific gravity acid which in turn produces a higher voltage.
Taking 1 Ah out of a 100Ah battery should not produce a significant change in the rested open cct voltage so your readings support my initial conclusion above.
I would stick a headlamp bulb taking about 5A on it, which in theory - as it's a 100Ah battery, almost certainly on a 20 hour rating - should last 20 hours to about 10.5V on load to be nominally fully discharged.(yes, use on load voltage - rested open cct volts on an old battery will not give you a realistic guide to its actual capacity.)
I suspect you will find it will drop off a lot quicker.
Click to expand...

Great info thanks.
Will give it a go.
Here is the battery:
https://www.osibatteries.com/p-2300...ed-lead-battery-12v-101ah-front-terminal.aspx

I was hoping that even at 10 years old (I may have that wrong - they are designed for a 10 year lifespan) that a second hand one may have some life left in it for use in the garden with solar and windturbine experiments. They cost about £500 plus new I think !
 
Sandmartin said:
I have always found that batteries discharge twice as fast as any book, guide or website says. Working on that principle I have never been disappointed. Same with the solar panels, I assume ½ the output the book says.
Click to expand...

But I rather doubt you've ever connected a boat battery to a book, guide or website, so how are they going to tell you anything specific to your installation and use? I've found that my batteries, on various boats over many years, behaved pretty much as anticipated.
As to expecting specified wattage from solar: could you tilt the panels? If not, were you fairly close to a tropic? If neither, what else did you expect? (A book, guide or website might have given you some idea.)
 
Sailingsaves said:
Great info thanks.
Will give it a go.
Here is the battery:
https://www.osibatteries.com/p-2300...ed-lead-battery-12v-101ah-front-terminal.aspx

I was hoping that even at 10 years old (I may have that wrong - they are designed for a 10 year lifespan) that a second hand one may have some life left in it for use in the garden with solar and windturbine experiments. They cost about £500 plus new I think !
Click to expand...

OK that is a top spec industrial battery, to the same sort of specs as the stuff I used to work on.
It is rated at 100Ah on an 8 hour to 1.75VPC (that's the on load voltage) at 20 degrees which is a stringent spec and I'm sure would have met it when new.
I don't think that range has been around for 10 years but not sure about that.
Of course the 10 year life is on the basis of being on standby, never discharged except for testing occasionally, correctly commissioned and correctly float charged and kept in a cool environment. Oh and correctly stored before installation of course. Failure will (or should!) be defined as capacity down to 80% of nominal.
You may be lucky, it all depends on how they have been treated.

Catalogue http://www.enersys-emea.com/reserve/pdf/EN-VFT-RS-017_0615.pdf
Performance data http://www.enersys-emea.com/reserve/pdf/EN-VFT-PD-010_1012.pdf
Installation, operation and maintenance http://www.enersys-emea.com/reserve/pdf/EN-V-IOM-005_1012.pdf

Have a good look at the initial commissioning requirements and the float/charging requirements. Bear in mind these are standby not cycling batteries.
Also note the short circuit current rating - 2100A. Don't do it!

Note these warnings in the manual:
Deep discharge will produce a premature deteriation of the battery and a noticeable reduction in the life expectancy of the battery.
Operation of valve regulated batteries at temperatures higher than 20°C will reduce life expectancy. Life is reduced by 50% for every 10°C rise in temperature. (I'm sure VicS will confirm that's basic chemistry. An extra 10 degrees doubles the reaction rate. As these batteries are expected to fail through long term corrosion, that's the effect on life. Of course if they are being cycled this relationship may become secondary.)
 
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VicS said:
Firstly the chap with the caravan got his arithmetic about right to begin with ... then someone challenged it and he changed his mind


My rule of thumb is
Very roughly
12.7V (or more) rested is fully charged.
11.7 rested is to all intents and purposes completely discharged.
Click to expand...

I don't recall where I first saw it, but years ago I read:

12.7 in heaven

12.5 still alive

12.2 charging due

11.8 it's too late
 
Plevier said:
OK that is a top spec industrial battery, to the same sort of specs as the stuff I used to work on.
It is rated at 100Ah on an 8 hour to 1.75VPC (that's the on load voltage) at 20 degrees which is a stringent spec and I'm sure would have met it when new.
I don't think that range has been around for 10 years but not sure about that.
Of course the 10 year life is on the basis of being on standby, never discharged except for testing occasionally, correctly commissioned and correctly float charged and kept in a cool environment. Oh and correctly stored before installation of course. Failure will (or should!) be defined as capacity down to 80% of nominal.
You may be lucky, it all depends on how they have been treated.

Catalogue http://www.enersys-emea.com/reserve/pdf/EN-VFT-RS-017_0615.pdf
Performance data http://www.enersys-emea.com/reserve/pdf/EN-VFT-PD-010_1012.pdf
Installation, operation and maintenance http://www.enersys-emea.com/reserve/pdf/EN-V-IOM-005_1012.pdf

Have a good look at the initial commissioning requirements and the float/charging requirements. Bear in mind these are standby not cycling batteries.
Also note the short circuit current rating - 2100A. Don't do it!

Note these warnings in the manual:
Deep discharge will produce a premature deteriation of the battery and a noticeable reduction in the life expectancy of the battery.
Operation of valve regulated batteries at temperatures higher than 20°C will reduce life expectancy. Life is reduced by 50% for every 10°C rise in temperature. (I'm sure VicS will confirm that's basic chemistry. An extra 10 degrees doubles the reaction rate. As these batteries are expected to fail through long term corrosion, that's the effect on life. Of course if they are being cycled this relationship may become secondary.)
Click to expand...

Wow, thanks for the info. I bought it as opposed to a new Banner 110Ah (£120) as a gamble that the industrial one would have been better engineered and looked after.

link - er - I appear not to be able to paste a link ?! stick www. in front of this lot:

leisurebattery.net/?gclid=COPm77a0o84CFe0y0wodM1cEXw#!product/prd1/3401475875/banner-energy-bull-95901-115ah-(k20)-135ah-(k100)



Reckon I made a mistake, especially as I was not told they were standby batteries, but were ok for solar installations, but will have fun experimenting. What you have told me does tie up with what I have found though. If discharged to 12.4V over 24 hours (running LEDs, cctv etc) a 20W panel and a windturbine do not put the amps back in to bring it up to full charge; in fact the current allowed into the big yellow brick is being retarded somehow because when I swapped it for a different battery, the watt meter showed A LOT more amps being allowed into the battery.

Thanks for the info
 
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I would consider them totally inappropriate for solar use, or cycling. Sounds a dishonest sale. Can you return them?
What was claimed to be the history?
 
Tilting panels is not much use on a boat either sailing or on a swinging mooring as it will rarely be tilting in the right direction. It's feasible if tied up to a marina pontoon but you'd probably have mains available.
macd said:
As to expecting specified wattage from solar: could you tilt the panels? If not, were you fairly close to a tropic? If neither, what else did you expect? (A book, guide or website might have given you some idea.)
Click to expand...
 
ghostlymoron said:
Tilting panels is not much use on a boat either sailing or on a swinging mooring as it will rarely be tilting in the right direction. It's feasible if tied up to a marina pontoon but you'd probably have mains available.
Click to expand...

Yes, of course, ghostly. I was actually disputing the nonsense written in post #10 rather than suggesting how panels should be mounted.
 
macd said:
Yes, of course, ghostly. I was actually disputing the nonsense written in post #10 rather than suggesting how panels should be mounted.
Click to expand...
Thats ok then. I always rise to the bait when tilting and swivelling panels are mentioned. My practice is to use semi flex panels installed near horizontal. That way you may not be optimized but will always be getting some output (when its light) - sailing or moored, on board or at home.
 
ghostlymoron said:
My practice is to use semi flex panels installed near horizontal. That way you may not be optimized but will always be getting some output (when its light) - sailing or moored, on board or at home.
Click to expand...

I think, and observations of other liveaboard boats seems to bear this out, that much depends on how much wattage you have room to point at the sky. We're lucky in having sufficient (340W) to supply our needs without faffing about with their orientation. Inevitably that means their output is low at the fag ends of the day, but that's no problem. If it were, we might have to re-think.

Of course the gentleman sailor should always be mindful that some of this faffing always seems to be needed at precisely the time G&T is served ;)
 
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