simonfraser
Well-known member
Is there a way to calculate this if you know the voltage under load and the amount of amps used ?
It’s a steady load over hours. Wet lead acid.
It’s a steady load over hours. Wet lead acid.
Battery state of charge calculation
- Thread starter simonfraser
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Neil_Y
Well-known member
You just need the amps its giving out and the battery capacity.
If you assume you start with a charged battery 12.70V at rest (it changes a bit with temp)
And you know the battery capacity say 100A/hr
If it's providing 1 Amp it will be flat in 100 hrs
If you assume you start with a charged battery 12.70V at rest (it changes a bit with temp)
And you know the battery capacity say 100A/hr
If it's providing 1 Amp it will be flat in 100 hrs
RichardS
N/A
If you know the load in amps and the voltage under load you need a different table to the one above as that is a "Rested" version.
I'll find it shortly.
https://www.dropbox.com/s/3owyixbxcn74sf5/Battery Voltage SOC-V.pdf?dl=0
Richard
I'll find it shortly.
https://www.dropbox.com/s/3owyixbxcn74sf5/Battery Voltage SOC-V.pdf?dl=0
Richard
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macd
Active member
Placing a known steady load (say 5A) on a battery until it reaches half charge is a common way of calculating actual Ah capacity. Half charge is normally determined by hydrometer readings. You could take the battery to 0%, but this is harmful to most batteries.
GHA
Well-known member
Based on voltage under load alone then unfortunately no, not with any degree of accuracy anyway. Far too many variables, temperature being a bit one, age of batteries, type of batteries.
For some graphs see here but they are way out for my T105's .
https://pssurvival.com/PS/Batteries/Lead_Acid/Lead-Acid_Battery_State_Of_Charge_Vs_Voltage_1993.pdf
Mistroma
Well-known member
RichardS and GHA beat me to it. It is worth noting that the Voltage also varies with temperature and type of battery.
The following figures are Voltages on a Trojan T105 at 26.7C (80F)
SOC (%)....Rest.........Steady 20A
100.........12.73.........12.41
95...........12.675......12.355
90...........12.62........12.3
85...........12.56........12.24
80...........12.5..........12.18
75...........12.435......12.115
70...........12.37........12.05
65...........12.305......11.985
60...........12.24........11.92
55...........12.17........11.85
50...........12.1..........11.78
40...........11.96........11.64
30...........11.81........11.49
20...........11.66........11.34
10...........11.51........11.19
I'll try to fix the formatting shortly.
At 10C the 100% Voltage under 20A load would be 12.18V instead of 12.41V. I imagine that this test was carried out under ideal conditions with a pretty new battery. Simple Voltage measurement will give an indication of SOC but isn't the best method to use.
The following figures are Voltages on a Trojan T105 at 26.7C (80F)
SOC (%)....Rest.........Steady 20A
100.........12.73.........12.41
95...........12.675......12.355
90...........12.62........12.3
85...........12.56........12.24
80...........12.5..........12.18
75...........12.435......12.115
70...........12.37........12.05
65...........12.305......11.985
60...........12.24........11.92
55...........12.17........11.85
50...........12.1..........11.78
40...........11.96........11.64
30...........11.81........11.49
20...........11.66........11.34
10...........11.51........11.19
I'll try to fix the formatting shortly.
At 10C the 100% Voltage under 20A load would be 12.18V instead of 12.41V. I imagine that this test was carried out under ideal conditions with a pretty new battery. Simple Voltage measurement will give an indication of SOC but isn't the best method to use.
GHA
Well-known member
But that's in only way to do an accurate capacity test if done in a temperature controlled bath, not really harmful if done once in a long while as long as the battery is charged back to full straight away. Possible some benefits >
http://www.cruisersforum.com/forums/f14/lifeline-battery-capacity-test-190768.html#post2476669
LadyInBed
Well-known member
Not State of Charge but associated - Energy Audit
simonfraser
Well-known member
If you know the load in amps and the voltage under load you need a different table to the one above as that is a "Rested" version.
I'll find it shortly.
https://www.dropbox.com/s/3owyixbxcn74sf5/Battery Voltage SOC-V.pdf?dl=0
Richard
Perfect
that gives me a good guide !GHA
Well-known member
Perfect
Sadly not really. No mention of battery capacity. 5A from a 1000Ah batt will be completely different to a 50Ah. No mention of temperature either. Very likely miles out and you won't know how many.
If it was that easy and accurate we'd all be doing it
GHA
Well-known member
Do you have a link to the source please? Ta
lw395
Well-known member
Sadly not really. No mention of battery capacity. 5A from a 1000Ah batt will be completely different to a 50Ah. No mention of temperature either. Very likely miles out and you won't know how many.
If it was that easy and accurate we'd all be doing it
If you know roughly the capacity in Ah, you can get a reasonable indication of SOC from the voltage at a known charge or discharge current.
Yes it varies with the exact chemistry of the battery. And the health of that battery.
But I view it as an indication, not something accurate to a fraction of a %.
It will also vary according to 'history', e.g. if you're drawing 5A now, but 10 minutes ago you were drawing 20A, that would likely skew the result.
There are published graphs in terms of a steady discharge at rates of 0.1C and so forth.
RichardS
N/A
Sadly not really. No mention of battery capacity. 5A from a 1000Ah batt will be completely different to a 50Ah. No mention of temperature either. Very likely miles out and you won't know how many.
If it was that easy and accurate we'd all be doing it
1000Ah battery in this context? Yeah, right.
Assume the battery is within the usual range of 50Ah - 100Ah and it's as close as you're going to get from a generic table.
Richard
Mistroma
Well-known member
Sorry, it was from a spreadsheet I picked up several years ago and I don't have a link to the source. I believe it was based on data published by Trojan and other sources. It has sheets for SOC vs. Specific Gravity and also Voltage (resting & 20A discharge).
Data sources:
1) Trojan data based on factory specification observed 30/7/2011
2) Cold weather observations 1/1/2012.
3) 100% value measured after equalisation @ respective temperature
4) Temperature correction calculation from http://www.dekabatteries.com (No idea where though)
I can email the file if you think it might be of some use. I use it as a reference when measuring temp. corrected SG and Smartgauge readings. The Voltage readings are of some use as the data is specifically for Trojan T105s and covers a wide temperature range.
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simonfraser
Well-known member
1000Ah battery in this context? Yeah, right.
Assume the battery is within the usual range of 50Ah - 100Ah and it's as close as you're going to get from a generic table.
Richard
+1
GHA
Well-known member
Thanks, sent PM.
So how do you find the smartguage against guessing SOC looking at the voltage? Am I about to get shot down in flames here
From logging IMHO it seems there's a lot of over confidence going on regarding batteries in general. Doubt if anyone actual knows their capacity, they take a good few cycles to ramp up max capacity when new then start declining. Has anyone ever actually done a proper temperature controlled capacity test?
Not that it matters much in the real world for the vast majority, if the volts are heading down closer to 12 then on goes the engine
simonfraser
Well-known member
Thanks, sent PM.
So how do you find the smartguage against guessing SOC looking at the voltage? Am I about to get shot down in flames here
From logging IMHO it seems there's a lot of over confidence going on regarding batteries in general. Doubt if anyone actual knows their capacity, they take a good few cycles to ramp up max capacity when new then start declining. Has anyone ever actually done a proper temperature controlled capacity test?
Not that it matters much in the real world for the vast majority, if the volts are heading down closer to 12 then on goes the engine
That’s exactly the point, just cos it reads 12V does not mean your batteries are at 50% if you are switched on
Looks like you can run them down to 11.8V or so, depending on temp and load etc
Sandro
Active member
Quote:” it’s a steady load over hours”.
If a patient watch can be kept, recording amps and duration of each spell of discharging or charging at constant current, adding and subtracting all the amp x hours, and knowing the start situation (e.g. fully charged) then the state of charge of a battery of known capacity can be reckoned.
I was told about this system by a submarine sailor of WW2 time, who used ammeter (central zero), clock, pencil and notebook – one made of paper I mean.
Nowadays a battery charger maker could add an integrating circuitry/software and display the value. Of course a zeroing pushbutton would also be needed.
Sandro
If a patient watch can be kept, recording amps and duration of each spell of discharging or charging at constant current, adding and subtracting all the amp x hours, and knowing the start situation (e.g. fully charged) then the state of charge of a battery of known capacity can be reckoned.
I was told about this system by a submarine sailor of WW2 time, who used ammeter (central zero), clock, pencil and notebook – one made of paper I mean.
Nowadays a battery charger maker could add an integrating circuitry/software and display the value. Of course a zeroing pushbutton would also be needed.
Sandro
Mistroma
Well-known member
Thanks, sent PM.
So how do you find the smartguage against guessing SOC looking at the voltage? Am I about to get shot down in flames here
From logging IMHO it seems there's a lot of over confidence going on regarding batteries in general. Doubt if anyone actual knows their capacity, they take a good few cycles to ramp up max capacity when new then start declining. Has anyone ever actually done a proper temperature controlled capacity test?
Not that it matters much in the real world for the vast majority, if the volts are heading down closer to 12 then on goes the engine
Smartgauge was pretty good initially but began to behave oddly in 2017. Pretty certain a Voltage spike upset it as a couple of other items also had problems. Manufacturer was interested as I had detailed data and they said it would require odd values in specific registers to get the results I reported. It was pretty much calculating SOC based on measured Voltage and close to resting figures from XLS I emailed.
They confirmed that I was correct and replaced the innards for a reasonable price. Smartgauge went back to normal after that.
I'm not saying that Smartgauge always gives perfect results. If you understand how it works when charging then the results aren't a big surprise.
Charging is mostly:
1) Odd engine runs during the day (or morning) when moving around
2) Solar (+some wind) averaging 130Ah/day most of the summer
Smartgauge reports 100% early as it is only predicting on Voltage & presumably a lookup table of typical values.
Once discharge phase starts it begins to give a reasonably accurate reading pretty quickly as it is then using the HF pulse method of calculating capacity. At least I think that's what it's doing based on a White Paper I read years ago (wish I could find it again).
Result of this is that slightly undercharged batteries can read 100% and then drop rapidly to 90% before levelling off close to the correct value. Values overnight and by next morning are a good indication of actual battery SOC.
Not perfect, but it is very easy to install and nothing to calibrate. Absolute battery capacity isn't relevant as it only reports a percentage result. So 60% doesn't mean 300Ah left in a 500Ah bank. It just means 60% of whatever capacity your batteries can store. Older 500Ah bank might only manage 400Ah and it would be 60% of that figure.
The only thing I use it for is to let me know if I need to put some more charge in or if batteries still have a decent SOC.
I actually did carry out tests on my previous set of batteries and it was very labour intensive over 4 days. I monitored Voltage, Amps load, SG and temperature hourly from very early in the morning for about 18 hours each day.
Temperature control wasn't a big deal as I stuck each battery in a big water bath overnight before testing and during the test. I don't think temp. varied by more than a degree C at most.
Load control was a bigger problem but I did manage reasonably well with a variety of small bulbs and a larger one. I noted actual current and included that in the final calculations so that Ah wouldn't be far off.
It proved that 2 batteries were still pretty good, 1 a bit low and the last one likely to need replacing fairly soon. It was enough to convince me that replacing 4 leisure batteries (I mean lorry start
) with T-105s before leaving the UK was a good idea.I would find it impossible to repeat the exercise whilst living on board. I now have a system which reports Ah or Wh from 2 sets of panels and a wind generator. I also get total Ah and Wh in and out of all batteries from all sources (wind, solar, alternator and mains). I can make a reasonable estimate of actual capacity using available data.
The most important information is that I do generate enough vs. usage to believe that my batteries do now get to 100% a lot of the time and only drop to 85-88% most mornings. SG measurements are usually close to value I expect.
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GHA
Well-known member
Of course, it's not really that complicated in principle, just getting accurate SOC that's not so easy. But most people still don't like the voltage going so low, some instruments don't like it and the fridge switches on and off from low voltage.That’s exactly the point, just cos it reads 12V does not mean your batteries are at 50% if you are switched on
Looks like you can run them down to 11.8V or so, depending on temp and load etc
Smartguage seems the most accurate simple way to get SOC, on discharge anyway.
Hands up though, from some data send by Mistroma , think I have been too negative about the possible accuracy, though it only applies to trojan t105's and age of battery/state of health make a difference as well. Some testing will hopefully show more.
The dropbox link doesn't come near the data, it's garbage.
How would you maintain a constant current over time? Not an easy thing to do , here's some real world current / voltage from a few days ago >
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