Which Battery C number do i use

Ako · April 2017

I use a maximum of around 3000 Wt/Hrs in 24 hours , about 1500 between 9am and 5pm when the input from my array is always far greater than that so batteries charge well and go from Bulk into Absorption before 12am . Between 5pm and midnight i use about 1500 Wt/ Hrs then from midnight until charging resumes the only load is the Inverters self consumption and im left with voltage around 24.5v or greater .

Looking at the chart to see what DOC i have im not sure which curve to apply ,

My ts-60 shows I have around 140 Amp/Hrs coming if from my array daily which i hope im right is assuming that would be approximately 20 Apm/Hrs greater than my consumption and the diference is the loss through the various drains explained to me in replies to another thread .

I record my Kw/Hrs coming in on the ts-60 and going out on my inverter , in a 24 hour cycle im getting approximately 4 Kw/Hrs in and 3 Kw/Hrs out , they appear to roughly confirm my thinking , am i interpreting everything correctly .

Estragon · April 2017

The rate changes through the day. In the evening, you're using say 20a/hr. If you have a 400ah bank, that's about a 5% or C/20 rate average over the 7 hrs. Within the 7 hrs the rate will fluctuate as loads turn on and off. If a well pump came on for example, the rate could be C/5 momentarily.

jonr · April 2017

24.5V in the morning suggests about a 60% state of charge. 1500WH is about 60 AH. If 60AH is using 40% of your battery capacity, that suggests it is a 150AH battery. This doesn't seem right, perhaps someone else can correct it.

Estragon · April 2017

If bank is 150ah (maybe 4xGC -220ah?) he'd be around c/8-10 average in evening.

Ako · April 2017

Sorry i only screenshot the lower part of the Tech sheet so i should have said , batteries are 4 x 6v , wired in series giving 24v 605 Amp/Hrs rolls S-605 about 3 months old , its the nightime DOD Im more concerned about as its the only time there is more going out than in , i assumed the C number would apply to the 24 hour cycle as i keep reading to use the C20 or C24 figures to make any calculations as its appropriate to the normal 24 hour cycle and the reason its often quoted by battery manufactures .

I am assuming on the screenshot bottom right where is has 4 different lines shown the corresponding key VOLTAGE ? HR then they mean C hour as the lower voltage line would give a lower DOD . Seems back to front to me , surely the longer the period im taking a set amount of power out the less DOD , if my voltage was 24 volts for sake of making easy calculation at C20 i would have a 55% DOD but at C1 only about 25% DOD

Estragon · April 2017

These are 605ah at 100hr rate - 468ah at c/20.

I think the chart you're looking at is showing how the rate of discharge affects voltage. Faster discharge pulls voltage lower faster.

What happens as you increase the rate of discharge is apparent usable capacity drops. Thats why the capacity rating above is higher at 100hr than 20hr.

Assuming you have no loads overnight, and don't turn stuff on before your 24.5v reading, you may have an issue. As a starting point I would suggest taking specific gravity readings on the bank when it gets to float.

Also, it would help if you could check charge controller settings and post same.

If you could add your system specs (panels, etc) in a signature for your posts it would help us understand your setup.

Ako · April 2017

Sorry but im totally confused now , rate of discharge affecting voltage , why would that information be useful for anything if it wouldent be DOD , sounds like the same thing . I have been taking my early morning voltage ( 24.5 ) and looking at where it intersects on the graph thinking if i know which C number to apply then it would give me the DOD hoping to keep around 30% or less .

Took your advice Estragon and added the info , see now why so many already had it showing . After midnight up until array is sending power about 8am only about 30 watts is being drawn , thats when i look at the reading as it seems to be the most appropriate time with such a small draw the batteries have had a long rest period . Looking at my 1 minuite logs the voltage goes up and down 1/10 to 3/10v at night resulting around 1/10v higher at 7am than midnight when we turn everything off . I put this down to the batteries increasing when settling coupled with the inverters self consumption .

The SG readings are generally about the same , some days after several hours in Absorption they go into float other days they dont but the SG readings will be around 1.280 - 1.290 , whichever the charge state the Amps going in will be cut right down to from the 40+ available in Bulk to around 5 to 7 amps mid afternoon .

Issue , ? thats not good i guess . Where could the issue be ?

Estragon · April 2017

The issue I was thinking of is what jonr said - at 24.5 resting and minimal loads something didn't add up. That resting voltage seems a bit low for ~60ah out of a ~450 ah bank. That use suggests SOC should be around 80-85%, but the 24.5v suggests more like 60%.

Voltage alone isn't a particularly good SOC indication though, as the voltage reading itself could be off some, and the bank is pretty new. It may take some cycling to get up to capacity.

If your SGs 1.28+ it's probably nothing to worry about.

Ako · April 2017

Its not a 450 ah bank its 600 .which i guess make it worse however 24.5 is the lowest so worst case scenario , looking at my logs 24.7 would be my average . At least im happy knowing either way im less than 50% DOD

Estragon · April 2017

The bank is 468ah at the 20 hr rate, which is the rate we generally use for planning. It will even lower for a cool bank. Most off-gridders wouldn't use a bank at a sustained 20hr rate though, so the planning should end up being conservative.

Anyway, 24.7v average resting in the morning sounds okay.

Ako · April 2017

Most off-gridders wouldn't use a bank at a sustained 20hr rate though, so the planning should end up being conservative.

Could you explain why , were you referring to what you wrote earlier that the C rates changes throughout the day depending on load etc .

Estragon · April 2017

Yes. Offgrid we may draw at higher rates to run a pump or whatever at times, but we usually try to size things so we can go a day or two with lousy weather before starting the generator. That would mean sustained average load that ends up being more like a 50 hr rate rather than the 20hr rate we use for planning, so the planning is conservative. Being conservative offsets factors that take capacity down, like cold batteries, occasional heavier draws, or aging batteries.

Vic · April 2017

Hi Ako,

Your Surrette batteries have a 20-hour C of about 445 Ah (due the fill electrolyte having an SG of 1.265. Using 450 Ah for the purpose of calculations will be just fine.

We tend to use the 20-hr figure for calculations, and for comparisons of different battery models, as it tends to make comparisons more fair.

Your SG readings seem a bit high for your batteries, which have a target SG of 1.265 when fully-charged. Here is an article on Measuring SGs, from Surrette
http://support.rollsbattery.com/support/solutions/articles/4347-measuring-specific-gravity

What brand or type of Hydrometer are you using?

Sometimes, SG readings can run a bit on the high side, when batteries have not been cycled very deeply. Most Flooded batteries do best, when cycled to 90% SOC, or lower.

YES, when trying to use voltage as an indication of SOC, the batteries should be Rested (NO charge or discharge for about 5 - 12 hours). Believe that the Surrette chart on V vs SOC is for Rested batteries.

As mentioned, battery temperature can have a fairly large effect on battery voltage.

Edit, to add;
Your PV array of 1050 STC Watts would generally produce no more than about 75% in good sun, on a clear day, so, it would seem that you might see no more than about 30 A produced, and possibly available for battery charging, using a standard rule of thumb, this is a bit on the low side, for a 450 Ah nominal battery bank.

Agree, that a morning battery voltage of about 24.5V (at 25 degrees C, and no charging and only inverter idle loads) seems a bit low for batteries that have had only inverter idle current for the prior 6 - 8 hours. Do you know the battery temperature at this time of a typical morning? And, are you certain that the meter that you are using for this voltage reading is accurate?

More later, Vic

jonr · April 2017

Ako,

Might be helpful to turn off the inverters for one night and measure a really accurate no-load voltage (and SG and temp) first thing in the morning.

Using the Rolls formula and accounting for the fact that after subtracting your loads, you might only be charging at 24A, I get a required 8 hour absorb time. Not getting to full charge would be consistent with being at a lower than expected SOC in the morning. More panels would help (more charging amps reaches absorb quicker and reduces the absorb time - Rolls recommends charging at 45A).

http://support.rollsbattery.com/support/solutions/articles/428-state-of-charge-charging-flooded-lead-acid-batteries

Estragon · April 2017

8 hr absorb? I might get a few in June if I turned end amps way down. I'm usually down to <1%C after 2-3hrs of absorb. No way I'd get 8hrs absorb most of the year without a bunch of generator run time.

Ako · April 2017

Will try to cover all the points mentioned but in no particular order .

Charging rises quite quickly to around 35 - 40 amps in bulk although it is occasionally up to 45 amps and always into Absorption by midday.after approximately three and a half hours . I zero my amps counter every night as part or the process of understanding the behaviour of the system , the forum has been invaluable for interpenetrating the results . Im getting a total Amps input of around 90 amps at that point . I have a large Tudor Glass Hydrometer with a glass float , its about 36cm long not including the rubber tube at the end i have checked it against others and im confident its accurate , i came with a long glass thermometer which seems to consistently agree with the the RTS on my ts-60 .

I see Float late afternoon some days and have not adjusted any settings on the program that decides when it goes into Float mode , i understand that it has to have something or other for less than 30% for a cumulative hour or 2 hours if the voltage over night has dropped below 25v . I dont fully understand how that works , seems odd to me that it is based on a theoretical computation rather than include the batteries , car chargers reduce the input current as the resistance in the batteries build up , i would have thought that would be a more accurate method as its in real time .

I run the logger function on msview set to take readings every minute between midnight and 8pm daily . I take the voltage reading from there .

Tonight i plan to turn the inverters off and take voltage and SG readings around 7am tomorrow .

Temperature at night around 17c or 18c rising to 22c - 23c in the day

Estragon · April 2017

The transition to float I think you''re talking about is the pwm duty cycle. The cc pulses the batteries with current with progressively less time pulsing and more time pausing as the batteries get more fully charged and less current is needed to hold the absorb voltage setpoint. The effect is less current flows per unit of time. Transition to float in this case happens an hour after the cc has reduced the "on" pulse to 30% of the time, "off" pause increased to 70% of the time.

The batteries are included in this process in the sense that their SOC determines how much current is needed and the duty cycle required to deliver that current.

This can work fine, but loads can prevent the duty cycle from dropping enough to trigger float without something like a MN WBjr to track current going to batteries only, ignoring loads.

jonr · April 2017

Ako,

If you can't get your charge controller to match Roll's charging requirements, consider running a generator every week or two to provide a complete absorb charge. Say 6 hours every two weeks - only 156 hours/year.

Ako · April 2017

Makes perfect sense when you explain it that way , thanks Estragon .

Which Battery C number do i use

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