More PV Math Questions

Re: More PV Math Questions

Battery voltage is not a very accurate method of measuring battery capacity if done under load (or under charge). You need to let the battery sit for a few hours (no load/no charge) to measure state of charge via voltage.

To estimate how long until you have discharged the battery by 50% is not hard by math...

I am not sure how 15 batteries divide evenly into a 48 volt bank... Lets assume you have 12 batteries (3 banks of 4 battery series strings). That means you have 100 Amp*Hours * 3 parallel strings = 300 Amp*Hour of storage capacity.

300 Amp*Hours / 0.3 Amps = 1,000 Hours for 100% discharge

Or 500 Hours for 50% discharge.

500 Hours / 24 hours per day = 20.8 days

At this low drain, you may end up with self discharge of your batteries leading to you having to charge them more often than just the math with the inverter...

Also, given that batteries that sit below ~75% state of charge (for longer than hours or a day or so)--leads to sulfation which prevents the battery from properly recharging (harden sulfates do not reconvert back on recharging).

AGM batteries have lower self discharge (and some brands say they don't sulfate down to 20% state of charge)--and may be a better solution for your needs.

Not quite sure what your usage is (sounds like lots of battery for a very light load). Lots of batteries may need a lot of solar panels to properly charge (especially if these are flooded cells).

-Bill

Re: More PV Math Questions

Since you are grid tied--and just dealing with the inverter usage (I am not sure that 12 watts would be drawn from the battery bank when the inverter is GT connected--but SG would be a better person to answer that part of the question).

In any case, at around 77F (standard temperature) the typical battery is 100% charged at 2.12 volts per cell (flooded cell battery). Or 50.8 volts for your bank.

So, anything above 50.8v is "charging" the the battery (50.8+1.2=52 volts "sell voltage" for flooded cells).

You need to look up the long term float voltage for your AGM cells (and may even want to confirm with a calibrated DVM) that you are not putting any more voltage in your standby batter bank than you need (sell mode of inverter, its backup battery charger, etc.) for normal operation (which is basically "standby"). Cycling, continuous charging, etc. will tend to age the batteries more (my humble guess).

By the way, "Watts" already has a unit of time in it... And "Watts Per Hour" is redundant (really, it is wrong as Watts is Joules per Second--but hours are used because we would be dealing with 3,600x larger numbers for our power bills) (and yes--it is confusing, and I have made the mistake before -- because we all know Miles Per Hour).

That amount of power 12 watts in 24 hours will use is:

12 Watts * 24 Hours = 288 Watt*Hours or 0.288 kWH (how your home meter charges for power)

-Bill

Re: More PV Math Questions

Assuming your Xantrex XW inverter is setup correctly and always in sell mode (at least when both the sun and the grid are "up")...

Then the settings on the C40 just always need to be higher than the Inverter's Sell voltage (you don't want the inverter to be "not selling" because the "batteries are not charged" and you don't want the C40 at float because it thinks the "batteries are charged" and is cutting back on the solar array current.

Basically, you want the C40 pouring all of the available current into the battery bank and the XW to act like a diversion load and removing all current (not needed by battery for "float") to sell.

From what Solar Guppy (involved in the design and testing of the XW family) has recommended before (reading a bit between the lines om my part)--basically, what is the 100% charge of your AGM battery voltage (not the float charger voltage setting) and then add 1.2 volts to that--and that should be your sell voltage setting. Keeping the batteries charged and not cycling should give you the longest life.

The batteries will remain at 100% capacity (assuming they are already charged) and virtually no energy will go into harmful reactions inside the battery (no hydrogen/oxygen gas generated, no excessive over charging current causing plate material to transfer, catalyst caps may last longer because of less H2+O2 to recombine, etc.).

With flooded cell batteries--periodic equalization (to "boil" the cell with generated gas) to mix the electrolyte (especially on tall batteries) is still needed. For AGM, there is no mixing issues--but a very light equalization (basically maximum recommended charging voltage) once or twice a year can help equalize the state of charge between cells (most cells at 100%, a few at 5%--this appears to be a "newer" recommendation--older recommendations where to never equalize an AGM battery; which is true--you never crank up the voltage to 15.5 volts to equalize an AGM--you will generate too much gas and vent the cell).

Clear as mud?

-Bill

PS: It would still be good to have the C40 set to reasonable maximum voltages--to act like a backup charge controller if the XW Inverter or the Grid goes down. More of an issue when you install a larger array.

Re: More PV Math Questions

You would have to find a table for an AGM at 48 volts--but assuming the AGM is very similar to flooded cell (voltage wise)--From the Battery FAQ:

State of Charge      12 Volt battery      Volts per Cell
100%     12.7     2.12
90%     12.5     2.08
80%     12.42     2.07
70%     12.32     2.05
60%     12.20     2.03
50%     12.06     2.01
40%     11.9     1.98
30%     11.75     1.96
20%     11.58     1.93
10%     11.31     1.89
0     10.5     1.75

So, a 10% drop in capacity is roughly a 0.04 volts per cell (2.12-2.08 ), times 24 cells = 0.96 volt drop from 100% charge to 90% charge (rough numbers based on flooded cell).

I have read elsewhere here, that one of the BIG batteries (telecom or something) that the battery voltage is directly proportional to acid specific gravity. And, other than at the extreme ends of the above chart--you can see that the voltage drop is pretty much linear with decrease in stored capacity with each 10% drop in state of charge.

So, what would I do? Not my money (;)) but--I would make sure the batteries are fully charged, inverter charger is off (it is not trickle charging at night) (and if you want to be exactly accurate disconnect one connection from a parallel string--disconnect one string so that there are no issues with restarting the inverter/charger because of no battery bank connection--but I would be just as happy to leave everything connected because a 12 watt load is almost self discharge level of loading) measure the bank resting voltage after 3 or so hours of non-operation. Then use that as your never to go below voltage (add a few 0.0x volts if you want for the 12 watt load) for your bank... And you should be sure that you are never discharging the batteries even a little bit.

In any case, you can cycle down to 90% or even near 80% and still not damage the batteries if they stayed at those levels for long periods (days to months) of time (that I know of--I could be wrong with Lead Acid/AGM's). Obviously, that is a significant amount of sorted power "lost for use" in an emergency) by having the batteries at that low of level--but if you have solar and generator backup--it is not the end of the world.

So, you could be down 10%@~0.96v or even down 20%@(24 cells * (2.12-2.07))=1.20 volts before I would start to worry about battery damage...

Anyway, that gives you some pretty good numbers to play with and see how they work with your system.

-Bill