epiphany...correct me if I'm wrong

t00ls
t00ls Solar Expert Posts: 245 ✭✭✭
I was thinking about my batteries and was wondering , how many watts they have.

It's odd I have never hears this question anywhere. You can find the amp hours usually expressed in a 20 hour period, but when you have a grid interactive hybrid such as the xw 4548....and the matching charge controller.....wouldnt it be nice to match your panels to the battery size to get the maximum sell.

so I have 3 105 Ah 12 volt batteries in parallel tied in series 4 times for 48 volts.

take the 105 divide it by 20 hours = 5.25a

multiply by 12 batteries = 63a

multiply that by 12 volts= 756 watts

it just struck me as odd that my system likes to put out in between 700 - 800 watts on a normal day.

I have 6 190w evergreen panels, 2 strings of 3 for 1140w. Given derating values...I end up with about 900 watts

I have my sell watts at 5 which sells up to around 1100w

I think I am trying to sell to small of a battery bank (which I know its smaller than 440 amp hours recommended ) I just think since I'm selling back I need to match the bank size to the panel wattage

does any of that make sensehttp://forum.solar-electric.com/images/smilies/embaressed_smile.gif

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: epiphany...correct me if I'm wrong
    t00ls wrote: »
    I was thinking about my batteries and was wondering , how many watts they have.

    It's odd I have never hears this question anywhere. You can find the amp hours usually expressed in a 20 hour period, but when you have a grid interactive hybrid such as the xw 4548....and the matching charge controller.....wouldnt it be nice to match your panels to the battery size to get the maximum sell.
    The 20 Hour Rate is sort of old fashion... Assumes a constant current discharge--Electric motors (driving fans and such) and filament lamps are probably the original loads.
    so I have 3 105 Ah 12 volt batteries in parallel tied in series 4 times for 48 volts.

    take the 105 divide it by 20 hours = 5.25a

    multiply by 12 batteries = 63a

    multiply that by 12 volts= 756 watts

    it just struck me as odd that my system likes to put out in between 700 - 800 watts on a normal day.

    Remember that Watts is a rate (like Miles per Gallons or Gallons per Hour)... 756 watts for how long gives you watt hours. I am not sure if you took out the 20 Hours on purpose, or wanted to talk about a XXX Watt constant load here.

    To figure out how many Watt*Hours you have in your battery bank (assuming 48 volts nominal voltage)
    • 105 AH per string * 4 strings * 48 volts = 20,160 Watt*Hours @ 20 hour rate
    • 20,160 WH / 20 hour rate = 1,008 watt 20 hour rate for your bank
    The above 1,008 20 hour rate is just a guess--you cannot directly "do the math" that I did because:

    Remember that, for many people, their loads are now AC inverters which are constant power devices and not constant current devices.

    Say you have a 2kW load on a 48 volt battery bank... Really ~52 volts right off charge and near 42 volts when dead. Assuming 85% efficiency for the inverter:
    • Power=Voltage*Current
    • Current = Power / Voltage
    • Current = 2,000 watts * 1/0.85 * 1/52 volts = 45.2 amps full charge
    • Current = 2,000 watts * 1/0.85 * 1/42 volts = 56.0 amps near dead
    I have seen a few batteries (those that seem to be advertised for use in UPS's -- which are just inverter/chargers) that have advertised their XX Minute constant power discharge capacity in Watt*Hours (at different rates, but even down to 15-30 minute discharge rates).
    I have 6 190w evergreen panels, 2 strings of 3 for 1140w. Given derating values...I end up with about 900 watts

    Well, we talk about the 5%-13% Charge Rate of 20 Hour battery rating. If you multiply the 20 Hour rate (in Amp*Hours) by Bank nominal voltage, you will get ~Bank Capacity in Watt*Hours. A 20 Hour rate is basically the 5% discharge or charge rate.

    And, the little I understand batteries, the Peukert factor applies to both discharging and charging (i.e., the faster you discharge, the lower your apparent bank capacity. And the faster you charge, the less efficient the charging is).

    But, in any case, we use the 5%-13% rule of thumb as the "optimum" charge rate for most batteries and most cost effective solar RE / PV system. It also makes it easier to size a system when people come here and ask what size system they should build without going into a bunch of details which gets everyone lost and sort of becomes too much information for too little return on knowledge.
    I have my sell watts at 5 which sells up to around 1100w

    Does not make sense to me--but I do not know the programming details of your XW system.
    I think I am trying to sell to small of a battery bank (which I know its smaller than 440 amp hours recommended ) I just think since I'm selling back I need to match the bank size to the panel wattage

    If you are limiting your inverter to a total output of 1,100 watts--that is one thing. If you are limiting your inverter to 1,100 watts of sell and not limiting the peak output (say you have a 3kW electric load--inverter output is 4.1 kW) and you are certainly stressing the battery bank.
    does any of that make sense
    I will ask you the same question about my ramblings. :confused:;)

    The 100 AH per 1kW of 48 volt XW inverter output is really a minimum battery requirement/limitation:
    • 1,000 Watts * 1/48 volts * 1/100 AH = 0.21 or 1/4.8 hour discharge rate
    C/4.8 is a high amount of load for a battery bank... C/8 (for flooded cell) is the usual recommendation for a continuous load.

    Certainly the XW in Grid Tie Sell Mode is not "taking" C/4.8 from the battery full time--but, as I understand, there are transition times when it will either take that energy, or the solar array will be charging a "full battery bank" while the XW is twiddling its electronic thumbs (monitoring battery bank voltage/state of charge, utility input power quality, and just ramping power up/down over a dozen seconds or so--probably to avoid dimming/brightening lights in the house, changes in motor sounds, etc.).

    And, there is the off-grid (utility power fail) that you have to design the bank to support -- Both solar array charging and inverter draw...

    Limiting just the sell wattage seems not to the address the issue (max power, max charge).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: epiphany...correct me if I'm wrong

    from bb,
    • Current = Power / Voltage
    • Current = 2,000 watts * 1/0.85 * 1/52 volts = 45.2 amps full charge
    now i know that what you wrote is technically correct, you may be confusing people by multiplying the inverse of the voltage and efficiency %.
    this may be better and is more as you wrote the formula as,
    current= (2000w/.85)/52v= 45.2a
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: epiphany...correct me if I'm wrong

    You should not be selling from batteries with only 315 A-H rated batteries. The wear and tear on batteries will more then consume the value of any electric pushed to grid.

    It is okay to sell excess solar power. This can be set up on XW to sell when battery voltage is above 52.8vdc to 53.6 vdc (float level).

    As to question about wattage and batteries. Question, as mentioned subsequently, should be how many watt-hrs.

    Realistically, you should not take more then 50% AH capacity if you want batteries to last a reasonable lifetime. You have 315 A-H rated at about 50vdc. Taking 50%, you have 157 A-H x 50 vdc or 7.8 kWH's of power. If your loads draw more then about 800 watts you will get less. At reasonable maximum battery draw of 25 amps per battery for a 105 AH battery, your max power draw should be kept under 3800 watts, which you could support for about an hour and half without damaging batteries.
  • t00ls
    t00ls Solar Expert Posts: 245 ✭✭✭
    Re: epiphany...correct me if I'm wrong

    I understand about not selling battery power....I only sell during the day

    my thing was, if you have battery backup as I do, and you sell the power back to the grid, I was thinking the battery bank total watt hours should be at the rated total solar panel watt hours

    one of the drawbacks to having a battery based system is having the batteries in the equation

    thanks for the math guys...just because it says new guy , doesnt mean I'm new to solar...although we could all learn more

    where I am located it wasnt feasible to hook the inverter out side up and run a separate sub-panel if the plan was to sell power back to the grid...tva uses 2 meters , one shows grid power to the house, one shows power from the inverter to grid and house....only time I use the inverter out side(not ac in ac out) is when the power goes out and I have to manually switch 2 breakers to get the power to flow to my home, one breaker shuts grid side off in case it comes back on, the other lets inverter ac out only go to the main panel.....much the same way as an automatic transfer switch would work , which I cant afford right now

    so the only time I sell is when the battery level is above 53.6 and stops selling when it reaches the lowest level of power that is set in the firmware for 54 volts sell, which is what the sell voltage is set at

    thats why I was saying that maybe I should have battery watt hours comparable to solar panel watt hours

    thoughts?
  • t00ls
    t00ls Solar Expert Posts: 245 ✭✭✭
    Re: epiphany...correct me if I'm wrong
    niel wrote: »
    from bb,
    • Current = Power / Voltage
    • Current = 2,000 watts * 1/0.85 * 1/52 volts = 45.2 amps full charge
    now i know that what you wrote is technically correct, you may be confusing people by multiplying the inverse of the voltage and efficiency %.
    this may be better and is more as you wrote the formula as,
    current= (2000w/.85)/52v= 45.2a

    niel...that equation makes more sense to me., although both would be correct
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: epiphany...correct me if I'm wrong
    t00ls wrote: »
    I understand about not selling battery power....I only sell during the day

    my thing was, if you have battery backup as I do, and you sell the power back to the grid, I was thinking the battery bank total watt hours should be at the rated total solar panel watt hours

    Except that the batteries act like a buffer between the solar panels and the GT inverter... And apparently (from reading here) GT inverters (at least the Xantrex ones) have a fairly long time constant (3-10 seconds or at least?) -- so the batteries need to be able to absorb/supply current during quickly changing solar conditions (clouds, bird flying over, etc.).
    thanks for the math guys...just because it says new guy , doesnt mean I'm new to solar...although we could all learn more

    We try (or are trying :roll:;)),
    so the only time I sell is when the battery level is above 53.6 and stops selling when it reaches the lowest level of power that is set in the firmware for 54 volts sell, which is what the sell voltage is set at

    thats why I was saying that maybe I should have battery watt hours comparable to solar panel watt hours

    As I understand--it is sort of both (solar panels and inverter) that you are sizing the battery bank for... And since one usually tries to match solar panel size to GT inverter size -- then the batteries would match the needs of both.

    Battery sizing issues are sometimes difficult to diagnose... Here is a thread with a high voltage DC fault. It appears there may be something else happening (controller communications issue?)--But I am not sure that there ever has been a resolution (battery bank issue or other hardware/software fault).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset