Using battery bank while system is charging, how does the bank deplete?

CVN-71
CVN-71 Solar Expert Posts: 54 ✭✭
I've always wondered this, if the system is charging from the sun, and at same time I'm using the inverter to power something, if watts in is same or less than watts out, how does this deplete the batteries (or does it while it's charging) and how does that effect cycle life of the batteries? For example I have two refrigs running continuously off my inverter. They don't draw much power and most of the day the system has enough sun to more than power them. How does this scenario effect the battery cycle life?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Usung battery bank while system is charging, how does the bank deplete?

    It's the ol' power balance thing again and how it averages through the day.

    Basically if the panels can supply what the batteries and the loads require you're all right.
    If the panels can only supply the battery charging needs (as in a system with minimal panels) you're in deficit charge.

    This is why we need to remind people that the actual charge rate is what comes from the panels minus what goes to the loads. So the rate from the panels may work out to be the 5% minimum, but if the loads aren't factored in the net charge rate is lower than that.

    At any given time: 20 Amps from panels to 200 Amp hour battery bank = 10% charge rate. Loads drawing 5 Amps means the charge rate is 15 Amps or 7.5% at that time. It is normal for loads to turn on and off through the day and for panel output to vary depending on insolation, SOC, and the load draw. What matters is how that works out at the end of the day: if Absorb has been achieved and finished and the batteries have reached Float (even briefly) you have a fully-charged battery bank. If not, one day won't make much difference. Several days happening repeatedly and you're in for shortened battery life.

    This is the reasoning behind the rules-of-thumb design like "shoot for 10% peak charge current". It does not mean you will have or need that much current, but it does usually result in having enough panel power to recharge batteries and supply loads. Most of us end up with "surplus" power; potential from the panels that is never realized because the batteries are full and the loads are low so the array only produces a small percentage of what it could if the demand was there. That's when we start "load shifting" to take advantage of power that would otherwise go unharvested.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Usung battery bank while system is charging, how does the bank deplete?

    basically all of your power is being diverted to the loads rather than the battery and when there isn't enough to keep the battery charged, a bit of power is also taken from the batteries.

    now technically, if you have say 100w coming into a controller from your pvs and your load is also 100w you will have a losing proposition there because of the small amount of power being used by the cc and inverter if that's there too.
  • CVN-71
    CVN-71 Solar Expert Posts: 54 ✭✭
    Re: Usung battery bank while system is charging, how does the bank deplete?

    Thanks guys I understand that portion, however as far as cycles go and life of batteries, if power used is less than power in, how does this effect # of cycles the batteries are good for? For example a battery that is discharged 10% then recharged, will last 5x longer than a battery that is discharged 50% then recharged. So, if for argumant sake I used 200w and take in 250w, is the battery "cycled" or is it a wash?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Usung battery bank while system is charging, how does the bank deplete?

    It's an "end of the day" situation.
    The more time spent charged or charging, the longer the battery life.
    The more time spent discharged or discharging, the shorter the battery life.
    Since the depth and frequency of discharge plays an important roll and is specific to the battery there's no rule-of-thumb guide here.
    The cycle times seen on battery charts are designed around consistent cycles: discharge evenly for so many hours then recharged completely as per specification. Like all the other numbers they are only a guideline; there's just too many factors that affect battery life to give an absolute answer.

    Deep cycle batteries, in fact, don't achieve full capacity until they have been cycled a few times.

    To adapt your example, if you use 200 Watts (on loads) you won't "take in" 250 Watts (from the panels) unless there's somewhere for it to go. In this case the "loads" would include system use and losses. If the panels are able to supply all the power necessary for all loads and losses and no power is drained from the batteries at all that day then the batteries will sit happily at "Float" and will not be considered to have used any portion of a discharge cycle. This is pretty much impossible in the real world.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Using battery bank while system is charging, how does the bank deplete?
    CVN-71 wrote: »
    For example I have two refrigs running continuously off my inverter. They don't draw much power and most of the day the system has enough sun to more than power them. How does this scenario effect the battery cycle life?

    Let's say its mid morning and your panels are pushing 1000 watts into the batteries. The fridges cycle on (drawing 400 watts) and now you are pushing only 600 watts into the batteries. Then the fridges cycle off and you go back to pushing 1000 watts into the batteries. This does not count as a battery cycle.

    Much more problematic is late afternoon and the panels are able to put out 200 watts. The fridges cycle on and you are drawing 200 watts from the batteries and 200 watts from the panels. Then the fridges cycle off and you begin charging the batteries with 200 watts. I really don't understand how these 'minicycles' affect battery life.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Using battery bank while system is charging, how does the bank deplete?
    vtmaps wrote: »
    Let's say its mid morning and your panels are pushing 1000 watts into the batteries. The fridges cycle on (drawing 400 watts) and now you are pushing only 600 watts into the batteries. Then the fridges cycle off and you go back to pushing 1000 watts into the batteries. This does not count as a battery cycle.

    Much more problematic is late afternoon and the panels are able to put out 200 watts. The fridges cycle on and you are drawing 200 watts from the batteries and 200 watts from the panels. Then the fridges cycle off and you begin charging the batteries with 200 watts. I really don't understand how these 'minicycles' affect battery life.

    --vtMaps

    Much better explanation of the question, vtMaps! :D

    These "mini cycles" have virtually no effect on battery life because they are short-term and not deep. The time they occur doesn't matter much either (during Absorb or Float for example even to the extent of pulling the charge out of the stage).

    Keep in mind that battery manufacturers determine the cycle life as I said before: all controlled conditions and steady discharge then recharge. In actual use this is not what happens. Instead the "mini cycling" is the norm. Yet we still get the years of service predicted by the manufacturers.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Using battery bank while system is charging, how does the bank deplete?

    There was a study done on micro-cycling of batteries and cycle life here: http://currentgeneration.co.nz/site/current/files/Partial%20AC-coupling%20in%20Minigrids.pdf
    The focus is on AC coupling but there's a big section in there about microcycling.