Regarding Charging (SOLAR PV) while LOADING 48V... does it affect SOD AND DOD?

ChanChan Posts: 45Registered Users ✭✭
What I do understand regading the battery life is that you should always only consume 30% (dod) of the charge to maintain a good battery life, But i also understand that to keep a battery life longer is to prevent too much (SOD) ...

Assuming I have a 3000W INVERTER, and during the day early morning I am getting about 1500W from the PV, while having a load of 1000W, will this affect my SOD?

DO PV actually generate the load from the PV to INVERTER for the LOAD or does it goes to PV to inverter to BATTERY then load?



MY Current setup is a HYBRID - GRID-TIE WITH BACKUP.

TEST.jpg 244.4K

Comments

  • BB.BB. Posts: 27,698Super Moderators admin
    There are several basic things that help extend battery life... Discharging to 50% maximum (do not go deeper if you can avoid it), recharge reasonably quickly (something like 10% to 13% rate of charge), when discharging, try to go to at least 25% discharging (75% state of charge--shallow discharging is not good for lead acid batteries), keep batteries cool (hot batteries, shorter life). Monitor electrolyte levels to prevent exposing plates to air (oxygen) and use distilled water (filtered rainwater, etc.) when refilling batteries. And avoid "over equalizing" your battery bank (equalizing generates heat in the battery bank, "uses water", the bubbles can erode the plates, and the oxygen which forms on the positive plates can cause plate and grid corrosion.

    When operating in Hybrid mode--The solar panels do supply power directly to the GT inverter--However, the battery bank does act like a "capacitor"--For single phase inverters, because your inverter is outputting power at 60 Hz (or 50Hz outside of north America), it is drawing power at a 120 Hz "pulse train" (really a sine squared wave form). If you measure the battery voltage, you will see the voltage change by a few tenths (or more) of a volt at 120 Hz. If your battery float voltage is too low, it is possible for the "discharge pulse" to take the battery bank into a "micro discharge", and then a "micro recharge"--Which will cause the battery to fail sooner.

    If your system does go Off Grid (utility fails), you discharge the battery bank (to 75% to 50% state of charge typically)--When the sun is up and your bank is charging (while still off grid)--Then your solar panels are supplying power to both your AC loads and Battery Charging at the same time--Nominally, for an off grid home, they usually charge the batteries during the day (sun is up), and discharge at night--So your solar panels quickly and properly recharge the battery bank (10% rate of charge nominal recommendation).

    If you have daytime loads (business, busy house during the day), then your "base loads" do take some percentage of your solar array's output and may prevent the batteries from being fully/completely recharged during the day (if your array is 10% rate of charge, and your base load is 1,000 Watts out of a 1,500 Watt array, then your charging current is roughly 3.3% of battery capacity. Way too low for a "comfortable" battery recharging current (long term grid outage or being full time off grid).

    Since batteries are expensive and solar panels are "cheap" these days--If you have a large base load and are trying to prepare for a long term grid outage/off grid event, then I would suggest your "ideal array" be 5-10%-13% rate of charge 10%+ rate of charge for full time off grid) + additional array wattage to carry your base load.

    Part of it depends on how deeply you cycle your battery bank--Lead Acid batteries really work well for daytime charging and night time loads. If your profile is reversed, you may get away with a 5% effective rate of charge (day time loads, very little night time loads, so battery bank does not cycle deeply during normal use). And when you have a couple cloudy days--You can always fire up the genset to assist with charging. (many people prefer to add solar panels and keep generator use/fuel storage to a minimum).

    Does this help?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ChanChan Posts: 45Registered Users ✭✭
    Thanks.
  • WaterWheelWaterWheel Posts: 254Registered Users ✭✭✭
    Bill,,,   In my case (off grid, fairly sunny area) I usually drop to about 1.227 SG ( about 73% SOC) by morning and charge up to about 1.263 SG (about 92% SOC).     I change the absorb time and voltage occasionally (spoke with battery manufacturer three times about this) to keep this ratio occasionally.    I EQ about every 6 weeks since the batteries are young and they are now staying fairly close in SG.      This is a typical Sept./Oct day for me.        Summers are similar.      Winters will dip a bit lower.    Usually I try to run washers ect during the daytime, especially during the afternoon after the batteries are well into the absorb stage (pulling less than 15 amps) or float.

    Are you suggesting that I run the washers (within reason) after sundown (when I expect good sun the next day) to drop my batteries a tad more in an effort to make them cycle a bit more?       I understand all systems are a bit different, especially batteries; but saying that do you recommend that I move some of my washing to the evening hours?     

    Conext XW6848 with PDP, SCP, 80/600 controller, and conext battery monitor

    18 SW280 panels on Schletter ground mount

    48v 790 amp/hr Crown battery bank

  • BB.BB. Posts: 27,698Super Moderators admin
    edited October 2016 #5
    Many people run the optional loads after noon or so, when when the system goes into the absorb/float (excess current from solar panels available). Or, over ~80-85% state of charge (when lead acid batteries tend to limit charging current--out of bulk stage).

    In your case, as long as your batteries are over ~90% state of charge--You appear to have enough energy/charging time to keep the bank happy.
    • 18 * 280 Watt panels = 5,040 Watt array
    • 790 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 6,053 Watt array for a 10% rate of charge
    • 5,040 Watt / 6,053 Watt = 0.83
    You are are about 8.3% rate of charge the closer you are to 10%+ rate of charge, the less you have to worry about daytime loads negatively affecting your charging (you typically have more current than needed to finish charge your battery bank and run the day time loads at the same time).

    If you were at 5% rate of charge, then (in my humble opinion), folks really need to be careful about finishing battery charging first (mostly) before they start to turn on optional/daytime loads.

    Again, I am no expert--But this is one way of I would suggest running an off grid system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterWheelWaterWheel Posts: 254Registered Users ✭✭✭
    Thanks Bill.

    Conext XW6848 with PDP, SCP, 80/600 controller, and conext battery monitor

    18 SW280 panels on Schletter ground mount

    48v 790 amp/hr Crown battery bank

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