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Nickel-Iron technology Off Grid Solar Power

Q18: Storage Battery Problems

One day the family relatives came for sleep over, we forgot about the consequences to the battery power house,so after watering for 4 hours (100 amp/hrs) the family opened and closed the fridge alot (20 amp hrs) and they all decided to take a shower or bath, this is another (100 amp/hrs).The bore pump kicked in I suppose twice, it has a float switch which is designed to give the bore pump a rest, so switches on for 4 hours each time. So one reckons the total consumption of power would have been (200 amp/hrs).So around 8pm, all the inverters were on low alarm, and the battery voltage was 18.5 volts.

The total power consumed in theory might have been

  • 420 amp/hrs.

    The solar panels would have helped for some of this load, but since the sunshine wasn't so clearless, one suspects the batteries did not get much charging. So we hooked up the bore pump and water pump to the 240 Volt Mains, for a week, to give batteries a rest.

    Testing battery capacity

    It's difficult to test battery capacity.

    I have been learning why battery voltages drop under loads. The good thing about Nickel-Iron, I know I can't do anything bad to them. So one just has to resolve the problem.

    From www: Why battery voltage drops?

    "As the battery runs out of power the voltage decreases.This happens due to the fact that as the reactants in the battery get less concentrated less reactions happen per second, decreasing the voltage. When you charge the battery you reverse the reactions bringing the reactants back to the original concentrations (or at least close) the voltage goes back up to what is originally was."

    So if you use less power during the day, the batteries should go to float or absorb faster.

    So today on Friday night , one begins testing:

    6:00 pm Voltage is 25.6

    6:30 pm Voltage is 24.5

    8:00 pm Voltage is 23.0

    6:00 am Voltage is 20.3

    One has to assume the SOC (state of charge) went from full to flat.

    The fridge was on all night (4amps when on), Let's assume on continously for 5 hours (20amp/hrs). The lights (1 amp for 5 hrs) is 5amp/hrs. The fan all night is 0.6 amps is less than 10amp/hrs. The water pump came on due to water controller from 5am lets say running for 2 hours, is 40amp/hrs. The total current removed from battery is no more than 75amp/hr. So the SOC should be 400-75/400 as percentage, 80% SOC. Assume full battery holds 400amp/hr.

    This should be a voltage of ??

    Well 80% of 24 volts is roughly 20 volts, what I am getting, but 20 volts is assumed to be close on flat?

    Something wrong?

    A picture of Zeus, red box shows last week, voltage is 20.53

    My first attempt at solution, is to change user defined voltages.

  • Absorb is 33V
  • Float is 29.5 Volts
  • Equalization (not used) is 34V.

    A picture of Trina, red box shows last week, voltage is 20.44

    The voltage differences, slight due to cable lengths, and various differences in impedance in fuse, etc.

    The visitors in the red box, showed us changing the load , removing the water pump and bore pump from the battery bank.

    So day 3, the battery took a while to recover, about 250 minutes in bulk, 360 minutres in absorb and 90 minutes in float.

    Thus with small loads the system coped.

    On day 2, is intesting.

    The minimum voltage is 23Volts, at night (24 Volts) this is a SOC of 23/24 or 95% SOC.

    One would expect that as the only loads are fan, lights and fridge, (not much).

    On day 1, I introduce the water pump, which came on at 5am to 6am, removing 40 amp/hr from battery.

    So my question is if the day before had 95% SOC without the water pump, and on the next day I introduce a load that removes 40amp/hrs, why does the SOC suddenly drop to 20 volts, the Inverter went low alarm?

    The water pump Inverter switches load off. Voltage lower than 20.0 V (was 18.7 V)

    I should have over 300amp/hr in my bank, and removing only 40amp/hr should not make the SOC voltage go from 24 V to 20V.

    Something is wrong .. with storage.

    Maybe one could try:-

    (1) Get new solar panels, with three MPPT boxes capable of three lots of 30 amps into the 500amp/hr bank.

    Purchase a energy in energy out monitor that tells you precisely the SOC, ie, the current that went in, and the current that goes out.

    This device sits between your batteries and your wires, so I was reluncant to introduce another voltage drop in my system. However one has no way to verify the battery storage.

    I am puzzled why it is taking over 4 hours for the batteries to charge, when only 75 amp/hrs was removed?

    At the moment from 7:00 am until 10.00 am, I have between 30 to 60amps coming in, I would expect the system to be on absorb by 11 am and on float by noon. But is not happening.

    In my older experience with Lead Acid batteries at say 85% SOC, they reached 100% SOC within 30 minutes, on my 800Watt system with 220Amp/hr battery storage. The load for one system was a 24VDC fridge, The load for the other system was LED lights and 24 VDC pumps, these are small loads.

    With this new system of Ni-Fe, using a fridge at 4 amps when on, LED lights of 1 amp, and fan 0.6 amps at night, these loads are small.

    Total consumption per night time is

  • Fridge 20 amp/hr (4 amps when on)
  • Lights 12 amp/hr (1 amp, only 2 in house)
  • Bed fan 8 amp/hr (0.6 A runs 12 hours)

  • TOTAL per night: 40amp/hr

    Voltage drop from 24.0 to 23.6V seems OK

    Thus 189 to 190 minutes bulk, plus 240 minutes absorb.

  • This is 180/60x50 = 150 amp/hr bulk,
  • plus 240/60x10 = 40 amp/hr absorb,

    Total charging 190 amp / hr

    Something wrong, excessive charging?

    What Victron Manual says:

    (1) Excessive charging - set to 0.1 C or 0.2C

    Mine is set towards low end, 0.1C (0.1x500) = 50amps. I get 30 to 60 amps, on low end.

    With another string of 30 AMPS, I should get 0.2C (C=500), the upper is 100amps of charging. Between 50 to 90 amps, high end.

    (2) Poor battery connection.

    Not applicable I hope? All my wires carry less than 30 amps, intentionally designed that way. In fact if you want more power use a 48 volt system and keep current under 30 amps. Cheaper wires and more efficient, less loss of power due to reistances.

    (3) Absorb V set too low.

    Might be cause, so testing back at 33V for absorb. Take a few days to see.

    (4) Not enough charging time

    Possibly a problem. Panels face for 7am to 2 pm sunshine efficiently. Around 6 hours of sunshine. Could install more panels, facing east and facing west angles.

    (5) Absorb time too short, caused by excessive charging current.

    Since my system is on low end, I should not have this problem.

    Going over webpage 15, I noticed since changing the user defined settings back to

  • Absorb is 33V
  • Float is 29.5 Volts
  • Equalization (not used) is 34V;

    that the system is remaining on bulk for over one day?

    This suggests to me the bank is not charged up, and will take a few days to reach this higher level of SOC.

    I suspect at 30 Volt Absorb, this might be 200 amp/hr storage, hence why adding 40 amp/hr loss, is considered a big deal.

    Now with 33 V Absorb, assuming I ever reach this SOC, I might get 500 amp/hr storage and hence 40 amp/hr loss is not a big deal.

    The obvious solution to test this is to add this third string of charging and get the system to 90 amps. This way I can guarentee charging between 60 to 90 amps.

    Well its the second day on bulk, so I reckon at 6 hours times 50 amps, is 600 amp/hrs with a daily loss of 50amphrs, is 550 amp hrs.

    On second day still bulk, another 550 amp hrs. Both water pump and bore pump still not connected to battery bank.

    Something weird? On the third day, if I am not in absorb or float, that will be over 1500 amp/hrs pushed into batteries, and obviously overcharging again.

    Will see what happens tomorrow.

    This week, the batteries never reached absorb set at 33.0V, though over the 7 days the battery voltage did rise towards 33.0V

    Thus the Amps coming in is not big enough for a days charging.

    So I see the voltages lower

    connected up both water pump and bore pump to the batteries again

    And until we get more solar panels, run the system at a lower SOC and try to get the system to cope.

    Now we have 31.5 V absorb, the MPPT has moved to Absorb, Not bulk. And the float is set to 29V.

    A couple of comments from other Ni-Fe users such as Mike:

    So, my off-grid experience living with NiFe batteries ( a large 48V, 800ah bank, single series string) is so far, mixed. I'm going through a lot of water, even in winter, with cloudy and short days. Shortly after sunrise, and way before the batteries get to absorb voltage, the battery shed sounds like a den of snakes, with all the bubbles hissing away. And when it hits absorb...... you nearly need earplugs.

    I have my major loads (large water pump for agricultural water) on a timer to only run on daylight hours, to not cycle the batteries, and do other load shifting, as anyone off grid would. So nights, the batteries are really only running the fridge and small amounts of lighting.

    Power has not gone out, but in cloudy weather, I've got a fair amount of generator run time, to keep the NiFe bank up, since it electrolyzes a lot of water compared to lead acid.

    So, I'm still reserving judgement, and when I have time, will break out the carbonate test kit, and see how the electrolyte is holding up.

    Mike

    Mike has 48 volt system on 800Amp/hr batteries using only 60Amp Morningstar MPPT.

    I see parallel problems, I too run water pumps not at night. Mike has not enough amps, Mike gets 30 to 60amps, when he should be getting 100 to 150 amps for his 800amp/hr system, as a minimum.

    Somebody else comments about misting as a problem.

    Misting... At least with my C6 "bubble tops", misting is a real issue - even more so than water electrolysis (I absorb at 1.60v per cell as my bank is now in UPS service). The mist that comes out of the batteries with the H2/O2 off-gassing settles back down and creates a highly alkaline, corrosive "dust" on the tops of the batteries. I have to wash these off every so often.

    A view of the laptop changing the user defined voltages.

    Notice the vertical plastic tubes should reduce misting and corrosion of my bars across the batteries.

    Mike says

    NiFe is Inefficient. You will need 30-40% more PV to charge them

    They chew through a lot of water, electrolizing it My 40 ea, 800ah cells go through about 8 or 9 gallons a month. Th e sound of a room full of bubbling cells is just the sound of nightime power

    They have high internal resistance, so you need a larger bank to supply surges and avoid inverter cutoffs from voltage sag

    And I think this is the last year I can get by on the electrolyte. Going to have to replace it next summer. really hard to store the spare chemical, you mix at time of using.

    Having used them the last 6 or so years, the only factor to use them is the ability to idle at any state without damage, LFP cells can do that, Not sure yet on their overall lifetime, but I might steer you toward that tech, seems to be starting to be more acceptance of it

    I suspect the generator is overcharging the batteries, and solar chargers are much better. Now my MPPT are running correctly, thanks to Victron update, I am not using much water. About 5 litres per month.

    So far I love my Ni-Fe batteries, yes one has to learn many things, but they are great. If I had Lead Acid batteries I would have spoiled them already on Inverters.

    Not so, with Ni-Fe, I just need to learn how to go about Ni-Fe technology. Shalom

    See how I go on 31.5V for absorb, this week, with only 60amps charging each day.

    Yesterday was tough, wife used water pump and bore pump also came on, but system was absorb, so while 39 amps went out, 39 amps came in, but 50 amps could have been available, but the MPPT was slowing charging process down. This is normal.

    At dusk, the water controllers came on and we went into 26 Volts, which is not good, but survived. Today at bulk, we have at 8.30 am

  • 43 amps coming in on Bulk, and nothing going out, voltage is 30V. This is great.

    So being careful, using water between 10 and 3 pm, we could manage on 30 to 60 amps charging.

    Next we add the third solar panel string to the power house.

    Ni-Fe battery technology

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