Below is a guide to using the batteries extracted from the internet. These instructions match my own experience with using them on my family house. If anyone knows how to post .pdf files I will add the Chinese guide.
CONSIDERATIONS
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Cost per cell
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Is it more cost effective to buy the cheapest deep cycle Lead-Acid cells and replace them when they fail?
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Can you afford to be without power while waiting for replacement cells?
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Do you need the best charge/discharge rate and cost is not a factor? Nickel-Cadmium cells are probably your best option.
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How many volts do your loads require
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For solar systems either 12 or 24 or 36 or 48 volts DC.
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How many cells to achieve the required voltage
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Effect battery cell count has on Charging Nickel-Iron Cells
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Nickel-Iron cells can be charged and need to be charged to significantly higher voltages than their rated voltage of 1.2 volts per cell. A Nickel-Iron cell can safely be charged at over 2 volts open circuit per cell (see about charging and open circuit volts at MISCELLANEOUS) which translates to around 1.65 volts per cell at full capacity. When charging ceases the cell voltage drops to around 1.4 volts and can be safely discharged to 0.8 volts per cell. If you are using 20 Nickel-Iron cells in series to give a nominal 24 volt battery (20 cells x 1.2 volts = 24 volts) you would have to charge the battery of cells at 20 x 1.65 = 33 volts to achieve a full charge. This is a high voltage for most 24 volt battery chargers as they are designed for charging Lead-Acid cells to a maximum of about 30 volts (12 cells x 2.5 volts = 30 volts). Also most devices that are powered by 24 volts dc have been designed to run on a voltage that can fluctuate between about 18 volts to 33 volts. So a battery that needs to reach 33 volts to get fully charged is going to be at the upper end of the range that most chargers can charge at and most 24 volt electrical devices can run off while charging is in progress. If charge voltages become a problem the way to overcome this is to reduce the number of cells in the 24 volt bank from 20 to 18 cells and reduce the charge voltage slightly. This will give an optimum charging voltage of 18 cells x 1.65 volts = 29.7 volts which brings the battery within the voltage range of both the battery charger and electrical loads.
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Effect battery cell count has on Usable Capacity.
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Reducing the number of cells in the battery bank from 20 to 18 will reduce the cost of the batteries by 10% but will also reduce the amount of usable watts stored by the same amount of 10% as you now have 2 cells less.
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Another thing to consider is the minimum voltage that 24 volt electrical load devices need, usually somewhere between 18 and 21 volts. With 20 cells and an electrical load device that needs a minimum of 18 volts, each cell can discharge to 0.9 volts (20 cells x 0.9 volts = 18 volts) before the loads lower operating voltage is reached. For 18 cells each cell can discharge to 1.0 volt (18 cells x 1.0 volts = 18 volts) before the loads lower voltage will be reached. Again a 10% difference.
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Having too many cells in the battery bank for the charge source voltage will mean the cells never get fully charged and would be wasting battery capacity which may increase over time as constantly undercharging will cause the cell capacity to slowly drift lower. This capacity loss can be reversed in Nickel-Iron cells by completing a few full 100% charge/discharge cycles to at least 1.65 volts per cell.
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In summary, if low charging voltages are a problem you would get better long term performance fully charging 18 x 1,000Ah cells than partially charging 20 x 900Ah cells and the cost would be similar.
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How fast will you be discharging?
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How rate of discharge affects Amp Hour capacity
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Battery capacities are measured in Ah (amp hours) or how many amps they can discharge at a constant rate over a specified period of time while remaining between the fully charged and discharged voltages. The rating system assumes you know the voltage the battery will read when charged and when discharged. Nickel-Iron cells are considered fully charged at 1.4 volts and discharged at 1.0 volt. Nickel-Iron batteries are usually rated over 5 hours (often written as C/5, where C is capacity divided by hours). As an example a fully charged 500Ah Nickel-Iron cell will measures 1.4 volts at its terminals and can discharge 100 amps for 5 hours, at which point the voltage at its terminals will measure 1.0 volt. Deep cycle Lead-Acid cells are usually rated over 100 hours (C/100) or even 120 hours (C/120). Because all batteries give up more amps if discharged slowly and less amps if discharged quickly, it becomes important to know how fast you will be extracting the amps from your battery bank. If Nickel-Iron batteries are discharged evenly over 100 hours an extra 20% more amps would be given out than if discharged over 5 hours. So a 500Ah Nickel-Iron battery would be rated at 600Ah if using the Lead-Acid standard of C/100. Similarly a 500Ah Lead-Acid battery rated at the C/5 standard performs poorly by comparison where a drop of 40% in total amps discharged occurs giving a C/5 rating of 300Ah.
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How rate of discharge affects cell Voltage
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All batteries have a maximum rate they can discharge their amps at. The higher the rate of amp discharge the faster and lower the cell voltage drops - sometimes well below the cells rated voltage. For instance starting a car can temporarily drop the battery voltage from 12 volts to 6 volts while the engine is being cranked. Once cranking stops, the voltage will then recover to a little under the voltage it was prior to cranking the engine - even if the engine didn't start.
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A large load causing a high rate of discharge on a fully charged battery will cause a smaller voltage drop than the same large load on an almost discharged battery.
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How many amps will you need?
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Is there a possibility you will need to get additional capacity in the future? You can add more Nickel-Iron cells of the same capacity to a Nickel-Iron battery bank with no adverse effects. Lead-Acid cells not of the same age and usage pattern in the same battery bank will conflict and shorten both the new and old cells life expectancy.
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Depth of discharge (DOD) or how much battery capacity can be used is also important to consider in assessment of battery capacity. Nickel-Iron batteries can be regularly discharged to 100% of their rated capacity or 100% DOD with no harm. For Lead-Acid batteries regular 50% or greater depth of discharge can significantly shorten the battery life to 1/4 of the maximum. Further reductions in Lead-Acid battery life can be expected if the discharge is faster than the C/100 rating. To maximize Lead-Acid battery life a load with a slow discharge rate not exceeding the C/100 rating to a depth not exceeding 10% of the rated battery capacity in amp hours is the optimum.
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In summary higher amp hour capacity cells can deliver higher rates of amp discharge with slower, shallower voltage drops than lower amp hour capacity cells. Make sure the cells can, at a minimum
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Deliver the maximum amps your electric load device/s will draw.
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Have enough amp hour capacity to keep the electrical load device functioning for the time you need it to function.
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And maintain a cell voltage above the minimum required by the electrical load device.
