Thanks for the information! I have a considerably different application for my system. Would you be willing to advise me if I shared with you my intentions?
I want to run a single phase to three phase convertor.(variable frequency), for 1 hour every 7-10 days. I run this convertor (well pump) on line voltage and want a back up system when I loose grid power. I can pump 25 gallons per minute into a 1500 gallon storage container then gravity the water to my home. The convertor draws 23a (4200 watts) max and idles at 5watts.
Can I design a system for this using my existing LifePo batteries (currently have 120 amps @ 24 volts, ) by adding more batteries and using two inverters of 1500 watts (I don't need to run the pump with full power since it will flow 35 gallons a minute with full load).
Then I was going to install 2300 watts of solar to keep the batts charged. Any hope for this working?
You have a few things working against you at the moment. First your 24V, 4 x 40Ah cells must mean that you are connecting these together in a 2P2S configuration (two sets of two cells in parallel then those two sets in series) Is that correct? That would make a 24V battery with 80Ah. Using the formula for power (watts), power(W) = volts(V) * amps(A), Wh = 24V x 80Ah = 1920Wh. That means you only have 1920 watt-hours of power in your battery to do anything. That would require that you drain your battery 100% everyday. As described, you need 4200 watts to run you well pump for one hour a day, so that is 4200 watt-hours. So the first problem is that you can't store enough power in your small battery to do the work you have described.
Second, your more detailed description says that you have 120 amps @ 24V. Where did this come from? Is this a different battery or the same battery you described at the beginning? 4 x 40Ah doesn't make a 24V battery with 120Ah. It either makes a 48V battery with 40Ah, or a 24V battery with 80Ah, or a 12V battery with 160Ah. I assume we are still talking about the Valence RT series U1-12RT batteries you described in your first sentence, and those are 40Ah 12.8V batteries.
Third, your Valence RT series U1-12RT batteries have a continuous output current of only 30A, meaning that you can only use 30A continuously. Given that I am assuming you are using them in a 2P2S configuration described above, the maximum continuous output from your battery would be 60A. Using the power equation again, P = VA, power in watts = 24 volts times 60A =1440 watts continuous. Since they are 40Ah batteries (80Ah in parallel), and you can only drain them each at 30A continuous (60A in parallel), you would have up to 80 minutes of runtime at their maximum output. So you can only pull 1440 watts maximum from those batteries in a 24 volt configuration, and you can only do it for 1 hour and 20 minutes. As you described above, your well pump needs 4200 watts of power to run, and as a well pump it will have a large surge load at start up. Pump surges are typically calculated as 3 times their running power. So in this case 3 x 4200 watts = 12600 watts. Since your cells have a maximum surge load of 80A each, your battery will only have the ability to surge with 2 x 80A = 160A for 30 seconds (according to their specification). That means again power in watts = 24 volts times 160A = 3840 watts. So we now know that your battery is too small to do the job you want them to do. You need 4200 watts of continuous power for at least one hour. You have only 1440 watts of continuous power. You need 13200 watts of surge power, and you only have 3840 watts of surge power. So the batteries don't even have enough surge power to meet your continuous power needs to run the well pump.
Fourth, two 1500 watt inverters are insufficient to run a 4200 watt well pump. First they don't have enough power to start the well pump, second they can't provide 4200 watts of continuous power to run the well pump continuously. You need an inverter with at least 4200 watts of output power to run a 4200 watt well pump, and it needs to be able to surge to 12600 watts for at least 5 seconds to start it. Even if you aren't running it full load, you will be at 72% load pumping 25 gallons per hour with a maximum capacity of 35 gallons per hour. That is still over 3000 watts and two 1500 watt inverters won't cut it. You are rounding down and the inverter manufacturer is rounding up. It needs to go the other way. You need inverters with more power than you plan to use, especially with reactive loads. You must take into account your losses with the 3 phase converter and power factor of the pump.
Fifth you are doing this in Seattle in the winter.
Now if you get a 6500 watt inverter with 100% surge capability for 5 seconds, or a 9000 watt inverter with at least a 50% surge capability for 5 seconds, and then get 16 x 100Ah cells and put them in a 48 volt configuration, and then add 5000 watts of solar panels and can get 1 hour of full sunlight a day in the winter, then you have a chance. At least until your batteries start to wear out. This is all borderline too. You need more of everything (at a minimum battery and solar panels) to ensure you can cover days you don't get enough sunshine.
I think your most cost effective solution is to buy a 13kW dual-fuel or tri-fuel generator. You can buy one of these for $1500-$3500. You could either fuel it with gasoline, propane, or natural gas. If you go the natural gas route you wouldn't have to worry about filling it up all the time. But since it would only run one hour a day, that might not be a big deal. You need to calculate the cost of using natural gas again the cost of gasoline or propane. Running generators on natural gas is more expensive than most people think, but again in your case you would only be doing it one hour a day.
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