How Many kWh Should I See in Real World Conditions?

[Sunshine22]

Hi Everyone!

I'm a noob and didn't test my 12.8v LiFePO4 batteries individually prior to putting them in series to make a 48v pack. I did individually charge each battery to max and put them in parallel for several days prior to putting them in series. Below is what I'm using:
Growatt SPF 3000TL LVM-ES 3KW
USE2
Bulk 57.5V #19
Float 55V #20
DC Cutoff 46V #21
Batteries - 4 x Zoom 12.8v 200ah LiFePO4 in series

I decided to do a battery capacity test yesterday. I allowed solar to fully charge the battery pack. In the evening (no solar or utility) I plugged a watt meter into an outlet powered by the inverter. I used an old school, oil filled, radiator style space heater to draw power. I ran it on high (~1500 watts) for 1/2 hour then turned it to low (~ 600 watts) for the remainder of the test. The reason I didn't leave it on high is because the Growatt's fans were really screaming, and I didn't want to leave it like that running in my shop overnight.

Results:
This morning (before sunrise) I found the system completely off (no lights, no power). The watt meter showed I used 8.46 kWh. Does that seem right for this system, or do you guys think I may have one or more defective batteries? FYI the temperature in the shop started at 55 degrees and was around 50 degrees this morning. The whole system including the batteries are in the shop.

Thanks!

 

[MaikaiLifeDIY]

I'm a little new, but volts multiplied by amps equals watts, watts is actual power.

So my math says your total watt-hours are 10,240 between all four of your batteries.

You have both wires and inverter efficiency that takes some power, plus the inverter itself uses power for operation.

So dividing the capacity by what you used shows you got about 82% of your available energy, and I believe most systems turn off when the voltage from the batteries reaches some threshold to protect the batteries, and or some batteries have the same protection in them.

 

[Sunshine22]

I'm a little new, but volts multiplied by amps equals watts, watts is actual power.

So my math says your total watt-hours are 10,240 between all four of your batteries.

You have both wires and inverter efficiency that takes some power, plus the inverter itself uses power for operation.

So dividing the capacity by what you used shows you got about 82% of your available energy, and I believe most systems turn off when the voltage from the batteries reaches some threshold to protect the batteries, and or some batteries have the same protection in them.

That's all correct. I'm wondering if that efficiency is standard, or sub-par? I have the DC Cutoff set to 46V. I'm not sure what the efficiency of the Growatt SPF 3000TL LVM-ES 3KW is - on average?

 

[Sunshine22]

UPDATE: Sun started to rise and the SPF 3000TL LVM-ES 3KW came back to life. It showed pulling in 100 watt solar. The voltage was 49.8v. I used a multimeter to check the pack - it matched = 49.8v, BUT when I checked the voltage of the individual batteries one was 0.5v lower than the other 3. Three were at around 12.6v and the low one was 12.1. Could this indicate it's a bad battery? What do you guys recommend?

 

[MaikaiLifeDIY]

Do you have a picture of the way you have your batteries installed?

 

[fastline]

I fail to see the issue here. There are a lot of assumptions being made here. One being that the battery 200ah spec is legit. Keep in mind most OE tests take a battery to it's knees so they can make that claim. Is that 200ah at a 20hr rate? Did you discharge for 20hrs? Was there actually charge still left in the batteries? What typically happens is resistance through everything shows a lower voltage than what the batteries actually are. IE, there may be more capacity in them.

You ran through an inverter. That will have efficiency loss. You also are betting on this NASA calibrated watt meter, yes?

For more real world specs, I usually assume 90% of what is published. That is 9.2kwh.

 

[Sunshine22]

Do you have a picture of the way you have your batteries installed?

I didn't take a picture, but the batteries are connected in series with approximately 6" sections 2 AWG copper cable with tinned copper lugs. The battery pack is connected to the Growatt with 6' sections of 4 AWG copper cable.

 

[Sunshine22]

I fail to see the issue here. There are a lot of assumptions being made here. One being that the battery 200ah spec is legit. Keep in mind most OE tests take a battery to it's knees so they can make that claim. Is that 200ah at a 20hr rate? Did you discharge for 20hrs? Was there actually charge still left in the batteries? What typically happens is resistance through everything shows a lower voltage than what the batteries actually are. IE, there may be more capacity in them.

You ran through an inverter. That will have efficiency loss. You also are betting on this NASA calibrated watt meter, yes?

For more real world specs, I usually assume 90% of what is published. That is 9.2kwh.

All in all I think it is ok. I realized I have my DC cutoff at 46v. The battery manual lists 10.8v as being 1%, so I probably had around 5% charge remaining at cutoff. BTW - What's up with "NASA calibrated watt meter"?

 

[fastline]

All in all I think it is ok. I realized I have my DC cutoff at 46v. The battery manual lists 10.8v as being 1%, so I probably had around 5% charge remaining at cutoff. BTW - What's up with "NASA calibrated watt meter"?

I simply mean that you are putting all the faith in the fact that the meter you used is correct. I am not saying it is wrong but if you use 3 different ones, you will probably get 3 different readings. Has to track voltage and current perfectly.