Vevor 24v/3500W inverter issue

[GedKealmen]

Hello everyone. I recently set up a simple solar system for my home out in the sticks. Almost all the parts were bought off AliExpress at very nice prices and have been working great for the most part. I am using 4 LiFePo4 12v/100Ah batteries in a 2P2S config for a 24v/200Ah bank, charging them with 1350W of solar input through a PowMr-m60-Pro charger, configured to boost charge at 29v, and float at 26.6v.

However, the inverter is not correctly calibrated for measuring DC input voltage. It systematically reads almost 1v above real voltage. And since the inverter has an autoshutoff protection built in for 30v, when I charge the bank at 29v or 29.2v (as recommended by the battery manufacturer) once the bank gets close to full charge the inverter sometimes shuts off briefly because it detects 30v+ incorrectly. This sometimes happens even when I boost charge at 28.8.

I highly doubt it's a cable issue. Voltage measured by the charge controller at PV in, by my JuncTek KH140F shunt on the bank negative out, and by multimeter at the battery bank terminals AND the inverter DC terminals all coincide, it is simply the inverter misreading DC voltage.

Does any knowledgeable person know if there is some way to recalibrate the inverter so it measures voltage correctly?

(I noticed a small blue adjustable potentiometer on it but I have no clue WHAT that might adjust. I'll add pictures of the inverter innards so people can hopefully let me know if they recognize what it could be for)

 

[42OhmsPA]

Hello everyone. I recently set up a simple solar system for my home out in the sticks. Almost all the parts were bought off AliExpress at very nice prices and have been working great for the most part. I am using 4 LiFePo4 12v/100Ah batteries in a 2P2S config for a 24v/200Ah bank, charging them with 1350W of solar input through a PowMr-m60-Pro charger, configured to boost charge at 29v, and float at 26.6v.

However, the inverter is not correctly calibrated for measuring DC input voltage. It systematically reads almost 1v above real voltage. And since the inverter has an autoshutoff protection built in for 30v, when I charge the bank at 29v or 29.2v (as recommended by the battery manufacturer) once the bank gets close to full charge the inverter sometimes shuts off briefly because it detects 30v+ incorrectly. This sometimes happens even when I boost charge at 28.8.

I highly doubt it's a cable issue. Voltage measured by the charge controller at PV in, by my JuncTek KH140F shunt on the bank negative out, and by multimeter at the battery bank terminals AND the inverter DC terminals all coincide, it is simply the inverter misreading DC voltage.

Does any knowledgeable person know if there is some way to recalibrate the inverter so it measures voltage correctly?

(I noticed a small blue adjustable potentiometer on it but I have no clue WHAT that might adjust. I'll add pictures of the inverter innards so people can hopefully let me know if they recognize what it could be for)

View attachment 232901
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I would stop charging at that high of a voltage. No need to go over 28V (3.5V/cell) bulk.
It's likely detecting a higher voltage because the BMS is disconnecting due to a runner cell...

I don't know how to recalibrate it.

 

[Mattb4]

The runner cell is even a greater chance of happening with 2-12vDC batteries in series. Your 2S2P is undoubtedly unbalanced. You shoulda bought cheap 24vDC batteries.

 

[GedKealmen]

I would stop charging at that high of a voltage. No need to go over 28V (3.5V/cell) bulk.
It's likely detecting a higher voltage because the BMS is disconnecting due to a runner cell...

I don't know how to recalibrate it.

Thanks. I will lower the charging voltage, I was simply following the battery manufacturer advice for charging.

But the issue is consistent at all voltage states. If the shunt/charger/multimeter read 27v, the inverter shows 28, it always shows 1v over real DC voltage. I would still like to try to correct that.

 

[GedKealmen]

The runner cell is even a greater chance of happening with 2-12vDC batteries in series. Your 2S2P is undoubtedly unbalanced. You shoulda bought cheap 24vDC batteries.

They are using a battery equalizer and measurements across all of them are always 0.01v off from each other after the equalizer does its thing. The only time the equalizer starts blinking is during boost charge. During bulk it stays off, at boost it blinks for a few minutes to equalize them, and after that all 4 batteries measure exactly the same or 0.01v off. BTW, the bank is in 2P2S, not 2S2P. Not sure if that makes a difference. Noob here.

 

[GedKealmen]

The runner cell is even a greater chance of happening with 2-12vDC batteries in series. Your 2S2P is undoubtedly unbalanced. You shoulda bought cheap 24vDC batteries.

BTW, I assume a "runner cell" means one of the cells in the batteries is charging to higher level than others, if I assume correctly and such a cell were causing higher voltages wouldn't that show on the multimeter readings at inverter DC input terminals?

 

[Mattb4]

BTW, I assume a "runner cell" means one of the cells in the batteries is charging to higher level than others, if I assume correctly and such a cell were causing higher voltages wouldn't that show on the multimeter readings at inverter DC input terminals?

What happens is the BMS cuts off and the voltage from your SCC spikes upwards causing a high voltage reading at your Inverter.

Edit: I must admit, after thinking a bit more about it, having the 2 sets of series batteries in parallel should prevent one BMS cut out from affecting the SCC output voltage. It would take both sets to cut out. Little less likely.

Could be you have call to contact Vevor about a faulty inverter.

 

[42OhmsPA]

Thanks. I will lower the charging voltage, I was simply following the battery manufacturer advice for charging.

Don't trust their advice. Take a look at a charge curve for LiFePO4 cell and how quickly voltage climbs at the top of the curve / knee.

But the issue is consistent at all voltage states. If the shunt/charger/multimeter read 27v, the inverter shows 28, it always shows 1v over real DC voltage. I would still like to try to correct that.

Are you verifying with the meter at each connection point or only at the main terminals? You've checked all connections are clean and tight, optimally with a thermal camera?

BTW, I assume a "runner cell" means one of the cells in the batteries is charging to higher level than others,

Correct

if I assume correctly and such a cell were causing higher voltages wouldn't that show on the multimeter readings at inverter DC input terminals?

No. Pack voltage will still be normal because the other cells will be lower...
3.5 + 3.5 + 3.5 + 3.5 = 14
3.45 + 3.45 + 3.45 + 3.65 = 14

 

[GedKealmen]

What happens is the BMS cuts off and the voltage from your SCC spikes upwards causing a high voltage reading at your Inverter.

Edit: I must admit, after thinking a bit more about it, having the 2 sets of series batteries in parallel should prevent one BMS cut out from affecting the SCC output voltage. It would take both sets to cut out. Little less likely.

Could be you have call to contact Vevor about a faulty inverter.

I did, they offered no help at all. Vevor is notorious for bad aftersale service... they did however refund me the cost of the inverter! So now I have a free inverter which misreads voltages but otherwise works great, it effortlessly puts out 2500W when needed and I seldom need more than that.

 

[GedKealmen]

Don't trust their advice. Take a look at a charge curve for LiFePO4 cell and how quickly voltage climbs at the top of the curve / knee.

Are you verifying with the meter at each connection point or only at the main terminals? You've checked all connections are clean and tight, optimally with a thermal camera?

Correct

No. Pack voltage will still be normal because the other cells will be lower...
3.5 + 3.5 + 3.5 + 3.5 = 14
3.45 + 3.45 + 3.45 + 3.65 = 14

Gotcha. I already lowered the bulk/boost voltages. Hopefully there will be no more inverter overvoltage protection issues.

Yup, I do verify voltages all over the system. I check each battery at its terminals, each 2P set, the whole 2P2S bank, and then at the inverter DC in terminals. I check the busbars too. All show the same voltage.

I also happen to own a 160x120 thermal camera and there is no heat showing at any connection points. The highest temperature I've seen when pulling over 80 amps from the bank has been arond 38C/100F, which shouldn't be an issue. I crimped all cables carefully and all points seem to be handling current correctly.

 

[adamantium]

Gotcha. I already lowered the bulk/boost voltages. Hopefully there will be no more inverter overvoltage protection issues.

Yup, I do verify voltages all over the system. I check each battery at its terminals, each 2P set, the whole 2P2S bank, and then at the inverter DC in terminals. I check the busbars too. All show the same voltage.

I also happen to own a 160x120 thermal camera and there is no heat showing at any connection points. The highest temperature I've seen when pulling over 80 amps from the bank has been arond 38C/100F, which shouldn't be an issue. I crimped all cables carefully and all points seem to be handling current correctly.

What did you do to lower the bulk/boost voltages and did that help? Was it the potentiometer that you show in the image at the top?

 

[matttelz]

this is soo funny. I have the same MMPT controller, I've had my eye on the Vevor 24V inverter, and I've got 400ah 12V batteries. You can always change the Calibration voltage on the MPPT controller to show a higher voltage than your battery pack so it'll stop charging the batteries at any voltage lower than the full 14.6V per pack. I've got mine to stop charging at 14.4V instead of 14.6V

 

[robbob2112]

Were it me, it is free - so carefully with one hand in a pocket and using a non-conductive tweaker (they are called that - little plastic thing that looks like a pencil with just a small metal tip) ... use the tweaker and twist the pot a bit - keep exact track of how many turns so you can return it to the original position. If turning it a couple times doesn't do anything to the voltage being read put it back to the original position and assume the adjustment is probably some of the soldered on components and you can't adjust without a schematic.

NOW - here is another thought - lengthen the positive cable by a foot or two - it will increase the impeadence of the pack by a tiny amount and cause a lower voltage to be seen at the inverter. Will also tend to prevent spikes being sent when things cut on and off. I have seen some use a 10ft cable for this purpose because.

 

[matttelz]

FYI I have a different inverter but the pot is almost in the same location as yours. The pot is right next to the vertical board which on my unit I believe controls the fans and temperature control. If I adjust the lower pot seen here it'll trim the ac voltage. Stock my unit came with 122.4v output. I can adjust it down to 119v or upto 125v is as far as I've pushed it.