ANT BMS VDC WTF? - Voltages dont match multimeter

Greetings oh scions of solar. Virtuosos of voltage. Buffs of the BMS. I have a situation that is driving me wacky trying to troubleshoot. Perhaps someone has an idea for me ?- thanks in advance.

My Setup: Growatt 48v SPF 3000TL LVM, 16s 176AH LiFePO4, ANT SMART BMS

So after a month of waiting for my ANT SMART BMS to arrive (ordered off eBay from a Chinese supplier) , I was excited to get it all hooked up (and replace a DALY I had installed temporarily). The setup was pretty simple and straight forward. {Once you figure out to temporarily short the two black wires to get it to start ;-) } Everything was rocking and rolling and I was feeling good.

Well, for a moment.

My Growatt was saying the battery voltage was 52-53V but the fancy LCD screen with the BMS was saying it was 56V. I assumed maybe there was some voltage drop between the battery and the inverter but it is pretty much 2AWG wire from battery to inverter so that didn't seem likely. I then took a multi-meter to each cell and they were all pretty much 3.3V but the BMS screen was reporting about 3.5V.

Anyone have any ideas what is happening here? Can it be a wiring issue if the BMS is reporting higher than the actual voltage. (.2V higher per cell and between 3-4V higher for the pack). The whole system is tied to the same negative so zero should be zero, right? The only thing I did notice when bringing everything only was that before the BMS was turned on I was getting approximately 4V between the battery positive and the C- lead on the BMS. I assumed that should have been 0V with the BMS still off but with everything sharing the same negative why would that make a difference.

To use a movie analogy, it seems like that scene in DieHard 2 when they reset sea level to -200ft so the ground wasnt really the ground.

Have you checked calibration on your DMM?

MisterSandals said:

Have you checked calibration on your DMM?

Click to expand...

Assuming DMM is the mulimeter, no but the values is both inline with what the inverter said (the full bank voltage of 52-53V) and before I put in the ANT BMS I had a BattGO ISDT bg-8s attached to each 8 cells so I could see the individual cell voltages (since the DALY didnt have any such a function) and the cells were showing voltages inline with what the multimeter said. I also have another meter connected to the Pos and Neg busbars in my system and it also agrees with the multimeter. So 3 sources agree and the BMS is the odd man out.

Seems like you have already answered your own question. I'd be contacting the people you bought the BMS from.

Thats what I was hoping wasnt the next step I dont see this going well. Such a shame. This BMS showed so much promise. LCD screen, bluetooth, so many bells an whistles including 4 temp sensors and even the wiring harnesses felt like quality. Damn you flawed voltages. Too much to hope for that there is a setting that changes it from Chinese to US voltage?

pangrande said:

Thats what I was hoping wasnt the next step I dont see this going well. Such a shame. This BMS showed so much promise. LCD screen, bluetooth, so many bells an whistles including 4 temp sensors and even the wiring harnesses felt like quality. Damn you flawed voltages. Too much to hope for that there is a setting that changes it from Chinese to US voltage?

Click to expand...

LOL .... I think that language is universal.

I don't know that BMS .... so maybe there is some magic setting ... but I really doubt it.
Give it a while and see if other ANT users check in.

I saw another voltage related ANT BMS thread but it was in the opposite direction. Voltages was lower. I have a magic BMS where the laws of physics dont apply and it can create extra voltage. Now that I think of it, maybe i actually got a bargain.

LOL .... Well, the good news is that since it things the voltage is higher than it actually is .... it's not going to cause any damage to your battery while you get it sorted out.

That is true. Thanks Bob.

Just saw this buried in the eBay listing for this BMS ....

Protection board calibration:
1. Single cell voltage calibration: You can adjust the "system reference voltage" (the default is about 3.0) to slowly increase the size of 0.001, turn down, you can change the unit voltage value until accurate!

Have you tried adjusting your reference voltage? Also, set the number of actual cells and cell type?

from the manual:

Protection board calibration:
1. Single voltage calibration: you can adjust the "system reference voltage" (default 3.0 or so), the amplitude of 0.001 to slow the transfer large, small transfer, you can change the voltage value until accurate!
2. Total pressure calibration: you can adjust the "total pressure calibration parameters" (default 3330 or so), slowly adjust the size of the value, you can change the total pressure until accurate!

3. Current calibration: You can adjust the "current sensor range", slowly adjust the size of this value, you can change the current until accurate!

Click to expand...

Hah. Yes, I set the proper cell count to 16 but I just discovered the "system reference voltage" line in the instructions. Very much the DieHard scenario. Going to try that now. Thanks. Stay tuned.

Which ANT BMS did you get? 7S-16S or the larger one?

I got the 10S-24S. Pretty slick unit.
Ok, think the reference voltage was the key. I had it set to 3.2V (I originally thought it may have been referring to cell nominal voltage. Spoiler alert, it wasnt). I knocked it down to 3.0 (to drop the per cell voltage by .2V) and now the voltages line up with the multimeter. So it looks like we may be good in the hood (that's not an expression, I actually kind of live in the hood).

Few notes for anyone who finds themselves going down this road;
-My first thought was why was the reference voltage anything more than 0, let alone 3V so I set it to 0. Bad idea. All cell voltages dropped to 0 and everything shut off. At least I know the BMS works.
-When I then set the reference voltage to 3v everything looked good but the inverter wouldnt start up because the BMS was throwing a PreCharge error. I figured I could fix it by pre-charging my inverter, which I did, but that didnt work. Still got the BMS PreCharge error. I then turned off my battery disconnect to prevent the inverter from pulling anything. Then started the BMS. Saw no PreCharge error. Then reconnected the inverter. That worked. Makes me wonder if this system can recover in the event of a BMS disconnect but since I am using the "common" lead for charge and discharge, if the BMS ever shuts off for voltage reasons I cannot see a scenario where that would self-correct so I guess I will need to intervene anyway.

Thanks all.

I guess the WAS a voltage translation thing .... LOL

Hah. Yeah. I guess it is like time zone thing. China +3V = US VDC. I'll let Wikipedia know.

pangrande said:

... before the BMS was turned on I was getting approximately 4V between the battery positive and the C- lead on the BMS.

Click to expand...

That would be leakage currents through the MOSFETS. With the BMS OFF, there is no active LOW drive on the Gates, so they are perhaps floating a bit. Regardless, since the input impedance of the meter is so high, it's probably not a lot of current. In fact, it is an equivalent impedance of ~5X that of the meter!