How to modify my BMS settings to avoid power cut off ?

[missoo]

Hi everyone,

I have a 4S 280A evecells with a 120 A overkillsolar BMS.
I am using 2 inverters :
- an edecoa 1000W (2000Wc) dedicated to my fridge
- a victron 400W (900Wc) that I use for charging computer, batteries and other equipments ...

These past days I had a lot of power cut off while my fridge was working. I noticed that when my refrigerator compressor was working the voltage was dropping to 11,4V - 11,9 V following by a power cut off (when the fridge stops working the voltage goes back to 13V...). I guess it's because my BMS settings are not right but I don't know how to modify them.

The undervoltage is set up to 2,5V and the undervoltage release to 3V. Should I change the undervoltage release to a lower value ?

Attached , the settings I am using right now.

Thanks for your help.

 

[DThames]

You have a problem that you should not try to fix by changing the BMS settings.

With a battery the size that you have, if the battery is at 13.0v and well charged, you should see very close to 13.0v while your frig is running.

Are you sure the battery is properly and fully charged?

 

[DJSmiley]

Voltage dropping to 11.4V should not happen, unless the cells are empty

Where did you read this?
- At the BMS? Check the terminals and wiring
- At the inverter? Check the wires and fuses. (Battery voltage should remain constant)

Most common issues with voltage drops are bad crimps, too thin wires. Another very common issues is the use of cheap automatic breakers (black with red levers)

Settings look fine to me, it's only the voltage drop which is concerning and should not be happening. (My previous 280Ah pack dropped 30mV under a 100A load)

 

[missoo]

I've just measured the voltage on my battery, it is 13,15V.

I read the voltage dropping under 12 V on my battery monitor.

For the inverter edecoa 1000W I am using 16mm2 wires. Is maybe the wires of my fridge not big enough ?...
For the inverter victron 500 W I am using 10 mm2 wires.
I have less than 1m cable lenght.

I am using this kind of fuse.

 

[DJSmiley]

16mm2 is fine for 1000W (altough I would prefer 25, but that doesn't make a massive difference in terms of voltage drop)

If there is >1V voltage drop between battery and inverter it must be pretty easy to spot, something should get pretty hot.

Try pulling a 500W load on the 1000W inverter for like 5-10min and you probably can feel something heating up significantly. A thermal camera would be even better.
Or just use a voltmeter and start from the battery to see at which point the voltage is dropping to <12V.

The fuses you're linking to are fine, as long as, off course, all cables and lugs are made properly (and are torqued to spec)

 

[missoo]

I tried a very quick test, and I measured a voltage drop from 13,2 V to 12,9V directly on the battery connectors... Is this also not normal ?

I can't put a high load on my inverter as the wires are directly connected to the fridge thermostat (they work together).

 

[tinyt]

I tried a very quick test, and I measured a voltage drop from 13,2 V to 12,9V directly on the battery connectors... Is this also not normal ?

I can't put a high load on my inverter as the wires are directly connected to the fridge thermostat (they work together).

Maybe poor bus bar connections between cells of your 4S battery. Just guessing.

 

[DJSmiley]

I tried a very quick test, and I measured a voltage drop from 13,2 V to 12,9V directly on the battery connectors... Is this also not normal ?

I can't put a high load on my inverter as the wires are directly connected to the fridge thermostat (they work together).

How do you mean? You can temporary unplug the fridge and connect a load to the inverter

I hope the thermostat is controlling the inverters power switch, and not the 12V feed to the inverter (Quite sure a thermostat can't do 50A+ on its contacts)

 

[missoo]

Yes, my thermostat is controlling the inverter power switch only. The wires are welded.

Here is a picture of my battery. What do you think ?

 

[TomC4306]

Perhaps the battery is not charged.
You don't mention how you are charging.

 

[chess-equality]

- an edecoa 1000W (2000Wc) dedicated to my fridge

Is this the Edecoa inverter + charger? Overall are you satisfied with its performance? Where did you buy it and would you be kind enough to post pictures of it?

I've seen an inverter + charger version of it from the Edecoa official store in AliExpress. I'm interested to know if it's any good at all.

 

[missoo]

My battery is at 13 V charging with 430W solar panels but with the heat wave in France I am staying in a shady place right now (my solar panels getting 1,4A right now).

Chess-equality, no I just have the simple Edecoa inverter that I bought on aliexpress. It works fine.

 

[DThames]

Missoo, do you know how to take a DC volt meter and follow your series circuit looking for a voltage that might appear across each connection?

 

[missoo]

DThames, I believe I do.
I began to measure a drop on my battery connectors and I wondered if I already spotted something wrong there.

I believe maybe you could explain it better...

 

[DThames]

DThames, I believe I do.
I began to measure a drop on my battery connectors and I wondered if I already spotted something wrong there.

I believe maybe you could explain it better...

While your frig is running, amp are flowing from the battery to the inverter.....if there is a bad connections it will have voltage across that connection. You need to look for voltage across each connection, where your battery cables are inside the battery, bus bars, battery to battery cable, battery cable to inverter post.....all connection points. For an example, the battery ring terminal to the cell post under the battery should be right at zero volts. If it were 0.3v that would take your 13.0v battery to 12.7v. Someone also said, it should create a warm spot where you have a bad connection. Maybe you could detect that or maybe not. It would depend on the amps and how bad the connection was, how warm/hot it would get.

Can you charge your battery from AC power to make sure it is fully charged?

 

[missoo]

Thanks for this really clear explanation. Indeed, I misunderstood, I didn't know I should measure the voltage across one connexion.

Right now I am right camping with my RV so I can't charge the battery.
However I guess it's not fully charged as I have got only 12,9V on my battery.

 

[DThames]

Thanks for this really clear explanation. Indeed, I misunderstood, I didn't know I should measure the voltage across one connexion.

Right now I am right camping with my RV so I can't charge the battery.
However I guess it's not fully charged as I have got only 12,9V on my battery.

Do you have a phone app interface to your BMS so you can see each cell voltage in real time?

Edit additional....If you can put a volt meter on one cell while under load, that cell voltage x 4 should give approx battery voltage. If that voltage is the low voltage you see on at the inverter battery terminals, that is a good sign of a low charge and a good sign your don't have a bad connection somewhere.

 

[missoo]

Yes I have an app to check cell voltage.



Also by looking at my installation, I noticed one of the "screw" used to tighten the positive wire on the edecoa inverter was untighten. (It's hard to tighten it by hand, maybe I should replace it).
I hope the issue was coming from there and that everything is gona be solved.

 

[DJSmiley]

Cell voltage is pretty low, and 87mV difference with only 2.3A load is massive.

Did you topbalance the cells properly before install? Also I would suggest checking the busbars and connections (and torque) and charge the cells, they seem pretty empty.

 

[DThames]

Yes I have an app to check cell voltage.

View attachment 103380

Also by looking at my installation, I noticed one of the "screw" used to tighten the positive wire on the edecoa inverter was untighten. (It's hard to tighten it by hand, maybe I should replace it).
I hope the issue was coming from there and that everything is gona be solved.

View attachment 103379

During normal discharge when you get down to below 3.1v there is not a lot left in a LiFePo4 battery. You need a charge. I would look at the top balance when you have time.