Question about MN3024DIY/Daly power loss from battery bank while connected to grid

[100degrees]

Setup: MN3024DIY plugged into the grid -> Daly BMS (8S running 2P at the moment) -> LEV60F LFP cells @ 28V collectively (3.5V per cell.)

I noticed that my bank would discharge down to 26.9V and then charge up again to 28V and repeat that process several times a day. Based on what I've learned here there are several possible causes of that lost energy: the batteries could be self discharging, the Daly could be misbehaving or the Chinese Midnite branded charge controller could be doing something odd. Since I'm still building my battery capacity tester, and haven't fully figured out how to use the badly translated Daly UART interface beyond making sure that it's basic cell voltage and balance settings look right, I started with the MN3024DIY.

I got connected to the Daly UART interface and saw that it reported a ~.3 amp power draw with the MN3024DIY plugged into the wall. On charge, amperage readings on the Daly match up with what the Midnite is set to charge at so I think it's fairly accurate. Then I disconnected the negative lead of the battery bank from the MN3024DIY. Amperage dropped to nothing, which doesn't really tell me much other than the Daly isn't discharging .3 amps through the tiny monitoring wires used for balancing since disconnecting the negative lead created an open circuit. The battery voltage did stop dropping though so the cause of my lost power isn't the Daly balancing circuit but something on the main load.

My assumptions are that the MN3024DIY shouldn't be drawing power from the battery bank when connected to the grid. Also, the Daly BMS is supposed to draw about 100uA during normal operation. Am I right or wrong to think that neither of these loads should be pulling 8 watts an hour?

I have a killowatt meter, if that can do a running read of consumed power maybe I can figure out how much power the combined system is running through in a day, then if I deduct the expected power draw I can at least find out how much power this thing is wasting.

Any thoughts on how I should go about troubleshooting this? Anticipating one of them: I'm poor, this is what I can afford, buying better stuff isn't an option available to me. If it were everything I own would probably be blue.

 

[MisterSandals]

I noticed that my bank would discharge down to 26.9V and then charge up again to 28V and repeat that process several times a day.

This is known as micro cycling. The battery charges, settles, charges, settles. Its pretty much jus thrashing but the idle draw from the inverter probably makes it more frequent.
At 26.9V, your battery is 95% full and has lost the surface charge and a little more.

What is the absorb stage voltage, tail current and time? You might be able to but a tiny bit more in with a gentle charge termination in the absorb stage.

Do you have access to cell level voltages? Can you confirm its terminating charge and not a BMS disconnect?

Can you post your charge settings? There might be some other settings to smooth this out.

Unless you are seeing something else i wouldn't worry that there is something wrong.

 

[Checkthisout]

Set your float voltage lower.

Your DALY's millivolt setting on its shunt doesn't go down low enough to get accurate current measurements at low current draws.

 

[100degrees]

This is known as micro cycling. The battery charges, settles, charges, settles. Its pretty much jus thrashing but the idle draw from the inverter probably makes it more frequent.
At 26.9V, your battery is 95% full and has lost the surface charge and a little more.

What is the absorb stage voltage, tail current and time? You might be able to but a tiny bit more in with a gentle charge termination in the absorb stage.

Do you have access to cell level voltages? Can you confirm its terminating charge and not a BMS disconnect?

Can you post your charge settings? There might be some other settings to smooth this out.

Unless you are seeing something else i wouldn't worry that there is something wrong.

View attachment 199205

Thank you for the reply and information. I don't know what absorb stage voltage or tail current and time translate to in my settings.

I have the batteries connected in 8S/3P. Assuming the cell voltages in parallel are the same, which is my understanding, then yes, I have that information. I'm posting all my settings minus the Daly error log, which seems to be bugged. I cleared it, immediately read the data and it just sat at "Reading the data..." for half an hour before I gave up.


MN3024DIY settings that could be relevant:
Battery boost charge voltage: 28V
Batter boost charge time: 120 Minutes
Battery floating charge voltage: 28.4V
Battery overdischarge voltage: 24V
Battery overdischarge delay time: 5 Seconds
Battery discharge limit voltage: 23V
Power saving mode: Enabled

According to this https://midniteftp.com/forum/index.php?topic=5671.0 power saving mode is supposed to only draw around 2W, though I can't find anyone who can confirm that or anything in the manual that gives a watt value to power saving mode. According to my meter the unit drew 390 watts over the last 24 hours with no load on the inverter in power saving mode and a -.3 amp constant draw on the Daly save during charge.

Attached are the Daly screen shots. If a field is blank the Daly didn't populate it with any data when I tried to read it.

 

[100degrees]

Set your float voltage lower.

Your DALY's millivolt setting on its shunt doesn't go down low enough to get accurate current measurements at low current draws.

I will try that tomorrow. thanks for the suggestion.

While I don't doubt that the Daly is reporting a less than accurate current draw, I've manually confirmed that the battery bank is discharging with nothing plugged into the inverter and with the charge controller connected to the grid. I have more research to do based on what MisterSandals said. It still seems odd to me that a charge controller would discharge backup batteries at all while connected to the grid with nothing connected to the inverter, but I guess that's incorrect.

 

[100degrees]

Thought I'd add, in power save mode the MN3024DIY is only drawing 3-4 watts from the grid according to my kill-a-watt, except when it's recharging the batteries it drained, which is several times a day.

 

[UpNorthMan]

Battery float charge at 28.4? Is this a typo? It's higher than the boost charge voltage.

 

[MisterSandals]

Battery floating charge voltage: 28.4V

Yea, this NEEDS to be lower than your boost. The float is the between-charge resting state. I'd try around 3.4Vpc which is a nice conservative number. If the battery is going to be stored, it should be even lower still, perhaps 3.2Vpc for long term (many months).

 

[100degrees]

Battery float charge at 28.4? Is this a typo? It's higher than the boost charge voltage.

Not a typo, I'm just ignorant. I did read the manual but I obviously didn't understand the manual. Based on your earlier advice I lowered it to 27.6V last night. Thanks for correcting my error.

 

[100degrees]

Yea, this NEEDS to be lower than your boost. The float is the between-charge resting state. I'd try around 3.4Vpc which is a nice conservative number. If the battery is going to be stored, it should be even lower still, perhaps 3.2Vpc for long term (many months).

Thanks. At the moment I've got it at 27.6, if my math is right @ 3.4Vpc it would be 27.2V on the MN3024DIY (8S arrangement) or 25.6V @ 3.2Vpc. I'll go lower it to 27.2V now. These batteries are going to be sitting until I have a power outage, which happens for about eight hours every year. In that use case what voltage do you think should I go with for the floating charge?

Thanks for teaching me what this setting does. Is there a book or something online I should read to understand more what I'm doing? Before I bought any of this I read the manuals and some of what's on this forum but I either didn't read the right things or missed something vital because I didn't know what float charge was until you told me. I learned how to spec a solar system and a lot about various battery chemistries but seems like I'm missing the basics.

 

[MisterSandals]

These batteries are going to be sitting until I have a power outage, which happens for about eight hours every year. In that use case what voltage do you think should I go with for the floating charge?

Thats the rub. By lowering the voltage for "storage except for 8 days", you are being gentler on your battery but also diminishing you state of charge for those 8 days you need it. I'd try to figure out how much you NEED and set your state of charge to match that. The difference in battery life is probably very low compared to having enough power when you need it.

Some folks vary their strategy based on storms on the horizon and bump up their float for maximum charge when an outage is more likely.

 

[100degrees]

Thats the rub. By lowering the voltage for "storage except for 8 days", you are being gentler on your battery but also diminishing you state of charge for those 8 days you need it. I'd try to figure out how much you NEED and set your state of charge to match that. The difference in battery life is probably very low compared to having enough power when you need it.

Some folks vary their strategy based on storms on the horizon and bump up their float for maximum charge when an outage is more likely.

It's actually just 8 hours, and 200 watts/hr for that eight hours, the battery bank is rated for about 3x that. So yeah, given the relatively small amount of power I'm pulling from my battery bank I think I'm fine to lower the voltage to something very conservative. Thanks for explaining and for those graphs of capacity and charge you posted. Power only goes out during either high winds or big snow storms so I may kick it up those days just in case it might be more than 8 hours but otherwise I'll keep it at a lower voltage.