Testing - Top Balancing Experiment - 4 cells at a time

[PeteW]

No BMS as yet (More on this below.), so I'm doing some top balancing 4, 280Ah cells at a time (As opposed to building 12V batteries and pre-charging). If you see this I'd be interested in your opinions on this.
I began doing some top balancing 4 cells at a time yesterday. I only did them for an hour per set of 4 cells. I did two sets yesterday and two sets today, 4 cells per set, 16 cells altogether. I did take notes about cell voltages before, during and after. I took notes on PS and multimeter voltages during and PS amperage reading at the beginning and end of each top balance session. I was pretty conservative. I turned the amperage up to 10A approximately, but kept the voltage at 3.55V. I numbered each cell when I first received them, so I've kept a log. I checked the voltage of each cell before starting so I could have something to reference. I won't get into a long explanation, preferring to wait for questions. I mostly want to make sure I'm not doing any harm. This is mostly a learning exercise for me. When I began the voltage ranges between my 16 cells were 3.271V - 3.290V. Current resting voltage between 4, 4 cell banks is 3.291 and 3.294.

 

[John Frum]

Please post a picture of the ac/dc distribution panel in your trailer.
I want to determine if you have 30 or 50 amp panel.
Also want to determine the amp rating on the ac to dc converter.
Finally I want to see the breaker and fuse maps.

You mention a hybrid inverter.
Hybrid usually means a grid tie inverter which is probably not what you want.

I suspect you want an inverter/charger.
Please provide product links from the Victron website to the inverter.
If you have a 50 amp panel this one is the Cadillac option.

https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-II-3kVA-2x120V-EN-.pdf

If you have a 30 amp panel this one is very nice, a little cheaper and probably easier to source.

https://www.victronenergy.com/upload/documents/Datasheet-Multiplus-inverter-charger_2kVA-and-3kVA-120V-US-EN-.pdf

Both of those units are 3000VA which is roughly equivalent to 2400 watts.

To charge via the tow vehicle alternator the orthodox choice is an isolated dc2dc charger.

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-Smart-DC-DC-chargers-isolated-250-400W-EN.pdf

How much spare alternator capacity do you have?

Lets address the top balance and bms topics in separate posts.

 

[PeteW]

I've moved my BMS questions to a new thread.

 

[PeteW]

Please post a picture of the ac/dc distribution panel in your trailer.
I want to determine if you have 30 or 50 amp panel.
Also want to determine the amp rating on the ac to dc converter.
Finally I want to see the breaker and fuse maps.

You mention a hybrid inverter.
Hybrid usually means a grid tie inverter which is probably not what you want.

I suspect you want an inverter/charger.
Please provide product links from the Victron website to the inverter.
If you have a 50 amp panel this one is the Cadillac option.

https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-II-3kVA-2x120V-EN-.pdf

If you have a 30 amp panel this one is very nice, a little cheaper and probably easier to source.

https://www.victronenergy.com/upload/documents/Datasheet-Multiplus-inverter-charger_2kVA-and-3kVA-120V-US-EN-.pdf

Both of those units are 3000VA which is roughly equivalent to 2400 watts.

To charge via the tow vehicle alternator the orthodox choice is an isolated dc2dc charger.

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-Smart-DC-DC-chargers-isolated-250-400W-EN.pdf

How much spare alternator capacity do you have?

Lets address the top balance and bms topics in separate posts.

Hi John, Thanks for your help, I've moved my BMS questions and my response to your questions here. https://diysolarforum.com/threads/b...use-thoughts-on-the-victron-12-200-bms.48801/

 

[John Frum]

3000VA is ~= 2400 watts
2400 ac watts / .85 conversion factor / 12 volts low cutoff = 235.294117647 dc amps
235.294117647 dc amps + 50 amps for the dc distribution center = 285.294117647 service amps
285.294117647 service amps / .8 fuse headroom = 356.617647059 fault amps.

4x 12 volt batteries each with a 120 amp overkill bms will be able to handle that load
or 2 x 24 volt batteries each with a 100 amp overkill bms
or 1 x 48 volt battery with a 100 amp overkill bms

 

[John Frum]

BTW I meant separate posts not separate threads.

 

[PeteW]

BTW I meant separate posts not separate threads.

First, due to My misunderstanding we're now discussing the wrong topics in each thread. We're discussing BMSs in the top balancing thread and top balancing in the BMS thread. It doesn't matter to me me, I leave it up to you.

3000VA is ~= 2400 watts
2400 ac watts / .85 conversion factor / 12 volts low cutoff = 235.294117647 dc amps
235.294117647 dc amps + 50 amps for the dc distribution center = 285.294117647 service amps
285.294117647 service amps / .8 fuse headroom = 356.617647059 fault amps.

4x 12 volt batteries each with a 120 amp overkill bms will be able to handle that load
or 2 x 24 volt batteries each with a 100 amp overkill bms
or 1 x 48 volt battery with a 100 amp overkill bms

I can follow the math and I can see where your going with it related to the BMSs, but I don't entirely understand what it means. Are you able to simplify it any for me or point me to something to read.

I'm taking it that you like the Overkill BMSs. I also saw what Sunshine eggo showed me about the Victory 12/200.

I've been reading about BMSs, but it's a lot to take in so I'm still digesting the information. Among the articles and papers I've read, I've read a number of filterguy's papers. Regarding BMSs I've only read his paper on BMS functions, I've looked at the one on Voltage settings guides for BMS, Chargers and Loads but was having trouble with it. What do I need to read first that might make it clearer.

 

[time2roll]

kept the voltage at 3.55V.

I mostly want to make sure I'm not doing any harm.

As long as voltage stops at 3.55, the cells will not be harmed.

 

[John Frum]

I can follow the math and I can see where your going with it related to the BMSs, but I don't entirely understand what it means. Are you able to simplify it any for me or point me to something to read.

What is the first point that you don't understand.
I don't think I can simplify it anymore than I have but I can expand on things.
Conversion factor is inverter efficiency.
12 volts low cutoff is used because as the voltage goes down the amperage goes up.
The working voltage range for LFP cells is 3.0 to 3.4 volts so we use 12 volts as the low cutoff for a 12 volt system.
service amps is the maximum current that the system should see in normal operation.
We want to add some headroom over service amps so that we don't have nuisance trips of our over current protection so we divide by .8 to give some headroom.

The bottleneck of each battery is its BMS.
The 12 volt overkill BMS are rated for 120amps.
So 4 batteries in parralel can theoretically handle 480 service amps.
In reality they batteries and the wires that connect them to the system will not be perfectly matched so we don't want to push that to the limit but they should be able to 285 amps comfortably.

I'm taking it that you like the Overkill BMSs.

Yes.
The forum has a lot of experience with them.
I have the 8s version in my own system.

I've been reading about BMSs, but it's a lot to take in so I'm still digesting the information. Among the articles and papers I've read, I've read a number of filterguy's papers. Regarding BMSs I've only read his paper on BMS functions, I've looked at the one on Voltage settings guides for BMS, Chargers and Loads but was having trouble with it. What do I need to read first that might make it clearer.

Sorry I don't have a primer for that.

A BMS has 2 main functions.
1Protect the cells from operating outside their safe envelope of voltage and current.
2. maintain the top balance of the cells

I'm sure you will have more focused questions.
Will be glad to answer them.

 

[PeteW]

As long as voltage stops at 3.55, the cells will not be harmed.

Thanks, I never went past that nor did I completely top balance the cells. This was more of a practice exercise. I've been informed that it's not good for them to try and keep them continually top balanced, thankfully I haven't, and I was encouraged to get my BMSs before doing more. Working on that. Trying to get my head around the BMS. I've read Filterguy's BMS functions paper, but I'm going to read it again.

 

[John Frum]

This was more of a practice exercise. I've been informed that it's not good for them to try and keep them continually top balanced

That is not what I said.
At least its not what I meant.
Leaving them hooked up to a charger for monthes at 3.65 is even worse than what I was talking about.

All top balancing means is filling each cell full to the brim.
Once you remove the charge voltage the cells will settle to a lower voltage but they will still be full.
LFP cells self discharge very slowly.
Just leaving them full at settled voltage for extended periods is an unnecessary stressor.

 

[S Davis]

What size cells?

 

[PeteW]

That is not what I said.
At least its not what I meant.
Leaving them hooked up to a charger for monthes at 3.65 is even worse than what I was talking about.

All top balancing means is filling each cell full to the brim.
Once you remove the charge voltage the cells will settle to a lower voltage but they will still be full.
LFP cells self discharge very slowly.
Just leaving them full at settled voltage for extended periods is an unnecessary stressor.

Okay, I understand. At this point I haven't done that. And will make sure not to. I've had the cells for a year and felt I should at least do something to boost them a bit. As I mentioned I'm rebuilding an Airstream and I'm the first to admit I might have a few to many irons in the fire.

 

[PeteW]

What is the first point that you don't understand.
I don't think I can simplify it anymore than I have but I can expand on things.
Conversion factor is inverter efficiency.
12 volts low cutoff is used because as the voltage goes down the amperage goes up.
The working voltage range for LFP cells is 3.0 to 3.4 volts so we use 12 volts as the low cutoff for a 12 volt system.
service amps is the maximum current that the system should see in normal operation.
We want to add some headroom over service amps so that we don't have nuisance trips of our over current protection so we divide by .8 to give some headroom.

The bottleneck of each battery is its BMS.
The 12 volt overkill BMS are rated for 120amps.
So 4 batteries in parralel can theoretically handle 480 service amps.
In reality they batteries and the wires that connect them to the system will not be perfectly matched so we don't want to push that to the limit but they should be able to 285 amps comfortably.

Thank you, your expansion was very helpful. Where might I learn more about how you arrived at the percentages used above? Not quite sure why the BMS will allow 480 service amps, but we only want to use 285 amps ( And I guess we're talking about 285A at 12V). Also, trying to understand the difference between Service Amps, Fault Amps. (see below, I'm quoting you again for better understanding) Sorry I know this must be awful for you.

3000VA is ~= 2400 watts

I'm going to put down my understanding and you can get me straight.

I'm guessing this is the efficiency factor due to the conversion of 12VDC to 120VAC. If the later is wrong I'm not sure why 3000W becomes 2400W.

2400 ac watts / .85 conversion factor / 12 volts low cutoff = 235.294117647 dc amps

Same as above kind of the loss of power is due to conversion of 12V to 120V and as the battery losses power the inefficiency increases.

235.294117647 dc amps + 50 amps for the dc distribution center = 285.294117647 service amps.

This is where I really start getting lost. I don't understand the addition of 50 amps, but I think I understand that service amps are usable energy.

285.294117647 service amps / .8 fuse headroom = 356.617647059 fault amps.

So fault amps refer to it being fused 20% higher to keep fuses / breakers from blowing / tripping at the slightest overage.

Yes.
The forum has a lot of experience with them.
I have the 8s version in my own system.

So a 24V system I gather.

Sorry I don't have a primer for that.

That's okay. I know this must feel like you're pounding nails into concrete. But hey, in my case, once they're in they usually stay put.

A BMS has 2 main functions.
1Protect the cells from operating outside their safe envelope of voltage and current.
2. maintain the top balance of the cells

Yes, that much I actually understand. Hey, that's something I guess.

I'm sure you will have more focused questions.
Will be glad to answer them.

I'm working on it. Be careful what you ask for. Seriously though, I'm thrilled you are willing to help me.

 

[PeteW]

What size cells?

280Ah Keheng cells. Thanks for pointing this out, I've added it to my opening statement.

 

[John Frum]

Thank you, your expansion was very helpful. Where might I learn more about how you arrived at the percentages used above? Not quite sure why the BMS will allow 480 service amps, but we only want to use 285 amps ( And I guess we're talking about 285A at 12V).

In theory 4 batteries each with 120 amp bms can pass 480 amps aggregate current.
Your inverter and your dc loads can pull 285 amps max.
The maximum design load = service amps.
Service amps + fuse headroom = fault amps.

Also, trying to understand the difference between Service Amps, Fault Amps. (see below, I'm quoting you again for better understanding) Sorry I know this must be awful for you.

see above ^

I'm guessing this is the efficiency factor due to the conversion of 12VDC to 120VAC.

Yes.

If the later is wrong I'm not sure why 3000W becomes 2400W.

Victron inverters are rated in volt amps.
3000 volt amps ~= 2400 watts.

This is where I really start getting lost. I don't understand the addition of 50 amps, but I think I understand that service amps are usable energy.

235 amps to cover the inverter.
Plus 50 amps is to cover the dc load center.

So fault amps refer to it being fused 20% higher to keep fuses / breakers from blowing / tripping at the slightest overage.

Correct.

So a 24V system I gather.

Yes.

 

[John Frum]

280Ah Keheng cells. Thanks for pointing this out, I've added it to my opening statement.

I've never heard of Keheng.
Is this some aliX or Ebay vendor?

 

[John Frum]

I explained above that top balancing just means filling each cell full.

q: Why do we top balance?
a: We top balance so that the capacity of the pack is determined by the lowest capacity cell.
Which is the best we can get.
If we don't top balance we will get less than the capacity of the weakest cell.

 

[S Davis]

280Ah Keheng cells. Thanks for pointing this out, I've added it to my opening statement.

So if they were shipped 30%+- charged you have another 72-76 hours of top balancing per four cells.

 

[John Frum]

So if they were shipped 30%+- charged you have another 72-76 hours of top balancing per four cells.

To flesh this out with math...

280 amp hours * .7 depth of discharge / 10 amps charge current = 19.6 hours per cell

Probably a bit longer because of the constant voltage phase.