Axpert settings for LiFePO4

[Coulomb]

7 hours ago, ojeysky said:

my PIP-3024GK which suddenly threw up error 32. I know the float bug affects that as well, do you by chance have the patch for those? 

No. Rarely a 3 kVA model, and so far never a 450 V MPPT model.

Quote

error 32

I can't even find that in VM III firmware or manuals. Did you find out what it was about?

[weber]

11 hours ago, ojeysky said:

It was quite sunny today and I didn't hit float all through the day. Do you think I should still change the charge current to 30A or I should just maintain the 40A currently set?

It's only on cloudy days that it might matter. You can wait and see if you get premature float on cloudy days, and if so, drop setting 02 to 30 A.

[ojeysky]

8 hours ago, Coulomb said:

I can't even find that in VM III firmware or manuals. Did you find out what it was about?

It's documented on the GK manual. The error is due to lack of communication between remote display and inverter. I have no idea what caused it. I reached out to MPP staff since it's still within warranty. They will be sending me replacement boards for the remote/com display (at my own shipping cost). Hopefully that resolves the problem.

[Coulomb]

23 minutes ago, ojeysky said:

It's documented on the GK manual.

Oh! Warning 32. You said error 32. Warnings are very different from errors / fault codes.

Edited February 8, 2020 by Coulomb
Added " / fault codes"

[ojeysky]

3 hours ago, Coulomb said:

Oh! Warning 32. You said error 32. Warnings are very different from errors / fault codes.

Oh my bad....sorry about the confusion created

[ojeysky]

Hello, so far so good with LFP. I like to get clarification about  a behaviour I have observed. Here is the link to my battery voltage log. Between 2pm and 4pm I noticed that the battery voltage would spike up to 28.3v and then come down almost within few minutes to around 27v... This happened for quite a while. Is this a normal behaviour? Perhaps the BMS was trying to balance up the battery or is this a symptom that I should be worried about?

Edited February 8, 2020 by ojeysky

[weber]

When I click your link I get "Dataplicity is unable to connect to your device. It may be offline." Can we see battery current as well as battery voltage?

It doesn't sound harmful, since it is merely cycling between absorb and float voltages, and not spending much time at the higher voltage. But I do not understand why it is happening. You could try dropping the absorb voltage setting [26] down to 28.2 V to see if it eliminates this behaviour.

Normally, after achieving float, an unpatched 24 V Voltronic inverter would not return to bulk/absorb until the battery voltage had fallen 2 volts below the float setting [27], i.e. 24.9 V in your case.

But that does remind me to tell you that, if you find that your battery voltage does not fall below 24.9 V over night, or when you boil the kettle first thing in the morning, and therefore it does not go back to absorb voltage on the following day, but stays at float voltage, then you may need to raise your float voltage setting a little, i.e. to 2 V more than whatever voltage it does get down to. But preferably no higher than 27.6 V.

It is quite the balancing act, to get an inverter/charger designed for lead-acid to work sensibly with LFP, particularly when it has the Voltronic Premature Float Bug. Now I'm worried that this one has some other bug as well. The behaviour may be caused by the BMS, but I can't figure out how. I'll be able to comment further when I've seen the battery voltage and current curves.

Edited February 9, 2020 by weber

[ojeysky]

2 hours ago, weber said:

When I click your link I get "Dataplicity is unable to connect to your device. It may be offline." Can we see battery current as well as battery voltage?

It should be up now, and yes I see battery voltage but the inverter does not report battery current

 

3 hours ago, weber said:

 

2 hours ago, weber said:

and not spending much time at the higher voltage.

Cool that's good to know

2 hours ago, weber said:

would not return to bulk/absorb until the battery voltage had fallen 2 volts below the float setting [27], i.e. 24.9 V in your case.

Perhaps mine is patched "to some extent" because I hit float as fast as few minutes after sunset and my voltage is still at 26.3v by then. You can look at my dashboard here

3 hours ago, weber said:

I'll be able to comment further when I've seen the battery voltage and current curves

Earlier shared the battery voltage log. I do not have that of current but I do log charge current which can be found in the URL I earlier shared if that helps

[weber]

The best hypothesis I can come up with is that the battery was sitting on the shelf for a while before it got to you. And in that time the cells got out of balance by maybe 2%, so that when the battery voltage gets to 27.6 V you have 7 cells averaging 3.41 V each and one cell at 3.73 V. The BMS would have switched on the bypass resistor for that cell when it passed about 3.5 V, but it can only bypass maybe 1 A, and you're charging at maybe 30 A, which is why it got up to 3.73 V. And then to save that one cell, the BMS open-circuits the whole battery using a solid-state relay so the charge current goes to zero. That's what I think is happening when the voltage suddenly jumps up to the absorb voltage setting of 28.4 V.

After a few minutes of disconnection, the bypass resistor pulls the voltage of that cell below maybe 3.6 V and the BMS reconnects the battery, and the cycle repeats. As it's doing this, the high cell is coming more into balance. But if it was 2% ahead of the others, that's 4 Ah for a 200 Ah battery. And if the bypass is only 1 A, then it will take 4 hours to balance that out. So it may continue these crazy oscillations for several (sunny) days, but if I'm right, they will eventually reduce and finally cease.

If you want to speed the process up a little, and be a little kinder to the battery, you could wait until the problem starts occurring each day, then set the max charge current [02] down to its minimum of 10 A, and then set it back to 30 A or 40 A each evening. But you don't need to do that. You can just let it go as it is now.

Please let us know how it is going in say a week's time.

Edited February 9, 2020 by weber

[Adri]

After some more fiddles, settings are now :

2    =    20A

11   =    20A

12   =    25.5

13   =    25.5

26   =    29.2

27    =    29.0

29    =    24.0

 

( setup :  4  SolarWorld SP150, AXPERT 1K-24,  2  AKKUTECH 25V 100Ahr)

I see battery voltage rises to 29.0V before charge current tapers off [to float value?] but seldom exceeds 29.1V (measured on Mooshi).  Now on SOL (setting 01), reads 140W (29.1V,  0A) with fridge load, running, TV, decoder.

Am I being risky? Label reads "CC at 0.2C to 28.4V".   Are they not deliberately down-specifying as 'excessive protection limit'?  I would wonder at what voltage the capacity is measured (100Ah)?

 

Edited February 10, 2020 by Adri

[weber]

@Adri, I already answered most of your questions, starting here:

https://powerforum.co.za/topic/4614-axpert-settings-for-lifepo4/?do=findComment&comment=68570

The capacity of LFP is usually measured between 2.5 V and 3.6 V per cell (28.8 V). By stopping at 3.4 V per cell (27.2 V) you only leave about 2% of the capacity unused, but you extend the life of your cells a little.

For voltage versus SoC, see Figure 2 of: http://www-personal.umich.edu/~hpeng/DSCC2013_Weng.pdf

For aging versus temperature and voltage/SoC see Figure 2 of: http://jes.ecsdl.org/content/163/9/A1872

Click the link to the Full Text (Free) on the right.

Edited February 11, 2020 by weber

[Adri]

On 2020/02/07 at 4:38 AM, weber said:

There's another issue. Current sharing. When you have two batteries in parallel you have to ensure you have the same resistance in the link between their positive terminals as between their negative terminals, and the takeoff cables must come from the positive terminal of one battery and the negative terminal of the other (diagonal takeoff), with the same lug-stacking order (e.g. takeoff lugs on top). And the batteries should be stacked beside each other and have the same thermal environment, as internal resistance goes down as temperature goes up, and so a small current imbalance can become a larger one.

 

I have 2 x AKKUTECH 25.5V 100Ah in parallel using the Anderson  "parallel" plug (with 8mm2 cables throughout). Do you think it's necessary to re-wire so that it's according to your idea or can I continue in this way?  I would like to balance the cells for longevity. What's your feeling here?

[weber]

Can you post a photo?

[weber]

@Adri What you have may be fine. Diagonal takeoff isn't the only way to balance currents between two batteries in parallel.

It will be fine if you are using one of these:

https://www.redarc.com.au/anderson-parallel-cable-2

provided the cables going from it, to each battery, are the same length.

 

[Adri]

Yes, Weber, using those same Anderson plugs with 12mm2 cables.  One pair direct connection from Batt1 to Axpert terminals = 1.7m.  (Added  1.5mm2 wires at each end of the black cables for current reading and a red wire for V for my Mooshi  - I can monitor the battery behaviour easily at the kitchen table.)  Then same kind cable/Andersons from Batt1 to Batt2 (0.9m).

Read your good stuff about the  proper way to interconnect, but figured with such heavy cables it may not be necessary.  Please correct me if I'm wrong/comment.

 

 

 

[weber]

@Adri, Sorry to take so long to respond. Using heavy cables isn't sufficient, because much of the resistance is in the crimps, the plug contacts and the bolted connections. If you have the same numbers of those on the path through each battery, then you may be OK.

http://www.smartgauge.co.uk/batt_con.html

As Chris Gibson shows in the above, there are only two ways to do it properly, his method 3 and method 4. He shows 4 batteries, not 2 as you have, but his method 4 is just "diagonal takeoff" applied hierarchically. Diagonal takeoff only works for 2, 4, 8, ... batteries (powers of 2) and, strangely, 5 batteries. But method 3 can work for any number. Your present arrangement may be the equivalent of method 3, or might easily be made so. I still don't find your description entirely unambiguous.

[Adri]

A very good reply weber and much thanks for it.

You're quite right that the right option is to properly balance the batteries with suitable connection philosophy. 

In my case charge current is mostly less than 10A from either source, and load current is max. 16A ever, so I wonder if my imbalance is SO important in my installation.  (Perhaps I'm lazy in my old age..?)  

I wonder if I alternate the main supply/take-off line from Axpert from first of the two batteries (call it "prime" battery) to 2nd battery ("slave") as internal connection of the 2 x Anderson plugs are parallel-connected inside the battery housing, say, once a week, we may achieve the right balance after some cycles?

(Thanks, again.)

 

 

[weber]

Alternating the connections would be better than nothing. But you will need to swap it once a week (or once a day) for the entire life of the batteries. Otherwise one will age and die long before the other.

[ojeysky]

On 2020/02/07 at 1:47 PM, weber said:

Please let us know how this goes. It is quite possible that I'm completely wrong. The explanation for the strange behaviour may lie elsewhere.

So far i still get bounces between 28.4 and 27.6v though not as much as before, i guess that was because i usually reduce the charging rate once the initial charge drops to zero. I also tried to deplete the battery and noticed the BMS kicked at 25v so i am guessing that would explain the possible cell imbalance that you were referring to. I am going to be having it checked by the seller. Meanwhile i am intending to increase my capacity and found a good deal of unit cells, but they are 150Ah, will there be any major issue if i parallel a 150Ah lithium with 200ah , and both of them are 24v? 

[weber]

On 2020/02/24 at 7:04 PM, ojeysky said:

Meanwhile i am intending to increase my capacity and found a good deal of unit cells, but they are 150Ah, will there be any major issue if i parallel a 150Ah lithium with 200ah , and both of them are 24v? 

Successfully paralleling strings of identical new cells is difficult enough. It requires extreme attention to detail. Successfully paralleling different makes, models or ages of cell is almost impossible. One string will end up carrying all the current during the middle of charge or discharge, and the others will carry all the current at the beginning and end. So they will all be abused, So it's pointless, unless the second lot of cells are free, or nearly so.

More information here:
https://powerforum.co.za/topic/2736-the-multiple-string-battery-riddle/?tab=comments#comment-43501

 

[ojeysky]

On 2020/02/27 at 7:29 AM, weber said:

Successfully paralleling strings of identical new cells is difficult enough. It requires extreme attention to detail. Successfully paralleling different makes, models or ages of cell is almost impossible. One string will end up carrying all the current during the middle of charge or discharge, and the others will carry all the current at the beginning and end. So they will all be abused, So it's pointless, unless the second lot of cells are free, or nearly so.

More information here:
https://powerforum.co.za/topic/2736-the-multiple-string-battery-riddle/?tab=comments#comment-43501

 

Thanks @weber I had thought the negative effect of paralleling batteries is much lesser with lithium compared to lead acids. From your comment it seem even if I get a new 200Ah lithium to add to my existing 200Ah lithium, I will still have a challenge. 

About a real life scenario, LFPs are quite pricy and for a battery that will last 5 to 10 years, it's natural the power needs may increase within the same period hence the need to increase battery bank. If one will need to decommission existing bank and get all new cells of total capacity needed then that will significantly increase the overall cost of LFPs.

[weber]

Yes. It's important, when designing a battery system, to try to predict the maximum storage requirement over the life of the batteries, and include enough capacity right from the start, preferably without paralleling.

When this hasn't been done, some workarounds are:

(a) Connect the new batteries to a separate inverter/charger and separate solar panels, and parallel the AC outputs of the two inverters. Unfortunately you can't do this with two Axpert inverters. To be able to parallel their AC outputs, they must be connected to the same battery.

(b) Connect the new batteries to a separate inverter/charger and separate solar panels, and use them to power separate loads, e.g. separate light and power circuits.

(c) Parallel the batteries but measure their current separately and deliberately add resistance, in the form of extra cable length, to approximately balance the currents in proportion to the capacities.

[ojeysky]

@weber so am back on the float bug workaround....tweaking the max charge settings either gets me to early float or continuous bulk (thereby ending up with not eventually having a well charged battery by sunset). So i remove max charge limit and today when i got an early float, i restarted the inverter and the system boot back into bulk mode which was great. The question now is whether there is a way to reset the inverter charging algorithm without having to restart the inverter?

[weber]

If this inverter accepts the same serial commands as its big brother, then I think it will reset its charging algorithm if you send it a command to change the float voltage or absorb voltage. You only need to change it by 0.1 V and then you could change it back. You could automate this, using Node-RED on a Raspberry Pi. The commands are PBFT and PCVV.

http://forums.aeva.asn.au/uploads/293/HS_MS_MSX_RS232_Protocol_20140822_after_current_upgrade.pdf

[ojeysky]

10 hours ago, weber said:

If this inverter accepts the same serial commands as its big brother, then I think it will reset its charging algorithm if you send it a command to change the float voltage or absorb voltage. You only need to change it by 0.1 V and then you could change it back. You could automate this, using Node-RED on a Raspberry Pi. The commands are PBFT and PCVV.

http://forums.aeva.asn.au/uploads/293/HS_MS_MSX_RS232_Protocol_20140822_after_current_upgrade.pdf

While I still figure out how to setup Node-RED on my RaspberryPi, I do have a tool that I use to change float/bulk settings (it's called solpiplog). However I have reduced and increased the voltage before without the reset, until I restarted the inverter.