Jump to content
Thank you for the great forum, Safe Driving over the weekend. Sincerely Jason
Adri

Axpert settings for LiFePO4

Recommended Posts

5 minutes ago, ojeysky said:

So I have set-up my BMV. It gives me 100% SOC on my battery. Can you confirm if you re-synchronised yours since you indicated you were using it for a different battery before.

Great to hear .

Yes it needs to go through a few  charge/discharge cycles with the correct peukert settings to work out where 100%

And when you initially connect power it will show 100% SOC

Share this post


Link to post
Share on other sites
Posted (edited)
On 2020/04/06 at 11:56 AM, 20VT said:

Great to hear .

Yes it needs to go through a few  charge/discharge cycles with the correct peukert settings to work out where 100%

And when you initially connect power it will show 100% SOC

After over a week of integrating BVN, here is my observation:

1. My battery BMS kicks low battery by the time am at about 130Ah depletion status out of 200Ah.

2. Voltage of about 3 strings among the 8 series were 0.04v lesser than rest of the string. I am thinking this is probably why am not getting full capacity and why I can't charge beyond 27.5v, it seem the cells are not balanced.

Can Daly BMS balance a cell voltage difference of 0.04v? How do I balance the cells without having to disconnect and parallel all the cells?

CC: @weber

 

Edited by ojeysky
Just to tag someone who could help with my question

Share this post


Link to post
Share on other sites
On 2020/04/27 at 12:48 AM, ojeysky said:

After over a week of integrating BVN, here is my observation:

1. My battery BMS kicks low battery by the time am at about 130Ah depletion status out of 200Ah.

2. Voltage of about 3 strings among the 8 series were 0.04v lesser than rest of the string. I am thinking this is probably why am not getting full capacity and why I can't charge beyond 27.5v, it seem the cells are not balanced.

Can Daly BMS balance a cell voltage difference of 0.04v? How do I balance the cells without having to disconnect and parallel all the cells?

CC: @weber

 

1.How did you previously verify the battery capacity ?

2.I have a Daly as well on some of my LFP cells and also noticed that the total voltage doesn't go past 27.5 - i suspect it happens either when on cell goes past the indivual overcharge voltage protection and it stops everything but I am quite happy with that as a maximum system voltage . Other thing I noticed is that the Daly doesn't really balance and if it does the current is so small (30mA) that it really doesn't make a difference

Share this post


Link to post
Share on other sites
Posted (edited)
9 hours ago, 20VT said:

1.How did you previously verify the battery capacity ?

2.I have a Daly as well on some of my LFP cells and also noticed that the total voltage doesn't go past 27.5 - i suspect it happens either when on cell goes past the indivual overcharge voltage protection and it stops everything but I am quite happy with that as a maximum system voltage . Other thing I noticed is that the Daly doesn't really balance and if it does the current is so small (30mA) that it really doesn't make a difference

1. Just the words of my battery supplier and other friends that got supplies from him, it is now that I have BMV in place that am also trying to verify the battery capacity independently.

2. Did you get your expected capacity before the BMS low voltage tripped? Perhaps I may just go the difficult route, by paralleling all the cells. Was just hoping for balancing option of the BMS without disconnecting the batteries.

Edited by ojeysky

Share this post


Link to post
Share on other sites
Posted (edited)
10 hours ago, 20VT said:

How do I balance the cells without having to disconnect and parallel all the cells?

One way is to use one or more car headlight bulbs with alligator-clip leads, and a multimeter. With the battery on charge, and close to full charge, clip the bulb(s) on to the cell(s) with the highest voltage(s), provided that voltage is greater than 3.4 V (for LFP cells), to burn off some charge and let the other cells catch up. You can leave the bulb on a cell until its voltage drops below that of the lowest voltage cell.

You can also work in the other direction. If you have an adjustable-voltage current-limited power supply (a lab power supply) you can set it to 3.6 V, and when the whole battery is being charged and some cells are over 3.4 V, connect it to the lowest voltage cell until it goes above the voltage of the highest voltage cell.

If there happens to be two to four high cells next to each other, you can clip one bulb across the lot to burn off charge faster.

You should put each bulb in a porcelain cup or mug to stop it blinding you and melting things. This is a @Coulomb innovation. 😀

Likewise if you have a number of low cells next to each other, you can adjust the lab power supply to (n × 3.4 V) + 0.2 V. But this is more dangerous. You must monitor the individual cells often to ensure none goes over 3.6 V.

3.4 V is an absolute minimum for balancing. The higher you go, the more accurate the balancing will be, but you shouldn't let any cell go over 3.6 V.

I note that connecting LFP cells in parallel will not balance them unless you charge them to more than 3.4 V while they are connected in parallel.

Edited by weber

Share this post


Link to post
Share on other sites
1 hour ago, weber said:

One way is to use one or more car headlight bulbs with alligator-clip leads, and a multimeter. With the battery on charge, and close to full charge, clip the bulb(s) on to the cell(s) with the highest voltage(s), provided that voltage is greater than 3.4 V (for LFP cells), to burn off some charge and let the other cells catch up. You can leave the bulb on a cell until its voltage drops below that of the lowest voltage cell. You can also work in the other direction. If you have an adjustable-voltage current-limited power supply (a lab power supply) you can set it to 3.6 V, and when the whole battery is being charged and some cells are over 3.4 V, connect it to the lowest voltage cell until it goes above the voltage of the highest voltage cell.

If there happens to be two to four high cells next to each other, you can clip one bulb across the lot to burn off power faster.

You should put each bulb in a porcelain cup or mug to stop it blinding you and melting things. This is a @Coulomb innovation. 😀

Likewise if you have a number of low cells next to each other, you can adjust the power supply to (n × 3.4 V) + 0.2 V. But this is more dangerous. You must monitor the individual cells often to ensure none goes over 3.6 V.

3.4 V is an absolute minimum for balancing. The higher you go, the more accurate the balancing will be, but you shouldn't let any cell go over 3.6 V.

I note that connecting LFP cells in parallel will not balance them unless you charge them to more than 3.4 V while they are connected in parallel.

Thanks for this @weber, just one or two clarifications, isn't headlamp designed for 12v and will putting it on a 3.4v battery work? If it will, I have a 12v DC fan, can I use that instead as that is readily within reach as it has alligator clicp.

You also said I should do this while charging is still on, how about once the battery is supposedly full (the first cell to hit 3.6v) and I then disconnect charging and then discharge the high cells to bring each cells to the same level, won't that work? 

Share this post


Link to post
Share on other sites
Posted (edited)

This is not mere theory. Coulomb and I do this with every new battery (set of cells). The typical 55 W headlight bulb draws about 4 A at 13.8 V, so you might expect it would only draw a quarter of the current at a quarter of the voltage, so about 1 A at 3.4 V. But in fact it will draw about 2 A at 3.4 A. This is because the filament has a much lower resistance when it is only glowing a dull red compared to when it is white hot.

A fan will behave in the opposite way. i.e. it will draw much less than a quarter of the current at a quarter of the voltage. I doubt that the fan will draw enough current from a single cell to be useful in balancing the cells in a reasonable time. But it can't hurt to try it.

If you only do this manual balancing after you stop charging, then the cell you connect the load to, and in fact all the cells, will drop below 3.4 V almost immediately, so you will only have a very short time of balancing. You would have to repeat this many many times.

Edited by weber

Share this post


Link to post
Share on other sites
13 hours ago, ojeysky said:

1. Just the words of my battery supplier and other friends that got supplies from him, it is now that I have BMV in place that am also trying to verify the battery capacity independently.

2. Did you get your expected capacity before the BMS low voltage tripped? Perhaps I may just go the difficult route, by paralleling all the cells. Was just hoping for balancing option of the BMS without disconnecting the batteries.

Ok fortunately my cells were quite close to the advertised capacity at 23.2V  so i didn't have any issues and am happy with them. Unfortunately also have the balance issue but haven't gotten down to doing them properly as I have recently noticed that the Daly doesn't really do it. 

Share this post


Link to post
Share on other sites

I once converted a mains-type lead light (the workshop type, for working under cars) with a standard headlight bulb where the low-beam filament had burned out, with croc clips.

Turned out to be the best 'rough' load for many 12V (and less) applications, like testing batteries and chargers. Particularly because of the steel wire lamp protection cage-design.

Share this post


Link to post
Share on other sites
Posted (edited)
On 2020/04/29 at 2:26 AM, weber said:

This is not mere theory. Coulomb and I do this with every new battery (set of cells). The typical 55 W headlight bulb draws about 4 A at 13.8 V, so you might expect it would only draw a quarter of the current at a quarter of the voltage, so about 1 A at 3.4 V. But in fact it will draw about 2 A at 3.4 A. This is because the filament has a much lower resistance when it is only glowing a dull red compared to when it is white hot.

 

@weberSo I got 2 bulbs connected to the 2 cells that moves faster than the rest, but the challenge is that by the time the 2 cells are at above 3.45, the rest will still be at 3.3+ and after a few minutes one of the 2 cells would hit over 3.6 and the BMS stops charging. I recall you mentioned that I can only balance at above 3.4 but in this case the other cells are not getting to that required voltage.

When charging is disconnected, the voltage comes back down below 3.4 with the 2 cells having higher voltage than the rest (3.35+).

What do you advice I do as a work around about this?

Edited by ojeysky

Share this post


Link to post
Share on other sites
Posted (edited)

Only one cell needs to be above 3.4 V for it to be worthwhile doing balancing. It doesn't matter if the others are greater or less than 3.4 V. And you should stop (or the BMS stops you anyway) when any cell goes above 3.6 V.

So you need to extend the time between the first cell going over 3.4 V and when it goes over 3.6 V. To do that, you need to reduce the charging current. Ideally, you would reduce it to the same as (or a little less than) the current that you are able to bypass around a cell by connecting a headlight bulb across it, which is typically 2 A for one filament.

With the 5 kVA Axpert inverters, if you are able to charge from the utility, you can use the lowest utility charge current setting [11] of 2 A. This is ideal. If you are not able to charge from utility, you will need to set the absorb voltage [26] down to whatever will reduce the current to about 2 A by the time the first cell exceeds 3.4 V. This could be as low as n × 3.36 V where n is the number of cells. As you progress with balancing, you will need to keep raising setting [26] to keep the charge current close to 2 A.

And if a cell does go over 3.6 V and so you have to stop charging, (or it stops automatically), then you leave the headlight bulb on that cell until its voltage goes below that of the lowest cell, then you start the charge cycle over again, and keep repeating this until you get all cells between 3.45 V and 3.6 V at the same time, and within 0.05 V of each other.

Hopefully, after you've done that once, you'll be able to set your charge voltage and current settings back to normal, and the BMS will be able to keep them balanced from then on.

I note for other readers, that these voltages only apply to LFP cells. Also, Ojeysky, I wonder if you have the correct Daly BMS — i.e. whether you have a model designed specifically for LFP cells and not other lithium-ion cells. Other lithium ion cells have a more linear voltage vs SoC curve, where LFP has some very flat plateaux. With a linear curve, the BMS can balance at any SoC, and can therefore get by with a few tens of milliamps of bypass current. This will not work with LFPs. With LFPs, such a scheme will actually unbalance the cells due to inaccuracies in voltage measurement. With the LFP cells you can only balance when they are near 100% charge (or less conveniently, when they are below 30% charge) and because the time available is so much shorter, the bypass current must be so much greater, typically half an amp or more.

Edited by weber

Share this post


Link to post
Share on other sites

Thanks @weber will give this a trial, my axpert clone does not have option to set current to 2A, the least possible is 20A so I will go the voltage route by reducing the bulk voltage, I guess I will need to reduce the float as well right? Cos my current settings is 27.6/27.2 bulk/float.

Yes the battery uses a Daly BMS for LFP and it's rated 60A charge current

20200412_195618.jpg

Share this post


Link to post
Share on other sites

Yes. If you need to set the absorb voltage lower than your usual float voltage then you should set absorb and float the same. You typically won't be allowed to set absorb voltage lower than float voltage, so you'll probably have to set the float voltage down first.

Share this post


Link to post
Share on other sites

Hello @weber I noticed that the cell that has the main positive to the inverter has a higher voltage than the rest of the cells, at times it can go as high as 3.7v is this a normal behaviour?

Share this post


Link to post
Share on other sites

No. You should not let it go to 3.7 V. Use a headlight bulb to bring it down to where the others are.

Share this post


Link to post
Share on other sites
2 hours ago, ojeysky said:

do you happen to have the firmware to resolve the early float bug issue?

There is no patched firmware for any 3 kVA models.

Share this post


Link to post
Share on other sites
2 hours ago, Coulomb said:

There is no patched firmware for any 3 kVA models.

Looks like we the 3kv users are stuck then. Guess I just keep on with my 27.2 float settings

Share this post


Link to post
Share on other sites
Posted (edited)
On 2020/03/10 at 12:53 AM, 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

@weber pardon my DIY skills, could you kindly show me with a picture sample of how to run the command via a terminal? and will be good to know how to do the automation on node-red? I already have node-red running on my pi. This seem to be my only option to work around the early float bug on my GK

Edited by ojeysky

Share this post


Link to post
Share on other sites
Posted (edited)
11 hours ago, ojeysky said:

the command below it changed on the inverter but it didn't reset the system back to bulk, is there something else I should be looking at?

Sadly, it's not possible to directly force the charge state (float, bulk, etc).

But you can get it to bulk stage indirectly. As Weber mentioned, changing the float voltage slightly and putting it back if desired should cause the solar charge controller to reset. I'm not clear on whether it will always start in bulk mode soon after; my experience is that it sometimes doesn't. When in absorb stage, the charger will go to zero charge when the battery voltage falls consistently below a point that is 2.0 V (for 24 V models) less than the float voltage for 10 seconds with no exceptions (500 measurements in a row @ 50 Hz). So to be sure, set the float voltage quite high (you might have to set the absorb/bulk/CV setting high first, since often the float voltage setting can't be set higher than the absorb voltage setting). Once at zero charge for about 15 seconds, it should start a ~1 A charge, then ramp up to a normal bulk charge, i.e. it is in bulk stage. At this point, you could put the float and if necessary the absorb voltages back to what you want them to be long term. Probably waiting 60 seconds would suffice.

The above are based on analysis of 48 V firmware, but I believe that the 24 V firmware is similar (but with voltages halved, obviously, including the 2.0 ΔV).

Edited by Coulomb
Reworded taking into account that small changes will reset the SCC, but may not restart in bulk.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


Topics

Contact Us - Power Forum South Africa

×
×
  • Create New...