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AXPERT MKS PLUS DUO 5KW battery charging issue


Syed

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Hi Syed

There is an Aussie chap who frequents this forum who can help you making sure that your Invertors have the right software - keep your eye out for Columb...
That is just the start...
For now, don't start blaming the Axperts - As cheap as they are, they do a reasonably decent job unless completely broken... So, start by doing a proper investigation of your batteries. Axperts are not an online system, so there are many factors that can reduce their efficiency/productivity compared to what you expect. And unfortunately batterries are the most likely cause. The number of times an Axpert switches on/off load is far higher that most hybrid systems, and this can kill batterries quickly if not set up correctly (read my battery blogs to show the smoke!).

My personal check procedure to ascertain the fault would be as follows: (But I am not an "Axpert" expert)

1.  Take your Solar Arrays completely out the loop, and charge your batteries fully from Mains before any testing
2. Take a basic voltmeter and make sure that all the batteries are still at similar voltages (if one batt reads 11 and another reads 13 you have battery issues before you start)
3. Feel the batteries, if one or two/six feel MUCH warmer than the rest then you have again a battery issue with dead or non-performing cells
4. If you think there are no issues with the batteries, then continue to next...
5. At this point you should either have a DC electrician or at least understand DC/AC power properly, if not consult...
6. Test your load power draw on AC and compare it to your power draw on your Axperts without Solar attached. A large power draw close to limits of the Axperts is a bad idea, they don't run well close to maximums. Although they are great at handling large intermittant loads (even up to double capacity), they shouldn't run at close to maximum continuously... They run great at about 40% for each (pushing further is a challenge)
7. Double check that your Axpert parallel setup is correct - I have heard/read on forums that a simple wire can cause one to be providing a far larger load that ness and not spreading the battery and solar input.
8. From your diagram, your seem to have a mismatch of charge, keep in mind that an Axpert can only charge the batteries connected to itself (irrespective of it's parellel setup), so you have a discrepency in the charging. This is unless you have something in place that is balancing the batteries that isn't in you diagram.

For now, thats the best I can offer... Happy to try assist further if still battling to isolate.

Regards
KLEVA

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Hi KLEVA,

This forum is terrific. Thanks for your time and effort clarifying lot of things and still looking further...

Actually I have tried charging the batteries just putting an external charge controller Outback FM80 in between the battery and SCC and instantly he charging current is as expected and charges the battery bank whole Sun Day

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Hi @Syed

The Axpert uses battery voltage to monitor state of charge which is notoriously inaccurate - today my Axpert system went from bulk charge (57.4) to float charge (54.4) when the Axpert says the batteries are at 100% but my BMV702 is reporting SOC as 80% - the amps going into the batteries now is so low that it will never get the BMV up to 100% even though I have ample spare PV capacity available.

My solution that I intend to implement right now is to set my float voltage to 56.7v with my bulk charge voltage 57.6v which should then easily get my batteries up to 100% SOC

This may sound dodgy but in my case I don't believe so. My battery data sheet (Ritar OPZ-490) gives the following spec for charge voltages;

  • Float use            2.25v - 2.3v           x24 batteries          = 54v - 55.2v
  • Cycle use           2.37v - 2.4v           x24 batt's                = 56.7 - 57.6

As I am using the batteries in "cycle use" it seems that there is no problem to jack the Axpert float voltage up to 57.4v :unsure:, if the batteries are used in a UPS environment where they will spend most of their life on float then 54v would be the way to go.

Have a look at your battery data - you may be able to implement a similar solution.

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30 minutes ago, pilotfish said:

Hi @Syed

The Axpert uses battery voltage to monitor state of charge which is notoriously inaccurate - today my Axpert system went from bulk charge (57.4) to float charge (54.4) when the Axpert says the batteries are at 100% but my BMV702 is reporting SOC as 80% - the amps going into the batteries now is so low that it will never get the BMV up to 100% even though I have ample spare PV capacity available.

Hi Pilotfish

are you using the vanilla firmware? There is a bug in it which can cause the unit to go to float prematurely. @Coulomb's firmware solves this problem.

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Hi Chris,

Yes I am using stock firmware, I haven't got around to doing the update yet.

I will do the update this weekend and then set my float back to 54.4 to see how it goes - if the SOC improves at day end I will leave it there, but from the battery data sheet I don't see using 56.7v as a problem :unsure:

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VRLA batteries can handle some gassing the valves opening between 70 and 130 millibar depending on manufacturer  Most of the gas recombines so losses can be anything between 1 and 5 %. Remember that this is gas loss not electrolyte loss otherwise VRLAs would be condemned to early failure. Those deep discharges should be avoided. I had 2 in 2 years both of them accidental. Such is life in a off-grid situation.

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I think I should have taken all the money invested in the PV setup and gone to the casino!

So I rechecked the data sheet and then looked at some other manufacturers of OPzV battery data, it seems that for solar applications they all recommend similar charge voltages of 2.37v - 2.4v per cell at 25'C (there is no mention of a float charge voltage).

Based on the above I have decided to go with 57.6v (2.4v per cell) bulk charge rate and then 56.9v (2.37v per cell) as my float charge setting. The reason for the drop in float voltage is that I have noticed my system going to float around midday and I reckon battery temps will be higher in the afternoon so charge voltage can be lower (-4mV/'C/cell = -0.1v/'C).

With this system it wont be necessary to install the Coulomb update as an early switch to float wont be much of an issue. In fact the current average switching time is quite convenient, as luck would have it.

This all started with a question from @Syed and I kinda hijacked the thread - but from his pic it looks to me that he is using OPzV batteries, if so I hope some of the above is useful.

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@Syed

Your setting 12 (back to grid voltage) seems high to me at 50v unless you are aiming at at very minimal discharge, if your batteries are OPz as I suspect then you can safely go to 48v here which is around 50% SOC.

My batteries are currently discharging with the bank 90% SOC and voltage 50.5v - If I turn on a kettle the bank voltage will dip below 50v and then recover - your system will probably switch to grid mode in this event even though your bank is at 90%.

So with this change and the stuff mentioned in previous posts these are my suggested changes to your current setup;

  • #12 = 48v
  • #26 = 57.6v
  • #27 = 56.9v

The above changes will extend your discharge time and improve your charging rate. Hope it helps.

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My batteries are OPzS 2V blocks. Actually I dont understand why AXPERT Cant push enough current where as Outback FM 80 on the same system psh charge current at expected rate.

Is it, there are 2 Panels in series and the voltage is not enough? or its the AXPERT CC itself that ca not push current when the batteries are little charge 60%-70% and develops internal resistance  as usual?

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5 hours ago, Syed said:

Actually I dont understand why AXPERT Cant push enough current where as Outback FM 80 on the same system psh charge current at expected rate.

The Axpert is perfectly capable of pushing the current into the battery when there is enough solar power available (e.g. when the Outback charge controller would do so), it's just a logic error that makes it think that charging is over and it's time to float the batteries, not charge them.

Is it, there are 2 Panels in series and the voltage is not enough? or its the AXPERT CC itself that ca not push current when the batteries are little charge 60%-70% and develops internal resistance  as usual?

I run 2S of 72-cell panels with no issues that I'm aware of. Sometimes the MPPT voltage will be down to around 63 V, but it still seems to charge just fine. In summer I get nominal or even over nominal power for short periods of time. My lithium iron phosphate (LiFePO₄) battery charges at a few volts lower than lead acid (my CV voltage is set at 55.2 V), but I don't think that would affect solar charging significantly.

The charge bug issue is described in the "discussion post" about the latest patched firmware. The essence of the problem is that the Axpert judges the charge to be finished when the charge current drops below a certain value. That could be because the battery is full, or because there is a cloud. It can even happen with utility charging, with perfectly steady charge current, if you charge at a rate less than the threshold. For most cases, that threshold is a fifth of the maximum charge current setting (parameter 2) (the threshold is different for the LiFePO₄ patched firmware and/or with paralleled inverters). Prove it for yourself with a test utility charge: set parameter 2 (maximum charge current) to 120 A and parameter 11 (maximum utility charge current) to 20 A. The threshold will be 60 A, and the charge current will be 20 A, so after 10 minutes (after only 30 seconds with pre-72.70 official firmware) the charge will change from bulk charging at 20 A to float charging at a handful of amps (depending on the exact state of your battery). Details here.

You might not notice the charge bug, especially with official firmware version 72.70, because when you watch it, you might not get 10 minutes of uninterrupted cloud. It's harder to miss with the earlier versions, because they only needed 30 seconds of cloud to terminate the charge, but you could still miss it, especially if your weather has been like it has been in Brisbane Australia lately (weeks of nothing but blue sky; my rainwater tanks are empty). If you remove *all* solar input, the SCC will turn off, but you can probably simulate 11 minutes of cloud by turning off the isolators / breakers for most (not all) of your solar strings. Watch the battery voltage on your monitoring software or on the LCD display, and watch for the change from bulk/absorb to float charge, on your monitoring software or watching the charge LED on the front panel go from solid (bulk/absorb) to flashing (float).

It's really scandalous that this charge bug hasn't been fixed after all these years. Charging batteries is a major job for an inverter/charger, and a battery worth more (sometimes many times more) than the cost of the inverter may be ruined as a result of the bug, which is probably a single line of code to fix.

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10 hours ago, Coulomb said:

The essence of the problem is that the Axpert judges the charge to be finished when the charge current drops below a certain value. That could be because the battery is full, or because there is a cloud. It can even happen with utility charging, with perfectly steady charge current, if you charge at a rate less than the threshold. For most cases, that threshold is a fifth of the maximum charge current setting

It seems all manufacturers use a variant of current and some kind of time period to decide to go to float. For one, a fifth seems to me entirely too much. If my 200ah batteries are set to charge at 30 amps max (around 15% of capacity), then it will switch to float at 6 amps. That is way too high. My AGMs really should be held at absorb until it hits 2 ampere  at least. Otherwise you're going to spend absolutely ages in float. That's probably fine for a UPS, but not for daily cycling.

The Victron charge algorithm (it's called adaptive charging) sets absorb to 20 times bulk (I think). I used to think that is too much, it means that on most days we never go to float, but I learned that it's a very safe method for batteries that are cycled daily (and you can turn it off). Judging from the way my midpoint deviation fixed itself, this is a good thing.

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Hi Coulomb, 

Thanks for your detailed suggestion. I will try this with my installation. Now can you tell me the below phennmenon are for the same bug?

1. When the CC is not charging full and assuming to be in Float, when a load is turned on it starts feeding current from the panel.

2. Among the two MPPT one gradually dies and then the other, not simultaneously. Everyday I down load the log and find, on MPPT is 0A at around 11:AM in the morning while the other MPPT continues with somewhere around 10-12A till 12-1PM.

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On 17/09/2017 at 6:51 AM, Syed said:

Now can you tell me the below phennmenon are for the same bug?

1. When the CC is not charging full and assuming to be in Float, when a load is turned on it starts feeding current from the panel.

That's to be expected. When the battery voltage drops below the float voltage setting due to a load or any other reason, the charge controller will increase its output enough to maintain the float voltage, or if it can't maintain that voltage, it will supply all it can. 

Quote

Among the two MPPT one gradually dies and then the other, not simultaneously. Everyday I down load the log and find, on MPPT is 0A at around 11:AM in the morning while the other MPPT continues with somewhere around 10-12A till 12-1PM.

 

That is weird. I don't have a dual MPPT firmware to read, so I can't be sure. I would have expected that the DSP in the inverter, which calls the shots for a single MPPT machine, would also call the shots for a dual or triple MPPT machine. I'm pretty sure it sets a current set point, not a voltage set point (in the case of one MPPT). So I'm surprised by that behaviour, but it probably means that the multiple MPPT chargers work differently.

Sorry, it didn't dawn on me till now that you have a dual MPPT pair of machines, so you can't use the [ edit: existing 72.XXy] patched firmware to fix the charging bug, if it exists in your machines. [ Edit : sadly it does.] 

A colleague had a single MPPT machine that came with main firmware version 73.00, and it still exhibited the same charging bug. We were unable to extract the 73.00 firmware to examine or patch. However, we were able to downgrade it to 72.70b patched firmware without apparent issue. That suggests to me that 73.00 will still have the same bug when running on a multiple MPPT machine, [ Edit: I've since learned that 73.00 is a single-MPPT only firmware] but maybe the bug only affects one of the two or three MPPTs. Maybe one or two charge controllers are effectively independent, just packaged in the same box for convenience. That does sound crazy, I'll admit. 

So sorry, I don't have a solution for you; if the bug exists, and it seems likely that it does ,  you'll have to wait for Voltronic to fix it, and ask them or your supplier for an update. [ Edit: we have a patch for dual/tri MPPT models now; see Weber's message several posts below.] 

Edited by Coulomb
Various changes, as noted.
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On 15/09/2017 at 7:12 AM, Syed said:

In the morning the MPPT charge Contrller does charge 1-2 hours and then it goes to float and not charging. The battery SOC shows 80%+ where as the Sp. gravity is not relevnat to 80% SOC.

When you say the SOC shows 80%, is that according to the Axpert display (or equivalently from monitoring software), or from a BMV or similar external device? As others have mentioned, the Axpert SOC meter is voltage based, with only crude load compensation, so it's not very accurate. Plus or minus 20% for a lead acid battery (plus or minus 80% for LiFePO4). What does the specfic gravity suggest? 

Also, what maximum battery voltage is reached, and does charge termination correlate with cloud cover? 

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  • 1 month later...
On 9/14/2017 at 11:12 PM, Syed said:

My batteries are dying every day running 10-20 mins of load. can anyone help me out

Syed, on your schematic there are only 2 panels connected serial, is that ok?  For 48v system charging most of the controllers need higher input voltage. 3 in series would be optimal config... 

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Two in series is fine if they are 72 cell, but these are hard to find these days except in the very large and heavy 300 W panels. For 60 and the now very rare 54 cell panels, 3S is best. 

Assuming Axpert MKS [ edit: and 5 kVA] as per the topic title. 

It's easy to figure out how many cells a panel has; count the rows and columns. 10 rows x 6 columns = 60 cells. You can usually tell from a photo, when planning a layout. 

Edited by Coulomb
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It all depends on how the MPPT algorithm is written in his charging controller. If his Vmp is 37 v/panel ?, with two in series he would get around 70v out in best case. Many of the 48v charging controllers need higher input voltage even to start operate efficiently. Perhaps there in its user manual is some kind of diagram about!? It should be...

In the 48v systems I had build up in past decades the preferred solar inp. voltages were in range between 74 and 120 v DC. And the best efficiency was always around 85-95 v DC. But these systems are all  with stand alone PV charging controllers (Morningstar, Midnite, etc).

 

 

 

 

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  • 1 month later...
On 15/09/2017 at 7:12 AM, Syed said:

Hi Guys,

I am in a complete mess with one of my project.

In the morning the MPPT charge COntrller does charge 1-2 hours and then it goes to float and not charging. The battery SOC shows 80%+ where as the Sp. gravity is not relevnat to 80% SOC.

My batteries are dying every day running 10-20 mins of load. can anyone help me out

Hi @Syed. I know this comes a little late, but we now have patched firmware 75.31a which fixes the premature-float bug for the Axpert Duo and Tri inverters.

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