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Axpert King & Narada Charge settings


Roentgenzap
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Dear All

I have browsed though the forum, and as a newbie, I have to declare myself confused! As first disclaimer - I am a novice to solar with basic understanding of battery principles.

 

So, my setup:

3 x Axpert King 5Kva in parallel (working as single phase) (Firmware 71.9)

12 x Narada 200ah batteries (Narada 12ndt200) setup in 3 x 48V parallel

27 x 370W panels on the roof. 

 

Spec sheets of the batteries which states equalize and cycle voltages of 2.35V - 2.4v and float of 2.25V. Maximum charge current of 50A. 

I am concerned that my batteries are not being fully recharged.

When working in SBU mode, as soon as PV cannot supply house demand, the battery volts will drop about 51.5V. During the night the house load is about 750W - with the morning battery volts at about 48.5V. 

I am still getting to grips with the system, but I am worried that they do not stay at absorption charge not long enough. Looking at the battery voltage graphs, would stay in absorption  voltage of 56.4V for about 1 hour, sometimes less. 

 

Initial setup by the installer was set at:

Bulk charge 56.4

Float 54V

Low DC cutoff at 42V.

Back to grid charging voltage 46V

Max PV charge current 60A (that is combined across the 3 parallel banks, so in effect 20A per bank as I understand). 

 

Would a more appropriate setting not be:

Bulk charge 57V  (halfway between spec levels)

Float at 54V

Low DC cuttof is far to low for me. I would think something like 47V (thats about 40% SOC)

back to grid at 48V

Should the PV charge amps be upped to 90A (that would be 30A across each parallel string). 

 

I am looking into installing a BMV-712 to get a better understanding of battery SOC and usage. 

 

Any advise would be highly appreciated!

 

 

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On 2020/07/27 at 11:46 PM, Roentgenzap said:

I am worried that they do not stay at absorption charge not long enough. Looking at the battery voltage graphs, would stay in absorption  voltage of 56.4V for about 1 hour, sometimes less.

They should stay near the absorb voltage until the net charge current into the battery falls to below about 10 A per inverter (assuming that all 3 inverters have setting 02 (maximum total charge current) set to 50 A). If not, you are suffering from the premature float bug. Until there is patched firmware for Axpert Kings (it may happen if the factory firmware stops changing rapidly), you might consider using the timed absorb feature (setting 32, misnamed "bulk charging time" in some manuals).

 

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Initial setup by the installer was set at:

Bulk charge 56.4

That's OK, though you might consider higher values in winter.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Low DC cutoff at 42V.

That's way too low, as you say.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Max PV charge current 60A (that is combined across the 3 parallel banks, so in effect 20A per bank as I understand). 

If each inverter is set to 60 A, that's a total of 180 A max (you need good cables for this), and each string will be getting about 60 A each (since you have three strings of battery modules in parallel). But the battery modules are specified at 50 A max, are they not?

If each inverter is set to 20 A, then that works out to 20 A per string (x3 for 3 inverters, then divided by 3 for 3 strings of battery modules). That could well be the problem if that's the case.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Would a more appropriate setting not be:

Bulk charge 57V  (halfway between spec levels)

As above, OK in winter, I would put back to 56.4 V in summer.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Low DC cuttof is far to low for me. I would think something like 47V (thats about 40% SOC)

Yes.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

back to grid at 48V

If you leave setting 29 (battery cutoff voltage) at 47 V, then the effective minimum voltage for setting 12 is 49 V (2 V higher). An undocumented gotcha.

On 2020/07/27 at 11:46 PM, Roentgenzap said:

Should the PV charge amps be upped to 90A (that would be 30A across each parallel string). 

As above, all three inverters should be set to 50 A, or possibly 40/50/60 if some PV strings are stronger than others. They should add to 150 A.

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Hi Coulomb, 

Thanks so much for the detailed answer. 

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6 hours ago, Coulomb said:

If not, you are suffering from the premature float bug.

 

Ok, so from what I understand from your link, is that the Float bug occurs when the charging amps drop for more than 50 sec below (Max charge Amps /5). So if we set the max charge value as low as reasonable, this should (at least partially) negate that occurring. For instance, when Max charge current is set at say 50A per inverter, the float bug will occur when the charge current drops to below 10A for 50 seconds. But if set at 30A per inverter, this will only occur if the charging current drops to below 6A? 

The trick would be to get the max charge amps as close as possible to the realisticconstant achievable amps on that PV string per inverter. 

 

Quote
6 hours ago, Coulomb said:

Until there is patched firmware for Axpert Kings (it may happen if the factory firmware stops changing rapidly), you might consider using the timed absorb feature (setting 32, misnamed "bulk charging time" in some manuals).

 

I see that there is patched firmware out version 71.80 - would you recommend using these instead of the 71.90 I am currently on?

 

Quote
6 hours ago, Coulomb said:

you might consider using the timed absorb feature (setting 32, misnamed "bulk charging time" in some manuals).

 

If I use this feature, what would a reliable setting time be?  Is there still some form of monitoring in this stage to decide when to exit the Absorption state or is it purely time based?

 

Thanks again for the help!

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17 hours ago, Roentgenzap said:

Ok, so from what I understand from your link, is that the Float bug occurs when the charging amps drop for more than 50 sec below (Max charge Amps /5).

Almost: the battery voltage has to be at least the float setting (setting 27) less 0.5 V. So that means your lead acid battery will likely get to at least 50% SOC (wild guess, and it depends on many factors). The bug is that they use the wrong setting for the voltage test (they use the float setting when they should use the absorb/CV setting).

Quote

So if we set the max charge value as low as reasonable, this should (at least partially) negate that occurring. For instance, when Max charge current is set at say 50A per inverter, the float bug will occur when the charge current drops to below 10A for 50 seconds. But if set at 30A per inverter, this will only occur if the charging current drops to below 6A? 

Yes, that's true. It could help a little.

Quote

The trick would be to get the max charge amps as close as possible to the realistic constant achievable amps on that PV string per inverter. 

Yes.

Quote

I see that there is patched firmware out version 71.80 - would you recommend using these instead of the 71.90 I am currently on?

I'd forgotten that old patch. For anyone using or considering using the direct BMS connection features, I would NOT RECOMMEND regressing to lightly patched firmware version 71.80d. However, you have a lead acid battery, and so presumably you don't care about direct BMS connection, so for you, I think it would be fine. I haven't had a chance to check the changes between 71.80 and 71.92, but I suspect that they're mostly to do with direct BMS connection. The hassle is if you do discover some bug that you care about that was fixed in 71.90, you'll have to update to 71.92 or later, which I suspect will necessitate updating the removable display firmware, which it seems you can't do quite as conveniently with a USB stick and OnTheGo cable.

Quote

If I use this feature [ timed absorb ], what would a reliable setting time be? 

I couldn't guess. You'd have to try some values, starting with 60 minutes, and see how far the current drops by the end of the absorb stage. You will need some buffer for extra cloudy days, but you don't want to set it much longer than necessary because it could reduce battery life.

Quote

Is there still some form of monitoring in this stage to decide when to exit the Absorption state or is it purely time based?

There is no monitoring of current at all, purely time based. It doesn't even stop the timer when the battery voltage falls well below the absorb voltage setting (that's the second charge bug). It will attempt to keep the battery voltage at close to the absorb voltage setting; that's the extent of the "monitoring".

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