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The dreaded premature float bug (Axpert 3kva 24V)


Fox

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Good morning

Thank you for all the info I’ve gathered to date.

I have recently installed a 3kva Axpert inverter (24V) and am struggling with the premature float bug (as far as I know there is no firmware patch for the 3kva and setting 32 is not available to adjust the time spent at maximum voltage).

As far as I understand the premature float bug only occurs under certain conditions (voltage above float AND low charging current).

This morning I did a test with charger source priority set to utility (output SBU and solar panels disconnected). I discharged the batteries down to the back-to-grid voltage (24.5V) and I hoped that I would get my batteries up to the CV voltage (28.2V).

But again, it got to the float voltage (27V) and stayed there – never reaching CV voltage.

My back-to-discharge voltage is 27V and max charging current = max AC-charging current = 20A.

Any ideas? Is there any way to somehow trick it into fully charging the batteries?

Thanks in advance

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43 minutes ago, Fox said:

setting 32 is not available to adjust the time spent at maximum voltage

I don't suppose you have equalisation settings? There are half a dozen of them up to about 38. Those can also force higher charge voltage.

If not... It's really not fit for purpose if even a utility charge doesn't work properly! 🤔

Edit: and sorry, I don't know any other tricks.

Edited by Coulomb
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Hi Coulomb

Thanks for the quick response.

There are actually equalisation settings which I can configure from Watchpower.

Should I do something like reduce equalisation voltage to bulk charging voltage and set to equalize daily?

Your help is much appreciated

 

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On 2020/02/20 at 8:57 PM, Fox said:

Should I do something like reduce equalisation voltage to bulk charging voltage and set to equalize daily?

Yes, exactly. It won't stop charging automatically when it's full, so you have to be careful. If your battery is a sealed lead acid type, especially if it's not an AGM type, you need to stay under the gassing voltage, about 14.2V per 12 V module in summer. You mentioned 28.2 V, so that's fine. Now you get to play the "guess the equalise time" game. Too much and you risk overheating the battery and reducing life; too short and you chronically undercharge the battery, reducing life. It's probably better to err on the side of too much charging, rather than too little.

I wonder though if Watchpower really knows your inverter model properly, and really can perform equalisation. 

Edited by Coulomb
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  • 1 month later...

Hi 20VT

I increased my float charge voltage and lowered my expectations :)

I'm afraid I'm far from being a guru, so please take my advice with a pinch of salt.

Essentially, with my inverter, the bulk charge voltage is pretty arbitrary because it never gets there (due to the premature float bug). So my only lever is the float charge voltage. This should probably be in the 26.4 to 27V range but I've increased to 27.4V.

On some days when I'm at home all day (there are a lot of those lately), I do a manual bulk charge by increasing the float charge to 28.2V for a few hours in the morning.

I'm not sure if this method will destroy my battery life, but it seems to kind of work for me right now.

Good luck

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On 2020/04/09 at 9:09 AM, Fox said:

Hi 20VT

I increased my float charge voltage and lowered my expectations :)

I'm afraid I'm far from being a guru, so please take my advice with a pinch of salt.

Essentially, with my inverter, the bulk charge voltage is pretty arbitrary because it never gets there (due to the premature float bug). So my only lever is the float charge voltage. This should probably be in the 26.4 to 27V range but I've increased to 27.4V.

On some days when I'm at home all day (there are a lot of those lately), I do a manual bulk charge by increasing the float charge to 28.2V for a few hours in the morning.

I'm not sure if this method will destroy my battery life, but it seems to kind of work for me right now.

Good luck

Thanks for responding and sharing what you are doing to get around this issue.

I assume that you have Lead Acid batteries.

 

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  • 7 months later...
  • 5 months later...

Hi everyone, I have the same issue in that even after a deep cycle my batteries go straight back to float voltage of 27.6 (user defined) and never go above this for a bulk charge. I fear this is damaging my batteries. Does anyone have any suggestions on how to get the 3KVA 24V inverters to bulk charge? I have played around with the charge currents as suggested above but still no luck. My only other option is to manually set the float voltage higher for a period of time as described above.

 

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18 hours ago, Andy07 said:

my batteries go straight back to float voltage of 27.6 (user defined) and never go above this for a bulk charge.

Do you have enough solar power to charge them higher? [ Edit: Can you charge them from utility? With reasonable settings, you should not get the premature float bug biting with utility charging. Though some 24 V models have very low charge current capability, possibly also from utility. ]

Quote

I fear this is damaging my batteries.

It is indeed not good for lead acid to be chronically undercharged.

Edited by Coulomb
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  • 2 weeks later...
On 2020/02/20 at 3:40 AM, Fox said:

22am struggling with the premature float bug

 

On 2021/05/19 at 9:41 AM, Coulomb said:

It is indeed not good for lead acid to be chronically undercharged

Good day all

I have an 18 month till now fully operating Axpert 1 Kva 1000 W MPPT Inverter recently(?) suffering from the same issue.

I met this forum starting with Coulombs analysis on "code 03" and have never again had a good nights sleep.... he he

I am Of Grid so my routine since mid Autum (April-May in Chile ) Includes a "Top Up" with Internal combustion Generator ( 2000 W ) 2 hrs past Sun Down and at wake up in the morning 6-7 AM . If it is a too cloudy/rainy day I have to run the generator (Axpert 20 A Utiliyu charge ) for about 1/2 - 1 hrs every 3 Hrs. ( Mostly to keep the home ferigerator going ). I stop when chargin power descends to lower 100 w. ( Starts with 250 W app )

Since descovering code 03 and as Fox says "the threaded under....." , the only way I have managed to reach 14.1 V bulk charge voltage is by adding a selfmade "Charger" out of an PC UPS transformer + Rectifier bridge  which gives me 15A additional  + Car battry charger 4A on top of all.

But this lasts only a few minutes ( maybe 15 min ) and Axpert ( which is controlling the total amount of charge as I understand it) kicks back to float. My battery bank is 2 x 170 Paralell  Ah Narada AGM VRL with recommended maximum charge of 25 A a piece. I , of course monitor the whole process with self made Software with readings every 10 sec.

Apparently the Axpert 1Kva is done.....

We have decided to upgrade our system to 3Kva 24 V and because mostly of the low price I am obviously looking for an Axpert Good offerings ( plenty ) in Chile.

Problem beeing,  "How do I make shure I get a "Good One" ? Manufacturing date ? Serial Number ? ....

Most of the upgrade money will obiously go into new batteries.....

I thank you very much for past and future help. If ever possible.

It has been very interesting for me to read for hours throgh this forum. Thanks to the all the community again.

Greetings

Francisco

Chile

 

 

 

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On 2021/05/19 at 3:41 PM, Coulomb said:

Do you have enough solar power to charge them higher? [ Edit: Can you charge them from utility? With reasonable settings, you should not get the premature float bug biting with utility charging. Though some 24 V models have very low charge current capability, possibly also from utility. ]

Hey @Coulombsorry for the delayed reply, I did not see that you had replied to me. I have no solar so only charging from utility, these are my settings:

Float Voltage: 27.3

Bulk Charge: 29.0 (I've never seen it even close to this as it always goes back to float)

Cutoff: 23.7

Utility Charging: 20A

Recharge Voltage: 24.3 (although not needed without solar as far as I understand)

Total Charging 20A

The above is all in USE mode but even if I set to AGM i experience the same behavior in that it always just goes back to float voltage and never above)

Thanks for your help!

 

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On 2021/06/02 at 8:22 PM, frasenci said:

"How do I make sure I get a "Good One" ?

The main thing is not to end up with a clone, despite their lower price. Some clones might be OK, but you just don't know what you're getting, the the temptation to cut corners is always very high. Axpert quality is already so-so; I have replaced the capacitors and MOSFETs in mine (I have two, bought in 2015) to extend their life.

All the non-clones are basically made in the same factory to the same quality standard; there are a few odd brands that seem to get special treatment. Consider also that whoever you buy the machine from is probably the only place to get firmware updates, replacement comms boards, etc. You might be able to get firmware updates from a web site like this forum, but 24 V firmware update files are less common. You can sometimes get replacement boards from Ebay and the like, but it's almost impossible to be sure you're getting the exact right board (there are so many variations), and the quality is unknown.

 

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21 hours ago, Andy07 said:

Bulk Charge: 29.0 (I've never seen it even close to this as it always goes back to float)

As an experiment, perhaps set the float voltage to be 29.0 V (same as the absorb/bulk voltage). That will tell you whether the charger is capable of 29.0 V, and it will give your battery a much-needed full charge. Watch the charge current carefully (don't leave it unattended like this); when the net charge current into the battery reaches about C/20, change the float voltage setting back to normal. (If your battery is rated at 100 Ah, then C is 100, and C/20 is 5 A). Note how long it takes to get to that point, from when the voltage gets close to 29.0 V (if it gets there, of course).

Quote

Recharge Voltage: 24.3 (although not needed without solar as far as I understand)

Yes, you will presumably be in UTi output source priority (setting 01), which will ignore settings 12 and 13 (back to utility and back to battery voltage settings).

Quote

Total Charging 20A

It probably doesn't matter much, but I would set this to about 0.15C for your battery, or another value if your battery manual specifies a different maximum charge current. The inverter will actually pay attention to this value, even though you can't reach it with utility charging; it uses this value to decide when to switch from absorb to float stages. [ Edit: At least, that's what the 5 kVA models do. 24 V models may operate differently. ] Exactly your problem, right? But increasing this value will make it switch to float sooner. But try the above experiment first. Let us know the result.

Edited by Coulomb
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  • 9 months later...
  • 5 months later...

Hi all, 

I'm hoping someone can assist, maybe @Coulomb as he seems to be a bit of an Axpert expert 😁

I have a Mecer 1500VA 12 Volt Axpert inverter (not sure if these are clones or not) and a Mecer 12V 200AH LFP Battery with a LCD volt meter connected to always show the battery voltage.

I have tested the mentioned bug and I can see that I have to set the Float voltage to the actual charge voltage that I want, which for these batteries are recommended as 14.1 volts @ 20 Amps. 

My Bulk Voltage is set to 14.1 , but if I set the Float to the recommended 13.6, the display never shows that it is charging at 14.1 and the battery voltage also does not go beyond 13.4, only if I set the Float to 14.1, does the display show charging at 14.1 and the battery shows 13.7 

My question is, how can I fix this, or better manage this?

I don't want the float to be on 14.1 as the inverter just keeps on charging the battery seemingly forever (Fan keeps running and display constantly shows 1-8 amps going into the battery even after the battery is full, or at least what I can determine is full seeing as it rests at 13.4 {99-100% SOC for LFP} after charging for 6 hours and unplugging the AC cord)

If I let it charge to full at 14.1 Float (keeping in mind it still keeps charging, fans running) and then change the Float to 13.4, it stops charging and displays shows 0 Amps going into battery.

I'm at a loss here, because I have noticed that if I keep the Float on 13.6, the battery drops to 13.2 within a few minutes when load shedding hits, and then it goes to 12.9 (20% SOC) within an hour of just watching tv (120 Watt load shown on inverter)

I bought this battery to fend off stage 6 and up (4 hour load shedding) but I don't want to think that after 1 hour it drained by 80% with only a 120watt load, the battery is rated as 2400Wh, I am sure it has to do with the the Axpert charging issues.
 

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7 hours ago, RTG said:

I have a Mecer 1500VA 12 Volt Axpert inverter (not sure if these are clones or not)

If it's branded Mecer and sold in South Africa then I think you can be assured that it is genuine. Of course, a clone maker could just slap on a Mecer logo, but they don't seem to have stooped that low (yet).

Quote

with a LCD volt meter connected to always show the battery voltage.

I think you need to verify the accuracy of that LCD meter, say against a digital multimeter, preferably a half decent one. You seem to have a large discrepancy between the LCD meter and the inverter.
 

Quote

My Bulk Voltage is set to 14.1 , but if I set the Float to the recommended 13.6, the display never shows that it is charging at 14.1 and the battery voltage also does not go beyond 13.4, only if I set the Float to 14.1, does the display show charging at 14.1 and the battery shows 13.7 

It sounds like your inverter's battery voltage measurement is way off. I have no experience with the smaller models to know if that's common or not. When charging LFP with their very flat voltage versus SoC charge curve, it's more important to have an accurate battery voltage reading.

Quote

My question is, how can I fix this, or better manage this?

After verifying the accuracy of the LCD meter, I would attempt to calibrate the inverter's battery voltage measurement. Details are here. The problem is, I'm not sure that these tiny inverter models have the required command. If you tell me your main (DSP) firmware version (U1 on the front panel), I might be able to tell.

The other thing is that as long as the voltage readings are consistent (and don't vary with time and temperature), then you can just translate the reading and change the settings to whatever it takes to get the required bulk/absorb voltage. The premature float bug should not affect utility charging, and you have not mentioned solar charging at all. Though make sure that maximum charge current (setting 02) is the same or nearly the same as the maximum utility charging current (setting 11). If setting 02 is greater 5 times the setting 11, then the premature float bug WILL manifest with utility charging (that's the way to demonstrate it). But I don't think your model can even go that high.
 

Quote

I don't want the float to be on 14.1 as the inverter just keeps on charging the battery seemingly forever (Fan keeps running and display constantly shows 1-8 amps going into the battery even after the battery is full,

LFP batteries don't work that way. When they are truly full, then it only takes a tiny current to push them to 3.6 VPC (14.4 V for your 4S battery). 

Quote

or at least what I can determine is full seeing as it rests at 13.4 {99-100% SOC for LFP}

Again, LFP doesn't work that way. The upper voltage plateau is about 3.33 VPC, so that's 13.3 V for your battery. It could have that voltage and only be about 70% charged. You can't use voltage to guess the SoC of an LFP battery, except at very high or very low states of charge. 8 amps is only 0.04C for your 200 Ah battery, so it will take a long time to charge at that low rate. BTW, 20 A seems very low for a maximum charge rate for this battery. Usually LFP can charge at about 0.5C, which would be 100 A. Not that your inverter can probably charge that fast.

Quote

after charging for 6 hours and unplugging the AC cord)

That's the other thing about LFP chemistry; the voltage relaxes after charging. If it has Yttrium added, it may relax less, but I don't think that most manufacturers use that  additive any more. This does NOT indicate a loss of stored charge.
 

Quote

I'm at a loss here, because I have noticed that if I keep the Float on 13.6, the battery drops to 13.2 within a few minutes when load shedding hits,

There is another voltage plateau at about 3.30 VPC (from poor memory), which is about 13.2 V for your 4S battery. My guess is that you're not charging the battery enough.

You might be making the rookie mistake of believing the SoC reported by the inverter. That figure (when you're using an LFP battery) is only for entertainment. It's really rough if you have a lead acid battery; for LFP, it can barely tell you the difference between totally full and dead flat. Higher power models come with a way to talk to the BMS of the battery (if present), and that can give a much better estimate of SoC.

Quote

and then it goes to 12.9 (20% SOC) within an hour of just watching tv (120 Watt load shown on inverter)

That's about 3.22 VPC, which is just above the nominal voltage of the cell (3.2 V). Personally, I let mine go down to the equivalent of just over 12.8 V, which is about 20% SoC if it's being lightly discharged.

Remember: LFP ain't lead. You can't treat them the same.

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8 hours ago, Coulomb said:

Thank you @Coulomb for the detailed response.

If it's branded Mecer and sold in South Africa then I think you can be assured that it is genuine. Of course, a clone maker could just slap on a Mecer logo, but they don't seem to have stooped that low (yet).

 

Quote

Good to know, pretty sure it is genuine then, thank you

I think you need to verify the accuracy of that LCD meter, say against a digital multimeter, preferably a half decent one. You seem to have a large discrepancy between the LCD meter and the inverter.
 

Quote

I just use the LCD meter for a loose indication of what the battery is doing, when I take the measurements I do use a fluke 

 

It sounds like your inverter's battery voltage measurement is way off. I have no experience with the smaller models to know if that's common or not. When charging LFP with their very flat voltage versus SoC charge curve, it's more important to have an accurate battery voltage reading.

After verifying the accuracy of the LCD meter, I would attempt to calibrate the inverter's battery voltage measurement. Details are here. The problem is, I'm not sure that these tiny inverter models have the required command. If you tell me your main (DSP) firmware version (U1 on the front panel), I might be able to tell.
 

Quote

It could be, I have noticed a difference in the inverters Voltage display whilst in operation/battery draining.

ie. When load shedding starts, Battery would read 13.5 V but inverter display would show 13.2 V with 130W load. (AFAIK, LFP does not suffer from voltage sag under load, especially such a low load.)


The inverter does not have any option to change voltage reading.

The firmware Version is: U1 05 04

I am honestly not comfortable with flashing firmware but I am willing to learn, that's if that is even an option for my inverter.

 

The other thing is that as long as the voltage readings are consistent (and don't vary with time and temperature), then you can just translate the reading and change the settings to whatever it takes to get the required bulk/absorb voltage. The premature float bug should not affect utility charging, and you have not mentioned solar charging at all. Though make sure that maximum charge current (setting 02) is the same or nearly the same as the maximum utility charging current (setting 11). If setting 02 is greater 5 times the setting 11, then the premature float bug WILL manifest with utility charging (that's the way to demonstrate it). But I don't think your model can even go that high.

 

Quote

I am only using Utility, no solar connected.

I also read the PFB should not show under utility charging, so it is quite weird that the inverter wont charge at bulk voltage if float is below bulk.
I believe you are referring to below (stumbled upon when going to the link you provided.)

https://forums.aeva.asn.au/viewtopic.php?p=59896#p59896

I have now changed the settings accordingly

Max charge current (02) = 50 Amps

Max Utility charging (11) = 50 Amps

Bulk max  = 14.6 V

Float = 13.4 V (Ensuring more than 1 volt difference between float and bulk)

Will test to see what happens when charging

 

LFP batteries don't work that way. When they are truly full, then it only takes a tiny current to push them to 3.6 VPC (14.4 V for your 4S battery). 

Again, LFP doesn't work that way. The upper voltage plateau is about 3.33 VPC, so that's 13.3 V for your battery. It could have that voltage and only be about 70% charged. You can't use voltage to guess the SoC of an LFP battery, except at very high or very low states of charge. 8 amps is only 0.04C for your 200 Ah battery, so it will take a long time to charge at that low rate. BTW, 20 A seems very low for a maximum charge rate for this battery. Usually LFP can charge at about 0.5C, which would be 100 A. Not that your inverter can probably charge that fast.

That's the other thing about LFP chemistry; the voltage relaxes after charging. If it has Yttrium added, it may relax less, but I don't think that most manufacturers use that  additive any more. This does NOT indicate a loss of stored charge.
 

There is another voltage plateau at about 3.30 VPC (from poor memory), which is about 13.2 V for your 4S battery. My guess is that you're not charging the battery enough.

You might be making the rookie mistake of believing the SoC reported by the inverter. That figure (when you're using an LFP battery) is only for entertainment. It's really rough if you have a lead acid battery; for LFP, it can barely tell you the difference between totally full and dead flat. Higher power models come with a way to talk to the BMS of the battery (if present), and that can give a much better estimate of SoC.

That's about 3.22 VPC, which is just above the nominal voltage of the cell (3.2 V). Personally, I let mine go down to the equivalent of just over 12.8 V, which is about 20% SoC if it's being lightly discharged.

Remember: LFP ain't lead. You can't treat them the same.

 

Quote

Noted.
I can agree that I may not have allowed the battery too charge enough based on the difficulty of getting it to charge at the required voltage.


I have now changed the setting as above.

I have never paid any mind to the SOC display as it is inaccurate as you say, besides it seems to just basically measure the difference between current battery voltage and cutoff voltage. (display would show fuller or emptier based on the cutoff voltage setting)




 


I just hope I don't have to resort to changing the float voltage to what I want the charge voltage to be every time load shedding hits, and then back to what I want the float to be after the battery has been charged.

I can say that it carried a 400W (35 Amps/hour) load last night for the 2 hour session and the battery was at 12.9 (+- 20% SOC) before the power came back, this still seems as too much drain for the rated capacity of 200AH, so lets see what the changes bring and then I will report back.


Thanks again.

 

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  • 6 months later...

Hi all, I finally got the inverter to charge correctly. 

What worked on my side is to put the charge setting on 3 step charging instead of auto or 2 step. (setting 15 on my inverter). 

Battery setting is on "user" 

Charge voltages are set to:

Bulk = 14.1v

Float =  13.4v

Cutoff = 12v

It now charges at 14.1v and I can see the battery actually going to 14.1v

It also stops charging when the battery is full and "floats" the battery only when it drops below my setting

Not sure if this the fix all for the Premature float bug, but definitely works for me. 

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  • 1 month later...
On 2023/04/23 at 9:29 PM, RTG said:

Hi all, I finally got the inverter to charge correctly. 

What worked on my side is to put the charge setting on 3 step charging instead of auto or 2 step. (setting 15 on my inverter). 

Battery setting is on "user" 

Charge voltages are set to:

Bulk = 14.1v

Float =  13.4v

Cutoff = 12v

It now charges at 14.1v and I can see the battery actually going to 14.1v

It also stops charging when the battery is full and "floats" the battery only when it drops below my setting

Not sure if this the fix all for the Premature float bug, but definitely works for me. 

What firmware/model inverter do you have? Most of the Axpert inverters dont have a setting 15 in the menu. What is it related to?

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