Coulomb

[ NOTE: I thought that this was for the VM IV, but it's for a recent version of VM II (with old style display, non-removable, not the round display) with Twin outputs.
My humble apologies to the hundreds of people that downloaded this and wasted time.
It's also not possible as-is to use in a 1500W model. ]
Main firmware for the Axpert VM II Twin 3600/5600W. Do not use with any other model. Use at your own risk. This archive is missing the Arena reflash tool .exe file. I believe that you can use the ArenaINVReflashTool.exe from some other firmware update files. That's how it came. Hex (.inv) file is dated 28/7/2023.
Firmware upload instructions.
3624MT, 5648MT_DSP 28.13 -- remove GT setting-20231114T104317Z-001.zip

This is removable display firmware version 02.81 for Axpert King 1 and Axpert VM III, 48 V or 24 V, for the original STMicroelectronics branded microcontroller. Do NOT use with the more recent GigaDevice branded MCUs. Only use if the existing display firmware version (as shown in a monitoring program, NOT the display) is 02.xx (NOT 102.xx). Do not use with any other models.
[ Edit April 2024: I now believe that this is the first 02.xx display firmware that will run with both STI and  GigaDevice chipsets. However,  it is now obsoleted by display firmware version 02.83 . ]

mcu.hex is dated 2022/Aug/16.
Use at your own risk. Upload instructions here.

This is factory removable display firmware version 02.83 for the Axpert King 1 (not II) and Axpert VM III (older versions, NOT 4 kW or 6 kW or Twin). It has support for several BMS battery types, including LIB and LIC. The mcu.hex file is dated 2022-10-06, but as of April 2024, it is the latest such firmware I know of. This display firmware is "universal" in the sense that it can run unmodified on both STI (original) and GigaDevice (newer) ARM MCU chipsets. In other words, this can replace older display firmware such as 102.73, or any display firmware with the major version numbers 102 or 02.
Do not use with any other model. Works with either 24 V or 48 V inverters in the above models. Cannot be updated by USB stick; RS-232 and Windows PC (or emulator) required. Use at your own risk.
Firmware update instructions (covers DSP and MCU updating).

Main (U1) factory firmware for the Axpert Max II 8 kW non-twin, version 54.06, compiled 05/Sep/2023, for the '2809 DSP. Do not use with any other model, not even Axpert Max II 8kW with other than 54.xx existing firmware. Use at you own risk. Non-twin refers to models with a single AC output port; all Max models have dual MPPTs.
Firmware update instructions.

Deprecated! Use patched firmware version 156.05 instead, which patches MPPT2 as well as MPPT1. I will delete this download when 156.05 is tested.
This is temporary patched firmware version 256.05, based on main firmware version 56.05 for the Axpert Max II 10kW. it changes only two things (apart from the version number): it fixes the premature float bug, and changes FOR MPPT1 ONLY the current at which it switches behaviour from 0.50 A to 0.05 A. MPPT2 is not changed, so that some users with similar arrays connected to MPPT1 and MPPT2 can compare the effects.
If this works, I'll patch MPPT2 similarly. If if doesn't, I'll do the full set of "stuck at 90 V" patches.
This firmware is for the TMS320F2809 processor, which is the one with the two voltage regulators (see this post). Do not use with other inverter models. 
Original dsp.hex is dated 2022/Mar/16. Hence factory firmware version 56.05 appears to supersede Axpert Max 10kW firmware version 90.01.
Use at your own risk. 

I found this in my downloads folder recently, and it seems to be one answer for a number of problems, such as parts of the display flickering intermittently, especially models with the colour display and round LED ring. In a separate .docx document (not attached here), it says:
[ Edit: Axpert MKS IV models (under their various names) should have this modification provided from the factory from about August 2022, according to one reseller. ]
[ Edit 2: See also this post for a way to rearrange the wires to the MPPT inductor; it worked for at least one poster. ]
I note that the capacitor part number (a Voltronic part number, you can't order from an electronics retailer with this number) mentioned above is different to that of the PDF in this download, so I may be conflating fixed. However, the rest of the .docx document has almost verbatim the same text. The docx document mentions an 18μF 600 VDC capacitor, but that would be much larger than the one pictured. If you don't order the part from your supplier, I suggest about a 2.2μF 600 VDC X2 class polyester film capacitor.
DANGER!
PV voltage on these models is lethal! The charge on the capacitor will persist at a lethal level possibly for hours even after the PV is isolated, so ALWAYS test first with a decent multimeter. Do not touch the terminals if the voltage is above 50 VDC. If you don't know electronics, DO NOT ATTEMPT THIS!
This modification may make your inverter significantly more dangerous to work on. If you are in the habit of working on the PV wiring, you might want to reconsider performing this modification. Maybe the circuitry will quickly discharge the capacitor, I don't know as I don't own one of these models.
DANGER!
There is also talk of a metal shield, and possibly an inductor, but I can't quickly find this thread. Hopefully, this will help someone, or at least point them towards a solution.

This is main (U1) firmware for the Axpert VM II, and probably also the PWM versions thereof (probably called an Axpert VP II or just Axpert VP 3K-24 or similar). This file contains both version 20.13 (2018) and 20.14 (November 2022). It supports 5 kVA, 3 kVA and 1.5 kVA models, all PF0.8 or PF1 (selected by resistors at start-up), except that the 1.5 kVA model is PF0.8 only (1200 W). I can't recall where I found this firmware. Do not use with any other model.
Use at your own risk.
Firmware upload instructions.

This is main firmware version 71.10 for the Axpert MKS III or PIP-5048MGX, and other names. It is for the '2809 DSP.
Do not use with any other model, or with any model that comes with firmware other than 71.xx.
Beware of version numbering clashes; 71.xx firmware exists for the 5 kVA Axpert King 1, Axpert MKS II, and Axpert MKS 1 PF1 64V.
Use at your own risk.
Firmware upload instructions.

I found this service manual in my collection. There are a few partial schematic diagrams, but not much detail, as is usual with these manuals.
This is an EASun branded PDF, but it looks very much like it came from Voltronic, hopefully legitimately. EASun do seem to be a legitimate Voltronic reseller these days.

This is unpatched main (U1) firmware version 74.04 for the Axpert MKS IV. Do not confuse with the Axpert VM IV which looks similar, or the Infinisolar V IV which looks identical (possibly apart from the colour). This will not work if you happen to have a model that comes with the '28066 DSP chip; see this post for details.
Firmware upload instructions.
Use at your own risk.

This is unpatched main (U1) firmware for Axpert Max 7.2kW. dsp.hex is dated 2022/Sep/11. Like all factory firmwares for Axpert Max so far, it has the premature float bug and the stuck at 90 V MPPT bug. Provided here as a failsafe/backup.
Use at your own risk. Do not use with other Max models, clones, etc. Compatible genuine models include the Kodak OG-MAX 7.2, PIP-7248MAX, and many more.
Edit: Do not confuse with patched firmware version also called 90.19 (not mine), based on this factory firmware.

This is main (DSP, U1) firmware version 72.04 for the Axpert Max 1 8kW inverter with the TMS320F28066 processor. Only use this firmware if your existing firmware starts with 72, and it's an Axpert MAX 8kW with the removable display (there are several other firmwares starting with 72 for totally incompatible models).
Firmware upload instructions.
Use at your own risk. 

This is patched firmware version 281.05, based on factory firmware version 81.05, for the Axpert MAX II 8kW with a TMS320F28066 processor. It will display as 81.05 on the front display, but as 281.05 on monitoring software.
This firmware should simply not update if your inverter is the type with a TMS320F2809 DSP processor. Use only if you know that your processor is a '28066, e.g. if your main firmware from the factory was version 81.xx.
The arena reflash tool is included.
This patched firmware fixes the premature float bug, and implements PV patches to prevent the panels from being dragged down below 200 V (was 90 V). This allows the battery to fully charge from solar, and increases PV production, especially in cloudy conditions.
For upload instructions, see https://forums.aeva.asn.au/viewtopic.php?p=87767#p87767 .
As always, use at your own risk.

This is factory firmware version 56.04 for Axpert King 3kW Rack Mounted models with the '28066 processor. The nominal voltage is 48 V despite the "3.24" in the original rar file name. dsp.hex file is dated 7/Oct/2021.
Do not use with any other model, and only if the existing firmware is 56.xx. Note that there are several other models with 56.xx firmware, do not confuse them.
Use at your own risk. 

This is factory (not patched) removable display firmware version 102.73. Note that you can only see the leading "1" from monitoring firmware; from the front display, it looks like U2 02 73.
WARNING: Only use this if your existing display firmware version starts with a "1"; I suspect that the leading "1" is important, and indicates a difference in display hardware.
Ignoring the above may result in BRICKING YOUR display, which will necessitate getting a new one. Apart from the expense, delay, and hassle, it will be difficult to obtain the exact right display model, as they are many with subtle differences.
This firmware release has a rare factory release note: "More increasing the reliability of communication of Pylontech version "C" Li batteries with the display and VM III / KING converters".
This removable display firmware is ONLY for the Axpert King or the Axpert VM III, though these are known by different names by different resellers.
 

This is patched firmware version 291.70, based on factory firmware version 91.70, for the Axpert MAX 11kW with a TMS320F28066 processor. It will display as 91.70 on the removable display, but as 291.70 via monitoring software.
WARNING! This firmware will not work if your inverter is the type with a TMS320F2809 DSP processor. Use only if you know that your processor is a '28066, e.g. if your main firmware from the factory was version 73.xx. If you get this wrong, the upload will refuse to work, but it should not brick your inverter (just restart).
The arena reflash tool is included.
This patched firmware fixes the premature float bug, and implements PV patches to prevent the panels from being dragged down below 200 V (was 90 V). This allows the battery to fully charge from solar, and increases PV production, especially in cloudy conditions.
For upload instructions, see https://forums.aeva.asn.au/viewtopic.php?p=87767#p87767 .
As always, use at your own risk.

Main firmware for the Axpert MKS series 1-3 kVA, six models:
Rating W/VA/Bat V
800/1000/24
1000/1000/48
1600/2000/24
1600/2000/48
2400/3000/24
2400/3000/48
This firmware is for the Freescale HCS08 processor (8 bit). Ver0254.s19 (like a hex file) dated 29/Mar/2016,
Use at your own risk.

This is patched firmware version 156.05, based on main firmware version 56.05 for the Axpert Max II 10kW. It changes only two things (apart from the version number): it fixes the premature float bug, and changes the current at which it switches behaviour from 0.50 A to 0.05 A. The same change is replicated now in MPPT2.
This seems to work; if it turns out that it doesn't, I'll do a full patch set. See this post and those following it for test results of patched firmware version 256.05 (which had only MPPT1 patched).
This firmware is for the TMS320F2809 processor, which is the one with the two voltage regulators (see this post). Do not use with other inverter models. 
Original dsp.hex is dated 2022/Mar/16. Hence factory firmware version 56.05 appears to supersede Axpert Max 10kW firmware version 90.01.
Use at your own risk. 

This is patched firmware version 192.04, based on main firmware version 92.04 for the Axpert Max 1 8 kW, with the removable display.
This firmware is for the TMS320F28066 processor, which is the one with the single voltage regulators (see this post). Do not use with other inverter models. 
Original VP5K.inv (hex) is dated 2022/Sep/22. 
The patches change the version number (will still display as 92.04 on the display), fixes the premature float bug, and implements Georg's patch set for the "stuck at 90V" bug. The minimum MPPT voltage is set at 175 V. For a 200 V version, see patched firmware version 92.06. The MPPT voltage and firmware version are the only differences between this and 92.06.
Use at your own risk. 

This is Axpert MAX II 10kW main DSP firmware version 90.01, using the TMS320F2809 processor.
DO NOT USE with a machine that uses the newer TMS320F28066 processor, though the reflash tool should prevent bricking by not attempting a reflash.
Use at your own risk.

EXTREME CAUTION! Displays have been bricked!
This is Axpert Max 1 (NOT Axpert Max II with the fixed colour display) firmware version 112.19 (displays as U2 12 19 on the front panel). The leading "1" seems to be very significant. I suspect that it means that the display uses a GigaData MCU chip as opposed to a ST Microsystems MCU, but this is not yet confirmed.
ONLY CONSIDER USING THIS FIRMWARE IF YOUR EXISTING FIRMWARE IS 112.xx.

As always, use at your own risk. As of mid November 2022, this should probably only be used to attempt to rescue a bricked display.
This firmware should be usable (with the above caveats) on all Max models (7.2, 8, or 11 kW), but NOT other models, unless existing firmware is 112.xx.
I don't know what features or bug fixes this firmware contains, sorry.

Main (DSP) firmware for the 5kW Axpert King 1 (NOT Axpert King II) with 145 V max PV, version 72.00. The main difference from previous firmware version 71.97 is that several chunks of code were moved around in an important, time critical interrupt routine. So the effects of this change are hard to gauge.
dsp.hex is dated September 2022, but from the dates on other files, I suspect that this firmware was compiled in August 2021.
As always, use at your own risk, and don't use this firmware for any other model. But be aware that the Axpert King 1 5kW is called other names by various resellers, e.g. "PIP-5048MK" or "Kodak OG+ 5.48". Often the word "King" is in the model name, but of course no "II" (Roman numeral 2) after the "King".
Firmware Upload Instructions for Models with a Removable Display.
 

This is main (DSP, U1) firmware version 90.14 for the Axpert Max I  8kW (with removable display, not the Axpert Max II with the fixed colour display). So not use with other inverter models or models with other rated power (e.g. 7.2 kW or 11 kW), but be aware that the Axpert Max 8kW is sold with various names by various resellers. dsp.hex has the date 18/July/2022. Use at your own risk.

If you attempted to reflash your 8 kW MAX I with firmware version 91.04, then unfortunately the software in this file will not work. In that specific case, use the 90.14 recovery download instead.

DO NOT USE FILE "Max 8K 91.04 old flasher.zip"!
If you have downloaded it, delete it! If you have attempted to use it, I'm so sorry, it is possible that you have bricked your machine. It seems that 91.04 main DSP firmware, which claimed to be an Axpert MAX I 8kW firmware, is actually for an Axpert MAX II Dual output 8kW, and uses a different command to initiate reflashing.
If this has happened (you attempted to load 91.04 and now you can't reflash the inverter with any other firmware), download the recovery zip file, and run the recovery reflash tool. This should replace the incompatible 91.04 firmware with the latest factory Axpert Max I firmware that I have, which is version 90.14 (note the unfortunate similarity in version numbers). Do not use this download for any other purpose. Sadly, I don't have an Axpert MAX, so I can't test this procedure. The recovery reflash tool is identical to the old reflash tool, except that the reflash start command has been overwritten with the one that the Max II dual output firmware will be expecting.
If there are further problems, please let me (Coulomb) know in the forums or by PM.
My sincere apologies for any owners that have been inconvenienced by this mistake.
================= Earlier comment: ===============================
Axpert MAX 8 kW main firmware version 91.04. Copied VP5K.INV from another archive as dsp.hex; that archive had a new reflash tool with a virus in it. This combination should work, but is untested. Use at your own risk. This version from about August 2022 is supposed to have fixed some long standing problems with PV charging, e.g. pulling the panels down to about 90V and very little current under some circumstances.
Do not use with any other model, but as usual an Axpert MAX 8 kW can be branded many different names.

Main firmware update file for Axpert King II (NOTE: NOT for the original Axpert King with 145 V max PV). This version is supposed to fix "Can't charge the battery in ECO mode".
This is the version with dsp.hex dated February 2022; there is another version also called 50.02 with dsp.hex dated November 2021.
Use at your own risk. Firmware update instructions here.