Tsmithf

Please post in English on this forum.
Sadly, there are very few 82.xx firmwares about. I have not seen any new ones since 82.63.

Lower is usually better to increase the longevity of your batteries. 0.2C for Lithium is usually the best, but 0.3C is also not bad. A lower charge voltage also improves lifespan. This depends somewhat on what battery chemistry you have.

Thank you! I will talk to the installers as I am not supposed to be dealing with this, being just a hobbyist and not a professional! I cannot see any washers and all of the cables  that I have access to, at least, are indeed very tight.
The bad thing about our situation is that space is rather limited and is complicated further by a wooden column that sits between the two batteries, increasing their spacing perhaps needlessly. The inverter is on only one side of the wooden column. I am saying this with respect to the cable lengths. Or maybe there is some way in which the installers could make them equal lengths, we will see.

Thank you very much for the in-depth suggestions, as previously noted, I will talk to our installers and try to figure this out. 
 
This morning, when I looked at the state of the battery after last night's restart, the cells were still out of balance by about 260mV.
 
Hi there! The BMS should be either a Pace BMS or a Gobel BMS (from what I understand, it is something of a rebrand of Pace by Gobel, but I am not sure). The firmware, reported by PBMS tools is P16S150A-21545-1.05, which leads me to believe that the BMS is Pace P16S150A, because I was able to find such a model on Pace's website.
As of now, I feel like the issues are with the BMS itself (or maybe with some of the cells as well), because last night's restart did not make any sense - I find it impossible for the battery to go from 53.2 to 56 volts in a matter of a minute or two without something catching on fire  or maybe I am missing something fundamental in my understanding.

Welcome to the forum. In which country are you residing? Being an installer and master installation electrician in South Africa judging by your pictures above you have some serious installation errors. First of all you need a external disconnect switch apart from the protection offered by the inverter and batteries for one reason only safety in case of emergency. You are entertaining 20kw of DC energy. 
Secondly your dc cabling size (35mm²) is incorrect you need to have it changed to 70mm² suggested by @Tsmithf. The crimping on the battery and inverter terminals seems to be a problem judging by the main battery terminal connection on the inverter. I only use a hex crimper on dc cabling. The 2 battery's cabling must be exactly the same lengths and short as possible otherwise you will have different resistance values between the 2 batreries which is problematic for current sharing. The colour coding of the DC cabling would be a disqualification in our country and a COC not issued.
My suggestions to your installer.
1. Both batteries to common busbar.
2. Disconnect switch or DC curcuit breaker between both batteries.
3. Main Disconnect switch between busbar and inverter.
4. Use hex crimper for all DC power cabling with red cable for positive and black for Neg and 75mm² cable (20kwh power source.)



Above example of hex crimping.

Great! 35mm2 for 185 amps i think is a little to short, you could produce a lot of heat there. If the 185 are constant. I use 70mm2 for max of 200amp and works like a charm. 
 
you could try connecting like this: 
inverter + to master battery + to slave battery +
inverter - to slave battery - to master battery -
like the picture attached….
if in the future you are planning adding more batteries, i would go with busbar right away, you would need 2 busbar ( + - )  rated for the max discharge current you would use. 
inverter in the middle of both batteries. And just like someone told me, the effects are not instant, i could take several days to work correctly. 
 
also i cant see exactly by the pictures but check for any washer beneath the terminals. Also everything has to be thight.  
 
 
 

Great! 35mm2 for 185 amps i think is a little to short, you could produce a lot of heat there. If the 185 are constant. I use 70mm2 for max of 200amp and works like a charm. 
 
you could try connecting like this: 
inverter + to master battery + to slave battery +
inverter - to slave battery - to master battery -
like the picture attached….
if in the future you are planning adding more batteries, i would go with busbar right away, you would need 2 busbar ( + - )  rated for the max discharge current you would use. 
inverter in the middle of both batteries. And just like someone told me, the effects are not instant, i could take several days to work correctly. 
 
also i cant see exactly by the pictures but check for any washer beneath the terminals. Also everything has to be thight.  
 
 
 

You  have linked to the  display firmware, with mcu.hex. To fix the stuck at 90 V issue, you need the main (DSP) firmware, with VP5K.hex.
You want main firmware version 82.63; that's the latest available firmware compatible with your inverter that fixes the stuck at 90 V issue.
To  make it easier to find and download, I attach a rar file I found on the internet. I compared the hex files, and they are identical.
 
This is factory firmware version 82.63 for the Axpert Max II 10 kW '28066 non-twin. Do not use with any other model. Use at your own risk.
There are several reports that the MPPT logic in this firmware is the good one, i.e. it has fixed the "stuck at 90 V" issue.
Firmware upload instructions for models with a round display.
82.63 axpert max 2 10kW.rar

About 3 week ago I install more solar panels and decides to play around with the combination series and parallel in the end series was best so I also say series if you can 

Leave them in series. Parallel you have higher current which means double the voltage drop over the cables.