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Here's the Huawei ESM RS485 pinout:

Here is some c# proof of concept/sample code for connecting to the ESM batteries over RS485: https://github.com/TruthData/SimpleModbusCommander notice Device ID's for all these bats seem to be hardcoded to 214, and they are stubborn when establishing the first connection. More info is here:

I have a system using 2x ESM48100B3 bats in parallel. In the past I used them with growatt 5000W inverters, but with now RS485 / modbus connection. The Huawei bats use Modbus RU protocol but it's implemented a little out-side of the Modbus spec so I wouldn't expect any non Huawei equipment to be able to communicate with it. After hours of searching I ran into this support article describing a Huawei maintenance tool that can communicate with the batteries, read internal cell voltages etc. from a PC over a custom rs485 cable. Sadly access to the tool is quite restricted, and Huawei support just spun me round and round when I asked for it. Long story short I found someone with the tool, and was able to finally connect from my pc and read out the voltages while doing a serial log/sniff. As I have an EPEver XTRA charge controller & NP2000 inverter that work with the batteries now, I decided to give a go at creating a little c# app that lets me pull data out of the charge controller and the Huawei Batteries... The code is available here on Github: https://github.com/TruthData/SimpleModbusCommander This code is proof of concept only, and I can't vouch for its reliability... as the original poster said ... I have found no way to set the battery ID's... both of my batts seem to be hardcoded to "214". The batteries don't immediately respond to requests.. It seems you have to poll them a few times over a few seconds. before they start responding.... I use a stock EPEver USB-RS485 cable for communications with their charge controller, and followed the instructions on the support article above for building a custom usb-rs485 cable for the Huawei using a $3.50 USB to RS485 Converter FT232 RS-485 Max485 Adapter FTDI FT232RL I hope the above is a blessing to folks...

I have a similar setup - similar issue, 2 ESM 100AH 48v Huawei bats, Inverter & MPPT charge controller are EPEVER. system completely stable charging or discharging but when both charge controller and inverter are running I get the high power spikes... went back and forth with EPEVER support thinking it was their issue, but it looks like a combo. a chance in power draw by the inverter while charging with a charging voltage above 52.5 (ish in my testing) , somehow spikes the voltage enough that the ESM's BMS seems to disconnect momentarily shooting the line voltage way up. (I think my previous 5000W hybrid Growatt inverter fried because of this cycle of voltage spikes.) After a month of emailing with EPEver ... I put long leads 2+ meters between charge controller and bat. AND the main fix was to dial down my bulk & float charge to under 52.3. There remains some minor voltage spikes (54v or so) occasionally, but not often. I hope this helps