Hello,

From more then 3 month I'm fighting with Voltronic/Axpert inverter to set the discharge battery current. I tryed with LiB bms protocol, Soltaro protocol. The value are not used by inverter. They are ignored so in case of low SOC the new discharge value are not takein in conssideration.

Also serial command PBATMAXDISC is not supported. Do you have any idea if there is any way to setup this value dynamically?

Inverter firmware is : 76.05

Thank you

Alex.

On 2025/05/27 at 8:15 PM, alexam3001 said:

I tried with LiB BMS protocol, Soltaro protocol. The value are not used by inverter. They are ignored so in case of low SOC the new discharge value are not taken in consideration.

I haven't checked, but I suspect that this setting is only used when using non-BMS battery types (USEr, AGM, or FLD). Obviously AGM and FLD are no use when using a Lithium battery.

I don't know if there is a command that the BMS can send to the inverter to tell it to use more of alternative power sources to reduce the current draw from the battery, and if so, whether the Voltronic display/BMS firmware responds to it.

Inverter firmware is : 76.05

For BMS related problems, the display/BMS (U2) firmware version is also important.

Edited May 31, 20251 yr by Coulomb

from bms protcol pesrpective inverter is asking this discharge current:

Lib : 0x0073 2 Discharge current limit 0.1A

from log : ['0x2', '0x3', '0x0', '0x73', '0x0', '0x1', '0x75', '0xe2' - inverter requesr

'0x2', '0x3', '0x0', '0x1', '0x5', '0xdc', '0x16', '0xf0'] - bms response

Soltaro protocol

==============

ID 0x1872.

2:1 has maximum charge voltage

4:3 has minimum battery voltage during discharge (may not act on this value as yet)

6:5 is max charge current, tenths of an amp, total for all packs (similarly for Pylontech, LIB, or LIC)

8:7 is max discharge current, tenths of an amp, total for all packs (similarly for Pylontech, LIB, or LIC)

Send this message continuously for BMS to maintain "connected" status.

but inverter is not reacting to max discharge current value. i tested with this 2 protocols and no reaction.

with my software i can basically emulate almost any type of bms /protocol.

I had a superficial look at some 39.xx display/BMS firmwares. Both 39.21 and 39.27 process and send on the PMAXDISCHG command to the DSP. But in DSP firmware version 76.60 (the only 76.xx for VM IV that I have), this command is not processed. The VM models, including the VM IV models, have a smaller processor, so they may have to skip some commands to get the code to fit into flash memory.

I checked 39.21 first as it is supposed to be the first display/BMS firmware to support BYD protocol. It seems to replace Shoto protocol! Worse, the BYD support is missing in display/BMS firmware version 39.27, which is the latest one that I have. Also, LIA is replaced by BAK again. I have no idea why they do this. There seems to be plenty of flash space available in these processors.

I didn't look terribly hard, but I did not see any place where a BMS function set the bit that would transmit a PMAXDISCHG command.

So it looks like the pipeline is blocked in two places: the display/BMS code may not be heeding the discharge limit values, and is not sending a PMAXDISCHG command. But the DSP is not processing that command anyway (second blockage). That's assuming that the PMAXDISCHG command is the only mechanism for limiting the battery discharge current.