How to prevent apctest from shutting down the UPS immediately when performing a battery calibration?

Question

I am running an APC SMT1500RMI2U connected to a Linux server via serial cable. On the Linux server, the UPS is managed by apcupsd and the associated utilities. apcupsd communicates with the UPS using the MODBUS protocol.

From time to time, I'd like to perform a battery calibration of the UPS via apctest. But every time I do that, apctest shuts down the server (nearly) immediately. This really sucks because I stumble across that misbehavior every time I try it. I just have killed not only the server, but 6 running VMs with it due to that test.

What I am doing wrong? What do I need to do to perform the battery calibration without apctest shutting down the UPS immediately?

Additional information:

• Debian 11 (bullseye), amd64, vanilla (no custom kernel or something like that), up to date.
• apcupsd and the associated utilities (e.g., apcaccess) work without issues.
• The batteries are definitely not that worn out; when I pull the plug, the server runs at least 20 minutes without problems.
• However, as described above, apctest shuts down the UPS / the server immediately after it has started the runtime calibration.
• Of course, I always stop apcupsd before using apctest (apctest won't start anyway if apcupsd runs).
• The version of apcupsd is 3.14.14.

The following shows the output of apcaccess:

root@cerberus ~ # apcaccess
APC      : 001,039,0917
DATE     : 2023-08-08 17:17:11 +0200
HOSTNAME : cerberus
VERSION  : 3.14.14 (31 May 2016) debian
UPSNAME  : APCUPS
CABLE    : Custom Cable Smart
DRIVER   : MODBUS UPS Driver
UPSMODE  : Stand Alone
STARTTIME: 2023-08-08 17:16:37 +0200
MODEL    : Smart-UPS 1500
STATUS   : ONLINE
LINEV    : 233.2 Volts
LOADPCT  : 26.6 Percent
LOADAPNT : 20.1 Percent
BCHARGE  : 100.0 Percent
TIMELEFT : 51.0 Minutes
MBATTCHG : -1 Percent
MINTIMEL : -1 Minutes
MAXTIME  : 120 Seconds
OUTPUTV  : 233.2 Volts
DWAKE    : 0 Seconds
DSHUTD   : 0 Seconds
ITEMP    : 27.9 C
BATTV    : 27.1 Volts
LINEFREQ : 50.0 Hz
OUTCURNT : 1.31 Amps
NUMXFERS : 0
TONBATT  : 0 Seconds
CUMONBATT: 0 Seconds
XOFFBATT : N/A
SELFTEST : NO
STATFLAG : 0x05000008
MANDATE  : 2017-03-01
SERIALNO : AS1709331884
BATTDATE : 2023-04-15
NOMOUTV  : 230 Volts
NOMPOWER : 1000 Watts
NOMAPNT  : 1500 VA
FIRMWARE : UPS 09.4 / 00.5
END APC  : 2023-08-08 17:17:11 +0200