APC UPS with external batteries

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Updated – December 28, 2022

The Pro series from APC is suitable as an emergency power supply. It works with two 12 V 9 Ah batteries connected in series, i.e. 24 V, delivers 1,500 VA, corresponding to 865 W. However, it only delivers around 400 watts for 12 minutes (at exactly 432 W), or around 5 minutes at full load. Alternatively, it has a connection terminal for external battery packs.

Two 12 V 80 Ah batteries in series are an inexpensive battery extension. This gives you 89 Ah, which means around 50 minutes of full load or 100 minutes of 50 % load.

Charging technology

Since APC's charging circuit does not use the ultimate gentle charging algorithm, it is recommended to use a charging booster with an integrated 12 V / 24 V - DCDC converter Victron Orion TR Smart 12/24 10A with 240 W power to ensure a long life for the batteries.

The DCDC converter is activated via an ignition plus connection, which is connected to the H input after removing the existing cable bridge. The converter has an integrated delay circuit (default 120s), which only activates charging of the battery connected on the output side after the set time period has elapsed.

Switchover – shore power <> emergency power

A is used to automatically switch between shore power and UPS emergency power supply HTRONIC switching station, identical in construction to ELV MPC1000, used. Here the load is connected to the middle connection terminal (Load), the output of the UPS emergency power supply to the left terminal (Slave) and the shore power supply line to the right (Master).

In order to maintain the different phase positions of both power sources, an adjustable delay circuit is installed, which switches over after x milliseconds in accordance with the loss of one of the two sources.

The right (master/shore power) connection always has priority.

Applied voltage is signaled by an LED. If there is shore power and the UPS is switched on, both LEDs (master and slave) light up, including that of the load output.

If shore power is available, the Victron Blue Smart IP22 as a charger 24/161 charges the two 12V batteries connected in series.

A high current relay RL200/12V isolation relay When connected, ensures that the plus lead of the two batteries connected in series is connected to the plus pole of the APC connection terminal. With shore power supply, this prevents the APC charging circuit from charging the two external batteries - with the less friendly charging algorithm - and this is reserved for the DCDC converter while driving.

In this way, the battery power used by the APC is immediately buffered again and the supply duration of the UPS is significantly extended.

Consumers and downtime

The Mac Mini M1 installed here and the four 15.6″ monitors, including the two HDMI extenders, require a maximum of around 35 W, which should allow up to 18 hours of pure UPS operation without any recharging. Even operating the LASER printer is possible without any problems, which of course shortens the aforementioned time period.