Home UPS

Table of Contents

1. Overview

A UPS which can power my entire home is built using a Schrack GENIO Online USDD400 UPS connected to four customized stock battery boxes. The battery boxes are retired APC SYBT5 which originally are 120 Vdc nominal and contain 10 x 12V 9Ah sealed lead-acid batteries. The total energy capacity of one unit is a little over 1kWh.

2. HV DC side

2.1. Battery boxes

A single SYBT5 battery box can be seen below:


After lifting the cover the internals are revealed:


Apart from the batteries themselves the unit contains:

  • a safety DC switch used to disconnect the battery string and allow for safe work on the pack
  • a backplane connector
  • a controller board with a DC fuse

In order for the battery box to work with my UPS I needed to remove a single battery from the string leaving 9 x 12V 9Ah and a 108 Vdc nominal voltage. The rewiring process was a breeze thanks to the well engineered internal layout and excellent quality of the internal cables and connectors. Kudos APC!

2.2. Safety switch

A single feature worth mentioning for the safety switch is that it contains two pairs of contacts - high current where the batteries are connected and control contacts. The control contacts are connected to the controller board to sense the state of the switch. I have left them connected there for now.


2.3. Backplane connector

After some digging around I highly suspect that the backplane connector is an Anderson Power Products Powerpole PP15-45 Finger Proof hermaphroditic connector. The jagged edge on the connector means it's a Finger Proof variant. I wanted to buy some of these connectors and make myself a proper plug however I couldn't find any reasonable priced offers in my country. So I got rid of the connectors and just connected my own cables.


2.4. Controller board

The controller board contains a beefy fuse (no wonder, the battery unit is rated for UPSes up to 14 kVA), a current sense shunt and a PIC microcontroller.


I had no use for the controller board so I just left it as it is in the case as it allowed me to just use the original DC fuse.

2.5. Wiring box

I decided to wire up all 4 battery boxes in parallel and connect them to the original UPS battery input. The image below shows the junction box I used with just the screw terminals so that you can see better. I took care to carefully color-code all of the terminals and cables as a mistake at this point when all of the batteries are charged would have drastic consequences.


The 4 boards on the left are 4 current sense boards using the ACS758 Hall-effect sensor IC to monitor the current going in and out for all battery packs.

3. Inverter

The inverter is a 4 kVA single-phase inverter made by Schrack. Unfortunately, I was only able to find a manual for it in Polish. The manual contains (apart from the usual stuff) a pinout for the RS-232 connector with I/O controlling UPS shutdown or indicating running on battery power. The UPS can be controlled by a piece of software called PowerShield but I was not yet able to find or download it. The UPS has a manual bypass switch which allows for on-line battery replacement, this is described here.

4. AC side

As my entire house is wired with 3-phase AC grid power I needed a way to supply all 3 phases using a single-phase inverter. Fortunately, as I do not power any equipment which actually requires proper 3-phase power I can get away with simply feeding all phases from one source - the inverter. This is achieved using a custom-made C19 to standard 3-phase 16A plug cable.


The wires are colored according to IEC 60446 with blue being the neutral, green/yellow the PE and blue is neutral.

I have also added a PZEM-016 power meter to the mix to monitor how much kW my inverter is producing and feeding back into the house.

Author: Maciej Grela <enki@fsck.pl>