12V DC Uninterruptable Power Supply

The objective of this project is provide a 12V DC [PR] power feed to supply our Home Control System (HCS), Internet modems, routers, hubs and other key security systems in our home. This also includes the security elements such as alarms and sensors and safety lighting.

Our current home doesn't enable us to install a full 12V dc power network with both solar and wind charging, so this is an interim step to provide a degree of 'off grid' capability until we move home. The thinking is that much of the design work and components used here could be re-used with our planned, local 12V dc power generation capability and a large battery store, removing the reliance on mains power to keep the battery bank charged.

Design

It just so happens that we didn't really need to design or build much for this project as the required hardware already exists in the form of the picoUPS-100 board from mini-box.com. I bought mine in the UK from linitx.com.

The picoUPS-100 ensures uninterrupted power for electronics by automatically switching between a 15-18V DC input source and a lead-acid battery. The switching between the power sources is instantaneous, allowing a smooth and uninterrupted device operation. The picoUPS-100 also has a built-in, two-stage battery charging circuit. The picoUPS-100 has been specifically designed for uninterruptable small/medium power PC operation, where 'always on' operation is required.

This design uses a high current mains powered PSU to feed this circuit board. This will provide the required 16-18V DC power input and is rated at 8A continuous output. We also monitor this feed using our 1-Wire input board to detect mains power failure.

A deep-cycle lead-acid battery bank is used to provide power backup. Assuming we only discharge it to a minimum of 40% capacity, this should keep the main Home Control System (HCS) devices and components running for approximately two days.

One feature lacking from the picoUPS-100 is the detection of a low battery charge state and automatic disconnect to protect the battery from total discharge. Our planned approach to this is to monitor the backup battery voltage using one of the analogue inputs on one of our I/O boards. Once 12.6V (typically 40% of capacity) is reached the Home Control System (HCS) should automatically switch off, removing the only significant load attached to the output. An alternative (and simpler) approach that will be investigated is to simply switch off the Home Control System (HCS) after a defined time period following mains power failure detection.

One thing to note is that this board doesn't provide the required 12V DC regulation. The power supply used outputs a voltage of 16.3V, which is too high to feed out mini-ITX PC and the other components in our Home Control System (HCS). On power failure the lead-acid battery will output voltages around 13.8V which is OK but, still higher than we would want.

Voltage Regulation

To provide a 12V regulated output we are using another of-the-shelf piece of hardware. This bit of hardware can actually support quite a wide set of features and functions but, as it is shipped it provides a regulated 12V output from an input voltage ranging from 6V to 30V.

This device could be connected directly to a 12V battery bank and can easily handle the voltages that would arise during battery charging.

Power Distribution

We have used standard automotive electrical components for low voltage power distribution. 6.3mm blade connectors are typically used. The regulator +ve output is passed through an 8-way blade fuse box. This enables a separate fuse for each powered device or network and ensures the whole system cannot be bought down by a short in one power output. Most are on a 1A fuse but the mini-ITX PC uses a 3A fuse. The ground is distributed via a 6-way spade terminal block. The power cables between shelves are over-long, to enable them to be pulled out and worked on.

Earthing

We have got to say a few words on earthing because it is important. The above system is earthed through the mains PSU used and throughout the rest of the design and installation the UPS ground or 0V line is assumed to be earthed. We make no assumptions about any other power sources and devices that are not connected to this line and if if doubt, optical isolation is used. At no other point do we connect the 0V power rail to earth in our home.

Progress

Our picoUPS board has been installed. Initial testing was done with a small 7Ah 12V lead-acid battery, to provide faster discharging and testing of the automatic Home Control System (HCS) shutdown. This component has now been connected into a much larger 12V battery bank, which provides more than 2 days of backup power for the current loads attached.

The components have been mounted on a 'shelf' for installation in our 'node 0' custom rack. The thin red wire goes off to our 1-Wire input board to enable detection of the mains power.

These have now been moved into our 19" rack cabinet.

Conclusions

Some form of UPS is essential for any serious home automation system. The UPS not only protects the HCS processor from power failures but also protects it from damaging power surges.
The UPS needs to provide power to all safety and security critical components. In our installation, this includes all sensors, PIRs, alarms, etc and essential IP-network components such as a switch and IP netcams.
Ultimately, we want to connect the voltage regulator into our completed home power network, along with solar and wind power generation. The DC-DC converter will protect the Home Control System (HCS) components from the excessive voltages created by connection of solar panels, etc.

Updates

March 2012

This month we have replaced our old UPS bateries with a single, new 125Ah deep-discharge battery. This one weighs 23Kg.

The old battery voltage was down to 13.47V whilst on charge. This new battery was delivered with an output voltage of 12.80V whilst not connected. When connected into our UPS, it charged up fully over a period of over 3 days. When finally charged, the voltage across the battery was measured at 13.48V (when not charging).