Home Power System

I'm still writing this up so come back soon.

This project provides a low voltage (12V dc) power supply system in my home, capable of powering a wide range of 12V direct current devices and some lighting. It provides some independence from the 240V ac mains power supply network. The solution outlined here is just one possible configuration and it has been chosen to be extensible in the future, in terms of supply capacity and features. It could be used with inverters to generate 240V ac power but that is not part of my initial plan. The main goal of the project is education through practice, to better understand that electricity is a finite resource and to better understand the costs involved in generating and maintaining an independent supply.

Why 12V?

The main reason for choosing a 12V direct current (dc) power network is that the required parts are readily available and fairly cheap, being used in cars, caravans and boats. One thing to bear in mind though is that 12V batteries do not supply exactly 12V and some devices are sensitive to these voltage variations. A 12V battery can supply as much as 14.5V and if loads are designed for a precise exactly 12V, they can be damaged by these voltages. In these situations a voltage regulator is required to limit the voltage delivered to the load.

Another factor to bear in mind is that when being charged, the power source (e.g. a photovoltaic solar cell) can deliver as much as 21V into the battery and if a load is connected at the same time, it could be damaged by this higher voltage.

This means that I have categorised my loads:

  1. Require a regulated 12V supply (12V +/- 1%)
  2. Can be directly connected to a 12V battery (12-15V).
  3. Can be directly connected to a 12V battery whilst being charged (12-19V).
  4. Require a regulated dc supply less than 12V, e.g. 9V.
In reality, there are few devices that fall into category 3. Usually they have their own in-built power regulation, in order to handle being exposed to a wide range of supply voltages. Typical loads such as light bulbs are not going to last very long when exposed to these higher voltages.

The Design

The initial design assumes that there are various power generation sources including the mains supply. These are used to charge two battery banks via a charge controller. In this system we plan to use two battery banks so that first can supply power in isolation from the second, whilst it is being charged. When the second is fully charged, we use it to power the various loads and start recharging the first battery bank. This removes the need to support category 3 devices above.

In an ideal world, the change-over from one battery bank to the other is instant and there is no decernable break in the supplied power. In practise this is hard to achieve and this aspect is covered in more detail later.

The use of two seperate and isolated battery banks provides a degree of redundancy and also allows each to be upgraded in isolation at a later date.

The plan is to get to a point where there is enough power generation capability to ensure that the mains power supply unit is never required. Limiting the number of loads is another way to acheive this in the short term though. When both battery banks are full, any excessive power generated is sent to a diversion load.

Earthing

To avoid current loops, all earths are taken to one point and connected using a large, high current terminal block.

Switching

A SPDT (single pole, double throw) switch (Maplin part no. JK28F) is used to select the charge source (wind/sun or mains), with the central switch position being neither connected to the batteries.

An identical switch is used to select which battery bank is connected to the loads, with the central switch position being no loads connected.

A similar switch is used to select which battery bank is being charged (Maplin part no. JK27E).

Monitoring

The design outlined here is based upon historical and planned monitoring of usage and loads. Each load connected is checked to see how much current it draws in typical usage, to help predict the size of battery store and the power generation capability required.

Distribution

The following components are mounted on a board, close to the storage batteries and my Home Control System.

I've used two terminal posts (on both 0V and 12V lines) between the power input and the onward distribution (Maplin part no. N48AQ & N49AQ), to allow me to easily plug in a meter in series or parallel to measure current and voltage used. These are joined by a bridging plate in normal operation.

An automotive 8-way fuse box is used with a number of blade fuses to provide increased protection. Each fuse goes off to a seperate load:
  1. 10A fuse to Mini-ITX PC.
  2. 1A fuse to 1-Wire Network.
  3. 5A fuse down to study 6-way distribution face plate, shown below.
  4. 5A fuse to 12V lighting in loft.

Automotive high current (35A) cable is used to minimise voltage drop between the batteries and the various loads.

Power is distributed down to standard blank switch plates with power sockets mounted on them. These are standard low voltage DC power sockets 5.5mm in diamter and with a 2.5mm centre pin (Maplin part no. JK10L).

I've made up a number of leads to convert these sockets to car cigarette sockets, so that in-car adapters and chargers can also be powered. This makes connecting things like Nokia phones, GPS devices and my children's Nintendo DS Lite consoles much easier.