12V Battery Monitor

The objective of this project is provide a 12V DC [PR] power feed to supply some key elements of our Home Control System (HCS), whilst monitoring the state of the 12V battery bank. It also acts a a distribution point for this power, via an 8-way fuse box.

The plan is to power our 1-Wire network, all of the PIRs in our home, the emergency lighting, the safety lighting and the security alarms and strobe lighting.

Since we have the 1-wire network entering and leaving this module, we also plan to stick a DS1920 on the LT1083CP-12 voltage regulator to check it doesn't get overloaded and too hot. It is a low drop out (LDO) regulator, ideal for use with lead-acid batteries and can handle currents up to 7A when a suitable heatsink is fitted.

Design

The unit is mounted in an aluminum case, which also acts as a heat sink. A twin RJ45 socket is recessed into the case for 1-wire network in and out. Cables to the battery bank pass through a hole with a rubber grommet. A cable feeding the regulated 12V to the fuse box also passes through this hole, as does a regulated feed back for the 1-wire network, having passed through a 1A fuse.

Battery Monitor

Dallas have a 1-Wire device specifically for battery monitoring, the DS2438. It needs a bit of circuitry to be able to handle the higher voltages seen in this application. This bit is currently being written up.

Progress

This project is still in progress but bits of it have been built as follows:

Voltage Regulation

As mentioned before, we are using the LT1083CP-12 voltage regulator as it is a high current, low drop out (LDO) device. With no load, the device consumes 4.95mA so it can be left permanently connected to the battery bank.

Each connection to regulated 12V output is via an automotive multi-way blade fuse holder, to ensure that none of the loads draws excessive power in event of a failure or short circuit. Typically, we are using 1A blade fuses but some lighting may draw more current.

Conclusions

Having a single module serving this core set of functions makes sense and simplifies the wiring in our home. The design used to monitor the battery bank voltage will work regardless of capacity and the charging mechanism used.

Given the tight coupling of battery bank, charging and regulation, it makes a lot of sense to keep the main Home Control System (HCS) and control boards in close proximity.