Battery Backup System

These days we rely on so many small gadgets such as phones, radios, LED torches.

I heard a podcast presentation with Steve Harris at I thought I knew all about electricity and energy. But I learned quite a few things. So I decided to put it all together and setup a small scale battery backup system topped off with a solar panel.

I already had 2 12v 7.2amp hour SLA batteries I got on special which I planned on using for powering a portable HF radio. And old 4.5watt solar panel. Assuming only 50 percent discharge, these batteries should be able to store about 80 watt hours of energy.

All I needed was a small inverter and a solar charge controller.

You can get small ‘can sized’ inverters cheaply (this one was $45 and has a USB charge socket too). Quite handy on its own kept in the car to recharge phones/laptops etc. Plugs into the 12v cigarette lighter socket.

The solar charge controller is an 8amp unit and cost $50. This will handle a 100 watt panel in the future.

A couple of rapid AA/AAA chargers were added to the mix. In the picture all the devices can be charged up from depleted to full from the batteries.

Battery Backup Charging Devices

Battery Backup Charging Devices

The 4.5watt panel is just enough to keep the batteries topped off and supply about 20 watt hours on an average day. This should be ample to keep these portable devices running indefinitely during a power failure or when out camping.

The only downside is that the inverter draws about 350mA (4.5watts) when idling. Need to remember to switch it off when not in use. It will drain the batteries in about 20 hours.


Some devices and their energy requirements.

Device        wattH   Number Total wH
Mobile       4.8         2              9.6
Netbook    47           1              47
Radio         13           1              13
Ebook         5            2             10
AA               2.4        8              19.2
AAA           1             8               8

Total 106.8 watt hours

Vehicle Use

This sort of system can also be run off the car battery system. With the engine idling (alternator current approx 50amps) this could drive a 600 watt inverter without draining the battery.

Larger System

To be a bit more useful the battery and solar panel need to be upgraded. Around $300 will buy :

  • 100 amp hour 12v marine battery
  • 800 watt inverter

Of course if we want to boil an electric kettle, a 2000+ watt inverter will be required. A 100 amp hour battery could boil it about 10 times before 50 percent discharged.

If anything, actually hooking up some batteries and inverters helps make you more energy literate.

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