KOPITE
Executive Member
With Eskom in South Africa constantly shedding its load all over our economy, it is important for anybody who requires a reliable power supply to understand how to build their own 220V backup power supply, so as not to be ripped off by an ever increasing number of pseudoscientific power salespeople.
In this article we look at how you can cobble together a 220 Volt power supply using components you can buy at your local automotive store. But first it is important to understand a few basics about batteries, and how to convert 12 Volt DC battery supplied electricity into 220 Volt AC current that your household appliances can use.
A twelve Volt battery is a storage device that, when charged, stores electricity in chemical structures that can reverse and release electricity if required. Batteries are typically rated in Amp Hours (AH) - ie how many Amps for how many hours the battery will perform. An inverter is a device which can take the DC current from a battery and convert it into 220V AC current for your appliances.
To calculate how many batteries you need, and how big an inverter is quite simple. Investigate the specification plates on the back of each appliance. In the case of my laptop, the power supply is a 90W unit.
For an inverter one can work on a rule of thumb that to produce 100W will require approximately 10 Amps, hence in reality the lap top requires about 10 AH of battery life per hour. With a 90 AH battery one can therefore expect about 9 hours of power before the battery provides too little current to keep the inverter going. But it is not this simple.
Hence to power a more intensive system, one would add up the power requirements of each item:
Eg:
TV: 250W
Computer: 160W
Laptop: 90W
Total: 500W
Hence the system will require 500W divided by 10 Amps = 50 Amp Hours
So to run this system for 4 hours (during one of the many power outtages in the country) would require about 200 Amp Hours of battery life.
Again, when drawing current from a battery it is important to understand that the AH rating of a battery is calculated based on the battery being discharged over a 20 hour period. In other words for a 40AH battery, 2 AH per hour. If one draws more current from the battery a reduced output is achieved. This can mean that if the battery is rapidly discharged as little as half the expected AH can be provided.
The table below shows how if one discharges a 100AH battery over 20 hours one gets the rated AH capacity, but for example if you discharge it over 5 hours you only get 80% of the capacity.
Battery Capacity Hours of Discharge
100 20
90 10
87 8
83 6
80 5
70 3
60 2
50 1
Hence, in the above situation, we are drawing 200 AH over 4 hours, or 50 AH per hour. A 100AH battery ideally provides 5 AH per hour to achieve maximum life, one would require 10 100 AH batteries to run this system with little damage to the batteries and maximum efficiency. In reality this is very expensive. If one were to draw 50 AH from one 100AH battery however, it would, due to the heavy usage be only 50% efficient and discharge after an hour, as opposed to the two hours one would predict. So, ideally one needs to reach a compromise. In this case choosing to run the batteries at 80% efficiency would require 200 AH divided by 0.8 (80% capacity) giving us a requirement in reality for 250 AH of battery capacity. Standard deep cycle batteries in South Africa often come as 105 AH batteries, hence three of these batteries would be adequate to power this system, and, as the batteries decline with age a small amount of additional AH are available to cover this shortfall.
In the next section, we show how to build a battery bank and inverter with off the shelf components from an auto parts store.
You will need and inverter (in this case an 800W model), a plug strip set, batteries (ideally deep cycle batteries), insulation tape, a plug, a battery charger, a screwdriver and some scissors, or if you have one, a cable stripper.
Cut the two prong plug off the charger and the three prong plug off the plug strip.
Attach the two prong plug to the plug strip and tape the cable up neatly. Attach the three prong plug to the severed battery charger. This makes for a more reliable battery charger as two prong plugs often don't make good contact. Plug the two prong plug into the inverter output socket and connect the inverter to the battery/s. Turn inverter on. It should power and an indicator light somewhere will come on.
Attach battery charger to battery.
Connect to equipment to be powered. Note - in this photo only one battery is shown - to operate this system effectively for 5 hours one would require 5 100AH batteries. In this photograph, Dr Janice Limson, editor of ScienceinAfrica magazine, and an electrochemist based at Rhodes University powers some of her research equipment by the inverter system depicted in this series.
Good breakdown on what you need, but I find still confusing
