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Exploring a single battery, 12V system

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Firstly, it's vital to understand that theory is a just that! If you rely upon theory alone, then expect the room in which you mess with any, high current, black art, to burn, especially if the correct fuse is omitted! "Ethel! Why is the shed a'burnin'?"

Beware them Amps! Current produces heat and whatever current is needed at 230V, mains voltage, is multiplied by more than 20, at the 12V, battery side of the circuit. 5 Amps, to run a 1200W, mains travel kettle, would require, in theory, 100 Amps from the 12V battery. The conversion process may be only 85% efficient, so add a 6th to the required 12V current. 116 Amps! That demand will "cook" even the toughest of deep-cycle (designed to be run down) batteries in less time than it takes to say "Ethel! Hajoo spell fayur x-stinwisha?"

The above diagram shows a 12V, back-up system, based upon a lone, 12V battery (poor thing!) and inverter. A battery monitor has been included. Such is a relatively expensive, optional addition. The reason for including the battery monitor is to show where it fits into the circuit. In technical terms, it's based upon a serial, shunt resistor, placed between the battery negative and the inverter negative.

The battery numbers game

The point of the installation is to have mains power available during a grid outage. However, there are two limitations, which are the total energy stored in the fully charged battery and how quickly it may be used. A 115Ah is the energy available at a 20hour discharge rate. If the energy is drawn at a rate which would exhaust the battery in 5 hours, the capacity drops to 90Ah.

Let's assume the 5 hour capacity and not discharge the battery to below 50% of the 5 hour capacity. Those conditions allow a maximum current of 18A, for 2hrs 30minutes. However that's 18A at 12V, which is 216 Watts. If the inverter has an efficiency of 85%, then the maximum power rating of the 230V, mains appliance(s) may be a total of no more than 180 Watts, if run continuously, for just 150 minutes.

So! How much energy is half of the battery capacity? Energy = Power x Time. E = 216 x 150 x 60 = ~2MJ. Hmm! Let's put that into perspective. Draining half of the energy from a high capacity, deep cycle battery, over 2hrs 30mins, is equivalent to the energy required (theoretically) to boil ~4.5 litres (~1 gallon) of water; from 0 degrees. Oh dear!

Adding a battery monitor is an expensive option. The reason for its inclusion is simply to show how it fits into the system. It relies upon a serial shunt on the negative side. WTF that means doesn't matter. Just wire it right!

Below is a costed, 12V system. The reason for using a 1000W inverter is that, for short spells, it allows the use of higher power appliances. The next section will explore a 24V system, which involves 2, expensive batteries. Gulp!

12V components and cost

£89.0020A 12V Battery Charger
£230.99115Ah Deep Cycle 12V Battery
£79.9612V Battery Monitor
£239.001000W 12V Inverter
£29.955. cable & 100A fuse & holder
£5.148. 300mm x 25mm sq cable
£0.001. & 2. supplied with the charger
£0.003. & 4. supplied with the inverter
£0.006. & 7. supplied with the monitor
£674.04Total cost

A 24V system with 2 x 12V batteries in series

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For reasonable battery life, in terms of recharge cycles, the 12V system allows a maximum mains power of 180W for 2hrs 30mins, which is hardly spectacular. Employing 2 of the same capacity batteries,in a 24V system has several advantages. Assuming that the power output remained at only 180W, it would now take 10 hours to completely exhaust the batteries, therefore their capacity could increase fromthe 5hr, 90Ah capacity to closer to 100Ah. 1 battery of 90Ah capacity has increased to 2 with a total of 200Ah capacity. 180W is now availabe for well over 5 hours.

Of course, 360W becomes the 5 hour rate, so that power is available for 2hrs 30mins. Strangely, the 24V inverter, from the same company, was £70 less than the 12V inverter. Dropping the battery charger current, from 20A to 12A saved £5, which paid for the battery serial connector. Overall, doubling the capacity (an extra, £231 battery) added only ~20% to the cost of the system. Having the DC side drawing only 10x the current of the AC side, rather than 20x, is much cooler (literally!).

24V components and cost

£89.0012A 24V Battery Charger
£461.982 x 115Ah Deep Cycle 12V Battery
£79.9624V Battery Monitor
£149.001000W 24V Inverter
£29.955. cable & 100A fuse & holder
£5.148. 300mm x 25mm sq cable
£5.148. 300mm x 25mm sq cable
£0.001. & 2. supplied with the charger
£0.003. & 4. supplied with the inverter
£0.006. & 7. supplied with the monitor
£816.17Total cost