Thursday, 18 April 2013

Running an event system from batteries

My outdoor event Print and Viewing Stations run from a battery and inverter system.  The battery supplies the basic power in DC (Direct Current) form and the inverter converts this to AC (Alternating Current ie. mains) 230V that my system components (printers, screens, laptops, etc.) can use. I can also run the system from a standard generator if I wanted to but for one day events the battery solution has some plus points. I don't have petrol, diesel or any other explosive or combustible liquid to deal with, it is silent, it is far far cheaper to run and I can run it without worrying about any deadly fumes it may be putting out. However, unless your needs are very simple then the capital cost of a small (1Kw) no-name brand pure sine wave generator will be far less than your capital cost of setting up a battery and inverter solution.

A battery and inverter solution sounds pretty simple on the face of it but there are a ton of wotchas and gotchas to be aware of. Firstly, you will be dealing with mains voltages on the output from the inverter. If kW and VA don't mean a thing to you and RCD sounds like something your doctor jots down in your notes then get a qualified electrician to help you. Seriously, this stuff can kill you.

Sizing your system is an art and again if in any doubt get some professional help. A lot depends on your loadings and how hard each device is working. Eg. a DNP DS40 printer runs at 20W when idle but pushes up to 330W when printing. To size your inverter and batteries you need to know your peak loads (for the inverter) and your average power usage to work out the required battery capacity. It is also handy to know the start up power requirements of anything you are running as some devices can use a surprising amount of power just to get going before they settle down to their average running power requirement.

The type of battery used is nearly as important as the size of battery used. For our purposes a deep discharge leisure battery provides a good balance between capacity and cost. Specialist deep discharge batteries are available for a price but generally you will find it considerably cheaper to just stay within the limits of a good leisure battery. 

A typical leisure battery should ideally never be discharged to more than 50% of  its capacity. The capacity is usually quoted as A/hr (Amp hours). In broad terms a 12 volt 125 Ahr battery can deliver a current of 1 Amp at 12 volts for 125 hours. That is what the 12v 125Ahr is telling us. It could alternatively deliver a current of 5 Amps for 25 hours (5 x 25 = 125). That is the theory but don't forget we can only use half the available power or we damage the batteries.

Work out your average power usage for the day and once you have an average power usage figure then double it. This covers the 50% maximum DOD (Depth of Discharge). Over discharging the batteries will severely reduce their life. A battery rated at 200 to 250 cycles will probably only achieve half this or less cycles before failing if regularly discharged below 50% of capacity. Ideally, you should also make allowances for the inverter efficiency (typically 85% to 92%) and any cable losses. Quoted capacities for batteries are calculated on a 20 hr discharge rate. You will need to add another 20% (typical value) capacity if you are discharging in 8 hours. Batteries need to be recharged immediately after use or at the latest within 24 hours (this too will have a severe effect on the life of the battery).

Lets use a simple example. I have a printer and laptop and the average power consumption for the two is 60 Watts. Ohms law tells us that Power (Watts)  divided by Volts = Current  (Amps).  For a 12 volt system I can divide the 60 by 12 and this will give me 5 Amps of current being drawn. If I have a 125 Ahr battery  then I can divide this by two (to accommodate the maximum DOD of 50%) and then divide the result by 5 (my current). 

125 / 2 = 62.5 Ahr (usable battery capacity)

62.5 / 5 = 12.5 hours of system use before reaching the 50% DOD.

You should have a correctly sized fuse for the DC side just in case you accidentally short the cables or there is a problem with the inverter. Remember that your cable, inverter and fuse must be able to cope with peak power requirements as well as average power requirements. In our example (when the DNP DS40 printer is actually printing) the peak power requirement would be in the order of 350 Watts. That is nearly six times the average load and the cables, fuses etc. must be specified to handle this and have a margin of safety. Cable sizing is very important. Too thin a cable on the DC side and there is a serious risk of fire and you will lose a lot of power in the cables themselves.

I use a 24 volt system to keep cable diameters down (4 x 12 volt 125 Ahr batteries wired as two batteries in series in parallel with two more batteries in series). This then requires a 24 volt charger and 24 volt inverter. This gives me 24 volt at 250 Ahr capacity. I have a 1 Kw pure sine wave inverter which can handle peak powers of up to 2Kw.

For four reasonable quality leisure batteries , good cables, fuses and fuse holders, a good 24 volt 15A charger, an RCD breaker and a good 1Kw pure sine wave inverter you should budget in the order of a £1000 pounds.