Friday, 13 August 2010

Batteries for Our Remote Power System

Yesterday I re-read the ‘Operating Instructions’ for the batteries that are installed as part of our remote power system and was reminded of quite a few important things to keep in mind about them. To be specific we have 24 X 2 volt ‘valve regulated lead-acid’ “type 8 PVV 1200” batteries manufactured by the German company BAE Batterien GmbH. The purpose of our batteries is to store and provide direct current for conversion to 240 ac volt via the inverter.

The valves on the top of the batteries allow for release of hydrogen and must not be opened as doing so permits the access of oxygen which discharges the energy in the battery cells. Adequate ventilation is essential.

The batteries have to be kept out of direct sunlight and need to be installed and operated such that the ambient temperature difference between the cells or blocks within individual batteries is very small (less than ‘3 K’ … whatever that means!). The strings of batteries are best to be installed in specially designed cabinets for temperature evenness control and the connectors between the cells all need to be the same diameter and length.

During discharge (supply of an electrical current) the active materials in the batteries are converted to lead, sulphate and water. Curiously, the faster the rate of discharge the less amount of aggregate current ends up being available.

For long life the batteries need to be returned to full charge within a period not exceeding 4 weeks. Our batteries are classed as ‘deep discharge’ which means they can be totally discharged from time to time, however it is not our intention to do this. We have our back-up generator set up to auto recharge the batteries by 20 % from 65 % as and when this reduced charge level occurs (as mentioned in a previous blog post). I’m assured that the 65 % minimum is about optimal but have not seen any empirical data supporting this. Car batteries are not at all ‘deep discharge’ and will deteriorate rapidly if this occurs regularly.

If the battery voltage is permanently less than full it will discharge by itself resulting in ‘loss of capacity and possible sulphation of the electrodes’.

What I found particularly interesting is that when a battery is classed as ‘fully discharged’ this does not mean that it has zero voltage. By way of example, each of our ‘2 volt’ batteries are considered fully discharged (called the ‘final’ voltage) when their voltage falls to 1.8. Conversely a ‘full’ battery holds more than 2 volts … and for us this generally occurs when the voltage is 2.4.

The ideal operating temperature range for our batteries is 10 to 30 degrees Celsius. The rated maximum is 45 degrees C however they will handle temperatures for very short periods of up to 55 degrees C. High temperatures have the affect of reducing the operational life of a battery. I was unable to find information on the absolute minimum temperature batteries can handle and have noticed that cold nights do seem to reduce battery voltage more. Cold temperatures have the long term impact of reducing battery storage capacity.

As far as maintenance goes, during the whole life (estimated at 15 years) our batteries do not need to be refilled with water. “The electrolyte is diluted sulphuric acid and fixed as GEL made with micro porous SiO2.” It is recommended that every 6 months the voltage and surface temperature of a sample of batteries be checked and recorded together with the room temperature. Every 12 months the voltages and surface temperature of all batteries are to be measured and recorded.

If the batteries ever need to be stored for extended periods they should be left fully charged and in a dry frost-free room.

So … what does all this mean in the context of how our batteries are presently installed? In short our location and set-up augers well for good extended life and performance of the batteries. We experience very few frosts each year and having the batteries off ground level within a cabinet within a closed room means that they should never reach zero degrees C … especially given that each one weighs 70 kg and they are installed side-by-side effectively creating a 1680 kg block. Whilst our summers are hot it is not common for temperatures to exceed 45 degrees C and since climate records have been kept in the district the hottest day on record was 48 degrees C … well within the 55 degree C maximum. Added to this we do have an additional roof over the container where we keep the batteries, virtually eliminating the chance of temperatures getting higher than the external ambient within the storage room due to heating by sunlight on the roof and walls. The set-up of our back-up generator means that the batteries should never be discharged to high risk levels and the sunlight patterns are such that I’m confident the batteries will get to fully charged levels for short periods at least once every 4 weeks regardless of season.

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