Green Energy Storage Or Exploding Batteries?
So you have safely installed your solar panels or wind turbine and all the associated
electronic wizardry (inverters, isolators etc) and need to somehow keep the electricity generated when the sun was shining or the wind was blowing so that you can run your lights at night!
You have a choice to make as to whether you 'keep' or 'store' the energy and use it yourself later or sell it into the grid and then buy it back later when you need it. The decision here is based upon the practicality of connecting to the grid e.g. are you installing green energy generation in the middle of nowhere? There is also the issue of approvals and certification as some states & countries will require forms in triplicate before you go anywhere near their terminations. You perhaps also want to consider the potential benefit of Feed-in Tariffs ...can you benefit from them? If the answer is yes, then would your installation qualify if it is a DIY build?
Once you have gone through the process of deciding you are going to keep or store the electricity, you need something to put it in!
A battery (or more than one; known as a 'bank' of batteries)!
Alessandro Volta is credited with inventing the battery (an electrochemical cell) in 1800, although there is some evidence that he was beaten to it by the people of Baghdad by about 2000 years or so! They seem to have been using batteries to plate gold & silver jewellery. Anyway ...in order to store the excess electricity you generate you will need lead-acid batteries. There are other newer technology batteries around e.g.NiCad & NiMH but these technologies are still quite expensive and in truth are nowhere near as efficient as the good ole' lead acid battery. It is possible to obtain used NiCad battery packs from newer or better quality golf-carts or from telecomms back-up systems that will do the job.
When looking to store the power generated and then draw it off relatively slowly you need to use deep-cycle batteries. Car batteries, starter batteries or similar, are no good as they are intended to provide high current in short bursts to start your car or boat or whatever. They will not stand being discharged to a very low charge state and then given a full charge again. The ideal type of battery come from fork-lift trucks, golf-carts or telecomm backup systems. These are designed to provide power more consistently over a longer period.
Lead acid batteries are exactly that ...batteries made with lead and acid. The liquid electrolyte in the battery is a 35% sulphuric acid to 65% distilled water mix; the acid is potentially dangerous in that it can cause chemical burns to any skin contaminated with it. It is the thickness and surface area of the plates that determines the ability to provide short term, high current or longer term supply. More surface area gives a higher current capability. Hence car or starter batteries tend to have plates made of a lead 'sponge' giving much greater surface area, but these plates are much more likely to corrode and disintegrate, causing the battery to fail in a short time if given repeated deep cycles.
When charging lead acid batteries, hydrogen and oxygen are given off as the electrolysis takes place. This gaseous mixture is extremely flammable. Batteries should never be charged in a closed area. This is a critical factor in the design of your storage area. There MUST be sufficient air flow to prevent a build-up of this gas. Any spark caused by a connection or disconnection could cause an explosion.
As with all electrical work the best way to prevent any incident or hazard is to employ the services of a professional electrical technician.
The battery itself could explode if the electrolyte level falls below that of the plates inside, allowing a larger area for a build-up of the hydrogen gas, potentially ignited when connecting the battery.
All lead acid batteries are made up of cells. Each cell supplies about 2.14 volts. A typical car battery therefore has 6 such cells. You can see this by the six little caps to top up the electrolyte. So the battery voltage is 6 x 2.14 = 12.8 volts. When fully discharged the voltage would be approx. 1.75 volts per cell or 10.5 volts for the battery. This measurement should be taken with no load. The state of charge can be checked by either a voltmeter or a hydrometer (which checks the specific gravity of the electrolyte).
Most lead acid batteries are sized by specifying a capacity in amp-hours. This is the number of hours for which the battery would supply 1 amp of current or a factor of that. For example a battery might be specified as having a capacity of 44Ah i.e. it could supply 1 amp for 44 hours or 4 amps for 11 hours. For deep cycle batteries a rating often specified is the '20 hour rate' or the '100 hour rate'. This is the current the battery could supply for that period. Care does need to be taken here because of something called the Peukert Effect. This is that a battery supplying lower current will have a higher amp-hour rate than if the current is drawn at a higher rate; another reason to turn the lights off! (...to do with the internal resistance of the battery itself and becomes quite complicated!). Search on Wikipedia for 'Peukert's Law' for a really technical explanation!
When connecting the batteries together to provide the voltage needed care must be taken as the potential for electrical shock is high and the current supplied by a bank of 12v batteries could prove fatal.
A few years ago whilst working in the offshore industry, I was involved with internal x-ray of pipe welds. The larger of the 'crawlers' would go inside the pipe and used ten 12v sealed lead acid cells to provide the 120v required to power the x-ray tube. One of my colleagues got a bit cavalier whilst wiring one of these up and managed to short the live terminal to the earthed casing. Due to the current flowing, the heat was such that the cable welded itself to the case and ...within a few seconds the whole wiring harness melted and caught fire. A bit of a panic ensued ... fire extinguishers etc. Totally destroyed the crawler and caused a few red faces!
The moral of the story is just because it's a battery doesn't mean it's any less powerful or less dangerous than mains electricity!
Another safety issue to consider is weight! Lead acid batteries can be very HEAVY. Dependant on the capacity a typical car battery might weigh anywhere between 15 US lbs (7kg) to 33 US lbs (15kg). Fork Lift truck batteries might weigh 2000lbs (900kg). Care must be taken when lifting batteries of any size. Obviously dropping a battery may very well crack it allowing the electrolyte (acid) to leak out.
by: Pete Bumstead
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