Selecting the battery technology for PV-storage systems
Is it conceivable that our ancestors already knew about the great importance of batteries and power storage devices?
Archaeologists believe that the three artefacts – a ceramic clay jar, a tube of copper and an iron rod – that were found together near the capital of Iraq and that later became known as Baghdad batteries could date back as far as the first century BC. Corrosion of the metal parts and further tests indicated that an acidic agent such as wine or vinegar might have been in the clay jar. Later experiments using grape juice as an electrolyte did produce 0.5 V of electrical voltage. The exact purpose and use of the Baghdad batteries are unclear and disputed by the scientific community. It cannot be disputed though that the battery plays an increasingly important role nowadays.
Today's rapid progress in the development of new technologies also includes batteries. Electric cars or private photovoltaic systems rely on batteries for PV-storage. Modern PV-storage systems use lithium iron phosphate batteries that offer high capacity and are durable and robust. Alternatively lead-acid batteries can be used. Their performance parameters concerning charging cycles, lifetime and efficiency, however, lag behind the levels established by the new battery technology. So which battery is the best for power storage systems?
Cycle numbers of storage batteries
The number of cycles describes how many times a battery can be fully charged and discharged. This is the key difference between these battery types: While lead-acid batteries usually have 1200 to 1500 cycles within their service life, a lithium-ion battery can deliver between 4000 to 7000 cycles. The capacity losses for lithium iron phosphate batteries at the end of their service life are kept within limits: 80 % of the original capacity is still available after 10,000 cycles. After 15,000 cycles it is still 60 %. The capacities of lead-acid batteries degrade faster. They are also more sensitive to being charged at elevated temperatures, which limits the feasible number of cycles even further. The development of power storage devices is already well advanced. However, no battery technology can offer an unlimited lifetime.
When will the limit of a battery life be reached?
The older the battery, the more the accumulator's power and capacity will decrease. Eventually, any battery’s durability ends, even if it has not been charged. Lead-acid batteries have a service life of about 10 years. Lithium-ion batteries are expected to last twice as long, with a lifetime of approximately 20 years. Elevated temperatures and stress on the cells induced during charging have a negative impact on the performance and the maximum service life of the batteries. Commercial use of lithium iron phosphate batteries is much more advanced compared to lead-acid based batteries. Long-term studies are still conducted. Nevertheless, lithium iron phosphate batteries are a proven and solid battery technology. Naturally, a good and reliable energy management can extend a service life.
Depth of discharge and capacity losses
The depth of discharge of an accumulator is the percentage of the battery’s capacity that has been extracted in comparison to the fully charged accumulator. Lead-acid batteries manage between 50 to 60 % unloading without any impact on their service life whereas lithium iron phosphate accumulators achieve an impressive 80 to 90 % depth of discharge. Thus, they are better suited to compensate for unintended or temporary improper use with excessive discharge, limiting the impact on battery life. With only 5 % capacity losses are also significantly lower in lithium iron phosphate batteries than in lead-acid batteries, where they can reach up to 20 %.
Environmental compatibility of power storage systems
Storage batteries can be built with lead, as the main electrical active raw material, but also with nickel and cobalt. These heavy metals are toxic substances, which miners, especially in the cobalt mines in the Democratic Republic of Congo, are often exposed to without any safety precautions. The use of these materials in batteries is therefore controversial. Lithium iron phosphate, in contrast, is an anorganic compound that also occurs as a natural mineral. Batteries based on lithium iron phosphate are not only considered to be safe, but in particular not toxic. With their high degree of environmental compatibility they prevail over other battery technologies.
What is the efficiency of the batteries?
The efficiency of a battery describes the ratio of energy supplied to the energy extracted. It is impossible to extract the previously supplied energy completely, and thus, achieve 100 % efficiency. The efficiency of lead-acid based batteries hover between 70 to 85 %. But they are clearly outperformed by lithium iron phosphate batteries, which achieve efficiencies of 93 to 98 %..
Safety and maintenance: what needs to be done?
Lead-acid batteries require an annual inspection by the installer and a refill with fresh distilled water. As they can produce gases such as hydrogen during operation, they can only be installed with adequate ventilation. Lithium iron phosphate batteries, on the other hand, simply require a good power management to control the charge. Mandatory harmonised standards on battery safety have not been developed, yet, as research in this field is thin. However, if a manufacturer offers an extended warranty period, you can be assured that the battery will convince with a long service life and high safety standards. Suppliers continuously work on improving the safety of their products.
Which battery technology is the cheapest?
Considering their lower performance, the price benefit of lead-acid batteries compared to storage based on lithium iron phosphate may seem obvious. However, subsequent operating costs should also be considered. A fee for the annual inspection and maintenance is affordable. And as the service life of a lithium iron phosphate battery is double as long as that of a lead-acid battery, the cost of the two battery technologies will be the same in the long run.
Conclusion: Which battery offers more benefit?
Performance parameters of lithium iron phosphate batteries are impressive, which is reflected in the initial purchase price. However, if you consider the current expenses over the lifetime, the differences in price with other batteries level off. Looking at the overall picture, the evidence of superior performance data suggests lithium iron phosphate batteries as the right choice. Seek advice from an experienced specialist before you decide on a purchase.