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?
Clay jars were discovered near Baghdad, Iraq, which Archaeologists believed could date back as far as the first century BC. They became known as Baghdad batteries. Three artefacts were found together, a ceramic clay jar, a tube of copper and an iron rod. 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 0.5V 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 in today's times.

Today's rapid progress in the development of new technologies does not exclude batteries. Electric cars or private photovoltaic systems rely on batteries for PV-storage. Modern PV-storage systems use lithium iron phosphate batteries. They offer high capacity and are durable and robust. Alternatively, lead-acid batteries can be used. Their performance parameters, such as 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 cycle number describes the number of times a battery is fully charged and discharged. This is where significant differences between the battery types become apparent: While lead-acid batteries usually have 1200 to 1500 cycles before the end of life, a lithium-ion battery can deliver between 4000 to 7000 cycles. The capacity losses for lithium iron phosphate batteries after the end of 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. This limits the available 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 do batteries reach their end of life?

At some point, all batteries reach the end of life, even if they are not being charged. With increasing age, the accumulator's power and capacity decrease. 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 will negatively impact the performance and the maximum service life of the batteries. Commercial use of lithium iron phosphate batteries is much more recent when compared to lead-acid based batteries. The long term research is still ongoing. Nevertheless, lithium iron phosphate batteries are proven and solid battery technology. It is also clear that good and reliable energy management can extend service life. 

Depth of discharge and battery losses

The depth of discharge of an accumulator describes by how much a battery has been discharged. It is quoted as a percentage of the ratio of extracted to maximum capacity. From lead-acid batteries, between 50 to 60% can be withdrawn, without impact on service life. While lithium iron phosphate accumulators achieve an impressive 80 to 90% depth of discharge. They are better suited to compensate for unintended or temporary improper use with excessive discharge, limiting the impact on battery life. Capacity losses in lithium iron phosphate batteries are with 5% also significantly lower 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. Nickel and cobalt are also widely used for batteries. These heavy metals are known as toxic substances. Mining, especially of cobalt in the Democratic Republic of Congo, is often undertaken without any or only very limited health and safety precautions for the miners. The use of these materials in batteries is therefore controversial. Lithium iron phosphate, in contrast, is a substance that can be found in its chemical composition as a naturally occurring mineral. Batteries based on lithium iron phosphate are considered to be safe, and in particular, not toxic. They have a high degree of environmental compatibility with which 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 energy extracted.  It shows how efficiently the battery stores energy. It is not possible to extract all of the previously supplied energy and achieve 100% efficiency. Lead-acid based batteries have efficiencies between 70 to 85%. Lithium iron phosphate batteries achieve efficiencies of 93 to 98%. They clearly outperform their older lead based competitors.

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. They can produce gases such as hydrogen during operation and can therefore only be installed with adequate ventilation. Lithium iron phosphate batteries, on the other hand, simply require good power management to control the charge. Research on battery safety is thin. Mandatory harmonised standards on battery safety, have not been published yet. However, if a manufacturer offers an extended warranty period, you can be assured that the battery will convince with long service life and high safety standards. Suppliers continuously work on improving the safety of their products as a battery fire could have fatal consequences.

Which battery technology is the cheapest?

The performance characteristics of lead-acid batteries explain the lower purchase price compared with storage based on lithium iron phosphate. However, subsequent operating costs also need to be considered. A fee for the annual inspection and maintenance is payable. The service life of a lead-acid battery is only half that of lithium iron phosphate battery. In the long run, there will hardly be much difference in cost between the two battery technologies.

Conclusion: Which battery is most beneficial?

It cannot generally be said which battery technology is best suited for which purpose. Performance parameters of lithium iron phosphate batteries are impressive. This is reflected in the initial purchase price. However, if the subsequent running costs over the lifetime are taken into account, the differences in price with other batteries level off. Looking at the overall picture, the evidence of superior performance data points to lithium iron phosphate batteries as the right choice. Let an experienced specialist advise you before you decide on the purchase.