The choice of a battery is one of the most critical decisions that needs to be made when designing a grid-backup or enhanced self-consumption solar PV system.
The two main types of battery commonly chosen for solar PV systems are Gel AGM batteries and Lithium Ion with various specific types and products from many different manufacturers available on the market.
Lithium-Ion battery packs typically are supplied as self-contained units with a built-in battery management system (BMS). Gross capacities vary from about 2.4kWh up to 8 – 10kWh depending on the model and manufacturer.
Can be connected in parallel
Extended by adding additional battery packs
Come with a BMS (Battery Management system) need to be fully supported by the software in the battery charger/inverter
Most lithium-ion batteries can be used daily down to about 80% of their gross storage capacity with little or no impact on their lifetime in terms of number of cycles.
This makes the storage capacity available for daily 90% of the gross storage capacity.
Most lithium-ion batteries have a full cycle efficiency around 95% even for a cycle from their full depth of discharge up to full capacity making them ideally suitable for daily use applications like solar PV systems which need to use most or all of their retained energy in the evening/night and charge up again fully during the day.
A good quality lithium-ion battery may have a lifetime of 5,000 – 7,000 cycles which is considerably more than 10 years of normal usage.
The built-in BMS will ensure that the battery condition is always maintained in optimum condition and a full 10-year life may be expected.
The cost of Lithium-ion battery is more than other batteries, but will produce more power or usable capacity
These batteries last twice as long as the lead-acid or gel batteries, over a10 year period the lithium-ion will almost always be a cheaper option with no need to renew the battery after 3/4 years.
A good quality lithium-ion battery can weight up 15kg per kWh a residential battery pack can weigh from 40kg – 100kg requiring some consideration as to where to place it.
Lithium Ion is less impacted by moderate temperature ranging from of 15 – 40 degrees centigrade will have no significant impact on the battery.
The choice of battery type is not a simple decision with many different factors to take into account. It always good think about the total cost over the life of the system and not simply choosing the lowest initial cost option which more often will cost more money in the long run.
Equally critical is the size of the battery with one too small providing insufficient benefit and one too large being a significant additional unrequired expense.