Battery Guide

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 Lead Acid and Lithium Ion with various different specific types and products from many different manufacturers available on the market. The table below gives a summary comparison of the key attributes of these two different battery technologies.

Attribute Lead Acid Lithium Ion
Total Storage Capacity An individual lead-acid battery will typically have a gross storage capacity of 100Ah – 200Ah @ 12V or 1.2kWh – 2.4kWh. They may be connected in series for a higher voltage and/or in parallel for greater capacity at the same voltage. A typical lead-acid pack suitable for a residential grid-backup solution will be in the range of 8kWh – 25kWh depending on the length of time required to operate off-grid and the total power of the loads to be supported. Lithium Ion battery packs typically are supplied as self-contained units with a built-in battery management system (BMS). Gross capacities vary from about 2kWh up to 8 – 10kWh depending on the model and manufacturer. Some models may be connected in parallel, others may be extended with expansion packs and all need to be fully supported by the software in the battery charger/inverter chosen.
Daily Usable Capacity There is a close relationship between the amount of the total battery capacity that is used each day and the life of the battery as expressed by the number of cycles and typically it is recommended to only discharge a lead-acid battery down to about 50% of the total capacity of a lead-acid battery, this if referred to as a 50% Depth of Discharge (DOD). This makes the storage capacity available for daily use only 50% of the gross storage capacity. Most lithium-ion batteries can be used daily down to about 90% 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.
Full Cycle Efficiency Lead-acid batteries tend to get less efficient the nearer to full capacity they reach which either results in a low full cycle efficiency of less than 80% if they are re-charged near to their full capacity or designing the system to only use about 80% of their full capacity in order to maximise their efficiency. 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.
Lifetime (Cycles) The number of cycles that a lead-acid battery can be used for is directly related to the amount of energy charged and discharged in each cycle. With a system configured to utilise 50% of the gross storage capacity of a daily basis a typical lead-acid battery will have a lifetime of 2,000 – 2,500 cycles. Allowing for some degredation over the life of the battery a useful lifespan of about 5 years in a well designed system may be expected. 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 battery management system will ensure that the battery condition is always maintained in optimum condition and a full 10 year life may be expected.
Cost The initial investment cost of a lead-acid battery will be relatively cheap when expressed as Rand per kWh of gross capacity but all comparisons should always be done a Rand per kWh of usable capacity which makes a lead-acid battery twice as expensive as it may initially appear. The initial investment cost of a lithium-ion battery may be 2.5 – 3 times more expensive per kWh of gross capacity compared to a similar sized lead-acid battery but when comparing the Rand per kWh of usable capacity the difference will be typically about 1.5 times as expensive. The lithium-ion battery will however last twice as long as the lead-acid so over a 10 year period the lithium-ion will almost always be a cheaper option with no need to renew the battery after 5 years.
Weight A lead-acid battery may weigh between 70kg and 80kg per kWh of usable capacity so a typically 5kWh – 6kWh domestic battery pack may weight in excess of 350kg which may cause difficulty in locating a large battery pack in a residential property as a strong floor will be required. A good quality lithium-ion battery pack will typically weigh between 10kg and 15kg per kWh of usable capacity so considerably less than a equivilant lead-acid pack but a typically residential battery pack will still weigh 75kg – 100kg requiring some consideration as to where to place it.
Charge / Discharge Power Most lead-acid batteries can be charged and discharged relatively rapidly and when connected in parallel the total charge/discharge rate is in effect increased. In a typical solar PV system a lead-acid battery pack may be charged and discharged in 2 – 3 hours with a peak discharge rate much higher for short period of times. Most lithium-ion batteries have a relatively restricted charge/dischage rate often needing 3 – 4 hours to charge and a maximum discharge rate of between 1kW and 2kW for a typical residential system. A system utilsing lithium-ion batteries therefore needs to be designed to take care to only connect essential loads to the circuit that will be powered from the battery pack.
Operating Temperature Lead-acid batteries are significantly impacted by the ambient temperature and an increase from 20c to 30c can result in a 25% reduction in the lifetime as defined by the number of cycles and a 50% reduction in the lifetime as defined in years. Lithium Ion is less impacted by moderate temperature changes and ambient temperatures in the range of 15 – 30 degrees centigrade will not significant impact the lifetime nor performance of the battery.

  The choice of battery type is not a simple decision with many different factors to take into account but we would always recommend that a comparison is made using the above considerations and looking at the total cost over the life of the system and not simply choosing the lowest initial cost option which in many cases may be more expensive over the life of the system. 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. Detailed below are some of the factors that need to be considered when determining the size of battery required:  

Attribute Comments
Essential Load Energy Usage For a grid-backup solution the most important thing to consider is the loads that need to be supported when the grid has failed. It is not generally practical to consider powering all the loads in the property, e.g. an electric oven, geyser and pool pump will all consume considerable amounts of electricity and would require a very large battery to run even for a short time. A good way to consider this is to generate a list of essential energy loads to be backed up and the amount of time they’re needed in a typical day. An essential load is basically something energy must always be available for. This could be something normal like a freezer or burglar alarm, or something site specific like a fish tank. If no power was available, would it lead to loss of (fishy) life or just defrosted ice cream? In the UK, power cuts are relatively rare but for more remote locations or other countries it is definitely worth considering. A lot of loads won’t require their maximum power all the time, so you can add a factor to take that into account. Once that’s done, you’ll have an accurate baseline of energy consumption and be able to consider the appropriate battery capacity.

Load Power (W) Time Factor Daily (Wh)
Lights 200 5 1 1000
Fridge 150 24 0.3 1080
Freezer 150 24 0.2 720
WiFi Router 10 24 1 240
Phones 50 1 1 100
Fish Tank 30 24 1 720
TV 170 4 1 680
Other 100 24 1 2400
Total       7690
Battery Operating Time The next critical decision is to decide the number of hours that the system needs to power the essential loads for. Typically a planned grid outage due to load shedding will last for 4 – 6 hours whereas a failure due to a grid fault will typically last for between 1 and 24 hours. The decision on how many hours to allow for is largely driven by the budget available as the cost of the battery pack will be directly related to its size and its size will be directly related to the number of hours chosen. Usually a system will be sized to support the essential loads for between 12 – 24 hours.
Space Available Especially when choosing a lead-acid battery the space available to hold the installed battery and the strength of the floor may be a consideration that imposes a limit on the maximum size of the battery that can be installed. With a Li-Ion battery this is unlikely to be a major concern as a Li-Ion battery will be much smaller and lighter than a similar usable capacity of lead-acid battery.
Charging Time and Rate The battery will be charged from the surplus energy available from the PV system, this is the difference between the energy generated by the solar PV system and that used by the loads during the daylight hours. It is therefore important to ensure that the battery can be fully recharged during a typical day of sunlight, especially in the winter months. A battery pack which is too large relative to the PV system will not get fully recharged and therefore not be fully available to provide power in the event of a grid failure.
Maximum Depth of Discharge Each battery pack will have a recommended maximum depth of discharge, e.g. lead-acid might be 50% and Lithium Ion might be 90%. Having determined the total energy required to be generated from the battery pack with the equation : ‘essential loads energy in 24 hours divided by 24 multiplied by the required battery operating time’ then the gross battery capacity needs to be determined by dividing by the recommended DOD. e.g. 7690W / 24 * 12 hours / 90% DOD = 4272kWh.

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