Energy storage FAQs.
What is battery backup?
For most systems, there will be times during the year when the power generated by the solar PV system is greater than the power used. During those periods, for a system without storage, all the surplus electricity is exported into the grid. For systems with storage, some of the energy that would otherwise be exported can be retained for use later in the day.
Why do I need energy storage?
To allow stored surplus PV energy for use later in the day and to provide a backup power supply in case of a power cut.
The provision of a backup power supply is particularly relevant for sites that suffer regular power cuts (although this is infrequent for most locations in the UK, it has been rumoured that the frequency of these power cuts will increase in years to come) or for sites that need to guarantee they can run on the critical load at all times, e.g. hospitals.
What are Lead-acid batteries?
Lead-acid batteries were invented in 1859 and are the oldest type of rechargeable battery. They are relatively low-cost and are widely used throughout the world.
The design of a lead-acid battery varies considerably depending on the application. For example, automotive batteries tend to be constructed of many thin plate electrodes (to maximise the surface area for chemical reactions and to deliver high surge currents). In contrast, deep cycle batteries designed for solar applications have larger, thicker and more robust electrodes (for a longer life and a deeper depth of discharge). Automotive batteries are not generally suitable and are not advised for use in solar PV storage applications.
Due to their ready availability and low price, "deep cycle" lead-acid batteries currently remain the predominant choice in most off-grid PV systems.
How much power can I use?
The size/capability of battery storage systems will limit what can be run during a power cut, e.g. 1kW would run a 100W light bulb for 10 hours.
What is "Deep Cycle"?
Deep Cycle describes a battery that is designed to be regularly deeply discharged using much of its capacity.
What are Lithium-ion batteries?
Due to the rapidly growing electrical vehicle market, lithium-ion batteries have climbed to the point where they are increasingly the battery of choice for domestic or commercial grid-connected solar storage applications.
The advantages of lithium-ion over traditional lead-acid batteries include:
- - Better energy density
- - Improved efficiency
- - Improved depth of discharge (DOD)
- - Low self-discharge
- - Increased lifetime (better cycle life in deep-discharge applications)
- - Low Maintenance
There are a few disadvantages like the increased cost and other safety considerations.
Safety considerations for lithium-ion batteries include the potential for thermal runaway. Protective circuits are built into lithium-ion batteries to protect against the risk of thermal runaway.
What is Depth of Discharge?
DOD describes how fully a battery has been discharged during a discharge cycle. It is expressed as a percentage of battery capacity, for example, 60%. A discharge of around 80% represents a "deep cycle" operation.
What is nominal capacity?
This is provided by the manufacturer and describes how much energy the battery can nominally deliver from fully charged under a certain set of conditions. Battery capacity is normally described in Amp-hour at a particular discharge current, for example, 500Ah at a C100 rate. In general, the quicker the battery is discharged, the smaller the capacity.
What is Effective capacity?
Effective capacity is often used to describe the usable capacity of the battery. The effective capacity is less than the nominal (nameplate) capacity. For example, a battery with a 500Ah capacity on a system programmed to limit Depth of Discharge (DOD) to 60%, the effective capacity is 500 x 0.6 = 300Ah. Make sure it is the effective battery capacity you are comparing.
What is System (charge-discharge) efficiency?
All batteries are subject to losses during the charge, storage and discharge cycle. The charge-discharge efficiency describes how effective a battery is throughout the full cycle.
Ask us for an estimate of the charge-discharge efficiency for your system so you can have a good idea of how much energy you may lose.
System efficiency = power out /power in
What is Self-discharge?
Self-discharge describes a normal characteristic of all batteries to lose charge over time gradually. The degree of self-discharge varies with battery type, age and temperature. A typical lead-acid battery can expect to lose around 5% over a month.
What is stage charging?
A typical full battery charge consists of three separate stages. Stage 1 is Bulk, then Absorption, and the final stage is Float
What is Opportunity Charging?
Opportunity charging describes a charging regime that occurs whenever power is available, such as from the intermittent output of a solar PV system. Long-term, repeated incomplete recharge cycles can have a significant detrimental impact on battery lifetime. Lithium-ion batteries are more tolerant to intermittent partial charging patterns: they respond relatively well to partial charging and do not need a frequent full charge in the same way that most lead-acid batteries do.
Where the battery capacity is relatively small compared to the PV array, there is a higher chance that sufficient solar charge current can perform a complete recharge cycle.
What is mains (grid) charging?
Repeated poor recharge can impact on battery lifetime; therefore most systems will on occasion, draw power from the grid to complete the charge cycle. It is important to determine to what extent the mains (grid) may be used to recharge your battery pack. Ask us for an annual estimate.
For grid-connected PV systems, there is not the same imperative to have a large PV array concerning the battery because:
- The mains (grid) can be used to complete the charge cycle
- The mains (grid) can be used instead of the battery at times of low solar input
What is discharge control?
As well as needing control of the charging phase, control over discharge is also required. Discharge control functions typically include:
- Ensuring discharge is halted at a set battery
- Limiting the rate of discharge (discharge current)
- Providing temperature feedback (adjusting discharge profile to battery cell temperature)
The discharge control system may also control and restrict when discharge can occur. Examples include:
- Restricting discharge to certain time windows during the day
- Preventing discharge until certain recharge/battery voltage thresholds have been reached
What is a DC charge controller?
A DC charge controller is a direct interface between a DC source (such as a solar PV array) and the battery. It converts the variable DC input voltage coming from your solar PV array to the precise required DC charge voltage for the battery.
What is an AC charger?
An AC charger is designed to provide a controlled DC charge sequence from an AC source - such as from the mains (grid) or a generator.
