Integrating Battery Storage With Solar PV

When designing a battery storage system to work with a solar PV system, there are a number of parameters that must be considered with particular regard to maintaining the health of batteries in very varied conditions. The batteries are the most significant component in terms of cost and they need looking after to ensure they will perform well and for a long time. 

That is particularly challenging in a PV environment and some of the main considerations are discussed below.

Battery conditioning - overview

Batteries like to be charged to their near full capacity and then discharged to a ‘usable level’ (depth of discharge or DOD) to ensure a prolonged life. Solar energy varies considerably during the day and during the year and so it is not guaranteed that the batteries will reach their desired capacity under all conditions. 

Some systems will provide a little ‘top up’ charge from the grid if solar contribution has not been good and the batteries are running at a low level. Furthermore, some systems incorporate a ‘winter mode’ operation to shut the battery system off for a couple of months or so whilst maintaining a trickle charge to preserve battery health.

Sizing the battery system

Another important consideration is sizing the battery storage system according to the available surplus energy. Domestic PV systems typically range from 1kWp to 4kWp generating anything from 0kWh to 25kWh of energy in a day. Household energy usage also varies considerably and this needs to be factored into the design.

If the battery system is undersized, you will still be exporting energy to the grid once the batteries are charged and so not maximising your ‘self-consumption’ benefit. However, an oversized battery storage system will suffer if the solar energy is insufficient to charge the batteries or the house load is very high thus yielding little or no ‘surplus’ energy.

It is imperative to weigh up the amount of solar energy contribution against the load profile of the household when designing a suitably sized battery storage system.

Battery charge cycle

Battery charging is managed by a battery charge controller which is sometimes a standalone unit or maybe integrated within an inverter. Batteries like to be charged rapidly to start with from a constant current provided from the charge controller (bulk charge phase). Once they reach a certain threshold, they switch to constant voltage (rather than constant current) again provided by the charge controller. When fully charged, the charge controller will hold the batteries at a steady state. 

As mentioned in 4.2, due to varying solar energy, it is not guaranteed that the solar energy will be sufficient to provide the required charging energy, particularly during its ‘bulk’ charge phase. Batteries therefore tend not operate under their optimum or desired conditions in a solar PV environment.

Battery discharge cycle

Batteries have a capacity rating measured in Ah (Amp hours) which can be converted to Wh (Watt hours) or more usefully, kWh (1,000Wh) which equates to the units of energy we buy from the grid and earn from the feed-in-tariffs. However, to qualify the amount of usable energy, battery manufacturers will quote the capacity according to the ‘rate’ at which the batteries are discharged. It is important to know that exceeding this rate (known as C-Rate) can considerably reduce the usable capacity of a battery and impact on its life expectancy. 

From a practical point of view, this means ensuring that you do not expect to run numerous appliances (oven, computers, hair dryer) all at once and expect the batteries to provide all of the required energy, in trying to do so, the C-Rating of the battery will be compromised and you’ll have a very inefficient system. Better design is to limit the amount of energy taken from the batteries and take some grid contribution during peak loads. This is a consideration in the choice of battery inverter which will be sized accordingly and is typically smaller than the PV inverter.

Charge/discharge (round trip) efficiency

Batteries, charge controllers and inverters all incur losses. The amount of energy retained from a ‘charge, store & discharge’ cycle is referred to as the charge/discharge efficiency or ‘round trip’ efficiency. So a Lithium-ion system rated at 5kWh and 90% efficiency will only deliver 4.5kWh of usable energy taking account these system losses.

Depth of discharge

The depth of discharge (DOD) describes how much energy may be taken out (discharged) from the batteries and this becomes the usable capacity. There is a considerable difference in DOD in Lead-acid (50%) and Lithium-ion (90%). So a 5kWh total capacity provides only 2.5kWh usable energy from a lead-acid battery compared to 4.5kWh from Lithium-ion. It is imperative to know what you’re buying when battery capacities are specified.