Existing solar PV customers are expected to be the biggest market for the new wave of home battery products – but designing and installing a retrofitted storage system comes with a host of potential challenges.

If the predictions about the growth of the storage market are to be believed, retrofitting home battery systems will soon become a major growth area for solar installers. However, adding an energy storage system to an existing PV system is no simple matter. There’s a huge range of products entering the market and an endless array of potential combinations and configurations, plus it’s a given that every customer’s needs and expectations will be different. Here we look at some of the main design and installation issues to consider when approaching a battery retrofit.

DESIGN ISSUES

Knowing what the customer wants

Listening to the customer and asking the right questions seems such a basic thing, yet it’s something we can all forget, especially when we’re keen to expound the latest functions and virtues of energy storage. Seven years ago, when Selectronic’s SP PRO inverter charger was first released, the most common reason for on-grid storage was power security. Certain customers required backup power if the grid went down, whether because they were using sensitive equipment or they were in a storm-prone region, or simply because the grid was unreliable in their area.

When designing a system with UPS functionality, key design issues include allowing for the maximum load support (kW) along with the appropriate energy storage (kWh) and selecting which loads are to be backed up. A smarter backup system can also have discretionary loads that are only backed up while the battery state of charge (SoC) remains above a defined level. For longer grid outages, integrated generator control can start a generator when the SoC drops below a certain level.

Recently, there has been a growing consumer demand for self-consumption of renewable energy sources. A desire to utilise functions such as time shifting, peak demand, arbitrage and energy shifting have become the most common drivers for storage. When multiple benefits are incorporated in the one installation, the economic returns can be good. It’s not all about the dollar return; however, as people can have emotive reasons, be it a desire for independence and control of their own power, or even the option to disconnect completely from the grid.

Customer load profiles

With the rollout of smart meters in some states, customers are now able to gain greater access to their load profile. When looking at storage solutions, it’s important to use this and not just a published average. Comparing the load profile to the typical solar curve is key to sizing the system correctly. If the load profile is not available, a reasonable estimate can be made by finding out what size loads occur, when and for how long. Careful consideration should also be given to any planned changes to loads – including new appliances or perhaps

The right battery for the right application

What is the best battery for an on-grid storage system? The fact is, each type of battery technology has its strengths and weaknesses. Key characteristics of a battery include:

  • usable capacity (kWh
  • peak and continuous output (kW
  • maximum charge rate (kW
  • return loop efficiency (percentage
  • specific energy (kWh/kg
  • specific power (kW/kg
  • design life (years
  • number of cycles
  • safety
  • operating temperature range

 

Bear in mind that not all lithium batteries are the same. Not only do the different cell chemistries vary in their characteristics, but the quality of the cell can vary greatly, as can the capacity and quality of the battery management system (BMS). The cycle life vs design life must also be considered, as the wrong battery could run out of cycles long before the design life of the battery and vice versa. Where an installation requires peak lopping, a high peak power and high charge rate are crucial, along with a high cycle life. Return loop efficiency at these high rates is also important.

Reading the specs

The rating and model numbers for the inverter charger and batteries can be confusing – perhaps deliberately so at times. For example, an SP PRO model SPMC482 is rated at 7.5 kW continuous, meaning it can run at that rate 24/7. Other manufacturers may use a 30-minute rating in the model number, so you would need to dig into the specifications to find out the continuous rating. The 30-minute rating of the SPMC482 is 11.25 kW.

The other confusion in rating the inverter can be in the units chosen – kVA instead of kW. You will again need to look through the specs to find the kW output, which invariably will be less than the kVA.

The other challenge lies with battery specifications. The manufacturer’s specifications can be hard to obtain or light on detail. Make sure you find out what the maximum charge and discharge rates are with lithium batteries, as these can sometimes be quite low. Normal valve-regulated lead-acid (VRLA) batteries usually specify the number of cycles vs the depth of discharge (DoD). The IEC 896-2 standard allows for testing VRLA down to 40 per cent SoC (usable), meaning their DoD figures start at 40 per cent SoC. This can seem to overstate the capacity of the batteries, as the specified 100 per cent DoD is in fact only 60 per cent, and 80 per cent is actually 48 per cent, and so on.

Check carefully what equipment is included and integrated. If features like the contactor, AC meter and generator controller are all integrated, not only will you save on having to purchase additional parts but the installation will also be much simpler and more reliable.

When adding additional solar capacity, make sure you are not exceeding any imposed export limits. If more solar is required than what the export limits allow, then extra equipment may be required to keep the solar to the allowable limit. If the inverter charger can set the export limit, this may or may not be acceptable to the utility depending on whether they require the limit to be defined in the inverter model.

INSTALLATION ISSUES

DC wiring

When retrofitting batteries, DC power adds a whole new set of challenges. It’s really important that you check the installation of DC terminations, battery links and fuses. Be sure to use the correct cable sizes and battery fuses. Undersized cabling can result in unacceptable power losses and voltage drop, premature failure and thermal issues. Using pre-charge cabling between the battery bank and the inverter allows these losses to be measured. Any issues with the DC side will be reported by a good-quality inverter charger.

Even new battery cells can fail, so you need to measure across all cabling and under different load/charging conditions.

There was an example of a call to our support department where the installer couldn’t track the cause of a DC fault. Finally they found that one of the DC cables had been nicked by a tradesperson drilling into the wall.

Battery monitoring

Lithium batteries usually come with their own BMS, which usually allows for cell-level monitoring. Lead-acid batteries can be monitored using both pre-charge cabling <br/>and temperature sensor. This will allow for mid-point monitoring – indicating whether the battery bank voltage is different between the two halves and equalisation is required. Temperature compensation also allows the charge voltage to be adjusted based on the battery manufacturer’s requirements. Make sure the temperature sensor is located around the middle of the bank and mid-cell to give a representative reading.

AC-coupled solar

AC coupling, where the grid-tie inverter is connected to the load side of the inverter charger, is now common. DC coupling is still used, but less frequently. One mistake that can be made with AC-coupled solar is to install it on the AC source side. This means that when the grid fails, the solar generation also stops. This may be a legitimate design in some circumstances.

Commissioning checks

Any commissioning checklists should be followed completely to avoid the end user having to “˜test’ the system after you have left. Running the loads up and down, pulling the power to ensure a smooth transition to island mode and checking any voltage variations on the load and source is essential. You don’t want to wait for the first blackout to reveal any issues. Avoiding a return visit not only saves money but also produces a satisfied customer.

Future

If the needs of the customer change down the track, then being able to change parameters of the system to accommodate future requirements is important. Future developments of the product should also be upgradeable through firmware updates. If the customer has different load/generation requirements at different times of year, these should be programmed to automatically change during those periods. Peace of mind for both the customer and the installer means not only having the right products but also the right company to support them.

Keep in mind that the liability for any warranties starts with the installer. Risk management 101 would emphasise the importance for due diligence on any supplier.