A no-brainer or a huge headache?

Battery based solar power systems have been entrenched in the residential market as the most popular form of solar power system. Battery energy storage is a vital component in residential solar power systems, primarily because of the ability to store energy and supply power in the event of a Grid-power failure, but also where prevailing utility regulations do not support or incentivise exporting excess solar energy to the Grid. In the latter circumstance it is advantageous to leverage energy storage (battery) to store excess solar energy and discharge it later (solar self-consumption) and subsequently improving the performance of your investment.

The debate of lead-acid versus “lithium” technology batteries has long been put to bed, with lithium, and specifically lithium-iron phosphate (LiFePO4), arising not only as the superior battery technology but also as the better value proposition, especially in cases where improving solar self-consumption is necessary. This is supported by the fact that many leading lithium battery products are accompanied by an extensive warranty, often up to 10-years and/or several thousand cycles of the battery, which ultimately presents a better value proposition (Rand per warranted kilowatt-hour).

From a consumer’s perspective, the market appears to be flooded with a variety of different lithium battery products and with seemingly comparable warranties and specifications. So, it begs the question:
How do you choose a lithium battery for your power system?

Is the age-old adage true in that you get what you pay for, or are the differences indiscernible and you should be hunting around for the lowest price product?

GreenHouse has enjoyed a rich history in the field of lithium battery storage products and has enjoyed the privilege of experimenting with and evaluating a great variety of lithium battery products that are available in South Africa, including products from Pylontech, Freedom Won, BlueNova, SolarMD, Revov, BSL and even the Tesla Powerwall. While this does not constitute the entire range of “solar” lithium battery products in South Africa, it does represent some of the most popular manufacturers in the market and we’ve had the opportunity to assess these products, their data sheets and supporting warranty documentation.

As an authorised Victron Energy distributor and experienced installer, we have an appropriately setup and configured testing laboratory allowing us to comprehensively test and evaluate various lithium battery products using Victron Energy’s hardware and monitoring platform to provide high-resolution, accurate and reliable data.

Lithium-Battery Compatible
First and foremost, it is vital that your chosen inverter (and associated equipment) is lithium-battery compatible. Compatibility with lithium battery products is primarily dependent on communication between the Inverter and the battery. The communication protocol can take many forms which includes Can Bus, RS485, RS232 and USB-Serial, but is most common and widely supported in CAN Bus format. Communication between the Inverter and the battery allows the system to establish a Master-Slave relationship where the battery takes the lead as the master component and the Inverter obeys these commands as the Slave component. This relationship configuration is incredibly important because it allows the battery, and more specifically the battery management system (BMS), to enforce limitation(s) on the inverter and to subsequently protect itself (the battery) against harmful conditions.

Harmful conditions include out of range conditions like pack voltage, pack current, cell voltage, temperature, and state of charge. Ensuring that these parameters remain within their approved ranges is one of the fundamental reasons that lithium battery products can boast extensive warranties (e.g., 10-years).

Where inverter-battery communication is established, three primary setpoints are communicated: Charge Voltage Limit (CVL), Charge Current Limit (CCL) and a Discharge Current Limit (DCL). The regulation of these setpoints allows the BMS to effectively control the inverter. To provide an example of this control, when the BMS produces a CCL=0 command, then the inverter is instructed to stop charging (entirely) and equally when the BMS produces a DCL=0 command the inverter is instructed to stop drawing power from the battery bank. Most lithium battery products communicate a static CVL value (maximum charge voltage), and this prevents inappropriate or misconfigured inverter settings from overcharging the battery. Some battery products are taking this a step further and are deploying a dynamic maximum charge voltage control, allowing more refined control of the battery charge process.

In the absence of inverter-battery communication the BMS will be required to protect itself (and the battery) using, typically, a built-in electrical contactor to disconnect itself from the inverter. This is a crude mechanism and repeated operation/occurrence may cause damage to the connected equipment.

Discharge Capability
Our second point of recommendation is the evaluation of discharge capability. It is important that the lithium battery product can sustain the rated discharge capability of the inverter. In essence this would mean that a 5kW Inverter would require a lithium battery capable of delivering 5kW’s of constant power, however it is important to build in a bit of “headroom” into the consideration because the conversion of DC power to AC power is not 100% efficient. To provide an example of this, a particular inverter is delivering 3000W to a load – and because this inverter is up to 96% efficient the inverter will be drawing approximately 3125W from the battery to support this load, and the conversion requirements. The Pylontech range of batteries, an incredibly popular battery, are quite conservative in their rated discharge capability, offering just C/2 (half of their rated energy capacity) as a continuous discharge power rating. On the other end of the spectrum the Freedom Won Lite Home product boasts continuous discharge power ratings of 1C.

While C1 discharge capabilities would in theory allow you to pair an inverter to a battery at a 1:1 ratio (e.g., 5kW Inverter and a 5kWh battery), the battery may not be able to sustain that C1 discharge rate or provide that discharge capability at low battery state-of-charge (SOC). This is because battery voltage reduces as battery energy is depleted, and the maximum continuous discharge capability of the battery is expressed and regulated as a maximum current. The product of this maximum current value and the reducing battery voltage results in a proportionately reducing delivery of battery discharge power, at maximum output.

It is a common ‘rule of thumb’ practice to size the inverter to battery ratio at a minimum of a 1:2 ratio. We suspect that this practice was borne out of the early days where Pylontech lithium battery products were one of the first, and only, lithium battery products available on the market – and subsequently imposing their products C/2 maximum continuous discharge rating. Nonetheless, this rule of thumb is still an appropriate and safe guideline to be applying to inverter-battery sizing considerations today. This section serves to highlight one of the most significant and qualitative differences that may be present in competing lithium battery products.

Depth of Discharge (DoD)
Lithium battery products support different degrees of Depth of Discharge (DoD) and which may differ under different circumstances. We have identified three categories describing the available energy (expressed as Ah or kWh) from the battery and subsequently this defines the warranted depth of discharge (DoD) under various circumstances. It is important to note that these three category specifications are not always clearly defined by each battery product or using equivalent terminology. The first category is the nominal energy of the battery, this describes the raw amount of energy capacity packaged into the lithium battery product. The second category is the usable energy capacity, and this is the maximum amount of energy capacity that can be extracted from the battery. The third category defines the maximum depth of discharge that is approved for daily cycling – expected in a solar self-consumption application.

Many of the latest model lithium battery products are being advertised with improved depth of discharge ratings. A few years ago, 70% and 80% maximum DoD was the norm, but now products like the Pylontech US2000C & US3000C are supporting 95% DoD and even some of the products from BlueNova, Revov and Solar MD are advertised supporting 100% DoD. A greater depth of discharge affords the user more available energy relative to the nominal energy rating, and this characteristic of lithium battery products is one of the key advantages when comparing lithium battery technology to the old lead acid technology. It is important to familiarise yourself with the battery’s approved depth of discharge rating so that you can evaluate the real energy that is made available from the battery, rather than assessing based on nominal energy values.

Minimum Capacity
Onto the next and final aspect for consideration, Minimum Capacity, which is in our opinion the most over-looked and under-valued specification of a lithium battery product. Minimum Capacity refers to the minimum amount of energy that is available from the battery product at the end of its warranty period. This parameter is an important clause in the warranty document as it affectively defines the retention of energy by the product. As the consumer you want to experience the least amount of energy degradation over the lifespan of the product and this specification, Minimum Capacity, is the fundamental differentiator between products with equivalent warranty periods. While many of the lithium battery products commit to a Minimum Capacity in the range of 60% – 80% at the end of the warranty period, it is often not as clear-cut as that.

Several lithium battery products contain a composite warranty period (e.g., 7-years + 3-years upon registration) where the warranty specification is defined at these milestones (e.g., at year-7 and then again at year-10). This complexity has resulted in some ambiguity in the effective warranty specifications when comparing different battery products. We have come across a particularly popular lithium battery product, where the warranty commits to a Minimum Capacity of 60% after 7-years, and then a Minimum Capacity of 70% of the value after 7-years, after 10-years. Ultimately the interpretation is that the warranty commits to a Minimum Capacity of 42% after 10-years – far less than some of the conventional warranties committing to a Minimum Capacity of 60%-70% after 10-years.

It is particularly noteworthy that none of the product warranty documents, that we have evaluated, define Minimum Capacity for an intermediate-year claim. What this means is that capacity loss which is greater than the expected linear trend towards the Minimum Capacity value, may not be covered under the protection of the warranty document. While the rate of energy degradation is expected to be linear, most warranty documents do not provide explicit cover for non-linear energy degradation while the available capacity remains above the Minimum Capacity value.

Conclusion
Lithium battery products offer tremendous value as an energy storage solution coupled to solar power systems. Their battery management systems afford the product revolutionary protection against harmful conditions and are fundamental to the provision of extensive warranty periods, typically 10-years. Manufacturer and product variations have increased greatly over the last decade, and so the difficulty in selection of an appropriate lithium battery product has increased. Through the discussion of the pertinent factors that differentiate lithium battery products it is clear that not all batteries are made equal. Misleading product advertising leads to premature conclusions being drawn in the comparison of lithium battery products, and it is only through careful evaluation of the product datasheet and, most importantly, the associated warranty document that we are able to fairly compare the products. Solar power coupled to battery energy storage is a considerable investment where much of the system’s performance is hinged upon the reliability and performance of the battery product and it is for this reason that we recommend you exercise great discernment in the selection of a lithium battery product for your power system.