COVER STORY: HYBRID BATTERY COMBINATIONS The battery is behaving as we expected it to,” Stevenson says. The company installed its second lead acid-lithium ion battery in 2022 at its plant in Ebbw Vale, Wales, where it forms part of a microgrid that comprises a rooftop solar PV array. The battery is used for peak shaving and is able to store solar generated during the day to use later. At the Royal Mint’s Energy Centre in Llantrisant, Wales, a 200kW dual chemistry battery has been in operation since July this year. The lithium ion component’s capacity is 150kWh and the lead acid’s is 500kWh. Battery Energy’s lead gel-LFP combined battery is aimed at cost optimized energy storage systems, where the LFP services short duration power peaks and lead gel provides long duration reserve energy. “This is ideally suited to ad hoc demands necessitating both short and long-duration energy storage,” says Glenn. One application he talks about is telecom battery storage systems capable of also managing grid energy demands. “Telecom batteries provide reserve power in the event of an outage. In aggregate, many gigawatt-hours’ of lead acid telecom batteries are installed globally and so these could be
“Energy transferred from lithium to lead via a circulating current is effective at converting hard sulfate crystals and maintaining capacity, extending lead battery life.” — Michael Glenn, Battery Energy Power Solutions 54 • Batteries International • Autumn 2023
used to manage grid energy demands and be paid for doing so. “In this case, the dual chemistry provides a solution. Energy transferred from lithium to lead via a circulating current is effective at converting hard sulphate crystals and maintaining capacfy, extending lead battery life,” Glenn says. Technical considerations and challenges “There have been projects in the past and these were described as hybrids because they used two different batteries or storage devices in a system. But they were essentially separate because they were connected only on the AC side,” says Stevenson. GS Yuasa’s battery connects both the lead acid and the lithium ion batteries DC-to-DC, requiring only one power conversion system instead of two. Stevenson says: “When we developed the concept we had to be aware of two main issues or challenges when connecting terminals of lithium and lead acid batteries. One was around voltage and the other around current. “You can’t connect at the individual cell level because of the different voltages. Instead, you have strings of cells of lead acid and lithium ion so you can establish an overlap between the two, so that you can select the right number of lithium ion and lead acid cells with the same voltage.” The two different battery chemistries will accept, or give, current at two different levels, so GS Yuasa had to size the batteries to avoid over-current during charging and discharging. “Because GS-Yuasa’s LIM series of lithium ion batteries can operate at much higher currents than typical lithium ion storage cells — 6C versus 1C — it is possible to store most energy in low cost lead acid cells with a small lithium ion capacity providing the operational flexibility of dual chemistry systems,” Stevenson says. In Battery Energy’s approach, circulating currents between the two chemistries was an important technical consideration. “This is where one chemistry charges or discharges into the other at rest. Fortunately, we found that the energy transfer from lead to lithium is minimal at the bottom-of-charge. “This ensures that the lead acid chemistry is not over-discharged. It is also critical to set the nominal voltage of both chemistry strings, this requires connecting the correct number of cells in series,” says Glenn.
The Future Electric Vehicle Energy networks supporting Renewables (FEVER) project, will decouple EV charging demand from the UK electricity grid, by enabling 100% renewable energy to be used through a hybrid energy storage system, located at the FEVER charging station — Andrew Cruden, Southampton University Both battery chemistries in Battery Energy’s solution charge from a common bus voltage, hence, no electronic control is required, for example a DCto-DC converter. The company has also adapted the charge protocol from a lead acid remote area power system. The LFP chemistry’s battery management system required for protection did not require any bespoke modifications to facilitate dual chemistry operation. Commercial progress Having obtained valuable insights from both laboratory testing and field trials Battery Energy is working with clients to develop commercial lead gelLFP products based upon their specific requirements. Rollout is scheduled to start from the first quarter of 2024. Dual chemistry batteries are not applicable for more standard or straightforward applications. GS Yuasa works with customers to identify which type www.batteriesinternational.com