BATTERIES | LI-ION V VRLA ALL THINGS CONSIDERED Although lead-acid batteries are long-established, with a majority market share, lithium-ion is starting to pick up pace. Alex Emms, of Kohler Uninterruptible Power, compares the two technologies Bank of VRLA batteries being used to support a KUP PowerWAVE 9000DPA modular UPS system F or uninterruptible power supply (UPS) system builders and users today, two battery chemistries predominate: leadacid typically valve-regulated lead-acid (VRLA) and lithium- ion (Li-ion). While Li-ion has limited presence in the UPS market, it has been growing in popularity in other areas as a result of advances in technology and power output, plus a reduction in cost. Li-ion is finding large-scale use within motive power and electricity grid storage applications and, with its rapid response, is often found in wind and solar renewable energy systems. Li-ion batteries have a better power-toweight ratio than similarly rated VRLA types (see Table 1). They also discharge more efficiently than VRLA at high discharge rates, although this advantage becomes less important at lower rates (see Figure 1). Charging rates from a fully discharged state are also higher, as long as the charger can deliver the required power. Full recharging can be completed in three hours, compared with a typical 80% charge in six hours for VRLA. Another advantage is a very wide usable temperature range, although discharge rates and longevity can normally be optimised by operating at 23C 5K. Li-ion batteries have improved resilience to temperatures outside this range, with much better low-temperature discharge capabilities, than VRLA. This makes Li-ion much better suited to uncontrolled temperature environments where free cooling can be employed using the lower-temperature outside air. However, like VRLA, operating at excessively high temperatures significantly reduces Li-ion batteries useful life. Figure 2 gives more detail on the two chemistries relative temperature/lifetime profiles. Cost is another critical factor. Prices have fallen significantly up to 85% over the past decade, and these reductions naturally increase Liions appeal. Nonetheless, as Table 2 shows, Li-ion pricing is still a barrier. However, we are definitely in the early stages of adoption. While prices arent decreasing as fast as previously, they are still tracking down, creating a significant upturn in adoption. In Europe and the Middle East, there are lags in Li-ion adoption, but there is increasing deployment in North America and Asia. Figure 3 shows historical and projected future trends for battery pack manufacturing costs. Design life is another factor, for which manufacturers are quoting up to 15 years. Operational life is probably nearer 10-12 years, but is not yet proven. This compares with a real-life norm of 7-8 years for VRLA. Why not Li-ion? Li-ion enthusiasts point to the batteries longevity as an advantage offsetting its higher capital cost. However, Kohler Uninterruptible Powers experience shows that UPSs correctly installed in a suitable environment and properly maintained and supported are typically reliable for 15 years. This neatly matches two consecutive 7-8-year VRLA lifetimes, but raises replacement coordination issues with 12-year Li-ion batteries. Li-ion is also disadvantaged by the true costs of achieving suitable autonomy, which is traditionally 10-15 minutes for UPSs. However, 200kW N+1 modular system* Li-ion solution VRLA solution Autonomy WxD xH in mm Weight kg WxD xH in mm Weight kg Seven minutes 650 x 600 x 2,055 550 1,870 x 900 x 2,450 2,700 15 minutes (19 min Li-ion) 1,300 x 600 x 2,055 1,100 2,480 x 800 x 2,000 2,700 30 minutes (35 min Li-ion, 31 min VRLA) 1,950 x 600 x 2,055 1,650 2,490 x 875 x 1,900 4,900 Table 1: Comparison of Li-ion and VRLA battery dimensions and weights * N is the number of UPS units required to meet the design load demand, 1 indicates that a single UPS unit failure will not adversely affect meeting the load 44 September 2019 www.cibsejournal.com CIBSE Sep19 pp44-46 Battery.indd 44 23/08/2019 16:27