Rechargeable batteries have become a dominant power source in our everyday lives. Indeed, batteries have moved way beyond the laptop PC and mobile phone to power garden tools and even transit buses and industrial cranes. Batteries can bring many benefits to a system design — think convenience in tools or fuel efficiency in hybrid cars.
So it's important to make sure you at least contemplate the role a battery might play in your next project, even if the application isn't typically battery powered. The array of different chemistries and packages offered by battery makers today just might allow you to add a product differentiator in the form of convenience, low noise, energy cost or environmental impact.
Battery Apps Go Green
To get an idea of the breadth of battery applications today, consider a couple of unusual examples. Battery maker Gold Peak Industries has provided battery subsystems for more than 100 rubber-tired gantry cranes installed at the Port of Hong Kong, according to business development manager Patrick Huberty. The batteries serve in a hybrid-drive system to reduce fuel use and the associated emissions of the typically diesel-powered cranes. The cranes charge the battery pack during the down-cycle of a hoist operation using regenerative braking similar to the way hybrid autos operate.
At the other end of the spectrum, consider a surging application for Li-ion (lithium ion) batteries in the computing space — and we're not talking laptop PCs. Rather, engineers are designing mission-critical servers that use batteries to directly protect volatile semiconductor memory — both main memory and cache — to augment traditional UPS systems. Chris Turner, director of battery technology at Nexergy, claims that the typical server design specifies batteries that can protect critical memory for 72 hours.
Nexergy specializes in making custom battery packs using cells from major battery makers. The sever application requires a pack that can be mounted and charged within the server. Turner estimates that such server applications account for as much as 20 percent of the company's business. Moreover, battery cell makers Panasonic and Gold Peak also identified the server as a growing market for Li-ion cells.
Battery Selection Criteria
Regardless of the application at hand, engineers face a lengthy comparison of technical and logistical characteristics when choosing batteries. Power density and energy density are two key technical characteristics. Energy density is measured in W-hr/kg (watt-hour per kilogram) and generally is a figure of merit in terms of run time from a charge. Devices such as mobile phones and laptops need high energy density. Power density is measured in W/kg (watt per kilogram) and generally is a figure of merit in terms of the maximum power that a battery can instantaneously deliver. Applications such as battery-powered tools generally need maximum power density.
Of course price matters, and the latest lithium-based cells typically have the highest absolute cost. Engineers must also consider cost relative to useful cycle life and power or energy density. Rick Chamberlain, vice president of engineering at Boston Power, states, "The key factor is the sustained capacity."
Boston Power is a startup that is targeting the laptop PC space and has just begun shipping battery packs for select HP laptops. In fact, consumers can buy the packs on the HP website. The company touts its Sonata battery pack as usable for the life of a PC, whereas many users replace their laptop battery packs multiple times over the PC life. Boston Power hasn't revealed much about its secret sauce although Chamberlain states, "We haven't developed a new material." The company claims its cells retain 80 percent or more of the rated capacity for more than 1,000 charges — generally more than three years of use.
The most advanced battery chemistries offer an advantage in power relative to both size and weight, and generally cost escalates with those benefits. View full size
Getting back to the basics, chemistry is one of the first questions engineers face in a battery-based system design. The above graph provides a quick look at a energy density relative to weight on the X axis, and relative to size on the Y axis. Generally speaking, the lower cost options are found in the lower left quadrant and the higher cost options are found in the upper right quadrant. Not surprisingly, more energy relative to size or weight costs more.
Starting at the lower left, the lead acid battery will likely be around a long time for automotive applications. Lead acid has been the battery of choice in UPS applications where size and weight was originally of little concern. But NiMH (nickel-metal hydride) is moving into the UPS role. Gold Peak's Huberty claims the company refitted a telecommunications switching center with NiMH batteries and reduced the footprint of the batteries from a 51- x 2.5-ft row with lead acid to a 15.3- x 2.6-ft row with NiMH. That is significant, as today's data centers are rapidly running out of space.
Indeed most system designs today will use either a Ni or Li chemistry — and it's a fair question to ask if engineers should consider anything other than Li batteries. The battery manufacturers don't like to provide precise numbers for their battery shipments or design wins by chemistry. But off-the-record comments suggest Li batteries make up about 50 percent of all batteries shipped today. In terms of new projects, Li wins 90 percent of the time or more often.
Nexergy's Turner states, "Li-ion is spreading into other (non-laptop and mobile phone) applications. Every market it has entered it has won." He adds that it can take five to six years for Li-ion to displace another chemistry and notes the ongoing transition to Li-ion in the handheld power tool space. The Li-ion tools do carry a heavy price premium and may be overkill for some consumers. "The 36V DeWalt tools have more power than if you had the same tool plugged into the wall," Turner says.
Li-ion offers other advantages, as well. A charged Li-ion battery pack in a tool will stay usable a month a more. A charged pack sitting outside the tool will stay charged a year. Engineers with Ni-based tools at home know that those batteries don't offer such resilience.
Safety and the Environment
Despite the success of Li-ion, the chemistry is far from perfect. It still costs significantly more than Ni-based batteries. And there is the well-publicized fires and explosions attributed to Li-ion-based notebook batteries, leading to safety concerns (See "Environmental and Safety Concerns," below). And Li-ion isn't a completely environmentally friendly choice.
Indeed, there's sufficient opportunity for Ni chemistries that PowerGenix has based its company on a NiZn (nickel-zinc) chemistry. It's tough to call PowerGenix a startup since it has been around almost a decade, but it is a startup in the sense that it now plans to play in the high-volume battery market. The NiZn chemistry has a long history. "Thomas Edison developed NiZn for cars," says Joe Carcone, vice president of business development at PowerGenix.
Carcone claims that NiZn batteries offer a 30 percent weight and size advantage relative to NiMH, as well as lower internal resistance and better performance at low temperatures. But NiZn has been long associated with dendrite formation and shape change of the zinc electrode. The former leads to cell failure while the latter degrades performance over the life of a cell. PowerGenix claims to have solved the problem with a new electrolyte formulation and new electrode composition.
PowerGenix is now producing NiZn cells in both the AA- and D-cell sizes that are common to primary or disposable batteries. The company has signed a deal to supply battery packs based on the D cell to electric scooter maker Veloteq. Two 15-cell packs will replace three lead-acid batteries at equivalent performance but half the size and weight. Carcone admits the NiZn packs triple the battery cost but points to a two to three times advantage in cycle life. Relative to Li-ion, Carcone claims the NiZn alternative offers half the cost and an advantage in safety and environmental concerns.
Li Cells and Logistics
Design teams will find that buying Li-ion cells isn't as simple as phoning in an order. For safety's sake, the technology requires precise charging circuits and temperature control. So the cell manufactures carefully vet customers, according to Dennis Malec, senior applications engineer at Panasonic's OEM battery division. In fact, outside of large customers such as laptop or phone makers, most customers buy Panasonic batteries from a roster of 12 prequalified battery-pack makers, including Nexergy. Gold Peak takes similar precautions with Li-ion sales. "With Li-ion we also like to design the charger," says Dane Russell, Gold Peak general sales manager.
Design teams ready to select a rechargeable battery also face the problem of choosing a cell size — and the form factor that's ultimately determined by both the chosen cell and the way the pack is assembled. In rechargeable batteries, you don't have a short list of standard products the way you do in primary batteries.