Software Defined Batteries Anirudh Badam1, Ranveer Chandra1, Jon Dutra1, Anthony Ferrese2, Steve Hodges1, Pan Hu3, Julia Meinershagen1, Thomas Moscibroda1, Bodhi Priyantha1, Evangelia Skiani4 1Microsoft Corporation, 2Tesla Motors, 3University of Massachusetts Amherst, 4Columbia University Abstract Battery Characteristic Units Different battery chemistries perform better on different Energy capacity Joule 3 axes, such as energy density, cost, peak power, recharge Volume mm time, longevity, and efficiency. Mobile system designers are Mass kilogram constrained by existing technology, and are forced to se- Discharge rate watt lect a single chemistry that best meets their diverse needs, Recharge rate watt thereby compromising other desirable features. In this paper, Gravimetric energy density joule / kilogram we present a new hardware-software system, called Software Volumetric energy density joule / liter Defined Battery (SDB), which allows system designers to in- Cost $ / joule tegrate batteries of different chemistries. SDB exposes APIs Discharge power density watt / kilogram to the operating system which control the amount of charge Recharge power density watt / kilogram flowing in and out of each battery, enabling it to dynam- Cycle count Number of dis- ically trade one battery property for another depending on charge/recharge cycles application and/or user needs. Using microbenchmarks from Longevity % of original capacity after our prototype SDB implementation, and through detailed N cycles simulations, we demonstrate that it is possible to combine Internal resistance ohm batteries which individually excel along different axes to de- Efficiency % of energy turned into liver an enhanced collective performance when compared to heat traditional battery packs. Bend radius mm 1. Introduction Table 1. A number of battery characteristics. These are of- ten in tension with each other – for example increasing The utility of a mobile device is often constrained by the ca- recharge rate compromises longevity. pabilities of its battery. Whilst integrated circuit performance has doubled every eighteen months according to Moore’s Such tradeoffs are present even within a given physical law, the same is far from true for battery technology. Bat- battery. For example, energy delivered by a battery in a sin- tery performance can be evaluated in many different ways gle charge-discharge cycle (energy capacity) is inversely re- (see Table1), but no matter which metric we look at, it has lated to the rate at which the battery is drained (discharge taken more than a decade to double performance. rate). This is because the resistance losses inside a battery Furthermore, the various properties of batteries are often are proportional to the square of the current. Similarly, a bat- at odds with each other. For example, batteries with higher tery’s longevity – its ability to perform consistently follow- power densities tend to have lower volumetric and gravimet- ing many charge-discharge cycles – is inversely related to ric energy densities, and vice versa. Similarly, making a con- the discharge and recharge rates. This is because higher cur- formable battery that fits a particular industrial design com- rents speed up the creation of fissures in the electrodes that promises its performance characteristics. reduce the amount of energy a battery can store. In summary, no single battery type can deliver the ever- growing list of requirements of modern devices: fast charg- Permission to make digital or hard copies of all or part of this work for personal or ing, high capacity, low cost, less volume, light in weight, less classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation heating, better longevity, and high peak discharge rates. on the first page. Copyrights for components of this work owned by others than ACM A growing range of battery chemistries are under devel- must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a opment, each of which delivers a different set of benefits fee. Request permissions from [email protected]. in terms of performance. We believe that combining multi- SOSP’15, October 4–7, 2015, Monterey, CA. Copyright c 2015 ACM 978-1-nnnn-nnnn-n/yy/mm. $15.00. ple of these heterogeneous batteries instead of using a single http://dx.doi.org/10.1145/nnnnnnn.nnnnnnn battery chemistry can allow a mobile system to dynamically 1 trade between their capabilities and thereby offer attractive Bendable batteries for long-lived wearable devices: tradeoffs. Bendable batteries are appealing for wearable form factor However, traditional methods of integrating multiple bat- devices. For example, thin, bendable batteries can be in- teries are not suitable for heterogeneous batteries. Simply stalled in the straps of a smart watch to augment a traditional connecting them in series or parallel chains does not provide Li-ion battery in the body, and significantly improve battery enough control over the flow of energy: batteries connected life. However, these batteries are much less efficient than tra- in series can only supply the same amount of current; bat- ditional Li-ion batteries because their rubber-like electrolyte teries connected in parallel must operate at the same voltage increases internal resistance. Using SDB, we develop a pro- and can only supply currents that are inversely proportional totype hybrid battery system using a bendable battery and a to their internal resistances. Li-ion battery, and develop an algorithm that a smart-watch We propose a new system, called Software Defined Bat- OS can use to minimize the inefficiency in such a system tery (SDB), that allows heterogeneous batteries with differ- based on user workload. ent chemistries to be integrated in a mobile system. SDB Supporting high power workloads: SDB enables two consists of hardware and software components. The hard- such scenarios. First, SDB enables a fast charging battery to ware enables fine-grained control of the amount of power be used in combination with a high energy density battery. A passing in and out of each battery using smart switching cir- device can then gain a good percentage of its charge in just cuitry. The charging and discharging hardware is designed to a few minutes, without losing out on total battery capacity be low-cost, and hence the algorithmic complexity of com- or the longevity of the battery back. Second, SDB supports puting how much power to draw from each battery, and how higher turbo modes for the CPU using a high power density to recharge each battery, is placed in the SDB software that battery in combination with a high energy density battery. resides in the operating system (OS). SDB helps the OS decide when to enable higher turbo modes Deciding how much power to draw from each battery, and based on workload requirements, and also to intelligently how to charge each battery is non-trivial. It depends on the manage the batteries. efficiency of each battery under different workloads, the age Battery management for 2-in-1s: In 2-in-1 laptops that of each battery, and also the user’s workload and usage pro- have a detachable keyboard, external battery packs under file. For example, if a high power workload is anticipated in the keyboard are typically used to charge the main internal the future, then it could be worthwhile conserving charge on battery. This however, reduces the efficiency and effective the battery that is more capable of handling such a workload energy capacity because of the losses involved in charging in an efficient manner. one battery from another. With SDB it is possible to reduce The SDB software component that resides in the OS this inefficiency and improve effective energy capacity by up implements a set of policies and APIs. The SDB software to 22%. uses simple APIs to communicate with the SDB hardware. We discuss these in more detail in Section5. The algorithms implemented by this software use various metrics for increasing the single charge-discharge duration 2. Background of the device, and the longevity of the batteries, and thereby decide the ratios in which to discharge each battery, and the Lithium-ion (Li-ion) batteries are commonly used in today’s ratios in which to charge them. We present the details of the battery-powered electronic devices. In this section, we first APIs and policies in Section3. present some basics of Li-ion batteries, and then describe The SDB design is cross-layer and involves new chemistries, how existing systems manage these batteries. additional hardware, and new OS components. Although 2.1 Battery Technologies an alternative SDB implementation can be hardcoded in firmware, our cross layer approach has two main benefits. A Li-ion battery contains a negative electrode (the anode), First, it opens up new battery parameters, previously unavail- which is usually made of carbon in graphite form, and a able to OS designers, for resource optimization. In existing positive electrode (the cathode), which is typically a metal mobile devices, the battery is usually treated as a black box, oxide. A separator ensures physical separation between the and is simply assumed as a reservoir of charge. As we show anode and the cathode to prevent shorting, and the battery is in Section5, OS techniques yield substantial gains in battery filled with an electrolyte composed of a lithium based salt usage. Second, this design allows a system designer to select whose ions can easily pass through the separator. Current any combination of batteries for an optimal design, includ- is discharged when the electrodes are connected externally ing new chemistries as they are invented, and developed. All over a resistive load while positive Lithium ions flow from of these can be enabled through a software update. the anode to the cathode through the electrode and separator.