Smart batteries and medical devices

Manufacturers in the medtech market are well aware of the issues associated with managing batteries in medical equipment. Medical engineering departments within hospitals often cite batteries as one of their top maintenance headaches.

Batteries can be viewed as living beings because their environment and treatment directly affect their performance. Overcharged batteries or ones operated at elevated temperatures often exhibit shortened lives, while those that are treated carefully can provide many years of maintenance-free performance.

In medical devices, the importance of battery management cannot be underestimated if their performance in products such as defibrillators, ventilators, and remote patient monitors is to be maximized. Smart battery technology can radically improve the way that medical equipment provides runtime information and can maximize battery life through careful management.

In order to charge batteries appropriately, it is necessary to understand the current charge state. This has to be balanced with extracting the maximum discharge and lifetime from the battery to offer value-for-money.

Appropriate battery specification is therefore of paramount importance to medical device designers. However, one type of battery solution – smart batteries, which have been on the market for a while now – can radically simplify the process of battery specification while dramatically reducing risk.
 

What are smart batteries, and how do they work?

Smart batteries are intended to be integrated into medical devices as part of a wider smart power management system. This typically includes a smart battery, a smart charger, and a systems management bus (SMBus) for communicating between the different elements.

In a traditional medical device setup, batteries are simply dumb, chemical power cells. Battery metering, assessment of remaining capacity, and decisions taken regarding power usage, are informed entirely by the readings taken by the host device. Such readings are largely guesswork, typically based on the amount of voltage passing from the battery through the host device, or (less accurately) through readings taken with a Coulomb counter in the host.

In a smart power management system, however, the battery is able to tell the host, with a high degree of accuracy, how much power it has left, and how it should to be charged.

The battery, smart charger, and the host device communicate with each other to maximize product safety, efficiency, and performance. For example, smart batteries only request charge when they require it, rather than placing a constant, steady drain on the host system, meaning they charge more efficiently.

A smart battery can also maximize the runtime per discharge cycle by telling its host device when to shut down – based on its own assessment of its remaining capacity. This method is vastly superior to dumb systems that use a fixed voltage cut-off.

Host medical systems that use smart battery technology can provide accurate, meaningful runtime information to users, which is obviously important in certain key medical applications where power failure is not an option.
 

Gauging and adaptation

Smart batteries constantly track their own capacity whether they are being charged, discharged, or stored. Battery capacity is reported in milli-ampere hours (mAh) to a resolution of 1mAh. Real capacity is reported in both mAh and as a percentage of the original design capacity and of the last time the battery was charged.

Certain correction factors are employed by the battery gauge to adjust for various changes in temperature, charge rate, and discharge rate. Smart battery gauges also tend to be adaptive, modifying the adjustments they make as the battery ages and its chemical properties and ability to hold charge decine.

As a result, smart batteries can usually predict their capacity to within ±1%, a major advantage when compared to the ±20% accuracy found in products employing dumb batteries.

Smart batteries can also extend their useful lives by modifying their charging algorithm based on changing conditions – batteries can be damaged if they are charged while they are very cold or very warm. Smart batteries will reduce the charge current while the battery is warm to reduce the potential for damage and prevent charging altogether if the battery is exceptionally cold or hot.

Future proofing Being system management bus (SMBus) and smart battery data specification (SBDS) compliant means that smart batteries comply with an open standard that is easily accessible by OEM device developers.

The smart battery system (SBS) specification, and the accompanying SMBus spec, were created by Duracell and Intel in 1994 and involve carrying battery information across a two-wire communication bus. Along with the smart battery charger specification (SBCS) and the smart battery system manager specification (SBSMC), the SBS standard describes all of the information that can be communicated between smart batteries, chargers, and host devices.

Because of its ongoing communication of status, charge profile, etc., a smart power system is able to future proof a product’s power supply. Should newer battery technologies be developed with higher storage capacities and different charging regimes, new batteries can be swapped out without having to replace the chargers in the field. The existing chargers will simply receive different charging instructions from the batteries and adapt accordingly.

Frequently, smart power systems are more economical in the long run, particularly in the case of expensive medical equipment for which maximized battery life is crucial – such as anesthesia workstations, ventilators, or remote patient monitors.
 

Easy integration, powering ahead

Many customers tend to imagine they’ll need to do a lot to make batteries work in their system. However, the reality is that electronic component manufacturers such as Texas Instruments and Linear Technology have produced excellent and largely reliable, highly integrated reference designs for their smart charger and power management ICs. These reference designs make the integration of a smart battery easy. In addition, the openness of the various communications protocols make it easier for software engineers to extract useful information such as remaining capacity, runtime to empty, and cycle life.

Despite coming with additional costs, smart batteries and charging systems tick the right boxes for many medical devices, with clear advantages in terms of life, charge, safety, and future proofing.

The market for smart charging systems is fairly mature. However, the option of implementing smart charging systems is often overlooked by designers. In addition, the battery life and level of adaptability that designers can expect to afford from specific smart power technologies are always changing.

Because of this, designers should engage with a credible third-party battery specialist to ensure that they specify the most appropriate smart charging solution for their device.

Accutronics
www.accutronics.co.uk

About the author: Rob Phillips is a managing director with Accutronics and can be reached at sales@accutronics.co.uk or 44.1782.566622.