Batteries for medical devices: Their characteristics and how to choose the right one

July 11, 2018 // By Neil Oliver
The United States is the largest medical device market in the world. Despite this, there are still issues that can make it difficult for design engineers to choose the right battery for their portable medical devices.

The US market for medical devices generates $180 billion in revenue, making it the largest in the world, according to the 2017 Global Market for Medical Devices report by Kalorama Information.

However, in my experience, there are still issues that original equipment manufacturers must address as we increasingly rely on portable, battery-powered devices. Variables such as battery chemistry, discharge profiles and the long-term effects of ageing, as well as charge rate, are all problems that continue to perplex many manufacturers.

 

Battery chemistry

Devices such as ventilators, infusion pumps, dialysis systems and anaesthesia machines have historically been powered by the mains AC supply, relying infrequently on their on-board battery as a backup only. This means that the battery often only receives a shallow discharge before being charged again.

While today's rechargeable batteries do not suffer from the memory effect that was common with old technologies such as nickel-cadmium batteries — where the battery would lose capacity if it wasn't fully discharged before being charged again — modern batteries such as sealed lead acid and valve-regulated lead acid still have a failure mechanism; a gradual rise in their internal resistance.

As the internal resistance increases, any sudden need for power, particularly from motor-driven devices such as dialysis machines and ventilators, can draw a lot of power, leading to a drop in voltage; not an ideal characteristic for a medical battery.

The most common mistake that I've seen equipment manufacturers make when matching a battery to a device is not selecting a battery with an internal resistance appropriate to the load. If the load current is high, or it has high pulses, and the battery has a relatively high internal resistance, then the voltage drop under load can be severe.

This will cause two problems. Firstly, the battery will heat up and waste energy and, secondly, the battery will quickly reach the device's cut-off voltage earlier than desired.

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