Primary Battery Design Considerations (1)
Do not forget the humble primary battery design!
What is a Primary Battery?
Primary, or ‘non-rechargeable’ are batteries where the reaction is one-way – once used they are not useable for anything else. There was a time when most consumer batteries were of this type, although for some applications they were quite large (the radio batteries from the 1950’s & 60’s for example.
Nowadays, many of the consumer applications are addressed with rechargeable batteries, even my portable radio has a LiION battery tucked away inside it. So in this day and age, where does the primary battery fit in?
Type of Application
Domestically, and most importantly, the smoke detector / alarm is probably the item in the house that typifies the role primary batteries are used in. Independent of any other power circuit, designed to operate for a fixed period of time without maintenance, low current drain and highly reliable. Primary batteries tend to have excellent energy densities but often poor specific power. The challenge for the battery is when it is asked to provide the higher current drain for the audible alarm. In the alarm, after many months use, the battery is asked to provide a high power peak when it is at its weakest. This is why the regular functional alarm check and battery replacement in line with the device manufacturers instructions are vital.
The electrical capacity provided by a primary battery is highly dependent on the rate at which it is being discharged. The higher the current, the lower the capacity. This often means that batteries are built with multiple cells in series and/or parallel to increase the voltage and share the current. Although there is no problem in doing this, there is a point at which additional components need to be introduced to keep the battery safe and functioning correctly. Proper design of these features is paramount with many of todays hi-tech lithium batteries. Matching battery type to the application is a key activity in product development, make sure you get it right!