Well quite, 4xAAA is a better choice in lots of cases. Just a point that it's still a very common design pattern, particularly in cheap devices. When you look at the capacities, 9V batteries are awful.
Bear in mind that 9V batteries also have far easier interfaces than AAA packs, requiring a single plastic/metal tab rather than an injection moulded retainer. In some cases, the price difference for both manufacture and battery cost makes it worthwhile.
As for current, plenty of devices run under 20mA happily. A microcontroller in active mode will probably use under 15. Most 9V devices aren't used continuously, my IR temp gun uses a single 9V for instance. However I've also seen smoke alarms and other home sensors that use them.
I'm not disagreeing with any of the criticism here, I would never use a 9V batt in a project, but there is good reason to consider it and people should understand why so many products still use them.
Stacked 9V batteries are typically the old Alkaline sort or rechargable.
In the name of strengthening the case for 9V batteries:
Not many people know this, but it's now possible to get lithium, secondary (aka "rechargeable") batteries. They really put out about 8.0V, but sub swell in many apps.
And have ~70mA constant discharge rate.
I've been enjoying using them in many applications, esp with a buck-switching regulator.
Bear in mind that 9V batteries also have far easier interfaces than AAA packs, requiring a single plastic/metal tab rather than an injection moulded retainer. In some cases, the price difference for both manufacture and battery cost makes it worthwhile.
As for current, plenty of devices run under 20mA happily. A microcontroller in active mode will probably use under 15. Most 9V devices aren't used continuously, my IR temp gun uses a single 9V for instance. However I've also seen smoke alarms and other home sensors that use them.
I'm not disagreeing with any of the criticism here, I would never use a 9V batt in a project, but there is good reason to consider it and people should understand why so many products still use them.
Stacked 9V batteries are typically the old Alkaline sort or rechargable.