Force quit then restart, all apps still active iOS 9

The list of apps shown after double clicking the home button is not only the list of open apps but also the list of apps most recently used. Or in other words: not all apps in the list are actually "open" (whatever this means on iOS anyway).

As long thought & finally confirmed once & for all by Apple's Senior vice-president, Craig Federighi, this week - it serves no purpose whatsoever to quit backgrounded apps.

Source : BBC News -

Apple customer goes to the top for iPhone battery answer

The head of Apple's operating system iOS confirmed that open but unused apps do not affect iPhone battery life after a customer emailed boss Tim Cook. The customer from Ohio, known as Caleb, asked Mr Cook whether closing down "multitasking apps" improved battery life and whether it was something the chief executive did himself.

Senior vice-president Craig Federighi replied "no and no".

However, other smartphone batteries can benefit from app closure.

Microsoft advises Nokia Lumia owners to close apps that aren't in use on a web page about extending battery life.

Android creator Google suggests identifying and closing apps that are not often in use but warns that frequent use of its Overview device manager to do this will in itself drain the battery.

"You can view and optimise your device through closing running apps and uninstalling unnecessary apps," said Samsung in an announcement about a new "smart manager" app for the Galaxy 6.

While many Apple users do shut down apps in the belief it extends the iPhone battery this is not advice explicitly given by the firm itself.

It only recommends disabling apps from carrying out background refreshes in a list of tips about saving power.