Why does my radio cut out after a few minutes with the engine off?

I have a Ford Focus. The manual states that with the engine off, I can listen to the radio for one hour. I can't, it cuts out after a few minutes. This seems to be a problem in general. Is this something to do with the battery, which may run down?

Asked on 30 October 2017 by David Hunt

Answered by Honest John
Modern systems are designed to shut off quickly to minimise drains on the battery because recharging the battery via an engine driven alternator uses a lot of fuel. If the alternator is regenerative (only works on the overrun) then it will not charge the battery adequately after a significant drain from something like a radio.
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