It happens that Dave Plowman (News) formulated :
In article ,
Andrew Gabriel wrote:
What would be _really_ nice is if they had a switch setting
to automatically start charging when mains is applied. That
could be the basis for an effective automatic emergency supply.
Sadly, they don't do this.

Yes. Wonder how easy it would be to modify?

Not that easy - the button needs to be pressed several times, depending
on the model, to set it for car battery charging. There are other types
which do start charging as soon as the mains come back on, but the down
side is that they discharge the battery if they remain connected whilst
the mains is off.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk