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I would be a bit reluctant to use a battery charger as it will give out more than 12 volts. You have to remember a car battery acts like a storage tank and shock absorber for any spurious voltages above its capacity.
Many of the professional standby savers work on 9 volts it takes a surprising little ampage to maintain any onboard memory - its in the milliamp range.
Believe it or not a standard PPŁ (volt) battery is sufficient to do the job - see link below for a professional type but I sure its possible to knock up a home-made one quite easily - although I think a tenner is quite cheap
Currently in madnoel10's garage:
Honda Civic 1.4l