Wishing

Hi - my elderly father in law owned an LC500h and sadly he has been seriously ill for most of this year and had been unable to drive the car. The AA have been out to charge the battery after it went flat due to lack of use. Is it possible to keep the battery 'usable' and charged by merely having the car engine running whilst stationary, or does it need to be taken for a drive to keep the battery charged ? My elderly mother in law not in a position to drive the car sadly. Thank in advance