A number of owners of hybrid vehicles from various manufacturers have found that the 12V battery can become excessively drained when the vehicle is unused for an extended period of time. Unfortunately this appears to be a decision that manufacturers have taken to fit small capacity batteries to save weight and cost with the downside being this issue. A solar trickle charger is one option to avoid this, as is fitting a larger capacity battery, but this would need to be done with care to ensure it still meets the required manufacturer specifications and does not void the warranty.