Is there a cheap device, or circuit I could make up, which alerts people to car doors being left open for more than X minutes, to avoid batteries being completely drained?
I know of a device which goes between one of the battery terminals and the cable which usually connects to that terminal, and when the battery power gets below a certain level it breaks the circuit, but it's not cheap and in some ways it would be better to deal with it much earlier (e.g. after 5 mins), and get the door closed for security & weather reasons.
Maybe just a buzzer which fits under the hood/bonnet, which detects power to the door light circuit (in some models), and beeps for a second every minute or so.
I have a Toyota Corona 1993 and we left a door open which flattened the battery a few days ago. Fortunately this model beeps when headlights, etc. are left on, so that's not a problem for us.