Car Battery Flat After Sitting Unused?

Flat Battery After Car Sat Unused

We get a lot of calls from people whose car has been sitting in the garage or driveway for a few weeks and now won't start. This is completely normal — car batteries are designed to be regularly recharged by the alternator while driving. Without regular driving, the battery slowly discharges through parasitic electrical draw (alarm, clock, computer systems). In Brisbane's heat, this process happens even faster. If your car has been sitting for more than 2-3 weeks and won't start, the battery has likely discharged beyond recovery and needs replacing. We see this a lot with second cars, holiday caravans, and vehicles belonging to elderly or hospitalised family members.

What to Do Right Now

Why This Happens

Modern vehicles never truly 'switch off' — even when parked and locked, systems like the alarm, central locking, tyre pressure monitors, and the body control module draw a small but constant current. This is called parasitic drain, and it typically ranges from 25-75 milliamps depending on the vehicle. Over 2-4 weeks, this drain can take a healthy battery from full charge to completely flat. Queensland's warm climate accelerates self-discharge too — chemical reactions inside the battery continue even without any electrical load, and heat speeds them up. Once a lead-acid battery has been deeply discharged, its plates begin to sulfate, permanently reducing its capacity.

How to Prevent This

• Start and idle the car for 15-20 minutes every week if it's not being driven
• Better yet, take it for a 20-minute drive weekly — idling alone may not fully charge the battery
• Consider a battery maintainer/trickle charger for vehicles that sit unused for extended periods
• Disconnect the negative terminal if the car will be unused for more than a month

Frequently Asked Questions