Car Batteries and Air Con in Mackay’s Heat

This guide is for Mackay drivers who rely on their car's air conditioning for most of the year and want their battery to keep up. It explains how the local climate puts load on both systems, what early warning signs look like, and which checks you can do yourself before the wet season.

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Mackay's climate in a nutshell

The Bureau of Meteorology describes Mackay as having hot, wet summers and mild, dry winters. Daytime highs are around 29 to 30 degrees in the wet season and about 21 to 25 degrees in the dry. Nights in the wet season stay warm, at around 23 degrees, and most of the year's rain falls between December and March. You can read more on the Bureau of Meteorology's Mackay climate page.

Those figures don't sound extreme next to inland Queensland, but the combination of warm days, warm nights and humid air means car air conditioners in Mackay work hard for months on end. Every hour the air con runs is an hour of extra load on the electrical system.

How air con and the battery are linked

Your air conditioning compressor is usually driven by the engine, but much of the system is electrical. The compressor clutch, the cabin blower fan, the condenser fans in front of the radiator, pressure switches and the climate control module all draw power. While the engine runs, the alternator supplies that power and recharges the battery.

Problems start when the alternator can't keep up or the battery is already tired. Short trips around town with the air con on high, plenty of idling in traffic, and heat soaking into the engine bay all add up. A battery in that situation slowly loses charge, and a weak battery can in turn cause odd behaviour from electronics, including the climate control.

Heat and battery life

Heat speeds up the chemistry inside a lead-acid battery, including the slow reactions that wear it out. In Mackay, the battery also sits in a hot engine bay for long periods while the car is parked in the sun after a drive.

Because warm weather makes it easier to crank an engine, a worn battery can keep starting the car well past its best. Then, one morning after the car has sat for a couple of days, it won't. Testing it before that happens is far easier than finding out in a car park. Our guide on how to test a car battery explains voltage checks and load tests.

Warning signs from the battery

Watch for these, especially in the months leading into the wet season:

• Slower cranking than usual, particularly after the car has sat for a day or two.
• Start-stop that stops working or only works occasionally.
• The headlights or cabin lights dimming when the air con or blower fan kicks in at idle.
• White or green crust building up on the terminals.
• A battery that is a few years old and has never been tested.

Warning signs from the air conditioning

Air con faults are not always refrigerant problems. Electrical faults are common and are often cheaper to fix. Look out for:

• Cold air at highway speed that turns warm at idle, which can point to a condenser fan not running.
• No air from the vents on some fan speeds, which often points to a blower resistor or fan control fault.
• A compressor that clicks on and off rapidly.
• A musty smell when the system first starts, which can mean a dirty cabin filter or moisture in the evaporator area.
• Water dripping into the footwell instead of under the car, which suggests a blocked condensate drain.

Humidity, mould and the condensate drain

In a humid climate, the evaporator behind the dashboard pulls a lot of water out of the air. That water drains out through a small tube under the car. Leaves, dust and insects can block it, and the water then backs up into the cabin. Wet carpet can damage wiring and modules that sit under the seats or behind the kick panels in some vehicles, so fix a leak early.

A clean cabin filter helps airflow and reduces smells. Many can be changed in a few minutes behind the glovebox. Our guide to common car air conditioning problems covers more symptoms and causes.

A pre-summer check you can do

Late in the dry season, before the hot and humid months settle in, is a good time for a quick check:

• Get the battery tested and note its age.
• Clean and tighten the battery terminals and check the hold-down.
• With the engine running and air con on, check that the condenser fans spin.
• Clear leaves and bugs from the front of the condenser with gentle water pressure, not a high-pressure jet.
• Replace the cabin filter if it's dirty.
• Run the air con for a few minutes regularly, even in the cooler months, to keep the system working.

What to leave to a professional

Any work that involves opening the air con system or handling refrigerant must be done by a licensed technician with the proper equipment. Don't use DIY recharge cans or vent refrigerant. On hybrids and EVs, the air con compressor can be driven by the high-voltage system, so anything beyond the cabin filter and the small 12V battery is for licensed professionals only.

For the battery, check whether your car needs an EFB or AGM type, and whether the new battery needs to be registered with a scan tool. Our guide to car battery types explains the differences. A mobile auto electrician can test the battery and charging system at home, whether you're in West Mackay or up the coast in Andergrove.

One last point: in Mackay's heat, never leave children or pets in a parked car, even for a short time and even with the windows down.

Parts for this job

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Questions Mackay drivers ask

Can a weak battery stop my car air conditioning working properly?

It can contribute. The air con relies on electrical parts such as the compressor clutch, fans and control modules. A weak battery and a struggling charging system can lead to low voltage, which can upset electronics. If you have air con faults and slow starting at the same time, get the battery and alternator tested first, as that is quick and inexpensive.

Why does my air con blow cold on the highway but warm at the lights?

At speed, air rushing through the condenser cools it. At idle, the condenser relies on its electric fans. If a fan, relay, fuse or fan control module has failed, the system can't get rid of heat when stopped, so the air turns warm. An auto electrician can check the fan circuit without opening the refrigerant system.

Is it safe to top up my own air con refrigerant?

Handling refrigerant is work for a licensed technician with the proper equipment. Adding refrigerant without finding why it was low also leaves the leak unfixed, and overfilling can damage the system. Many cooling problems are electrical rather than refrigerant faults, so it's worth having the electrical side checked first.

When is the best time to check my battery and air con in Mackay?

Before the wet season, when hot, humid weather and heavy air con use set in. The Bureau of Meteorology notes most of Mackay's rain falls from December to March. A battery test and a quick look at the condenser fans and cabin filter in spring can save a breakdown in the hottest months.

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