Frosty Mornings and Flat Batteries in Canberra

This guide is for Canberra and Queanbeyan drivers whose cars get sluggish, or refuse to start, once the frosts arrive. It explains what the Bureau of Meteorology's figures say about our winter mornings, why cold is so hard on batteries and starting systems, and what to check before June so you are not stuck in the driveway at 7 am.

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How cold Canberra mornings really get

Canberra's winters are known for hard frosts, and the numbers back that up. In its 2020 annual climate summary for the ACT, the Bureau of Meteorology noted that Canberra Airport averages 61 nights a year below 0 °C. That year was mild by local standards, with only 42 sub-zero nights, the fewest since 2009, and the coldest morning was still -4.9 °C on 5 June.

Mid-winter is the hardest stretch. In July 2014, for example, the Bureau recorded 18 nights at or below 0 °C against a long-term July average of 17, and Canberra Airport's mean minimum for the month sat just below zero. The Bureau's Tuggeranong station recorded -5.1 °C that month and -4.7 °C in August 2020. In 2018 the first sub-zero mornings at Canberra Airport and Tuggeranong arrived on 6 May.

You can read the Bureau's summaries for yourself on its ACT climate summary pages. The point for drivers is simple: for a big part of the year, your car is asked to start in temperatures that would test a battery anywhere in the country.

Why a battery that was fine in March fails in July

A lead-acid battery makes power through a chemical reaction, and that reaction slows down as the battery gets colder. On a -4 °C morning a battery can deliver noticeably less current than it could on a mild autumn afternoon.

At the same time, the engine is harder to turn. Cold oil is thicker, so the starter motor has to work harder and draw more current, for longer, from a battery that has less to give. A battery that is ageing, slightly undercharged or has a weak cell can cope in summer and then give up on the first proper frost.

That is why so many Canberra flat-battery call-outs bunch up in the first cold snap of the year. The battery did not suddenly fail overnight. The cold just exposed a weakness that was already there.

Signs your battery won't survive the next frost

Watch for these warning signs in autumn, before the frosts settle in:

• The engine cranks slowly on cold mornings, then starts normally later in the day.
• Interior lights or the dash dim heavily while the engine is cranking.
• The stop-start system stops working, or a battery message appears on the dash.
• The battery is more than three or four years old and has never been tested.
• White or green crust is building up on the terminals.

Test before winter, not after

The best time to check your battery is April or early May, while it still has a chance to pass. A simple voltage reading with a multimeter after the car has sat overnight tells you its state of charge. A load test or a modern conductance tester tells you whether it can still deliver its rated cold cranking amps (CCA). Our guide on how to test a car battery walks through both.

If you are replacing a battery, match or exceed the CCA rating your vehicle maker specifies, and fit the right type. Many newer cars with stop-start need an AGM or EFB battery, and some need the new battery registered to the car's computer. See car battery types explained before you buy.

Clean terminals matter more in winter too. Corrosion adds resistance right where the starter needs the most current. Clean them with the battery disconnected, negative lead first, and protect them with terminal grease.

Canberra habits that drain batteries

Canberra's road layout encourages short trips. A run from Gungahlin to the shops, or from a Belconnen suburb to the bus interchange, may only be ten minutes. In winter that short trip is often done with the heater fan on high, the rear demister running, the headlights on and the seat heaters going. The alternator may not fully replace what the start took out, and over a few weeks the battery slides further down.

A car that only does short winter trips will often benefit from a longer drive on the weekend or a top-up with a quality smart charger every few weeks. Choose a charger that suits your battery type, and charge in a ventilated space.

Cars parked under a carport or in the open also cool down fully overnight, while a car in a closed garage usually stays a few degrees warmer. If you have the choice on a cold night, the garage helps. If your battery keeps going flat even with regular driving, something else may be drawing current while the car is parked. Read why your car battery keeps going flat for the usual suspects.

Diesels, glow plugs and other cold-start parts

Diesel utes and four-wheel drives rely on glow plugs to warm the combustion chambers before cranking. A failed glow plug or glow plug relay may not be noticed in summer, but on a frosty morning it shows up as long cranking, rough running and white smoke until the engine warms up. If your diesel has a glow plug light, wait for it to go out before you crank.

Other parts get a workout in winter too. Rear demisters, heated mirrors and heater fans all run on fused circuits, and a fan that only works on its highest speed usually points to a failed blower resistor. Check that your headlights are aimed correctly and clean as well. Winter in Canberra means a lot of driving in the dark on both ends of the working day.

On a frosty morning

A few habits make winter starts easier on the whole electrical system:

• Switch off the headlights, heater fan and accessories before you crank, so all the battery's power goes to the starter.
• If the engine doesn't fire, crank in short bursts of a few seconds with a pause between, rather than one long crank.
• Never pour hot water on a frosted windscreen. The sudden temperature change can crack the glass. Use the demister and a plastic scraper.
• Make sure your washer bottle has a proper washer additive, not just water, and don't run the washers on a frozen screen.
• If you need a jump start, connect the leads in the correct order and check your owner's manual first, as many modern cars have dedicated jump points.

When it's more than the battery

If a fully charged, tested battery still struggles, the starter motor, the main cables or the alternator may be the real problem. A worn starter draws more current in the cold, and a weak alternator never fully recharges the battery. Our guide on whether it's the starter motor or the battery helps you narrow it down.

Mobile auto electricians across the city attend frosty-morning no-starts, and plenty of them test and replace batteries on the spot. Find local help through our pages for Tuggeranong, Gungahlin and Queanbeyan, or browse the other districts.

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Questions Canberra drivers ask

How many frosty mornings does Canberra get each year?

The Bureau of Meteorology says Canberra Airport averages about 61 nights a year below 0 °C. The number varies a lot from year to year. In 2020, a mild year, there were 42. Most sub-zero mornings fall in the colder months from May onwards, and July averages about 17 of them on its own. Readings around -5 °C are not unusual in a cold snap.

Why does my car start fine in the afternoon but not on a frosty morning?

Cold slows the chemical reaction inside the battery, so it delivers less current, while thicker cold oil makes the engine harder to turn. A healthy battery copes with both, but a weak or undercharged one may only manage once the day warms up. That pattern is a strong hint the battery needs testing, and possibly replacing, before the coldest weeks arrive.

Should I buy a battery with a higher CCA rating for Canberra winters?

Start with the cold cranking amps your vehicle maker specifies and treat that as the minimum. A bit more CCA can give you extra margin on frosty mornings, provided the battery is the correct size, type and terminal layout for your car. Stop-start vehicles usually need AGM or EFB batteries, so check the type before comparing CCA figures.

Is it okay to leave a battery charger on overnight in winter?

A quality smart charger designed to be left connected will switch to a maintenance mode once the battery is full, so it can be used overnight or longer. Older manual chargers can overcharge a battery if left on. Follow the charger's instructions, charge in a ventilated area, keep leads tidy, and make sure the charger suits your battery type.

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