Hybrids, EVs and 12V Batteries in Perth

This guide is for Perth owners of hybrids, plug-in hybrids and fully electric cars who have been caught out by a car that won't wake up. It explains the small 12V battery that every one of these cars still carries, why Perth driving and parking habits can flatten it, and where the line sits between safe owner checks and work for a licensed professional.

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Your hybrid or EV still has a 12V battery

It surprises a lot of owners. A hybrid or EV has a large high-voltage traction battery that drives the motor, but it also has an ordinary-looking 12V battery. That small battery runs the computers, door locks, lights, screens and, most importantly, the relays that connect the high-voltage battery when you press the start button.

If the 12V battery is flat, the car can't switch on its high-voltage system. You can have a traction battery that is nearly full and still be stuck in the driveway with a car that won't respond. On many models the remote key won't even unlock the doors.

The 12V battery is often small, and in some models it sits in the boot or under a rear seat rather than under the bonnet. Your owner's manual will show where it is and where the jump-start points are.

Why Perth owners get caught out

Perth has plenty of situations where a car sits still for days or weeks. Fly-in fly-out workers on a two-week or longer swing often leave a car at home or in airport parking. Families heading east or overseas leave the second car in the garage. Retirees in places like Mandurah and Rockingham might only use the car a couple of times a week.

A modern car never fully sleeps. Its modules wake up to check the alarm, listen for the key, look for software updates and, in many EVs, keep a mobile data connection open for the app. Each wake-up takes a little from the 12V battery. Over a long swing that small drain can be enough to leave it too low to start the car.

Short local trips can add to the problem. Most hybrids and EVs top up the 12V battery from the high-voltage system through a DC-DC converter, but only while the car is switched on and in its ready mode. A car that only does a five-minute run to the shops may not spend long enough in ready mode to fully recover after a long rest.

Before you leave the car for a swing or a holiday

A few minutes before you go can save a callout when you get back.

• Check the owner's manual or app for a storage, transport or long-park mode. Many EVs have one, and it cuts the background drain.
• Turn off features you won't need, such as remote climate pre-conditioning schedules and sentry-type camera modes, which can use a lot of power.
• If the car will sit at home, ask the manufacturer or your service centre whether it is safe to leave it plugged into its charger. Some EVs can keep the 12V battery topped up while plugged in, but not all do.
• For a hybrid in the garage, a quality smart maintainer suited to your battery type can be connected to the 12V battery or the jump points, if the manufacturer allows it.
• Leave a spare key with someone who can start the car part way through a long absence, and let it sit in ready mode for a while.

Know your battery type before replacing it

Many hybrids and EVs use an AGM 12V battery, and some newer models use a lithium 12V battery. A battery in the boot or cabin is often vented through a tube to the outside, so it must be the correct vented type. Fitting the wrong battery, or a cheaper flooded battery in the cabin, can cause problems with charging and with gases.

Match the size, type, terminal layout and capacity to what the manufacturer specifies. Some cars also need the new battery registered with a scan tool so the charging system knows it has been replaced. Our guide to car battery types explains the differences, and how to test a car battery shows how to check one with a multimeter.

Jump-starting a hybrid or EV

Most hybrids and EVs can be jump-started on the 12V side using the points shown in the manual. Usually a small lithium jump pack is enough, because you are only waking up the electronics, not turning over a big engine. Once the car is in ready mode, it starts charging its own 12V battery.

Be careful about the reverse. Many manufacturers say not to use a hybrid or EV to jump-start another car, because the DC-DC converter isn't designed for the current a starter motor draws. Read your manual before offering help in a car park. For the general steps, see how to jump-start a car safely.

The high-voltage system is not a DIY job

Hybrid and EV traction systems run at hundreds of volts. The cables are usually coloured orange, and they can hold a dangerous charge even when the car is off. Never cut, probe or unplug orange cables or anything with a high-voltage warning label. Servicing the high-voltage battery, inverter, DC-DC converter or charging hardware is work for licensed professionals trained and equipped for it.

What you can safely do yourself is limited to the 12V side: checking the battery's voltage, cleaning terminals, using a maintainer and jump-starting at the marked points. If a warning light appears about the hybrid or EV system rather than the 12V battery, have the car checked by a professional. Our guide on dashboard warning lights explains how to treat the different colours.

When the 12V battery keeps going flat

If the car goes flat after only a few days, the problem is probably more than normal standby drain. A tired battery, an aftermarket accessory such as a hardwired dashcam, a module that won't go to sleep, or a faulty DC-DC converter can all cause it. An auto electrician can measure the parasitic draw once the car has gone to sleep and work out what is staying awake. Our guide on why a car battery keeps going flat covers the general process.

Perth's spread-out suburbs mean a mobile auto electrician is often the easiest option when the car won't wake up at home. Owners in the northern suburbs can start with our Joondalup page, and those further south can look at Mandurah or Rockingham.

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Questions Perth drivers ask

Why won't my hybrid start when the main battery shows plenty of charge?

The high-voltage battery can't connect until the car's computers and relays switch on, and those run from the small 12V battery. If the 12V battery is flat, the car stays dead no matter how full the main battery is. Jump-starting the 12V side at the points shown in the manual usually wakes it up, but the 12V battery should then be tested.

Can I leave my EV plugged in while I'm away on a FIFO swing?

Often yes, but it depends on the model. Some EVs keep the 12V battery topped up while plugged in, and some only do so during an active charge. Check the owner's manual or ask the manufacturer. Also look for a storage or long-park mode, and switch off camera and remote climate features that keep waking the car up.

Can I replace my hybrid's 12V battery myself?

Many owners can, as long as they fit exactly the type the manufacturer specifies. Batteries mounted in the boot or cabin often need to be the vented type, and some cars need the new battery registered with a scan tool. Keep well clear of any orange high-voltage cables. If you're unsure, have an auto electrician do the swap and test the charging.

Is it safe to jump-start someone else's car from my hybrid?

Many manufacturers advise against it, because the hybrid's DC-DC converter isn't designed to supply the heavy current a starter motor draws. Some models do allow it under set conditions. Check your owner's manual first. If in doubt, offer to call roadside assistance or use a portable jump pack instead of your car.

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