About one in eight of our callouts now is a hybrid or an EV, and the owner is almost always confused about why they're calling a battery bloke at all. Fair enough. You bought a car with a massive battery under the floor — how can it possibly have a flat battery?
Because there are two of them, and the little one is the one that leaves you standing in a carpark.
Two batteries, two completely different jobs
| Traction battery | 12V auxiliary battery | |
|---|---|---|
| Voltage | 200-800V | 12V |
| Job | Drives the wheels | Wakes the car up, runs the electronics |
| Type | Lithium-ion | AGM, EFB or small lithium |
| Life in Perth | 8-15 years | 3-5 years |
| Who replaces it | Dealer, specialist | Us, roadside, 20 minutes |
The 12V runs the door locks, the computers, the contactors and the dash. On startup it's what closes the high-voltage relays so the traction battery can actually connect to anything. If the 12V is dead, those relays never close, so the 400V pack just sits there fully charged and completely useless. The car won't even unlock on some models.
Yes, this happens to Teslas in Perth
A flat 12V on a Model 3 or Model Y means no dash, no charge port release, and on some builds the doors won't open from outside. Same story on a Kona, an ID.4 or a BYD. It's the single most common EV roadside callout in Australia, and it has nothing to do with the expensive battery.
Why the 12V dies faster in a hybrid or EV
- There's no alternator. A DC-DC converter charges the 12V off the traction pack, but only when the car is properly awake — so a car that mostly does short trips or sits for a week gets less top-up than you'd think.
- Sentry mode, cabin preconditioning, app connectivity and over-the-air updates all keep systems awake and pull from the 12V.
- Manufacturers fit small batteries. There's no starter motor to crank, so the 12V is sized for electronics only — often 35-45Ah. Less capacity means less margin.
- Perth heat treats them exactly the same as any other lead-acid battery. Three summers in a Scarborough driveway will take a chunk out of an AGM regardless of what it's bolted into.
- Many hybrids put the 12V in the boot or under a seat where it's harder to notice corrosion or swelling until it's properly failed.
The warning signs are different from a normal car
There's no slow cranking, because there's nothing to crank. What you get instead is electrical weirdness: a cluster of unrelated warning lights on startup, the infotainment rebooting itself, the car failing to go into READY mode first press, keyless entry getting flaky, or the 12V battery warning appearing in the app. On a Toyota hybrid it'll often throw a genuine "12V battery" message a week or two before it strands you. Don't sit on that one.
The catch is that a lot of these symptoms look like a software fault, so people book a dealer diagnostic appointment three weeks out and get stranded in the meantime. A 30-second conductance test tells you straight away whether the 12V is the culprit. That's a standard part of our battery testing, and it's free whether you end up needing anything or not.
Can you jump start a hybrid or EV? Carefully
Yes — the 12V system is a normal 12V system and it takes a jump like anything else, but the details matter. Most EVs and hybrids have designated jump terminals under a bonnet cover rather than exposed battery posts, and you must never connect anything to the high-voltage system. Get it wrong on some models and you'll take out a DC-DC converter, which is a genuinely expensive mistake.
A jump also only gets you moving. If the 12V is 4 years old and has been deep-discharged, it'll strand you again within days. The proper fix is replacing it with the correct AGM or EFB spec — not a cheap flooded battery, which will fail early and can vent inside a cabin or boot location where it's mounted.
Fitting is fussier than a normal car
Modern hybrids and EVs need the new battery registered so the DC-DC converter charges it on the right profile, exactly like a Stop-Start petrol car does — the same job we cover on Stop-Start battery replacement. Skip it and the car keeps charging the new battery like a tired old one and you'll be replacing it again inside a year.
There's also a memory-saver step. Pulling the 12V on a lot of EVs drops settings, and on some it puts the car into a transport or service state that's a pain to clear. We keep the system alive on a backup supply while the swap happens, which takes about 20 minutes at your place through mobile battery replacement — no towing an EV that can't be put in neutral, which is its own nightmare.
If your hybrid or EV is throwing odd electrical faults or has flagged a 12V warning, get it tested before it leaves you locked out in a carpark. We come to you, we test it properly, and we'll tell you if it's the 12V or if you genuinely need a dealer.
Frequently asked
My EV won't start but the main battery is at 80% — how?+
The 12V auxiliary is flat. It's what closes the high-voltage contactors, so without it the traction pack can't connect to anything. The charge percentage on the big battery is irrelevant until the 12V wakes the car up.
How long does the 12V battery last in a hybrid?+
In Perth, typically 3-5 years — much the same as a conventional car, because heat is the main factor and it doesn't care what the car is. AGMs at the longer end, small flooded units at the shorter end.
Can I jump start my hybrid from another car?+
Usually yes, using the designated jump points rather than direct battery terminals, and never anything touching the high-voltage side. Check your manual first, because a few models specifically prohibit it. If you're not certain, don't guess — the DC-DC converter is not a cheap thing to replace.
Do you replace 12V batteries in Teslas and other EVs?+
Yes. It's a standard 12V AGM or lithium auxiliary depending on the model and build year, we carry the common ones, and we register it where the car requires it. What we don't touch is the high-voltage traction pack — that's dealer and HV-specialist territory.
Common service areas for this guide