Home Maintenance Electrical

12V Battery on the Smart Fortwo 453 EQ (Electric Drive)

Shop Workshop job View manuals →

Service interval: No scheduled interval — replace on failure or a persistent 12V-battery / Visit Workshop warning, not on a fixed schedule

Tools you'll need

  • Multimeter (the only thing you'd use yourself here — to read the battery's voltage at the posts without disconnecting anything)

Fluids & specs

No fluids or capacities apply to this procedure.

Torque specs

No workshop-manual-verified fastener torques are linked to this procedure. If a fastener needs a torque spec, refer to the workshop manual for your model and year.

What this is and why it matters

The Smart Fortwo and Forfour 453 EQ — the electric-drive models — run the motor from a high-voltage traction battery, but they still carry an ordinary 12-volt battery for everything else: the lights, the control units, the locking and infotainment, and the electronics that wake the car and bring the high-voltage system online [453 workshop manual, "14 Volt on-board electrical system"].

This is a third, distinct battery in the 453 family, and it's easy to confuse with the other two:

On the EQ, the 12V battery is its own failure point, separate from the high-voltage pack. The owner manual's instruction when the 12V battery is not charging is blunt — do not drive on, pull over safely, and consult a qualified specialist workshop — and the ECE manual adds that in that state "the engine may switch itself off after a short while" [453 EQ owner manual].

This page is a reference, not a how-to: the replacement itself is workshop work (the reason is below), so what follows is what the battery is, what you can safely check yourself, and where the line is.

What you'll need

For anything beyond looking, nothing — the replacement is a workshop procedure. The one thing you can do yourself is read the voltage: a multimeter across the battery posts tells you its state of charge without disconnecting it. Leave the leads on the posts and the ground line attached; measuring is fine, disconnecting is not (see below).

Why this isn't a normal battery swap

On a petrol car you disconnect the 12V battery, swap it, reconnect, and you're done. On the 453 EQ you must not do that. The workshop manual is explicit:

"The ground line for the 12 V battery may not be disconnected. Otherwise high-voltage battery balancing will not be performed." — and "the 12 V battery must not be disconnected on these vehicles." [453 workshop manual]

Disconnecting the 12V battery the ordinary way interrupts the high-voltage balancing the car manages through that ground path. That is why the factory replacement is a documented workshop procedure (AR54.10-P-0005SR), performed with the diagnostic system connected, rather than a driveway swap [453 workshop manual]. It is the single most important thing to know about this battery: there is a high-voltage reason it can't be treated like a normal 12V.

What you can safely do yourself

The useful boundary on the EQ 12V battery is recognition, not replacement:

  • If the dashboard shows a 12V-battery warning or a "Visit Workshop" message, treat it as real — the owner manual routes it straight to a workshop [453 EQ owner manual].
  • Read the resting voltage at the posts with a multimeter if you want a quick sanity check, but leave the ground line connected.
  • Do not start an ordinary disconnect-and-swap. The ground-line rule above is the reason.

Common gotchas

  • It's not the petrol aux battery, and it's not the main 12V. A 453 EQ throwing a 12V or "Visit Workshop" warning needs the right page — this one. The petrol stop-start AGM aux and the general 12V are different batteries with different procedures.
  • The 6-hour charge instruction is not for your car. The workshop manual's "charge the starter battery, charger connected for 6 hours" step is written for the non-EQ 453 (model 453 except 453.091/391/491). On the EQ, the manual's own rule is that the on-board battery is brought up "by using a battery charger" to more than 12.5 V under the diagnostic procedure — don't apply the petrol car's standalone charge step here [453 workshop manual].
  • Disconnecting to "reset" something is the classic mistake. On a normal car people pull the negative terminal to clear a fault or quiet the electronics. On the EQ that disconnect is exactly what the workshop manual forbids, because it stops high-voltage balancing.

When to skip DIY

Almost always, for the replacement itself — it's a workshop job by design, not by difficulty. If you're getting a no-start, scattered electrical faults, or a 12V / battery-monitoring warning on a 453 EQ, the next step is a qualified specialist workshop, not a parts-store battery and a wrench.

A note on what this page deliberately leaves out: the exact physical location of the EQ's 12V battery, its capacity and type, and a step-by-step replacement sequence are not in the service sources we hold — the factory R&R lives in a diagnostic-gated workshop annex. We've logged those for corpus acquisition rather than guess at them; this page covers what the sources actually support — the safety rule, the workshop routing, and how to tell this battery apart from the other two.

Manual references

Stuck mid-procedure?

SmartDiag-AI walks through this kind of job with you, weighted to community-known patterns for your exact model. The link below opens the chat with this topic and your chassis pre-loaded.

Ask SmartDiag-AI about 12v battery on the smart fortwo 453 eq (electric drive)

Related maintenance