How to Jump-Start a Smart Fortwo, Roadster, or Forfour

Tools you'll need

A set of jumper cables, or a portable lithium jump pack
A donor vehicle, if you're using cables rather than a pack

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

A flat 12V battery is the most common reason a Smart won't wake up — no crank, dash flickers, central locking goes dumb, or the car simply does nothing. A jump-start gets you moving again in a few minutes. It is worth being clear up front that a jump treats the symptom, not the cause: if the battery went flat on its own (lights left on excepted), it is usually tired or there is a charging fault, and you'll be back here next cold morning. See 12V Battery Replacement for the load-test and replace path.

The procedure below is the standard, safe boost sequence — it is the same on any petrol Smart. What changes between models is where the battery lives, and the electric 453 EQ has its own rule that you need to read before you touch it.

Before you start

Both vehicles switched off, keys/fobs out and away from the cars.
No open flames or sparks near either battery — a flat battery can vent hydrogen.
Eye protection is cheap insurance.
If the dead battery looks swollen, is leaking, or is frozen solid, stop — don't jump it; replace it.
A modern lithium jump pack is the simplest option on a Smart because the battery can be awkward to reach (see below) — no donor car needed, and the same connection order applies.

The safe connection sequence (petrol Smart)

This is the standard order that keeps any spark away from the battery you're charging:

Donor positive (+) to the dead car's positive (+).
Donor negative (−) to a solid, unpainted metal ground on the dead car — an engine bracket or a chassis ground point — not the dead battery's negative post. Grounding away from the battery keeps the final spark away from any vented gas.
Start the donor car, let it run a couple of minutes.
Start the dead Smart.
Once it's running, remove the clamps in reverse order — ground on the dead car first, then the rest.
Keep the Smart running, or drive it, so the alternator can put some charge back (a short idle won't do much — a 20–30 minute drive is better).

With a self-contained jump pack, the same order applies: pack positive to the car's positive, pack negative to a chassis ground, switch the pack on, start the car, switch off and remove in reverse.

Where to connect on your Smart

The 12V battery is not always where you'd expect, and on most Smarts it isn't under the hood. Locations (from 12V Battery Replacement):

450, 451 Fortwo and 452 Roadster: the main 12V battery sits under the driver's-side floorboard (passenger-side floor on right-hand-drive cars) — pull the mat and lift the floor panel to reach it.
453 Fortwo and Forfour: the battery is in the front engine-bay area, under the hood.

Because the floorboard battery is awkward to get clamps onto, many Smarts provide a dedicated remote jump/boost point under the hood so you don't have to lift the floor at the roadside. If your car has one, use it — it's there for exactly this. Check your owner's manual for its exact location and which terminal is the boost positive, rather than guessing; the position varies by model and year, and that's the one detail worth confirming against your own book.

The electric 453 EQ — read this first

The electric-drive 453 EQ is different and deserves caution. It still has an ordinary 12V battery for the lights, control units and waking the high-voltage system, but its handling is not a driveway job. Per the 453 EQ 12V battery reference, the factory rule is that the 12V battery's ground line must not be disconnected (the high-voltage balancing depends on that path), and the owner manual's instruction when the 12V battery isn't charging is blunt: don't drive on, pull over safely, and consult a qualified specialist workshop.

A jump connects to the battery rather than disconnecting it, but given the high-voltage caveat and the factory "consult a workshop" guidance, treat a flat 12V on an EQ as a specialist matter, not a casual boost — and never attempt to "jump" the high-voltage traction battery; that is not a thing you do with cables. If in doubt on an EQ, follow the owner's manual and call a workshop.

After the jump

Don't switch the car straight back off — let the alternator recover the battery (a proper drive, not a two-minute idle).
If it cranks fine after a drive but is flat again next morning, the battery or charging system is the real problem — go to 12V Battery Replacement and load-test it.
A jump does not need the 453 battery registration step (that's only for replacing the battery) — but if you end up replacing a tired battery, the 453's registration step still applies. Details on that page.
Repeated dead batteries with no obvious cause (lights left on, long lay-up) usually point at a parasitic drain or a failing alternator rather than the battery itself.

When a jump won't help

If the Smart won't take a jump at all — no response with a known-good pack or donor — the problem is likely past the battery: a blown main fuse, a bad ground, or a starter/ignition fault. The fuse box & fuse layout page is the next stop, and a persistent no-start is worth a guided diagnosis or a scan for stored codes rather than more cranking.

Manual references

Browse Smart manuals on smartcarmanuals.com — full catalog of 88 manuals filterable by chassis, year, region, or document type.

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 how to jump-start a smart fortwo, roadster, or forfour