12V Battery Replacement on Smart Fortwo, Roadster, and Forfour
Service interval: Replace when crank speed drops or load test fails — typically 4-7 years
Tools you'll need
- 10mm and 13mm sockets (sizes typical — confirm yours)
- Ratchet and short extension
- Battery terminal cleaner or wire brush
- Memory saver or OBD-II memory keeper (optional but useful)
- Star Diagnosis or compatible scan tool — required for 453 battery registration
- Floorboard trim tool (450/451 floor access)
What this is and why it matters
A tired 12V battery on a Smart causes far more weird behaviour than people expect. Slow cranking is the obvious one. The less obvious ones are intermittent module faults, paired voltage codes across multiple systems, transmission warnings on the 451, and ABS/ESP lights that come and go. Before chasing electrical gremlins on a Smart, load-test the battery. Half the time the "computer problem" is just sub-12V at the terminals.
Replacement is straightforward on the 450/451/452 — basic disconnect, swap, reconnect. The 453 adds a step: the BCM tracks the battery's age and state of health and adjusts the charging strategy accordingly. Drop in a new battery without telling the BCM and it keeps charging like the battery is still 5 years old. The new battery overcharges for weeks, the electrolyte cooks, and the new battery dies early. The fix is to register the new battery to the BCM with a Star Diagnosis or a compatible scan tool. Some independent scan tools handle Mercedes battery registration; others don't. Confirm before you assume.
What you'll need
A battery in the right capacity and chemistry. AGM if the original was AGM (most 453s, some later 451s). A flooded lead-acid replacement in an AGM-spec slot will work physically but will not last; the charging strategy is wrong for it.
Sockets — 10mm and 13mm cover most terminal and hold-down hardware on Smarts but sizes vary by year and trim. A memory saver plugged into the OBD-II port keeps the radio code, clock, and module memory alive while the battery is disconnected. Not strictly necessary on most Smarts but it saves a couple of relearn quirks.
For the 453, scan-tool access for battery registration. Star Diagnosis is the factory tool. Several aftermarket Mercedes-capable tools (iCarsoft MB-II and similar) advertise this function — verify it works on your specific 453 trim before relying on it.
Step by step
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Locate the battery.
- 450, 451: under the driver's-side floorboard (passenger-side floor on RHD cars). Pull the floor mat, lift the floor panel, the battery sits in a tray below.
- 452 Roadster: similar to 450/451 — floorboard access.
- 453 Fortwo and Forfour: under the hood / front engine bay area. Removing trim panels gets you in.
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Ignition off, key out (or fob away from the car for keyless variants). Optional: plug the memory saver into the OBD-II port.
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Disconnect the negative terminal first. Wrap the loose negative cable in a rag and tuck it well clear of the post. Then disconnect the positive.
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Remove the hold-down bracket. Lift the old battery out — it's heavier than you'd expect, especially the 453's AGM unit.
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Clean the tray and terminals. Surface corrosion on the terminals is a real cause of voltage problems; a wire brush takes it off in seconds.
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Drop the new battery in, hold-down bracket on, positive terminal first, negative terminal second. Snug — not over-tight; aluminum lugs round off easily.
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453 only — register the battery. Connect the scan tool, navigate to the BCM / energy management section, run the battery replacement / battery registration function. The tool will ask for capacity (Ah) and chemistry (AGM / flooded). Match it to what you installed. Save and exit.
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Start the car. Watch for warning lights. Some modules need a short drive cycle to clear post-disconnect codes — a 15-20 minute mixed drive usually handles it.
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Module relearns to be aware of:
- 451 with automated manual transmission: some 451s want a clutch actuator brake-pedal reteach after battery disconnection. If the dash shows a transmission warning that won't clear, that's the next step.
- All Smarts: power windows may need a one-time up/down with hold at the top to relearn the auto-up function.
- All Smarts: radio may ask for a code if your unit has anti-theft enabled.
Common gotchas
The 453 not getting registered. Skipping the registration step is the single biggest mistake. The new battery works fine for a few weeks and then starts misbehaving. Don't skip it.
Mixing chemistries. AGM-spec slots want AGM batteries. The car's charging algorithm is set for AGM characteristics; a flooded battery in there will boil and gas off.
Pinched cables under the floorboard panel on the 450/451. Easy to do on reassembly. Lay the cables flat and check the panel seats clean.
Brake-pedal reteach on the 451 if the transmission throws a fit afterwards. Cross-reference against the transmission warning page.
Memory loss surprises. Radio code, custom seat-position memory (where fitted), trip computer. The memory saver on the OBD port avoids this, or just write down the radio code before you start.
When to skip DIY
If you don't have a scan tool that handles 453 battery registration, this is a shop visit — or pay the local Smart specialist a small fee just for the registration step after you've installed the battery yourself. That's a real and reasonable approach. Plenty of owners install the battery DIY and pay $30-60 for the 10-minute scan-tool visit afterwards.
If the car has been sitting with a dead battery for weeks and now has a swarm of stored codes across multiple modules, the post-replacement diagnostic clean-up may take longer than the battery swap itself. A shop with the right tool clears it in minutes.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Main 12V battery — 450/451/452 (50-60Ah typical) | $120-180 | Search Google |
| Main 12V AGM battery — 453 (60-70Ah typical) | $160-240 | Search Google |
| Battery terminal kit (where corroded) | $10-25 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread.
Manual references
- Browse Smart workshop manuals on smartcarmanuals.com — model-specific reference manuals on the home page; pick your chassis code section for torque specs and detailed procedures.
How-to videos
Related fault codes
- U0100 Lost Communication with ECM/PCM on Smart Fortwo 451 / 453
- P0562 System Voltage Low on Smart Fortwo 451 / 453
- C1000 ABS / ESP Control Module Fault on Smart Fortwo 451 and 453