Auxiliary Battery Replacement on Smart Fortwo 453 and Forfour 453
Service interval: Replace when paired voltage / comm codes appear or load test fails — typically 4-6 years
Tools you'll need
- 10mm socket (typical — confirm yours)
- Ratchet and short extension
- Trim removal tool
- Star Diagnosis or compatible scan tool — only if your 453 build needs aux registration
What this is and why it matters
The Smart Fortwo 453 and Forfour 453 carry a second small 12V battery in addition to the main one. It's an AGM unit, typically around 10-15Ah, and it lives near the main battery. Its job is to support the stop-start system through engine restarts, and to hold standby loads (BCM, alarm, comfort modules) when the main battery is briefly disconnected or pulled down.
When this aux battery gets tired, the symptoms are scattered and confusing. Paired voltage codes across multiple modules. Intermittent communication faults on the CAN bus. Stop-start refusing to engage. Random module-not-responding flags that clear and come back. The car often gets blamed on the main battery first. Many 453 owners replace the main battery, see no change, and only later find the aux battery was the actual problem.
If you have a 453 throwing scattered electrical codes — and your main battery tests fine — the aux battery is the next thing to check, not the last.
What you'll need
A correct-spec aux AGM battery. Capacity in the 10-15Ah range; the original spec is on the existing battery's case label. Match it. AGM, not flooded.
Sockets — 10mm covers most aux battery hold-down hardware on the 453, but verify yours. Trim removal tools to get the cover panels off without breaking clips.
Whether you need a scan tool for aux registration depends on the build year and trim. Some 453 builds register the aux battery to the BCM separately from the main; others don't. The workshop manual on Manualslib for your specific year is the right reference. If in doubt, install the new aux battery first and watch for stored codes — if codes persist after a couple of drive cycles, scan-tool registration is likely needed.
Step by step
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Ignition off, fob away from the car. Pop the hood / front access panel and locate the aux battery — small, sits near the main battery, has its own hold-down.
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If you're replacing the aux battery and not touching the main, you can usually leave the main connected. If you do disconnect anything, do negative first, positive second, and reverse on reassembly.
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Disconnect the aux battery: negative terminal first, then positive. Tuck the cables clear of the posts.
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Remove the hold-down bracket. Lift the aux battery out. It's small and light — easy lift.
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Clean the tray and check the cable terminals for corrosion. White or green crusty deposits on the terminals is a real cause of intermittent faults — clean with a wire brush and replace the terminal hardware if it's badly gone.
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Drop the new aux battery in. Hold-down bracket on. Positive terminal first, then negative. Snug.
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Reassemble the trim panels.
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Start the car. Watch for warning lights and stored codes. A short mixed drive cycle will clear most post-disconnect codes on its own.
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If your build needs aux battery registration: connect the scan tool, navigate to the BCM / energy management section, run the auxiliary battery registration function, set capacity and chemistry to match the new battery, save and exit. If your scan tool doesn't have this function, the workshop manual for your specific year tells you whether registration is required. When in doubt, check.
Common gotchas
Replacing the main battery first when the aux is the actual problem. The most common 453 mistake. Test both, or at least replace the cheaper one first if the symptom matches.
Wrong capacity or chemistry. The aux is AGM. Don't swap in a flooded battery, don't undersize it.
Corroded aux terminals masking the real problem. Sometimes the aux battery is fine but the terminal connection is poor. Clean the terminals before condemning the battery.
Skipping registration when it was needed for your build. If post-replacement codes don't clear after a couple of drive cycles, that's a sign you needed the scan-tool step. Get the registration done — the symptoms will keep coming back otherwise.
Mistaking aux symptoms for something else. The intermittent comm-fault pattern often looks like a wiring problem or a failing module. The aux battery is a much cheaper first check than tracing CAN bus harnesses.
When to skip DIY
If you've replaced the aux battery, the main battery, and the symptoms still pattern-match an aux problem (paired voltage codes, intermittent comm faults), it's time for a Smart-experienced shop with a Star Diagnosis. There are voltage-monitoring quirks on some 453 builds that benefit from factory-level diagnostic data.
If your scan tool doesn't handle aux battery registration and your build needs it, pay a local Smart specialist for the 10-minute scan-tool visit. Don't drive around with a fresh aux battery the BCM thinks is something else — same logic as the main battery.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Auxiliary AGM battery (~10-15Ah) | $40-90 | Search Google |
| Aux battery terminal hardware (where corroded) | $5-15 | 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.
Related fault codes
- U0100 Lost Communication with ECM/PCM on Smart Fortwo 451 / 453
- P0562 System Voltage Low on Smart Fortwo 451 / 453
- B152014 F2K5 Starter Relay Fault on Smart Fortwo 453