Smart Car Battery Replacement Guide: DIY Steps, Cost & Best 12V Battery Brands
The 12V battery in your Smart Fortwo isn’t just a starting battery—it’s tucked under the passenger footwell, and on electric models, it’s the quiet culprit behind locked gear selectors and dead electronics.
Replacing it yourself takes about 30 to 45 minutes on most models, but the job comes with one hidden fastener and a battery type that matters more than the brand on the label. Here’s what actually works, what it costs, and which batteries Smart owners trust when the original finally gives out.
Key Takeaways
- AGM is the safe default for Smart Fortwo and EQ models—especially since the battery lives inside the cabin and venting matters .
- Budget roughly $120–$260 for the battery depending on type and brand; dealer OEM units can run $204 or more .
- Labor at a shop typically adds $50–$75, but DIY difficulty is rated very easy on a 1–5 scale .
- The Smart EQ (453) has a hidden Torx screw at the battery base that generic guides often miss—a long T40 extension is required .
- Brand loyalty is split between Odyssey, Napa, Duracell, and Exide, with many owners prioritizing warranty length over name .
DIY Steps: What the Job Actually Looks Like
The process is straightforward once you know the layout, but Smart didn’t make access generous. The battery sits in the passenger footwell, not under the hood.
Step 1: Prepare and Preserve Settings
Turn off the ignition and all electrical accessories. If you want to keep radio presets and clock settings, connect a memory saver before disconnecting anything . It’s a cheap tool that prevents the small annoyances of a full reset.
Step 2: Remove the Footwell Trim
Open the passenger door and slide the seat back. Unscrew the carpet holder, fold the carpet away, then remove the footrest—on the 451, this means unscrewing two fastening discs counterclockwise and pulling the footrest sideways out . The battery compartment is now exposed.
Step 3: Disconnect Terminals (Negative First)
Remove the positive terminal cover, then disconnect the negative (black) terminal first, followed by the positive. This order prevents a short circuit if your tool touches metal while loosening the positive side .
On the EQ, the positive terminal has a small clip that lifts off with a 10mm wrench. Leave the actual terminal clamp attached to the battery during removal—it’s easier to transfer to the new battery once it’s out .
Step 4: Release the Mount—Including the Hidden Clamp
On standard Fortwo models, unscrew the top battery bracket and pull off the vent hose .
On the Smart EQ (453), there’s an extra step that trips up first-timers: a clamp plate at the battery’s base, secured by a Torx screw. You need a T40 Torx bit with at least a 20cm extension to reach it from above the battery. One owner stacked four bit holders together and used a ratchet from the top because there’s no room to work at the base . Once unscrewed, fish the clamp out through the narrow gap with your fingers .
Step 5: Install the New Battery
Angle the old battery forward and lift it out. Transfer any vent hose or terminal hardware to the new unit. Place the new battery in the same orientation, reconnect the positive terminal first, then negative, and tighten the mount . Reinstall the footrest and carpet.
After reconnection, some EQ owners report everything works immediately with no fault codes. Others need to reset the clock. The memory saver prevents both scenarios .
Comparison Table: Battery Options for Smart Models
| Battery Type | Best For | Key Specs | Typical Price | Owner Notes |
|---|---|---|---|---|
| AGM (Group 47/H5) | Fortwo 451/453, EQ | 60–62Ah, 680 CCA | $180–$260 | Preferred for cabin placement; no acid spill risk |
| EFB (Group 47/H5) | 453 with start-stop | 60Ah, 540–680 CCA | $170–$260 | OE on many 453 models; AGM is an upgrade |
| OEM Smart/Mercedes | All models | 12V/60Ah, 680A AGM | ~$204 | Exact fit, no bracket adjustment |
| Odyssey Performance AGM | Fortwo 2005–2016 | 650 CCA, 105 min reserve | ~$500 | Premium price; strong cold-weather performance |
| Exide / Varta / Duracell AGM | EQ and 451 | 60Ah range | $105–$250 | Mixed owner experiences; warranty matters more than brand |
Best 12V Battery Brands: What Owners Actually Say
Brand debates on Smart forums get heated, but the consensus is pragmatic: warranty length and battery type matter more than the logo on the case.
Odyssey earns consistent praise from long-term owners. One forum member reported 4+ years on an Odyssey AGM, alongside 5+ years on a Napa Legend and 3+ years on a Duracell . The Odyssey Performance Series AGM for the Fortwo carries 650 CCA and a 105-minute reserve capacity, but it’s expensive—around $500 from specialist suppliers .
Exide has a following among 451 owners looking for value. One owner paid €105 for an Exide AGM for a 451 MHD and considered it a solid deal . Exide also appears in EQ discussions as a reliable mid-range option.
Napa and Duracell are the practical picks. The Napa Legend AGM hit 5+ years in one owner’s Fortwo, and Duracell (sold through Batteries Plus) delivered 3+ years with a 36-month warranty .
Varta gets mixed reviews. Some owners swear by it; others report early failures. One forum member had a Varta die completely after about 3.5 years of short-trip driving and switched to Banner . The EQ community has discussed Varta Blue Dynamic EFB N60 as a recommended option, but opinions vary on whether EFB or AGM is the smarter choice for the DC/DC converter’s longevity .
What to avoid: Flooded batteries that require venting. Because the Smart’s battery sits inside the cabin, an AGM—which is sealed and spill-proof—is the safer choice . The official Smart manual also specifies using a battery with a central venting cover to prevent corrosion damage .
Chart: Where the Money Goes on a DIY vs. Shop Battery Replacement
The chart below shows a rough cost breakdown for a typical AGM battery replacement. The battery itself is the bulk. Labor is the variable you control by doing it yourself.
Labor estimate based on Haynes manual range of £50–75 converted to USD. Battery costs reflect typical AGM and budget EFB pricing from owner reports and parts listings .
FAQ
What battery type does a Smart Fortwo need?
AGM is the safest choice, especially for EQ models and any Fortwo where the battery sits inside the cabin. It’s sealed, spill-proof, and handles deeper discharge better than flooded batteries .
How much does a Smart car battery cost?
Expect $120–$180 for budget options and $180–$260 for a quality AGM. Dealer OEM batteries list around $204 for a 60Ah AGM unit .
Can I replace the battery myself?
Yes. Haynes rates the Fortwo job as very easy, with 30–45 minutes of work. The EQ’s hidden Torx clamp adds time and requires a long extension, but nothing technically difficult .
Do I need to reprogram anything after a battery swap?
Generally no. Most owners report immediate function. A memory saver prevents radio preset and clock loss .
Why does my EQ need a special tool for the battery?
The base clamp sits at the bottom center of the battery, accessible only from above with a long T40 Torx extension. Standard short tools won’t reach it .
How long should a Smart battery last?
Average life is around 5 years for a quality AGM under normal use. Short-trip driving and cold climates shorten that. The original Fiamm in one EQ lasted 3 years before showing weakness .
The Battery You Choose Matters Less Than How You Treat It
Smart owners who get long life out of their 12V batteries share a few habits. They use AGM. They keep the terminals clean. And they occasionally put the car on a charger if their driving is mostly short city trips—the alternator or DC/DC converter can only do so much when the engine or traction system runs for ten minutes at a time.
The hidden Torx clamp is the only real surprise in the job. Once you know it’s there, the rest is a standard battery swap with one rule: negative off first, positive on first.
What’s the strangest place you’ve found a car part hidden—and did you have the right tool for it? Share your Smart maintenance story in the comments below.
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