# 5 common mistakes when changing a car battery (only a specialist detects) > Blog article — Málaga Baterías · Updated 2026-07-17 _The 5 most common mistakes we see when changing car batteries: reversed polarity, missed BMS coding, AGM replaced with conventional. How to avoid them._ Changing your car battery looks simple: two bolts, two cables, one new battery . But in the workshop we see, week after week, mistakes that only technical experience can spot and that cost the customer hundreds of euros in repairs or in shortened service life. Here are the 5 most frequent mistakes we see in Malaga, with practical solutions. Mistake 1: Reversing the polarity (the most expensive one) Connecting the red cable (+) to the negative terminal and the black one (−) to the positive. It is the most expensive mistake: it causes an instant short circuit that can: - Blow the main fuses (easy to fix, 5-30 €). - Damage the engine control unit (ECU) : 600-2,000 € to repair or replace. - Destroy the alternator. - Kill the digital instrument cluster. How to avoid it : before touching anything, identify the terminals by their markings or the colour code. Connect the positive first, then the negative. When disconnecting, negative first, then positive. On a modern car, better leave it to professionals. Mistake 2: Not coding the new battery on cars with a BMS Vehicles with a BMS (Battery Management System) , especially premium European brands (BMW, Mercedes, Audi, Porsche, Range Rover, Volvo, etc.), register every battery that is installed. If you fit a new one without coding it via OBD-II , the car keeps thinking it is the old one and applies an unsuitable charging profile. Consequence : instead of lasting 4-6 years, your new battery lasts 2-3 (even less in a hot climate like Malaga). Higher cost per kWh. Start-Stop and energy-recovery functions are progressively disabled. How to avoid it : if your car is from 2012 or later, mid/high range, demand that the workshop performs the BMS coding via OBD-II . At Málaga Baterías it is included in the price of the replacement. Mistake 3: Replacing AGM with a conventional battery to "save money" A new AGM costs 80-100 € more than an equivalent conventional battery. Some customers decide to save and fit a conventional one. It is an expensive mistake in the medium term : - The car charges with an AGM profile (higher voltage, higher current): the conventional battery gets overcharged and loses electrolyte. - Start-Stop stops working (the car detects insufficient capacity). - Service life reduced from 4-6 years to barely 2-3 years. How to avoid it : if the car came with AGM from the factory, fit a new AGM. Full stop. The price difference pays for itself in 2 years. Mistake 4: Not tightening the terminals properly A loose terminal causes micro-intermittent electric arcs every time you demand high current (starting, switching on the lights). Symptoms: - Visible voltage drops (flickering lights). - Intermittent electronics errors. - Accelerated corrosion at the contact. - In the worst case, terminal overheating and fire hazard. How to avoid it : tighten to 6-8 Nm with a torque wrench. If you do not have the tool, "firm by hand, without bending the material". Check after a month that it is still tight. Mistake 5: Not detecting parasitic drain before replacing If your battery drains by itself within a few days, the problem may NOT be the battery : it can be an abnormal parasitic drain (faulty alarm, dashcam with parking mode, GPS, a USB left plugged in, an interior light that never switches off). How to avoid it : before buying a new battery, measure the standby current draw . A modern car normally draws 20-50 mA with everything shut. If you read 200+ mA, there is your problem. We check it for free at the shop with a multimeter. Real case: the customer who changed 3 batteries in a year Eduardo, a taxi driver in Malaga, changed 3 batteries in 14 months. After the third one, he came to see us. We measured his parasitic drain: 380 mA in standby (it should be ~30 mA). A Xiaomi dashcam with parking mode enabled was draining the battery. We disabled the mode, installed a maintenance charger and the last battery has now been running for 22 months. We saved him 2 batteries and a lot of headaches. Why trust a Malaga specialist Replacing a battery properly requires: - Prior diagnosis (do not replace what does not need replacing). - Identifying the exact reference (correct technology). - Tightening the terminals to the right torque. - Coding the BMS where applicable. - Resetting consumption and maintenance counters. - Collecting and recycling the used battery with documentation. At Málaga Baterías we do all of this in 15-20 minutes. The replacement costs the same or less than elsewhere and we make sure you will not be back within 18 months. FAQ Can I change the battery myself? On simple cars without Start-Stop, yes (taking care with polarity). On modern cars with a BMS, better not: the risk of it turning expensive is high. What if I only need to jump-start the car with cables? That is safe if you do it right (positive of the good battery to the positive of the dead car; negative of the good one to the chassis of the dead car, NOT to the negative terminal). But it is only a temporary fix: come and get it checked. What happens if I connect it the wrong way round? If it is only a few seconds, it probably just blows fuses. If it lasts longer or you make repeated contact, ECU damage. If it happens to you, do not start the car: come on a tow truck or call us. At Málaga Baterías we do the replacement done right . +34 952 31 01 18 . Author: Francisco David Pérez Ochoa — Manager of Málaga Baterías. Over 18 years (since 2008). … ## Buy or change this battery in Málaga Málaga Baterías — official Varta distributor. No appointment needed. Free quote and recycling of your old battery. - **Phone:** [952 31 01 18](tel:+34952310118) - **WhatsApp:** [+34 952 31 01 18](https://wa.me/34952310118) - **Address:** C/ La Orotava, 100, 29006 Málaga - **Directions:** https://maps.google.com/?q=Málaga+Baterías+C/+La+Orotava+100+Málaga ### Areas we cover Málaga city: Centro, Teatinos, El Palo, Pedregalejo, Ciudad Jardín, Cruz de Humilladero, Carretera de Cádiz, Churriana, Campanillas, Puerto de la Torre, Bailén-Miraflores, Palma-Palmilla. Costa del Sol: Torremolinos, Benalmádena, Fuengirola, Mijas, Marbella, Rincón de la Victoria, Vélez-Málaga, Torrox, Nerja, Alhaurín. --- [See the full product page](https://malagabaterias.com/en/blog/5-common-mistakes-changing-car-battery-only-specialist-detects)