The car battery and charging system: what every part does

The 12 V battery caused 45.4 % of the 3,691,813 breakdowns ADAC attended in 2025, up from 35.7 % in 2015. This guide is the whole system in one place: what kills a battery, the three types and which one your start-stop car needs, how to replace one without confusing the car, what the voltage on your multimeter means, chargers and where the black clamp goes, what the alternator does now, how to jump-start by the manual, and what the law says about the old one.

Makers disagree on thresholds, so every figure here carries its maker's name. Where your owner's manual says something different, the manual wins.

1. Why batteries die

The damage is done in summer, the failure shows up in winter. VARTA ran a 12-month fleet test in Dubai: every conventional battery failed after 5 to 6 months, while the AGM batteries lasted the full year, and VARTA's point is that most failures surface in winter although the damage comes from summer heat. CTEK puts a number on heat: a battery at 27 °C discharges 33 % faster than one at 18 °C, and it cites a US supplier's average life of 51 months in cold northern states against 30 months in hot southern ones.

The second killer is the short trip. Each start takes a large burst of power, and a five or ten minute drive does not put it back, so the battery ends each trip a little lower than it began. Midtronics says trips under 20 minutes prevent the alternator from fully recharging the battery, idling charges far more slowly than driving, and short-trip cars should be tested every six months. Banner adds that charge acceptance needs a certain battery temperature, which is only reached after a journey of at least 20 km, vehicle-dependent. VARTA's answer is not a bigger battery: a larger battery does not change the charge balance, it only gives more reserve, whereas an AGM can run partially charged without a negative effect on its life. CTEK's answer is a mains smart charger rather than extra driving.

The third is the car itself while parked. ADAC measured 25 new cars locked and closed: 5 to 40 mA of quiescent current, roughly 2 to 12 months of standby on a healthy battery. With the boot left open, some cars still drew up to 0.45 A after two hours and could fail to start after five days.

How long does a battery last? ADAC says about 5 years, 6 to 8 or more with good maintenance and regular recharging, and recommends a check before autumn if the last replacement was more than three years ago. Exide says batteries more than 4 years old are more likely to struggle in winter. Midtronics argues against replacing on a schedule, because it throws away serviceable batteries and misses early failures: a marginal battery "will often continue starting the vehicle — right up until it doesn't." In a six-week CTEK workshop trial, the average incoming battery read about 12.3 V, 34 % needed recharging and 17 % needed replacement.

One rule before anything else: a battery with pushed-out ends, a cracked case or damaged terminals is replaced, not charged. VARTA says to replace any battery with signs of damage to the terminals, case or cover, and lists pushed-out ends as a sign of plate growth; CTEK says to inspect for cracks, leaks and corrosion before charging.

2. Flooded, EFB and AGM: the three types

A conventional flooded battery has liquid acid around the plates. An EFB is the same battery with a polyester scrim on the positive plate. An AGM holds the acid in a glass fleece, which makes it maintenance-free and leak-proof.

Start-stop is why the other two exist. VARTA says only EFB or AGM can cope with start-stop's demands: entry-level systems typically use EFB, advanced systems with braking-energy recovery use AGM. The reason is state of charge. With energy recovery the battery is held at roughly 70 %, and Banner calls continuous operation at about 70 % "a guarantee for the premature failure of a conventional battery", which the cyclic load can flatten within a few months; Midtronics says a standard flooded battery in a start-stop car typically fails within 6 to 18 months, with hundreds of extra charge-discharge cycles a day. AGM suits regenerative charging; an EFB is usually kept at a higher charge level and so stores little or none of the recuperated energy.

Cycle-life claims are maker-specific: VARTA claims twice the cycles for EFB and three times for its AGM, Bosch claims up to four times for its S5 A AGM and twice for its S4 E EFB, and Exide claims up to 4x for AGM and up to 3x for EFB. Exide prints a CAUTION label on its conventional ranges so they are not installed in start-stop vehicles.

Where the battery sits matters too. VARTA says only 58 % of batteries are in the engine compartment, 40 % are in the trunk and 2 % in the passenger compartment, and a modern start-stop replacement can take up to 28 steps. Any battery inside the vehicle needs a degassing tube, because it gives off oxyhydrogen; Bosch allows installation angles of up to 90° for its S5 A AGM but 0° for EFB and conventional batteries.

3. Replacing it, and telling the car

Like for like or better. A start-stop battery can only be replaced with a battery of the same type; upgrading EFB to AGM is possible and advisable; fitting a normal battery where an AGM or EFB was results in rapid deterioration. Banner's version: AGM to AGM in an identical housing and similar performance class, EFB to EFB or AGM, and no conventional wet-cell battery where EFB or AGM is original equipment, even if start-stop is switched off. VRLA is another name for AGM; AFB or ECM is another name for EFB. A more powerful battery, higher Ah or higher cold-start current, has no negative effect on the car.

Some cars are specific. The Volvo XC90 (MY2015) takes an H7 AGM 80 Ah / 800 A or H8 AGM 95 Ah / 850 A, and Volvo says the replacement should be done by a trained Volvo technician. The Mazda3 (MY2020) with i-stop takes only a Q-85 or S-95. The Mercedes G-Class (2020) is "equipped with a fleece battery", and the vent hose has to go back on the original side opening or gases or acid could escape.

Banner's order is negative off first, positive back on first. Its torque figures are max 4 Nm for the terminal clamps and max 15 Nm for the base hold-down, and a sensor on the negative terminal tells you it is a start-stop or battery-energy-management car. Refit the negative terminal cover so nobody clamps a charger or jump lead to the post. VARTA says the same: ground cable off first, on last, to prevent damage from an accidental ground with a tool. Renault adds a wait of at least five minutes after switching off the engine before disconnecting, and never with the engine running.

Then the part most people skip: registration. On a car with a battery sensor, the battery management system adapts charging to an ageing battery, and registration tells the engine control module a new one is in, which essentially resets the charging parameters. Skip it and you risk under- or over-charging, start-stop not engaging and a shorter battery life. It needs a scan tool that supports the car's BMS protocol; a basic OBD-II code reader typically is not enough. Banner sorts makers into three groups: self-learning systems (Citroën, Dacia, Fiat, Honda, Mercedes, Peugeot, Renault, Toyota, Volvo among them) that need between 2 hours of operation and up to 6 days; closed systems where registration is recommended or required (Audi, BMW, Jaguar, Land Rover, Mazda, Mini, Seat, Škoda, Volkswagen); and combined systems (Ford, Hyundai, Kia, Lexus, Nissan, Opel/Vauxhall, Porsche, Smart). Porsche: start-stop does not work without registering the new battery. The BEM code is a two-line code specific to maker and battery; it is not available for aftermarket batteries, so multi-brand tools generate it from capacity, manufacturer and technology. Without registration, Banner says fault messages may appear, comfort functions may be limited, and on BMW the battery may be overcharged.

What the makers' own documents say: BMW's bulletin says "Register the new battery using ISTA", and a different capacity means selecting "higher/lower capacity" and running the retrofit. VW's tech tip says a replaced battery or battery monitoring module "must be adapted to the vehicle", or the charging system is compromised, with premature battery failure and fault codes, and start-stop is disabled if the battery is below 12.1 V. Ford dealers run a BMS reset with the scan tool; Snap-on says the alternative is to let the system sit for eight hours to reset itself. Kia's 2024 manual says the sensor calibrates in about 4 hours with the ignition off, then the engine is started and stopped 2 or 3 times. Ford's sensor calibrates state of charge in sleep mode when total draw is below 300 mA, taking 4 to 6 hours; without the reset it keeps the old battery's age counter.

For the whole job step by step, see how to replace a car battery.

Expect things to reset after a disconnection. Kia lists power windows, trip computer, climate control, audio and sunroof; Hyundai lists power windows, utility view, climate control, clock and sunroof; Volvo says the windows' pinch protection must be reset.

4. Resting voltage: what the number means

Switch everything off, let the battery rest, then measure at the terminals. VARTA says a fully charged conventional battery shows about 12.8 V open-circuit, and below 12.4 V it should be recharged as soon as possible, because continuous low charge damages it through sulfation. Banner recharges at 12.50 V off-load and expects about 12.7 to 12.8 V after a recharge. Exide says fully charged is 12.7 V or above and recharge immediately below 12.5 V, measured key-off at the terminals. So the threshold is about 12.4 to 12.5 V depending on whose battery it is. ADAC's simplified rule covers all of them: over 12.4 V is over 60 % and fine; 12.0 to 12.4 V is 25 to 60 % and should be charged; under 12.0 V is under 25 % and must be charged. ADAC also says a voltage reading is only a first indication and says nothing about state of health. Under 12 V, in ADAC's words, nothing works.

The rest matters because of surface charge. VARTA says to switch on a consumer such as the headlights shortly before measuring; Midtronics says headlights on for 30 seconds and then rest a minute if the car was running; ADAC says a few hours standing, not straight after a drive; Exide prefers the car sitting overnight. Banner's figure is about 5 hours after charging ends, or at least 1 hour with no discharge.

Voltage is state of charge, not state of health. Midtronics: a battery reading 12.6 V can still have severely weakened internal conductance. And low charge is not neutral. CTEK says sulphation begins when the voltage falls below 12.4 V even though the car may still start, and Exide says operating at low charge can cause irreversible plate sulfation and permanent capacity loss. GS Yuasa scraps a battery below 11.00 V because that sulfation cannot be completely reversed by charging.

5. Cold and heat

Cold takes power away at the moment the engine wants more. Exide says a fully charged battery has lost about a third of its starting power at 0 °C, and at −18 °C about 40 % of the power remains. Midtronics puts rated capacity at about 80 % at 0 °C and 50 % at −18 °C, so a battery already at 70 % of design capacity may deliver 35 to 40 % on a very cold morning. Those are different measures; keep them apart.

Below 0 °C a lead-acid battery hardly accepts any charging current, so Banner says to warm it above 0 °C before charging. A fully charged battery freezes only at about −60 °C on Banner's charging page and about −70 °C on its FAQ; a discharged one at 12.0 V off-load freezes a few degrees below zero. Never jump-start a frozen flooded battery. Mercedes says the same from the driver's seat: if the instrument-cluster lamps stay off in the cold, the battery has probably frozen and you may neither jump-start the car nor charge the battery; Volkswagen says a battery that has ever been frozen must be replaced. VARTA's winter rule is simple: fully charged batteries can be stored well below zero, partially charged ones can freeze just below zero, so check and recharge below 12.4 V.

Heat does the lasting damage. Banner says self-discharge roughly doubles for each increase of about 10 °C, long exposure to high temperatures increases water consumption and grid corrosion, and charging must stop if the acid exceeds 55 °C.

6. Reading the label

Ah is the 20-hour capacity: a 100 Ah battery delivers 5 A for 20 hours, and if no K-value is printed, K20 is assumed. The cold-start figure is only comparable within one test standard. Under EN 50342 the battery is tested at −18 °C and must hold the current for at least 10 seconds before voltage falls to 7.5 V; the SAE test behind the US CCA figure is also at −18 °C but runs 30 seconds down to 7.2 V, so an "A (EN)" and a "CCA" figure are not the same test. VARTA says the standard (EN, DIN, SAE, JIS or IEC) is printed after the figure and must be entered in the tester, and a DIN value is only about 60 % of the equivalent EN value.

Date codes are mostly not for you. Banner says its production code is encrypted for internal tracking and the purchase receipt is the only warranty proof.

7. Testing it

A conductance tester sends a small signal and reads the response: no load, no heat, no sparks, and it works on a discharged battery as long as it has at least 8 V. The Midtronics verdicts are Good Battery, Good Recharge, Charge & Retest and Replace Battery. VARTA's guide says CHARGE + RETEST shown twice means replace. But the verdict is only as good as the inputs: VARTA found that in about a third of roughly 300,000 tests the technology, standard or CCA entered was wrong, and a "Battery OK" can still appear near end of life. Banner says quick-tester readings are approximate, and on REPLACE to check age and charge before buying.

With a multimeter you can read two more things. Midtronics: above 9.6 V while cranking means the starting system is OK; 13.3 to 15.5 V with the engine running means charging is OK, higher points to the regulator, lower to the alternator or wiring. Yuasa's cut-off is 9.5 V while cranking. An old-school load test is about three times the nominal capacity for about 10 seconds, so a 45 Ah battery at roughly 135 A, with no significant voltage fluctuation.

8. Charging it with a charger

A smart charger charges in stages and then maintains. The CTEK MXS 5.0 runs Normal at 14.4 V / 5 A for 14 to 160 Ah, AGM at 14.7 V / 5 A and Recond at 15.8 V / 1.5 A, with temperature compensation from −20 to +50 °C. NOCO's GENIUS5 runs 12V at 14.5 V / 5 A, 12V AGM at 14.8 V / 5 A and Repair up to 16.5 V, up to 120 Ah. The Bosch C7 runs Mode 1 at 14.4 V / 7 A and Mode 2 cold/AGM at 14.7 V / 7 A, for 14 to 230 Ah. So 14.7 to 14.8 V for AGM is the smart-charger convention; Exide's own guideline puts AGM absorption at 14.1 to 14.4 V, which is why the AGM button exists and why you follow your charger's manual.

Where the black clamp goes. CTEK: red clamp to battery positive, black clamp to the vehicle chassis "remote from the fuel pipe and the battery", mains last, mains off before the clamps, black off before red. CTEK's reason is the battery management system: an unpainted chassis point away from the battery lets the car's BMS recognise that charging is taking place, and only if there is no chassis point does the clamp go on the negative terminal. Banner's rule: battery removed or disconnected, charger minus to battery minus; battery still installed, charger minus to earth. NOCO says the same for negative-ground cars.

How much and how long. Banner: a current of about one tenth of the capacity (60 Ah → 6 A), max 14.8 V, float 13.5 V, and stop if the temperature exceeds +55 °C. VARTA: a full charge is usually achieved by charging overnight, but do not leave the battery on the charger for more than 24 hours, and charge every two months if the car is used sporadically. CTEK's table from empty to 80 %: 60 Ah in 12 hours, 110 Ah in 26 hours. A trickle charger keeps sending charge after the battery is full and can overcharge it; a smart charger goes into pulse maintenance instead, which is why CTEK says it can stay connected.

Recondition and repair modes come with warnings. CTEK's Recond is only to return energy to empty wet and Ca/Ca batteries, once a year or after deep discharge, at up to 15.8 V. NOCO's Repair is 12 V lead-acid only, can cause water loss in flooded batteries, and NOCO says to disconnect the battery before using it. Bosch's regeneration mode is 12 V only, with the battery disconnected from the vehicle first.

9. The car's own charging system

The alternator is no longer a fixed 14 V. BMW's Brake Energy Regeneration charges the battery when you brake or lift off, and decouples the alternator under acceleration. Mazda's i-ELOOP stores braking energy in a capacitor and steps it down through a DC-DC converter. VW Group's energy management watches the battery while parked, shuts down non-essential consumers when charge becomes critical, and can slightly raise the idle to boost alternator output. Ford's Smart Regenerative Charging sets the generator anywhere from 12.2 V to between 14.5 and 14.9 V, with a refresh phase that may briefly reach 15.2 V. That range is why a single "correct alternator voltage" does not exist for a modern car; read your car's figure against its manual.

Toyota says the red battery light indicates a malfunction in the charging system: stop in a safe place immediately and contact a dealer. Ford says to switch off all unnecessary electrical equipment and have the system checked immediately.

Cars also protect the battery from you. Mazda CX-3 interior lights switch off after about 30 minutes; Kia's turn off after approximately 10 minutes. A Renault Clio shows "Battery energy saving mode" after 5 to 30 minutes with the engine off and cuts lighting, radio and sockets, and lists short trips, urban driving, low temperatures and prolonged use of consumers with the ignition off as the causes of low charge. Volvo says to run the engine at least 15 minutes to recharge, that driving charges faster than idling, and to avoid ignition mode II with the engine off.

10. Jump starting, by the manual

The short version, with the clamp order on one page, is in how to jump start a car.

Kia, Mazda and Volkswagen all put the last clamp on a ground point away from the battery, not on the flat battery's negative post, and they give three different reasons. Kia: connecting to the discharged battery's negative terminal can overheat and crack the battery and release acid. Mazda: a spark there could ignite gas around the battery. Volkswagen: the electronics would then make an incorrect evaluation of the battery's condition. HELLA says cars with a battery management system must be jump-started via the jump-start points in the engine compartment, or the sensor can be damaged.

The order, in Volvo's words: red to the donor positive, red to the car's own positive jump-start terminal, black to the donor negative, black last to the car's own negative ground point; off in reverse, black first, and the black clamp must not touch either positive terminal. VARTA's generic version ends on the engine block of the stalled car, away from the battery. BMW says the body ground or a special nut acts as the negative terminal, and if the first attempt fails, wait a few minutes.

The details people get wrong. Both batteries must be 12 V; a 24 V supply, two 12 V batteries in series for example, can permanently damage the starter and ignition system. Volkswagen's cable spec is at least 25 mm² for petrol engines and at least 35 mm² for diesels, and the donor battery must have the same voltage and approximately the same capacity. The cars must not touch, the clamps must be fully insulated, and no mobile phone while connecting. Volvo runs the donor at about 1500 rpm for a few minutes before the attempt and says not to touch the connections during it. Volkswagen says to stop the starter after about 10 seconds and try again after about a minute. Kia runs the car at least about 30 minutes afterwards; Volvo says a flat battery needs at least 1 hour of charging at about +15 °C and 3 to 4 hours in the cold, and recommends an external charger instead.

Hybrids and EVs are not donors. Hyundai says jump-starting another car damages the hybrid's lithium 12 V battery; Renault says an E-Tech must not restart another vehicle; Volvo says never use the 48 V battery to jump-start another vehicle. Tesla says a Model 3 cannot be used to jump-start another vehicle. And no starting aid at all on a frozen, damaged or leaking battery.

A jump pack follows its own manual. The NOCO GB40 is for 12 V lead-acid only, red clamp first, black to the negative terminal or chassis, black off first, 20 to 30 seconds between attempts and no more than five in 15 minutes. It refuses a battery below 2 V; the manual override forces it on but disables spark-proof and reverse-polarity protection.

11. Safety

Batteries generate explosive gases during normal operation, so CTEK says charge in a well-ventilated area, never put the charger directly above the battery, wear eye and clothing protection, never charge a frozen battery and never charge a damaged one. VARTA: never lean over a battery while boosting, testing or charging, keep the terminals protected against shorting, and do not add water. Acid in the eyes is flushed with clean water for at least 15 minutes before seeking medical help. Mercedes adds that neither you nor the battery may be electrostatically charged, and that charging without a maximum-voltage limit can damage the on-board electronics. Filled batteries are transported and stored upright, secured against tipping and shorting.

12. Storage and winter

Store it full, cool and dry, and check it. VARTA's autumn checklist: charge completely, switch off all electrical devices, disconnect if possible, store in a cool, dry place, fully charged. Check before storage and charge below 12.5 V, then at least once every three months. Bosch: about 15 °C out of direct sunlight, check every 2 months, charge below 12.4 V, and below 10.5 V the battery is defective. Banner says a fully charged battery can be stored until it reaches 12.50 V, up to 12 months depending on temperature, checked after roughly three months and monthly if it reads under 12.60 V. A maintenance charger does the checking for you: VARTA says charge every two months if the car is used sporadically, because inadequate storage can lead to deep discharge and irreversible damage.

Before winter, VARTA says to have the battery checked at a workshop at least once a year, clean the terminal clamps to prevent contact resistance, and disconnect the negative first. For corroded terminals, VARTA says to scrub them with a dilute solution of baking soda and water and reapply anti-oxidizing grease before reconnecting; Banner says to clean them and lubricate lightly with acid-free grease.

13. The old battery: recycling and the law

A lead battery is about 60 % lead, 7 % plastic and 30 % acid, and EUROBAT puts the European collection-and-recycling rate at 97.3 % for 2015 to 2017, essentially a closed loop. Clarios says up to 99 % of the materials in its batteries can be recovered and that it turns nearly 200,000 old car batteries into new ones every day.

In the EU, Regulation 2023/1542 makes the seller take the old one back. Producers must take back waste starter batteries free of charge without obliging you to buy a new one, and distributors must take them back regardless of chemistry, brand or origin at or near the point of sale, with distance sellers offering take-back too, including at delivery. The waste chapter applies from 18 August 2025. You, the end-user, must keep waste batteries separate from other waste and take them to a designated collection point; that is what the crossed-out bin means. From 18 August 2025 every battery carries that symbol, with Pb underneath if it contains more than 0.004 % lead.

Germany goes one step further: § 19 BattDG makes the retailer charge a deposit of 7.50 € including VAT if you do not hand in an old starter battery at purchase, refunded when you bring one back; distance sellers refund against a proof of return no older than two weeks. Every German retailer must take back used starter batteries free of charge, and the Umweltbundesamt says vehicle batteries are recycled without exception; they do not go in the household bin.

On warranty: Banner's GB terms cover material and manufacturing defects and say the dated purchase receipt is the only documentation; East Penn and Interstate in the US exclude batteries that are merely discharged, frozen, incorrectly charged or not properly maintained. A battery that is merely flat is not covered by those warranties.

Do this

• Measure resting voltage after the car has sat for hours, headlights on briefly first. Recharge at about 12.4 to 12.5 V; under 12 V, charge now and test.
• Short trips only? Put a smart charger on it every couple of months. Black clamp on the chassis, red on the post, mains last.
• Replacing: same type, or EFB up to AGM; never a flooded battery where EFB or AGM was fitted. Negative off first, on last. Clamp max 4 Nm, hold-down max 15 Nm.
• Battery sensor on the negative post? Register the new battery, or let the car relearn by the manual.
• Jump-starting: last clamp on the ground point your manual shows, away from the battery. 12 V donors only, no hybrids or EVs, never a frozen battery.
• The old battery goes back to the seller, free. In Germany you get 7.50 € back.

Rev, the AI mechanic in GarageHub, logs the battery's age, each resting voltage you read and the last charge, and reminds you before autumn.