Charging a 12-Volt Car Battery: How Long It Really Takes

The simple formula for charging time

Charging time depends on three things: how much charge the battery has lost, the current the charger delivers, and the efficiency of the process. A starter battery is rated in amp-hours (Ah), for example 60 or 70 Ah, and a charger in amps (A).

Hours ≈ (battery capacity in Ah × share to be refilled) ÷ charger current in A × 1.2

The factor of 1.2 covers energy lost as heat and the slower final phase. Example: a 70 Ah battery at 50% charge needs about 35 Ah. With a 5 A charger: 35 ÷ 5 × 1.2 ≈ 8.4 hours.

The old version of this page gave a rule that is still useful: choose a charging current of about 10% of the battery's capacity, so 6.5 A for a 65 Ah battery and 8 A for an 80 Ah battery. At that rate a completely flat battery needs at least 10 hours. Battery University notes that manufacturers recommend no more than about 0.3C, which is 18 A for a 60 Ah battery, and that higher currents must be reduced as the battery nears full charge.

Charging time by charger current

These are our estimates with the formula above. Modern smart chargers reduce the current as the battery fills, so the last stage adds several hours in practice. Battery University gives 12–16 hours for a full charge of a lead-acid battery with a conventional three-stage method.

What happens inside: the three charging stages

A good charger does not push the same current all the way. It works in stages:

1. Bulk (constant current). The charger delivers its full current until the battery voltage reaches the limit. This stage brings a lead-acid battery to about 70% in 5–8 hours.
2. Absorption or topping. The voltage is held while the current falls. The remaining 30% takes another 7–10 hours. The battery counts as full when the current drops to 3–5% of its Ah rating.
3. Float. A low voltage holds the battery at full charge and makes up for self-discharge.

The voltage limit for a 12-volt battery is roughly 13.8–14.7 V, which is 2.30–2.45 V for each of its six cells, and at around 14.4 V a normal lead-acid battery starts to gas, splitting water into hydrogen and oxygen. A battery should not be held at topping voltage for more than 48 hours. Cold batteries need slightly more voltage and hot ones less; good chargers adjust automatically, by about 3 mV per cell for each degree Celsius.

How to check the state of charge

These reference values come from Battery University for a starter battery at about 26 °C. The voltage only means something after the battery has rested without load or charging for at least four hours; battery makers recommend 24 hours. Straight after charging, the reading is temporarily higher.

On a battery with removable caps, a hydrometer measures the electrolyte density in each cell. The old version of this page gave 1.26–1.30 g/cm³ for a healthy, charged battery, which matches the table.

Charger types compared

We give relative costs because charger prices vary widely between shops and countries.

Flooded, EFB and AGM batteries

• Flooded (wet) batteries have liquid electrolyte. Check the level every few weeks in heavy use and top up with distilled water after charging, not before, unless the plates are exposed.
• EFB batteries (enhanced flooded) are a stronger flooded design used in many cars with start-stop systems.
• AGM batteries hold the electrolyte in a glass mat. They are sealed and tolerate overcharging less well than flooded batteries, so use a charger with an AGM setting.

Cars with start-stop often have a battery sensor on the negative terminal and recommend charging through the jump-start points under the bonnet. The owner's manual shows where they are and whether the battery must be registered after replacement.

Step by step: charging a battery safely

1. Ventilate. Charge outdoors or in a ventilated garage. Hydrogen from a charging battery can ignite, and a spark when connecting a charger can cause an explosion.
2. Clean the terminals. Wipe the top of the battery and the posts; dirt and moisture can create a leakage path between the terminals.
3. Connect with the charger off. Red clamp to the positive terminal, black clamp to the negative terminal or the earth point named in the manual.
4. Choose the mode for battery type and size, then switch the charger on.
5. Wait for the charger to report full, or until the current has dropped to a few per cent of the Ah rating.
6. Switch off before disconnecting, remove the black clamp first, then the red one.

Avoid charging a frozen battery or one above about 49 °C. A deeply discharged battery freezes more easily in winter, which is one more reason to keep it charged during cold spells; our guide to driving on ice covers other winter preparation, and our advice on choosing winter tyres completes the list.

Common mistakes when charging

• Judging charge by voltage straight after charging. The surface charge makes the reading look better than it is; wait several hours.
• Using a high current to the end. Fast bulk charging is fine, but the current must fall as the battery fills, or water and plates suffer.
• Charging an AGM battery in the flooded mode. The higher voltage can damage it.
• Leaving an old charger without cut-off connected for days. It keeps the battery at gassing voltage and dries it out.
• Ignoring a battery that never reaches full. If the voltage stays below about 12.4 V after a complete charge and rest, the battery is probably worn.

Can driving recharge a flat battery?

Only partly, because the alternator charges best at steady speed, and a short trip mostly replaces the energy used to start the engine. Battery maker guidance suggests regular drives of 20–30 minutes on main roads to keep a healthy battery topped up, but a battery that was flat needs a mains charger.

After a jump start, a quick charge also has limits. A short burst of current may only charge the surface of the plates, so the car can fail to restart until the charge has spread through the battery over several hours. For reference, headlights left on draw about 6 A, so they can empty a 60 Ah battery overnight.

Keeping a battery healthy in storage

A lead-acid battery that stays partly discharged builds up lead sulfate crystals, which permanently reduce its capacity. For a car that stands for weeks, a 1–2 A maintainer is the simplest answer. Without one, charge a stored battery about once a month; Battery University suggests topping up flooded batteries in long storage at least every six months and AGM batteries every 6–12 months.

Starter batteries are not designed for deep discharge. Repeated flattening shortens their life considerably, and a battery that will not hold a charge after a full cycle is due for replacement. The old version of this page quoted a typical life of four to five years.

This guide covers 12-volt starter batteries. Charging the traction battery of an electric car is a different subject; our electric cars section and the Nissan Leaf battery guide explain those times and chargers. Terms such as amp-hour or AGM are also in our dictionary of automotive terms.