The alternator charges the car battery while the engine runs. Its voltage regulator holds the system at roughly 13.8 to 14.4 V, a little above the battery’s rested 12.6 V, so current flows back into the battery and replaces the charge used for starting. Off the car, a battery is recharged with a charger using one of a few battery charging methods: constant current, constant voltage, multi-stage smart charging, trickle or float charging, boost charging and, for flooded batteries only, equalising.
How the battery itself is built, and its chemistry, is on our companion page on the lead acid battery in automobiles.
What charges a car battery while you drive?
The engine drives the alternator through the accessory belt. The alternator makes three-phase AC, and a diode rectifier inside it turns that into DC. The voltage regulator then controls the rotor’s field current so the output voltage stays inside a set window whatever the engine speed or electrical load.
- Normal charging voltage: with the engine running and accessories off, a healthy system usually reads about 13.8 to 14.2 V at idle, and roughly 13.7 to 14.7 V across different conditions. Some makers specify higher.
- Temperature: the regulator raises the voltage when cold and lowers it when hot, because a lead acid battery accepts charge less readily in the cold and gasses more in the heat.
Older vehicles did the same job with a DC dynamo and a cut-out relay; the dynamo gave little output at idle, which is why it was replaced by the alternator.
Smart (variable-voltage) charging in start-stop cars
Cars with engine start-stop and brake energy recovery do not hold a fixed voltage. A battery sensor on the negative terminal reports current, voltage and temperature to the battery management system, and the ECU varies the alternator voltage. It charges hard when the car is coasting or braking (energy that would otherwise be lost), and eases off during acceleration to cut engine load. A voltage that swings between about 12.5 V and 15 V on a drive is normal for such a car, not a fault.
These cars use EFB or AGM batteries, which tolerate staying partly charged and cycling many times a day.
Battery charging methods used in the workshop
Figures below are for a 12 V flooded lead acid battery (six cells); follow the battery label if it differs.
1. Constant current charging
The charger holds the current fixed and lets the voltage rise as the battery fills. The traditional workshop rate is about one-tenth of the Ah capacity, so 3.5 A for a 35 Ah battery. It is simple, and several batteries can be charged in series on one charger. The drawback is that the current does not fall as the battery nears full, so the end of charge must be watched or timed, or the battery will gas heavily and lose water.
2. Constant voltage (constant potential) charging
The charger holds a fixed voltage, typically 13.8 to 14.7 V for a 12 V battery (2.30 to 2.45 V per cell). A flat battery draws a large current at first; as its voltage rises, the difference shrinks and the current tapers off by itself. The alternator works the same way. The risk is an excessive current at the start if the charger has no limit.
3. Constant potential with current limit (taper charging)
Most practical chargers combine the two: they cap the current at the start, then switch to a fixed voltage once the battery reaches it. The current then tapers down on its own.
4. Multi-stage (smart) charging: bulk, absorption, float
A microcontroller charger runs the battery through three stages. Battery University gives these figures for lead acid:
- Bulk (constant current): brings the battery to about 70% charge in roughly 5 to 8 hours.
- Absorption (constant voltage): holds 2.30 to 2.45 V per cell while the current tapers. This last 30% takes another 7 to 10 hours. The battery is full when the current falls to about 3 to 5% of the Ah rating.
- Float: drops to about 2.25 to 2.27 V per cell (13.5 to 13.6 V) and holds the battery full indefinitely.
This is the method to use at home.
5. Trickle and float charging
A trickle charger supplies a small, steady current to make up for self-discharge in a stored vehicle. A true float charger holds a fixed float voltage instead, so the current drops almost to zero once the battery is full. A cheap unregulated trickle charger left on for weeks will overcharge; a float or smart “maintainer” will not.
6. Boost (fast) charging
Boost charging pushes a high current for a short time to get a flat battery back to a usable state quickly, for example to start a vehicle in a workshop. It restores part of the charge, not all of it, and it heats the battery. Finish with a normal slow charge.
7. Equalising charge
An equalising charge is a deliberate controlled overcharge, about 2.50 V per cell or roughly 15 V for a 12 V battery that brings all cells of a flooded battery to the same state and breaks up light sulphation. Battery University suggests doing it only when the specific gravity differs between cells by 0.030 or more, checking the gravity every hour and stopping when it no longer rises. Do not equalise sealed maintenance-free, AGM or gel batteries; the lost water cannot be replaced.
Comparison of battery charging methods
| Method | What the charger controls | Main advantage | Main drawback | Typical use |
|---|---|---|---|---|
| Constant current | Current (about C/10) | Simple; charge time easy to calculate; batteries in series | Overcharges if not stopped | Old workshop chargers, initial charge |
| Constant voltage | Voltage (13.8 to 14.7 V) | Current tapers by itself | High inrush on a flat battery | Alternator, simple chargers |
| CV with current limit | Current, then voltage | Safe start and self-limiting finish | Slower than boost | Most bench chargers |
| Multi-stage smart | Bulk, absorption, float | Full charge without overcharge; battery-type modes | Costs more | Home and service chargers |
| Trickle / float | Small current or float voltage | Keeps a stored battery full | Unregulated trickle can overcharge | Stored cars, bikes, standby sets |
| Boost | High current, short time | Fast partial recovery | Heat; incomplete charge | Workshop emergencies |
| Equalising | About 15 V, controlled time | Evens out cells, reverses light sulphation | Water loss; flooded only | Flooded batteries with uneven cells |
Worked example: how long will it take to charge?
The simple formula is:
t = Ah to be replaced / (charging current x charge efficiency)
The efficiency term is there because not every ampere-hour pushed in is stored; some is lost to gassing and heat. For lead acid the coulombic efficiency is typically around 85%, so use 0.85.
Part A. A 35 Ah car battery is at 50% charge. How long on a 3.5 A constant current charger?
- Ah to be replaced = 0.5 x 35 = 17.5 Ah
- Without losses: t = 17.5 / 3.5 = 5.0 h
- With losses: t = 17.5 / (3.5 x 0.85) = 17.5 / 2.975 = 5.9 h
In practice a smart charger will take longer than this, because in the absorption stage the current tapers well below 3.5 A. Treat the formula as the minimum time, and plan on an overnight charge.
Part B. How long does the alternator need to put back one engine start? Our lead acid battery page works out that a 3-second start at 150 A uses about 0.125 Ah. If the alternator has 10 A spare for the battery after the car’s own loads:
- t = 0.125 / (10 x 0.85) = 0.0147 h = about 53 seconds
So a normal drive easily replaces a start. What a short drive cannot do is recover a deeply flat battery: 17.5 Ah at 10 A would need more than two hours of driving, which is why a battery flattened by headlamps left on should go on a charger.
Safe charging practice
- Ventilate. Near full charge the battery splits water into hydrogen and oxygen. Hydrogen becomes explosive at about 4% in air. Charge in an open, ventilated space with no flames, smoking or sparks nearby.
- Connect and disconnect with the charger switched off, so the last clamp does not spark at the battery.
- Temperature compensation. Charge voltage should fall by about 3 mV per cell for each °C above 25°C and rise by the same below it, which is about 18 mV per °C for a 12 V battery. On a 35°C Indian summer day, a 14.4 V setting becomes 14.4 − 10 x 0.018 = 14.22 V. Stop charging if the battery case gets hot; Battery University gives 49°C as the upper charge temperature.
- Never charge a frozen, bulging or leaking battery.
- Flooded batteries: check the electrolyte covers the plates before charging, and top up with distilled water only after charging, when the level has risen.
Jump-starting in the right order
Jump-starting is not a charging method; it borrows cranking current from another battery. With both engines off:
- Red lead to the flat battery’s positive (+) terminal.
- Other end of the red lead to the donor battery’s positive terminal.
- Black lead to the donor battery’s negative (−) terminal.
- Other end of the black lead to a bare metal earth point on the dead car’s engine or body, away from the battery, so any spark is away from its gas.
- Start the donor car, then the dead car. Remove the leads in the exact reverse order.
Charging AGM and EFB batteries vs flooded
- Flooded (including sealed MF): tolerate the widest range of settings; only open-vent types can be equalised.
- EFB: a flooded battery with stronger plates; it charges on the normal flooded or standard setting of a smart charger.
- AGM: a valve-regulated battery. It has low internal resistance and accepts charge fast, but it cannot lose water. Use a charger with an AGM mode, stay within the voltage printed on the battery, and never equalise or boost it with an unregulated charger.
In start-stop cars the battery sensor tracks the battery. Varta warns that an AGM car must get an AGM replacement; the wrong type leads to start-stop faults and short battery life.
How to tell when the battery is fully charged
- At a constant absorption voltage, the charging current has fallen to about 3 to 5% of the Ah rating and stays there. For a 35 Ah battery that is roughly 1 to 1.75 A.
- After a rest of several hours off charge, the open-circuit voltage is about 12.6 V or higher, or the hydrometer reading on each flooded cell matches the full-charge figure. Use the state-of-charge table on our lead acid battery page; do not read the voltage straight after charging, because surface charge makes it read high.
A note on alkaline cells
Only secondary (rechargeable) cells like lead acid can be charged by any of the methods above. An ordinary alkaline cell, the kind in a torch or car key fob, is a primary cell: zinc powder in a potassium hydroxide electrolyte against a manganese dioxide cathode. Trying to charge one can make it leak or burst.
For lectures on automotive electrical systems, see NPTEL. How the charged battery then cranks the engine is covered in our page on the starting system.
FAQs
What charges a car battery?
The alternator, driven by the engine through a belt, charges the car battery whenever the engine is running. Its regulator keeps the voltage at about 13.8 to 14.4 V so current flows into the battery. With the engine off, only an external charger or solar maintainer can recharge it.
What are the main battery charging methods?
Constant current, constant voltage, constant voltage with current limit, multi-stage smart charging (bulk, absorption, float), trickle or float charging, boost charging, and equalising charge for flooded batteries.
What is the difference between constant current and constant voltage charging?
Constant current holds the current fixed and lets the voltage rise, so it must be stopped at the right time. Constant voltage holds the voltage fixed, so the current falls by itself as the battery fills. Most modern chargers start with constant current and finish with constant voltage.
How long does it take to charge a car battery?
Divide the ampere-hours to be replaced by the charging current times about 0.85. A 35 Ah battery at half charge on a 3.5 A charger needs at least 17.5 / (3.5 x 0.85) = 5.9 hours, and longer in practice as the current tapers.