How long to charge a car battery? (Complete guide for UK drivers)

For most drivers, you won’t give your car battery a second thought until something goes wrong. A reluctant start on a cold morning, a flat battery from leaving the lights on, or a vehicle that’s been sitting without movement for a little too long. The first question that comes to mind: How long to charge a car battery? And the honest answer, it all depends on a few different things. 

Here, we break that question down for you. We look at battery size, how flat it is, and the type of charger you’re using, all of which can affect charging time. We also share some realistic timeframes so you know whether you’ll be waiting a few hours or need a full overnight charge. 

At Battery2U, we’re here to help drivers choose the right batteries every day. So whether you’re trying to revive a flat battery or just keep your car healthy between journeys, this guide will help you charge safely, efficiently, and without guesswork. 

Quick answer: How long to charge a car battery? 

A ballpark figure is: 

For most 12V car batteries, you can expect a charge from around three to four hours up to 24 hours or more to get a useful charge. The exact time will depend on: 

• The size of the battery 

• How discharged it is 

• The output of your charger 

• The type of charger 

For a very rough rule of thumb: 

• A typical 60Ah car battery on a 10A smart charger will usually take 5-6 hours to bring it from a very low to a nearly full charge. 

• The same battery on a small A2 trickle charger will easily take 24 hours or more if the battery is deeply discharged. 

Key factors that affect car battery charging time 

How long it takes to charge a car battery can depend on several factors. Here’s what every driver should know: 

Battery capacity 

Your car battery’s capacity, measured in amp-hours (Ah), indicates how much energy it can store, much like the size of a fuel tank. 

• A higher Ah rating means the battery can power electrical systems for longer before going flat. 

• However, this means it will take longer to recharge at the same charging rate. 

Most modern petrol and diesel cars use batteries between 40Ah and 80Ah, SUVS, and vans will typically have bigger batteries. 

How discharged the battery is 

Not all “flat” batteries are equally empty. The more a battery is discharged, the longer it will take to recover. 

• Maybe you left your headlights on for an hour. The battery’s voltage has dropped a little, but it still holds most of its charge. In this case, a smart charger may top it up in just a few hours. 

• If the battery has been sitting dead for days or weeks, it will take much longer to recharge. Deep discharge can cause sulphation, something that reduces the battery’s ability to hold a charge permanently. 

Charger output

A charger’s amp rating (A) determines how quickly electricity flows into the battery, think of it like how fast fuel is pumped into a tank. 

Typical charger outputs: 

• 2A: Gentle trickle chargers 

• 4-6A: Standard domestic chargers

• 8-10A: Faster smart chargers

• 20A+: Heavy-duty or workshop units 

1. Lower amps might be slower, but they’re also gentler on the battery. These are ideal for maintenance or classic cars. 

2. Higher amps are faster, but they can cause heat buildup and stress if overused or used on small batteries. 

Charger type 

Not all chargers are created equal. The type you use can impact charging speed and battery health. 

• Conventional chargers will deliver a fixed current. The user will need to monitor the progress and manually unplug when the battery is full to avoid overcharging. 

• Smart chargers will automatically adjust the charging rate as the battery fills up. Once full, they switch to a maintenance mode, making them ideal for everyday drivers. 

• Trickle chargers deliver a very low current to keep a battery topped up over long periods, ideal for stored vehicles or classic cars. 

• Boost chargers give you a quick burst of high current to get you started in an emergency. Handy if you’re stranded, but overuse can shorten your battery lifespan. 

Battery condition and age 

A new healthy battery charges quickly and efficiently. The older your battery, the longer it’ll take, and it may never reach full capacity. If your battery is struggling to hold a charge even after a full cycle, it might be time for a replacement. 

Temperature and environment 

Both cold and hot conditions can affect charging efficiency: 

• Cold weather will slow down the chemical reactions inside the battery, extending the charging time. 

• Hot weather can cause overheating and damage if you don’t monitor the charging. 

Simple way to estimate your car battery charge time 

Real-world charging time varies, but you can use a simple formula to get a rough estimate. 

The basic formula 

A quick rule of thumb is: 

Charging time = (Battery Ah / Charger amps) x 1.2 

That extra 1.2 factor accounts for energy losses during charging, as no process is 100% efficient. 

Example: A typical UK car has a 60 Ah battery, and you’re using a 5 A smart charger. So: 

(60 / 5) x 1.2 = 14.4 hours 

It would take roughly 14 hours to fully recharge from near-empty, though a smart charger may do the job quicker as it will optimise the charge throughout the process. 

Different ways to charge a car battery (and how long they take)

There are a few different ways you can charge a car battery, but not all methods are equally effective. Here’s what to know: 

Using a mains car battery charger 

For most, a mains-powered charger is the safest and most reliable way to charge a car battery at home. It gives you full control over the process, helps extend battery life, and avoids the risk of damaging electrical systems. 

Smart chargers are a great tool to have because they: 

• Automatically cuts off when the battery is full, preventing overcharging

• Offer reconditioning and desulphation modes to help revive weak batteries 

• Switch to maintenance mode for a safe long-term connection 

At Battery2U, we stock a wide range of car battery chargers for everything from small city cars to larger SUVs and vans, all designed for easy, safe home use. 

Charging a car battery by driving 

Your car’s alternator charges the battery whenever the engine is running. Converting mechanical energy into electrical energy, replenishing the battery as you drive. 

But the rate of charge depends on engine speed and duration: 

• A 20-30 minute drive on A-roads or motorways can restore a good amount of charge to a slightly low battery. 

• Repeated short trips or long periods of idling rarely provide enough time for a full recharge; this could lead to a drained battery over time. 

Jump starting vs proper charging 

A jump start or a set of jump leads can get you back on the road quickly, but it doesn’t fully recharge the battery. It gives you enough power to start the engine, then the alternator takes over. 

After a jump start, the battery will still be partially discharged, so it could struggle to hold a charge, and you could risk overworking the alternator. To recharge the battery, you’ll have to connect it to a mains charger as soon as possible. 

How to safely charge your car battery at home 

Charging a car battery at home is straightforward, and if you follow a few basic safety steps you can get the job done right. 

Before you start: 

• Park on a level surface and apply the handbrake. 

• Make sure the engine is off and the keys are out. 

• Keep good ventilation, don’t charge a battery near open flames or sparks. 

• Check your battery type and make sure your charger can be used with it. 

Connect the charger correctly

1. Identify the terminals: Positive is usually marked in red, negative is usually black. 

2. Connect the leads: Red clamp attached to the positive terminal. Black clamp to the metal earth point on the vehicle chassis. 

3. Set the charger: Choose the correct voltage, typically 12V for most modern cars. Select the correct mode, depending on the charger you have. 

4. Switch on the charger 

What to look for during charge

While charging, the indicator lights or display will show the progress. 

When to stop: 

• Most smart chargers show when charging is complete and automatically switch to maintenance mode. 

• If the battery feels hot or emits a strong smell, stop charging immediately and get it checked by a professional. 

How to tell when your car battery is fully charged

Knowing when your battery is fully charged is a safety issue; here’s how to check:

Visual signs from a smart charger 

Most smart chargers will tell you when charging is complete; they’ll either have a green light or the display will read “full”.

Using a multimeter 

For those comfortable with DIY testing, a digital multimeter can measure battery voltage: 

A fully charged 12V lead-acid battery typically reads around 12.6-12.8V at rest, after being disconnected for a few hours.  

When charging isn’t enough: Signs you need a new battery 

Sometimes a battery simply can’t be revived. Here’s when to know when it’s time to replace: 

• Slow cranking even after a full charge 

• Dashboard battery or charging warning lights 

• Frequent need to recharge or jump-start 

• Visible damage, swelling or leaking 

When to stop trying to revive a battery 

Most car batteries will last around between four and six years under typical UK driving conditions. If your battery: 

• Fails to hold a charge after repeated attempts 

• Shows signs of physical deterioration

It’s safer and more economical to replace it than risk breakdowns or alternator stress. 

How long does it take to charge a car battery… A roundup 

Charging your bar battery properly is key to keeping your vehicle reliable, especially in varying weather conditions. While there are many different factors that should be considered when it comes to charging a car battery, you can expect to need to charge a car battery for at least three hours to get a decent charge and be able to use your car for what it’s needed. 

If you find that charging your car battery is no longer working and need a replacement, then get in touch with us at Barry2U and we’ll help you find the car battery you need and get it delivered as quickly as possible. 

FAQs 

How long does charging take for a completely dead car battery? 

Charging time depends on the capacity of the battery - Ah - and the output of the charger. For a common 60Ah battery:

• 2A trickle charger: 20–30 hours
  

• 5A smart charger: 12–15 hours
  

• 10A smart charger: 5–7 hours 

Can I leave a smart charger on overnight?

Yes, smart chargers are designed to be connected in the long term. When the battery is fully charged, maintenance mode kicks in to maintain it at the full mark without overcharging. Thus, they're ideal for overnight charging and for stored vehicles.

How long should I trickle charge a car battery? 

Trickle chargers supply a very low current, typically 2A or less, to maintain a battery over time. For a partially discharged battery, you might expect:

• 20–30 hours for full charge (40–60Ah battery). 

You can safely leave a trickle charger connected for long-term storage, as it allows the battery to stay topped up without sustaining any damage from overcharging.

Which is better: fast charging or slow charging of car batteries? 

• Slow Charging: Lighter on the battery itself, reduces heat and stress; best for older batteries or sensitive ones.

• Fast Charging: Faster, however generates more heat and can shorten battery life when overused.

Rule of thumb: For regular maintenance, stick with a smart charger or slower charge, saving fast/boost modes for true emergencies.

How often should I charge a car that I don’t drive much? 

If your vehicle is used infrequently, charge it every 2–4 weeks with a smart charger or maintainer. Use a trickle charger to prevent the battery from going flat if the car is going into long-term storage.

Will idling my car charge the battery?

Technically yes, the alternator charges the battery while the engine runs, but:

• Short trips or extended idling are inefficient.
  

• 20–30 minutes of steady A-road or motorway driving can restore a small amount of charge to a slightly low battery.
  

• Driving is best for topping up, not for fully recovering a completely flat battery.

Can I overcharge my car battery with a charger?

Yes, you can overcharge a car battery with a conventional charger. If you leave it for too long it’ll overheat, cause water loss and even permanent damage.