How Long Does It Take To Charge A Car Battery?

What’s the most annoying sound in the world? It might be the one your car makes when you turn the key, and nothing happens.

We expect and count on that engine to roar to life every time we get behind the wheel. When it doesn’t, you can feel the frustration surging through your body – which is ironic, considering nothing is surging through your car.

The likely culprit is a dead battery. If you need to get on your way ASAP, you’ll need a set of jumper cables or a jump starter. But if you’ve got the time, you can hook up to a battery charger and set your battery right again.

But how long does it take to charge a car battery?

This article will explore the many variables that affect car battery charging time.

Before the end, you should have a good idea of whether you’ll be going to work late, or taking the day off.

Factors That Affect Car Battery Charge Times

Consider a swimming pool. If you want to know how long it takes to fill the pool up, you’ll need to know the volume of the pool, how much water is already in it, and how much water is coming from the hose.

All the same variables apply to recharging a car battery:

• How drained is the battery? – You can test this with a proper car battery tester.
• What is the capacity of the car battery?
• Has it been attached to a battery maintainer if not in use?
• What is the amperage of the charger?

Let’s take a quick look at each variable and see how they all fit together into a simple equation.

How Dead Is It?

A “dead” car battery isn’t always dead. It’s kind of like that scene in The Princess Bride when Billy Crystal explains that Carey Elwes is only “mostly dead.”

Just because a battery isn’t starting your car, that doesn’t mean you have a completely dead battery. How long it takes to recharge will partly depend on how depleted the battery is to begin with.

Big Batteries vs. Small Batteries

Just as not all cars are the same size, not all batteries are the same size, either. That makes sense; a compact car doesn’t need a battery big enough to start a V8 4×4 quad cab pickup.

So, if you had, say, a 40Ah and an 80Ah battery, it would theoretically take twice as long to charge the 80Ah one, assuming the same level of depletion and rate of charge.

Rate of Charge

Different chargers recharge batteries at different rates.

Obviously, how fast the power is flowing back into the car battery plays a part in determining overall charge time. A 25A charger will charge more quickly than a 12A charger or a 2A trickle charger.

Note on charge rates: Most car battery chargers these days are “intelligent” (i.e. smart chargers), meaning they adjust the rate of charge as the battery fills up. Therefore, any calculation you might make to figure out how long your charge time will be will likely be a bit off. This is because the charge rate will slow down as you approach 100% charged.

Other Variables

Other factors can impact the rate at which car batteries charge. These include the condition of your car’s battery, its ability to take and hold a charge, and the condition of the terminals.

To a lesser extent, the length and gauge of the charging wires will also affect charge times. Shorter, heavier gauge cables will charge faster than long, lightweight wires.

Let’s Do Some Math

Sorry, I know you were hoping to dodge the M-word, but it’s unavoidable in this situation. Here’s a basic formula to help you calculate your charge time, more or less:

Charge time (Hrs) = (Amp hours – (Charge percentage x Amp hours)) / Charger amps

Get your amp hours off your car battery (most car batteries provide this), and the charger amps off the charger. Again, the charger amps may vary, but just go with the maximum rate you’ll be using.

Next, determine your depth of discharge.

You’ll need a voltmeter or car battery tester, and our article for guidance on how to test a car battery for this, by the way. A fully charged car battery (12V) at rest should have a reading of about 12.7V. From there, the percentage of depletion goes something like this:

• 5V – 90%
• 42V – 80%
• 32V – 70%
• 2V – 60%
• 06V – 50%
• 9V – 40%
• 79V – 30%
• 58V – 20%
• 31V – 10%
• 5V – 0%

The manufacturer of your battery may provide an accurate chart, but this is a good guideline for most 12V lead-acid batteries.

Knowing the depth of discharge allows you to calculate how many amp hours you’ve used and how many are left.

Divide the remaining amp-hours by the amps of the charger and the result is how long, in hours, it will take to replenish the flat battery fully.

It’s not going to be 100% accurate, but it’ll be close enough unless you’re using this to time a roast in the oven.

Let’s put the equation to the test, shall we?

Imagine you have a 45Ah battery. You test it and get a reading of 12.2. According to our chart, that means the battery is at 60%. That means of 45Ah we have 27Ah remaining, so we need to put 18Ah back in. So, we hook up to our 12-amp charger (just pulled that number out of the air), and we’ll be back to 100% in about 1.5 hours.

Fast Charging Versus Slow Charging My Car Battery

Most likely, you want to charge your car battery ASAP. However, it’s easier on your battery to take your time.

Recharging a battery creates heat, and the faster you do it, the more heat there is. If you try to fast charge a deeply depleted battery, you’ll be generating heat for an extended period. This can damage the battery, or even start a fire.

Slow charging is best for a deeply depleted battery. If you need just a few amp-hours, go ahead and crank it up a bit for a short time.

Charging On the Road

If you don’t own a charger, you’ll need to get your car jump-started. You can do this with a jump starter, jumper cables, or by calling your roadside assistance service and letting them worry about it.

Once the car is running, the alternator will go about its usual task of recharging the battery. How long until it’s recharged?

If you let your car idle for about 20-30 minutes (depends on how many RPMs your car engine idles at) that should return enough charge to restart later without trouble. To fully recharge, take it for a spin for about an hour, and try not to use the radio, DVD, lights, or anything else that draws electricity.

Which Charger to Use – Standard Chargers, Trickle Chargers, or Maintainers? 

All three types of charger are useful but have different applications. Below, we provide a brief overview of 

Standard Chargers

Standard chargers provide higher voltage to the battery than other types of charger, thereby charging it much faster. In short, this is the type of charger you need to get a full charge quickly when your car battery is highly depleted or completely dead. 

Trickle Chargers

This type of smart charger is best for maintaining your car battery’s health over the longer term. They charge car batteries very slowly and switch off automatically when your vehicle’s battery is fully charged. This helps to avoid overcharging and overheating and saves you the hassle of waiting around until you have a fully charged battery. 

On the downside, trickle charging doesn’t deliver the power needed to resuscitate dead car batteries quickly if you’re in a hurry. 

Battery Maintainers

An idle battery can go from fully charged to dead as a dodo in less than two months.

If you tend to leave your car idle for long periods of time, this is the type of charger you need to avoid having to jump-start your car every time you start using it again. 

Maintainers are basically small chargers that drip-feed power into your car battery over longer periods of time, allowing you to fully charge the battery passively. This can help to extend battery life and will also ensure you won’t rock up to your car after an extended period of neglect and discover the battery’s dead.

Final Thoughts

In the end, the important thing is to charge your battery carefully to avoid damaging it. No point adding insult to injury by wasting hours only to end up needing to replace a faulty battery! (Or, adding injury to insult by hurting yourself with a dangerous recharge.)

Thanks for checking in with us today, and letting us share in your automotive journey. And all that new knowledge we gave you? Ironically, there’s no charge for that.