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 immediately, 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?

Close up of a car battery charging, still in the carThis 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 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” 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 it’s completely drained. How long it takes to recharge will partly depend on how depleted the battery is to begin with.

Big Battery vs. Small Battery

Just as not all vehicles 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 battery and an 80Ah battery, it would theoretically take twice as long to charge the 80Ah battery, 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 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 chargers these days are “intelligent,” 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 charge times. These include the condition of the 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 battery, 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 12V battery 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 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 your car battery charged up 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 battery charger, you’ll need toget your car jumpstarted. 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 vehicle is running, the alternator will go about its usual task of recharging the battery. How long until it’s recharged?

If you let your vehicle idle for about 20-30 minutes (depends on how many RPMs your 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.

Final Thoughts

So what’s the verdict? Do you have time for lunch before you’re ready to roll, or time to binge watch a season of your favorite show?

In the end, the important thing is to charge carefully to avoid damaging your battery. No point adding insult to injury by wasting hours only to end up needing a replacement! (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.

From childhood go karting and motocross, to collecting and obsessing over scalextric, matchbox and radio controlled cars, I've always had an obsession with cars. Learning through manuals, books, trial and error, and more knowledgeable family members, I've also enjoyed tinkering with the mechanics and electronics of any vehicles I've owned. Now, over 3 decades later, I've started this site as a place for me to share my knowledge, to teach others how to care for and maintain their vehicles themselves, at home, so they can get the most of their vehicles and save a pretty penny compared to always seeking out professional help.

Leave a Comment