How Long Does it Take to Charge a Car Battery? [Easy Calculator]

If you recently needed a jump-start to get your car running or things are acting a little sluggish when you turn the key, it might be the perfect time to hook your battery up to a dedicated charger for a bit to get your car battery back into optimal health.

The question that a lot of people have is how long it will take to charge a car battery so that they’ll either know when they can be back on the road or so they’ll know when to disconnect the charger so that they don’t ruin the battery from overcharging.

If you’re just looking for a quick and easy answer, I’ve got you covered below.

A weak car battery can be charged in 2 to 6 hours, whereas a severely depleted battery may take 10 to 15 hours to charge with an appropriately sized battery charger.

If you were looking to see how long YOUR battery will take to charge in YOUR particular situation, then I encourage you to keep on reading and to use the simple calculators in this article to find your answer. It’ll be simple, I promise!

If you were wondering how long it takes to charge a car while running your car at idle, you can check out the experiment I did here.

Without further delay, let’s get your questions answered!

3 Things to Identify to Estimate Battery Charging Time

For practical purposes, I’m just going to stick with the “big three” when it comes to what you need to know to estimate how long it will take to charge your car battery. We could get into the weeds with other things, but this will keep things short and sweet and you’ll definitely have a good ballpark estimate of how long you can expect to have your charger hooked up to your battery.

Three things we need to identify are the following:

  • Total capacity of your battery in amp-hours (AH)
  • Open voltage of your battery
  • The charging amps of your charger

In this article, we are going to cover 12-volt batteries (applicable to car, marine, golf-cart) and we’re going to need to get an estimate of how many amp hours your battery has. Unfortunately, car batteries are starter batteries and are not intended for prolonged use and deep discharges.

Car batteries are designed to give off a high amperage for a very short amount of time just to start your vehicle. A few seconds of use is all that they were designed for and then they are to be topped off by your car’s alternator as you drive.

Nobody seriously considers powering anything long-term off of their car battery and doing so would severely shorten the lifespan of your battery and no time. You would be lucky to get a dozen deep discharges or several dozen even at 50%.

Because they are “starter batteries” they are rated in Cold Cranking Amps (CCA) or Reserve capacity (RC). These are measurements that will give the user a better idea of how many amps they can put out to turn over the engine or how long the battery will last if the alternator goes out. It’s not uncommon to see both or only one of these ratings on the battery’s sticker.

If you’re lucky, you’ll find an amp-hour (AH) rating as well!

Comparing Cold-Cranking Amps or Reserve Capacity to Amp Hours is not cut and dry. They’re completely different measurements for completely different purposes. It would be like saying your car can go from 0 to 60mph in 7 seconds, so how many miles per gallon can you expect to get at 60mph?

Instead of just accepting my fate of never knowing even an estimation between one and the other, I decided to extrapolate charging data from the Schumacher brand battery charging company. In the same chart, they gave charging times for the same charging amps for batteries that were rated in amp hours and also charging times for batteries that were rated in cold-cranking amps and reserve capacity.

I figured out the charging efficiency that they were using for any given charge and when I reversed the equations I was able to come up with a formula to convert Reserve Capacity to Amp Hours and also Cold-Cranking Amps to Amp Hours based on the charge time and amps of the charger.

I have a full article here where you can see my math and I get way more into the weeds. This article was designed to stay quick and to-the-point.

In the first calculator below, type in your CCA rating to get your approximate amp hours.

In the following two calculators, there are 2 methods for converting reserve capacity to amp hours. If your battery just says the RC number with a tested amperage of 25 (or without any amp rating), feel free to use the first calculator since 25 is the assumed industry standard.

If your battery has a reserve capacity with an associated amperage different than 25, then use the second calculator.

Perfect! Now that you have your estimated amp-hour equivalency, we need to find out what your open voltage is of your battery. You can quickly find this out using a multimeter like this one seen on Amazon. Just connect the leads to the multimeter as shown, switch the dial to voltage DC (the symbol with a “V” and a dash with 3 smaller dashes or dots underneath), and put the black lead on the negative terminal of your battery and the red one on the positive.

A junk 12-volt battery that won’t hold a charge any longer.

If you don’t have a multimeter, you’re just going to have to estimate where you think you are. There’s no other way to read it. If your battery was struggling for 6 to 8 seconds to start your car, maybe you can type in 12.06 to simulate a 50% battery. That’s just a guess but you can still get an idea by plugging in different values.

Here is a chart that shows your battery’s voltage and the corresponding percent of charge that it has.

Percent of Battery Charge Remaining by Voltage

Car Batteries should not be discharged at a high rate. Anything at 50% or below will shorten the lifespan of your battery considerably. Charge immediately if your battery gets this low.

Okay, now that we have our batteries total amp hours and we have the estimated open voltage of your battery, you just need to identify how many charging amps you will have from your charger. This should be simple enough and you can simply look at the top of your charger and somewhere on the sticker it will tell you how many amps will be delivered.

4 amp charger for car batteries
One of my favorites is this 4-amp charger. It has had daily use for over 6 years and hasn’t let me down. This will charge most drained car batteries overnight without a problem.

The calculator below is for a charger that will only use one amperage. If you have a charger that starts at 8 and then scales down to two for the final 10% of the charging process, then this calculator still might give you a ballpark but it was designed with one amperage in mind.

In the calculator below simply type in your three values and at the bottom, you will see your estimated charging time. You can use this formula for 12-volt golf cart batteries as well as marine batteries as well for your boat.

How Many Amps Should a Car Battery Be Charged At?

Car batteries typically have a 50 to 70 amp hour equivalency and our best charged at 5 to 7 amps with a smart charger. Using 10 amps is the maximum size for charging most car batteries and anything above that increases the risk of damaging your battery due to overcharging.

At 5 to 7 amps, you can easily charge most car batteries overnight even if the battery is completely dead. If you are charger is having a hard time reading your car battery when it is completely dead, check out my article here for a fix.

If your battery is only 50% discharged or less, you won’t have to wait overnight and your battery should be good to go in a couple of hours at 5 to 7 amps.

It might be tempting to think that bigger is better when it comes to charging your battery and that more amps would result in a faster charge time. It’s actually a dangerous line of thought and can severely ruin your battery in a quick amount of time.

Remember that during the charging process we are taking electrical energy, passing it through the battery, and the electrolyte inside is converting the electricity into stored chemical energy. It can only do this at a certain rate efficiently.

The moment you exceed the battery’s ability to convert electricity into stored chemical energy it is overcharging and could be damaging your battery based on the severity and type of battery you have. Flooded batteries are more forgiving but require more maintenance.

The battery has to get rid of the excess electricity somehow and it does so by releasing heat. Excess electricity goes through a process called electrolysis and basically, the electrolyte inside the battery begins to boil.

When electricity passes through the electrolyte during electrolysis (overcharging) it is actually destroying the molecules of your electrolyte and evaporation occurs as a result of the heat buildup. The evaporated gases build-up, internal pressure increases, and your battery will release these gases in order to preserve the structural integrity of the battery.

Even sealed batteries, which can recombine the evaporated electrolyte up to a point, have a check valve that will allow gases to vent out if the pressures build too high.

If the battery cannot vent out the gases fast enough you will see the sidewalls of the battery start to bulge out and your battery will it become very hot to the touch. At this point, you have pretty much ruined your battery because there’s a good chance the lead plates inside of your battery are now compromised structurally, if not chemically as well.

Even if your sidewalls do not bulge and you allow your battery to undergo overcharging for too long, the electrolyte levels will decrease inside your battery and the lead plates will be exposed to air which will cause them to immediately oxidize.

This damage is irreversible and your battery has now lost the ability to hold a charge in this portion of the plates or to release a charge from that portion of the plates as well. Your Cold Cranking Amps are now diminished for starting your car on a cold morning.

If you let it go for too long and all of the electrolyte evaporates out, then your battery can undergo thermal runaway and eventually catch fire or even explode.

This can be especially true if you are using an old-school charger that is not equipped with all of the modern safety features found in microprocessor-controlled smart chargers.

Use a smart charger if at all possible to minimize most of the safety issues that can arise from charging, but you will still need to make sure you are selecting the correct charging amps.

It is best to stick with a charger that has an amp rating that is 10% or less than the total amp hour rating of your battery. Again, for most car batteries, this will be 5 to 7 amps since they are 50-70AH equivalent batteries.

You can certainly use fewer than 5 amps, it will just take longer to charge but you will not be doing any disservice to your battery by doing so. In fact, a slower charge means that your battery will be the most efficient in converting the electricity into stored chemical energy.

If you have a 10 amp charger, which is common in big box stores, I would not throw it away. Just use caution and monitor it a little more carefully than you would when using a lower amp charger. If you hear a loud rolling boil, whistling, or any other venting sound coming from your battery then unplug the charger and get yourself a charger with fewer amps.

Best and Safest Car Battery Chargers

You can’t go wrong with any of the following Chargers seen on Amazon. They are all smart chargers that enter into a “float” or “maintenance” mode when the charging process is over and can be safely hooked up for extended periods of time (indefinitely).

This charger is rated at 5 amps and will work with any type of 12-volt car battery (or 6-volt batteries as well). It can be left hooked up indefinitely your car battery, so if you’re going on vacation for a bit or you don’t drive during the winter season you can rest assured that your battery will be ready to go when you get back.

What I really like about this charger is that it will detect a voltage as low as 1 volt on your dead battery. 12-volt batteries are technically 100% discharged when they read 10.5 volts. If you have let a battery sit for long enough in a discharged state, it might get down into that 9, 8, or 7-volt range.

Most modern smart battery chargers require a minimum voltage to be detected in order to engage the charging process. It’s a safety feature and most need anywhere from 9 to 10-volts in order to engage. If your battery is below that number, then you are out of luck or you need to either hook up an old battery charger with no safety features on it or jump-start your battery with another vehicle in order to put a surface charge on it and then switch back to the charger.

This charger on the requires 1 volt.

This is another 5-amp charger from a company that I have been using daily for 6 years and have had zero problems with whatsoever. As far as I know, this charger does need a minimum voltage to detect a dead battery somewhere about 9 to 10 volts. However, this charger can service 12-volt or 24-volt batteries and can be mounted as well.

It is marketed towards boaters for this reason but is perfectly safe for your car battery.

Finally, this is the cheapest of the three chargers but is a solid choice regardless. It’s the 4-amp version of the charger listed above and will take care of your battery just fine. I have this one permanently hooked up for my home battery bank for emergency power and have also used it monthly to top off my car battery.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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