How Long Do Rechargeable Batteries Take To Charge?

The time it takes for the rechargeable batteries to be fully charged depends on the type of charger. However, if you use a regular charger for your AA batteries, you can expect one battery to be fully charged in six hours. So, simultaneously charging two batteries takes 7–13 hours. Meanwhile, AAA batteries take up to 6–9 hours to be 100% full.

How Long For Rechargeable Batteries To Charge

To know the exact time it takes for your charger to recharge your batteries fully, you should know the type of batteries you are dealing with, such as AA, AAA, NiMH, or NiCd. You must also check the battery’s capacity, measured in mAh, and the electric current output of the charger, measured in mA.

You can also calculate the rechargeable battery charging time by trying the formula below:

Milliampere – Hours of the battery (mAh) / milliamperes of the charger’s output power (mA) = Hours to charge

Let’s look at some more math to figure out the specifics of charging different types of batteries. If you are dealing with AA, AAA, C, or D-size rechargeable batteries, you must use this formula:

Hours = 12 x mAh


Hours = 12/1000 x mAh

Consider an example to understand it better. You are trying to recharge a 2,400 mAh NiMH battery of the AA size with 1.2-volt power and a 100 mA charger.

Hours = 12/1000 x 2400 = 0.012 x 2400 = 28.8 hrs.

So, it takes almost 29 hours with a standard 100 mA charger. But, if you have a trickle charger with 350 mA output power, your luck changes to:

Hours = [12/1000 x 2400] / 3.5 = [0.012 x 2400] / 3.5 = 28.8/3.5 = 8.2 hours

Since the 350 mA charger is 3.5 times more potent than the 100 mA charger, divide the final result of the first calculation by 3.5, and you’ll get the result. It takes over 8 hours to recharge a 2,400 mAh battery with a 350 mA power output charger.

How Rechargeable Batteries Work?

Rechargeable batteries start discharging when they are not being used. It is referred to as self-discharge. This means you must recharge it before using it because it happens quickly, too. A typical rechargeable battery gets fully charged in about six hours, and that’s the maximum time it takes even if the battery is dead.

If you are using NiMH batteries, storing them at full charge and room temperature will keep them functional for three to five years. Additionally, if you want to check whether your existing NiMH or NiCD batteries are still working, you need a battery tester. It can measure the voltage in a close circuit and tell you whether you have a good or a defective battery. You can also use a voltmeter, but it isn’t reliable regarding rechargeable batteries.

Is Overcharging a Concern?

Each time you leave the batteries in the charger even after they are fully charged, they lose their capacity a little bit. This usually happens because we are all in the habit of plugging devices into their chargers at night and leaving them overcharged.

When you overcharging rechargeable batteries, they lose their ability to reach 100 percent charge every time. Since you can recharge them only a limited number of times, it reduces the overall lifespan of the batteries by overcharging them. That is why when you know it is full already; you must discharge rechargeable batteries.

Today, some manufacturers have a timer that stops the charger when it hits the prescribed number of hours. These chargers can detect when the batteries are fully charged and stop charging on their own. They have a microprocessor circuit that calculates battery parameters like temperature, state of charge, and voltage to determine when to stop charging it. But, if you don’t have a smart charger, you need to know when to switch off the charger.

When working with rechargeable batteries, the battery will reach its peak voltage. That is usually towards the end of the charging process. This causes a small drop of about 5-20 mV in the voltage, which is what smart chargers detect and stop sending power to the batteries.

There is also the process of trickle charging which happens with a meager rate of charging current. The idea behind trickle charging is that the batteries used in devices like cordless phones never run out of charge, and you don’t have to worry as much about overcharging because the process takes place at a glacial pace. Trickle charging solves the problem of losing power due to self-discharge.

Frequently Asked Questions

Can You Overcharge a Rechargeable Battery?

Yes, you can, but it damages the battery a little bit. It won’t happen right away, and the damage won’t be visible. Overcharging a battery eventually loses its capacity to recharge to 100 percent. It has a high probability of reaching a finite number of times by recharging these batteries; since it doesn’t get 100 percent, the tendency would be to recharge them more times, which will hit the saturation point sooner than it should.

How Do You Know When Rechargeable Batteries are Fully Charged?

It would be best to look at the blinking colors while charging it. It served as an indicator if it was fully charged or not. Most chargers switch colors between “charging” mode and “charged” mode, so find its meaning in the manual.

How Long Does it Take to Recharge AA Batteries?

It usually takes about three to four hours to charge any AA battery. This is more efficient than regular chargers, which take about 8-10 hours to charge two NiMH batteries fully, three hours to charge Li-ion batteries and about eight hours to NiCad batteries. Furthermore, ultra-fast battery chargers can get four NiMH batteries to their full capacity in about 4-6 hours.

How Long to Charge Rechargeable Batteries the First Time?

Most rechargeable batteries come pre-charged from the factory. However, it is always best to charge them before use. It usually takes two to three hours to charge them for the first time. But, for optimal results, it is recommended that you charge your batteries as instructed by the manufacturer.

About the author, Phil Borges

Phil Borges is a battery aficionado. He's written extensively about batteries, and he loves nothing more than discussing the latest innovations in the industry. He has a deep understanding of how batteries work, and he's always on the lookout for new ways to improve their performance.