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How Long Does It Take For Rechargeable Batteries to Charge? 

 September 19, 2021

By  Phil Borges

There are many different types of rechargeable batteries and chargers in the market. Here’s a quick guide on picking the right charger for your batteries and how long it takes to fully recharge them.

This depends on the type of charger but if you are using a regular charger meant for your AA batteries, you can expect one battery to be fully charged in six hours. So, charging two batteries simultaneously (which is often the case) takes 7-13 hours depending on the charger. For AAA batteries, that is 6-9 hours.

But you need to know what type of batteries you are dealing with, their mAh values and the charger’s output power which is measured in mA. This will give you the exact time it takes for your particular charger to recharge the batteries at hand to a 100 percent. Here’s how that is done.

How Long Does It Take to Recharge a Rechargeable Battery

This is an easy question to answer no matter the type of batteries you are using. There is a little math involved in the process but that’s fairly simple.

The formula to calculate the charging time of batteries is:

Hours = Milliampere-hours of the battery (mAh) / milliamperes of the charger’s output power (mA)

The rechargeable battery’s capacity, measured in mAh, is usually written on its body on the side. Then you must check the type of the battery which could be AA, AAA, NiMH or NiCd to name a few. Then you must check the electric current output of the charger which is measured in mA.

Now, you must remember that charging 9-volt batteries takes a lot more time because these types of batteries can be charged only at a low current rate. But they are still preferred because this is an expensive type of battery.

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 Ah

OR

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

So, it takes almost 29 hours with a normal 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 powerful than the 100 mA charger, you just divide the final result of the first calculation by 3.5 and you get the end result. It takes a little over 8 hours to recharge a 2,400 mAh battery with a 350 mA power output charger.

How Rechargeable Batteries Work

Now, rechargeable batteries cost a little more than regular batteries for the obvious benefits they come with. A regular rechargeable battery can be charged at least 2,100 times which makes it worth the investment.

Most if not all rechargeable batteries are 1.2 volts. This is determined based on the active materials which fuel the battery’s circuit. Alkaline batteries have an approximate voltage of 1.5 volts which is the result of a reaction between the materials that make the anodes and the cathodes in the battery. In NiMH batteries, that voltage is 1.2 volts.

About the duration. The thing about rechargeable batteries is that they start discharging when they are not being used. It is referred to as self-discharge. This means you must recharge them just before you are ready to use them because it happens pretty 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. And if you want to check whether your existing NiMH or NiCD batteries are still working, you will need a battery tester. This can measure the voltage in a close circuit and tell you whether you have a good battery or a defective one. You can also use a voltmeter but that is not reliable when it comes to rechargeable batteries.

Is Overcharging a Concern?

You must also know when the batteries are fully charged so that you can remove them from the charger and get going. This is because rechargeable batteries can very well be overcharged.

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 to their chargers at night and leaving them so.

When you do this with rechargeable batteries they lose their ability to reach 100 percent charge every time. And since you can recharge them only for a limited number of times, you reduce the overall lifespan of the batteries by overcharging them.

Now, some manufacturers have a timer which stops the charger when it hits that prescribed number of hours. But there are plenty of smart chargers in the market now.

These chargers can detect when the batteries are fully charged and they stop charging on their own. They have a circuit of microprocessors that calculate 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, at some point in the process 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.

When you don’t have a smart charger, you have to manually calculate the hours it takes to recharge the battery to its 100 percent using the formula mentioned above. And closer to that time period, you can try removing the batteries and checking their capacity by placing them in an appliance.

There is also the process of trickle charging which happens with a very low rate of charging current. The idea behind trickle charging is that the batteries that are 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 and this damages the battery a little bit. It won’t happen right away and the damage is not visible. But if you keep overcharging a battery it eventually loses its capacity to recharge to a 100 percent.

This means each time it will recharge less and less till it eventually won’t recharge at all. The other way this could go wrong is that there are a finite number of times you can recharge these batteries. Since it doesn’t reach its 100 percent, you will need to recharge it more times and will hit the saturation point sooner than you should.

How Do I Know When My Rechargeable Batteries Are Charged?

The first thing to note here is to use the charger that the manufacturer recommends for your rechargeable batteries. Most chargers have a light that switches colors between “charging” mode and “charged” mode.

How Long Should It Take to Recharge AA Batteries?

It usually takes about three to four hours to charge any type of AA battery. But just to be clear:

Regular chargers take 8-10 hours to fully charge two NiMH batteries whereas ultra fast battery chargers can get four NiMH batteries to their full capacity in about 4-6 hours.

Regular chargers take about three hours to charge Li-ion batteries but some chargers can get it done in less than two hours.

Regular chargers take about eight hours to charge NiCad batteries.

About the author

Hi, I'm Phil Borges.. thanks for reading.. My wife says I can't shut up about batteries so to save my marriage I've started this blog .. where I'd be ranting about batteries! Please leave a comment if you have any questions, I'm happy to help!

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