How Many Hours To Charge A Car Battery With A Trickle Charger?

It takes 2-4 days to charge a car battery with a trickle charger. 

Ok, now to the nuances of that answer. Here are the reasons it might take longer.

Trickle chargers are a type of charger designed to maintain a battery and are not optimal for recharging a depleted battery. 

When I was a kid, my grandfather kept one on his boat motor, and one on his RV. He was always ready to get into the outdoors at a moment’s notice, even after the vehicle has sat for an extended period. 

Our family bought one as well and used it once to try to charge our van after the interior lights were left on overnight. It didn’t work in the time we had, and the next day we had to have a friend come to jump start the car. 

It was an eye-opening lesson on their limitation

The Purpose Of A Trickle Charge

When driving your car, the battery is recharged by the alternator. When you drive your car weekly, the battery is constantly being recharged and never dies. 

When a vehicle sits for a extended period of time, the electronics can create a slow drain on the battery, causing it to discharge to a point where it will not start. A battery tender will maintain the battery with a low amperage flow.

image of two boats in storage

Additionally, batteries have a slow, self-discharge that must be combated (although, electronics are generally the bigger problem.)

A 2 amp trickle is ideal  to keep a steady, slow charge in the battery to keep it from becoming so discharged that your vehicle will not start. 

A battery that has been overly-discharged can be completely damaged or have a shortened service life. These low amp chargers maintain your battery and prolong their lifespan. 

This is why RV owners, ATV owners, fisherman and even farmers like to use them to keep their batteries in good condition and ready for use. It also protects the batteries’ lifespan. 

Deep cycle and marine batteries are often used as a reserve power source and need a slower charging source to get them filled up. 

Classic car owners use them frequently. 

This also explains why used cars so frequently have dead batteries on the lot and need a battery replacement shortly after you buy them. After sitting for a long period of time, the battery has been discharged too much to every properly hold a charge. 

Did You Know?

Low Battery Can Void Warranty

Discharging your vehicle battery to under 10 volts can void damage the battery and void the warranty. This can happen when the battery is used to power small electronics systems such as a GPS tracker. Different manufacturers have different levels of discharge that they will warranty. If your vehicle is sitting for several weeks, either disconnect your battery, or add a maintainer.

How Many Amps Is A Trickle Charger? 

They are low amp chargers that generally produce between .8 and 2 amps. 

Some battery charges have both a 2 amp and a 10 amp setting so they can be used in a maintenance mode or charging mode. 

For automotive lead-acid batteries batteries, anything under 10 amps is considered a slow charge. 

20 Amperes or higher are fast chargers. 

How Long Does It Take To Charge A Car Battery? 

The length of time required depends on the size of the battery in amp-hours and how discharged it is.  

A charger with high amps can charge a car battery in less time. However, there is also a risk of damaging the battery if the amperage is too high. 

charging a car battery in the car

Shops like O’Reilly’s and Autozone offer a free, rapid-charge of batteries that only requires 45 minutes time to charge. These systems push a higher amperage through the battery but use a computerized system of safety checks to keep damage from occurring. 

How long it takes to recharge the battery depends on the types you are using. Here are some of the most common amperages.

  • A 1 amp charger can take 60 hours to charge a completely dead battery.
  • A 2 amp model would require 30 hours of charging time
  • At 10 amps it can charge a 12-volt car battery in about 4 hours charging time
  • At 20 amps it can charge a vehicle battery in about 2 hours
  • A 50 amp model can charge a truck battery in an 1 hour to 1.5 hours
  • The Largest 70 or 80 amp car battery chargers can charge a truck batter in less than an hour. 

Calculating Required Charge Time: 

1. Measure the voltage of the battery. At 12.2 volts a car battery is about 50% empty. A specialized automotive battery tester can tell you exactly how discharged the battery is. (some have this built in). 

2. Multiple the percentage against the total amperag-hours of the battery (not the amperage, the amp-hours). Find a close estimate of the Amp hours and reserve capacity by dividing the cold cranking amps by 7.25 

3. Add 20% to this percentage measurement to accommodate the internal resistance of the battery.

4. Divide that amount by the amp hour charge rate.

A 400 amp truck battery will have 55 amp -hours. At  50% empty it will take 

27.5 amp hours X (1.15% reserve) = 31amps/10 amp hour charge rate = 3 hours to a full charge on a 10 amp charger.

As you can see, what seems like a small discharge (12.2v) is actually a 50% discharge. And a 50% discharge for a large vehicle’s battery can take a full day to several hours to recharge using a standard over-the-counter model. 

A little larger 20-amp charger can do that in half of the time, or in 1.5 hours.

With a maintainer, it would take  over 15 hours to get that same charge!

The Difference Between A Trickle Charger and A Regular Battery Charger

mechanic testing a battery and voltmeter reads 12.78
An auto mechanic uses a multimeter voltmeter to check the voltage level in a car battery.

A trickle charger provides a continues “trickle” of a charge to keep maintain your battery “topped” off without the risk of  overcharging. They are designed to feed a low charge over a period of time to prevent discharge. However it should be removed to prevent over charging once the battery is completely charged. 

A maintenance charger is the new terminology to denote a smart 2 amp charger that can stay connected to the battery and will automatically cycle off if once the battery is topped off. 

Today, all of these are generally of the “maintenance charger” variety and old-timers tend to use the terms interchangeably or call them a “battery tender”. 

The Difference between these two is that the regular models has higher amperage settings. they also have the circuitry to shut off when the charge is complete, or if the battery is overheating. 

Fast Chargers For Car Batteries 

Automotive battery 20 amp chargers (or higher) are considered to be fast chargers. The larger amperage can reduce the length of time needed to reach a full charger.

Image of the Shumacher with 200 amp boost

Every personal garage should have one of these for charging your battery, (as well as a battery jumper pack for rescuing stranded vehicles). 

The fastest ones are found in the professional garage, and are available for purchase by individuals for as little as $200.

A professional garage often has a 50 amp or 70 amp high frequency charges for a quick charge in less than hour. These high frequency charges enable it to rapidly turn on and turn off the battery charging, cycling through different modes to help the battery receive the charge more quickly.

The high frequency or “multi-stage” models are more expensive. However by charging the battery in bursts, they can protect battery cells better than a linear  or continuous charger will. 

Sometimes these charges are called “intelligent chargers” due to how they “read” the battery, and adjust to the correct phase. These models will start off at a higher level and decrease the charge as the battery fills. 

40 amp smart chargers are affordable and are best for a quick charge. Gel and AGM batteries charge more quickly at higher amps than a lead-acid battery will. 

Repeated discharging and recharging of batteries can reduce their lifespan. However, if a battery is in storage and unattached to anything, you can simply top it off once a month with a standard car charger and achieve a similar result to a battery maintainer. 

Pros and Cons of Trickle Charging

Most of us are in a hurry to charge our batteries. However, when you have the time, the advantages of a lower charging rate is that it creates less stress stress on the battery. It is a softer charging process.

Advantages:

  • Better for the battery
  • Recovers deeply discharged batteries
  • Preserves battery lifespan
  • A cheaper option 

Disadvantages

  • Take days to charge a car battery
  • Fewer options for reconditioning batteries

Can a Trickle Charger Jumpstart A Car?

They deliver a tiny charge and needs a significant amount of time to recover a dead car battery. 

You’ll need to let the it work for a day or longer. 

It helps if you have a voltmeter to check progress. Your battery will need to read at least 12.4 volts before it can start your vehicle. 

Why Is Jumpstarting A Car So Much Faster?

When you use jumper cables to start a car, you are borrowing the cold cranking amps from another battery to start your car. 

It’s like instantly installing a fresh battery. 

Today, there are small battery packs with jump charges (as NOCO calls them) that can deliver that instant boost you need to get a car going. 

Some come with a 210 cold cranking amp “jump start” setting. This allows you to supplement a dead battery with an amperage boost to get it going again. 

Once the car is started the alternator delivers the voltage to recharge the battery while driving. 

If a car has just been jumpstarted, you should drive it for a minimum of 20 minutes to allow it time to restart. If that is not possible, let it idle for at least 40 minutes to allow time for the battery to be recharged. 

It is suggested to not use your lights or run the radio during this initial charge time, although I frequently break those rules without consequence. 

Recovering a Dead Car Battery 

A deeply discharged battery may become so damaged that it is unable to hold a charge. 

Maintenance chargers will sometimes work to revive these batteries if you leave them on for 3 or 4 nights. The slow charge will sometimes work when a faster charge will not. 

However, the new intelligent chargers sometimes have a battery reconditioning mode. This programming allows it to begin with a slow charge and recondition the battery so that it is more likely to hold a charge. 

That said, I’ve generally found that by the time a battery is in this poor of condition, you are generally running on borrowed times. 

Batteries only last about 4 years, and with all of the electronics on new cars, they tend to die even sooner. 

We’ve had some luck in drilling out the cells and topping them off with a mixture of epsom salts and water. This is probably not recommended, but when the budget it tight, fresh electrolytes and a slow charge will sometimes revive a dead battery. 

Voltage Doesn’t Matter, But It’s A Quick Way To Measure Battery Levels

It’s the amperage in a battery that will start your car — not the voltage. 

However, most of us don’t own the expensive amp-meters needed to measure a battery with 400 or more cold cranking amps. 

Using a multimeter and making sure it is above 12.4v is one of the quickest ways to check battery levels. 

Where Should You Connect The Charging Clips?

A battery will have a positive and a negative side. If the color coding is correct, your car will have a red wire to the positive side and a black wire to the negative side.

That said, I’ve seen cars with two black wires. 

Before beginning, you must find the “+” and the “-” printed on a battery. 

The + is the positive side. Connect the red alligator clip to the positive terminal. 

The – is the negative side. Connect the black clip to the negative site or to the ground cable on your vehicle. 

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