What Does Float Charge Mean on a Battery Charger? The Quick Helpful Guide You NEED

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What Does Float Charge Mean on a Battery Charger

What Does Float Charge Mean on a Battery Charger? Demystify this term to ignite your journey towards better battery health.

You may have heard that float charge is a good way to keep your battery charged and healthy. But do you know what float charge means and how it works? The answer is that float charge maintains battery capacity by providing a constant voltage.

But that’s not all. Float charge is not a regular charging mode, and it has some pros and cons. In this post, I will explain what float charge means on a battery charger. I will also explain how it compares to other charging modes. Additionally, I’ll explain when and how to use it.

Keep reading to learn more.

Bulk ChargeCharger delivers full current until battery reaches ~80% capacity. Voltage rises.
Absorption ChargeCharging current decreases while voltage is held constant. Battery reaches full capacity.
Float ChargeSmall constant voltage applied to maintain full battery capacity during storage.

Key Takeaways

  • Float charge maintains battery capacity by providing a constant voltage.
  • Different battery types benefit from float charging to prolong their lifespan.
  • Smart battery chargers often feature float charge mode for efficient charging.

So, What Does Float Charge Mean on a Battery Charger?

So, what exactly is a float charge? I’ll explain.

Basic Concept

Float charging is a method that keeps a battery at full charge by delivering a small current when the battery is idle. Your battery is ready to go at a moment’s notice. Neat, huh?

This is especially useful for batteries that remain unused for long periods, like those in RVs or boats. A float charger provides just the right amount of charge to stop the battery losing power.

Battery Chemistry and Float Charge

The chemistry of a battery plays a big role in float charging. Different types of batteries, like lead-acid and lithium-ion, have different charging requirements. The float charge can be adjusted to meet the specific needs, keeping the battery in good condition without being overcharged or damaged.

Voltage Levels and Charging Modes

Float charging involves three stages.

  • Constant Current Stage: Initially, the battery is charged with a constant current.
  • Constant Voltage Stage: Next, it transitions to a constant voltage stage.
  • Float Charge Mode: Finally, it enters the float charge mode, where the charger supplies just enough power to keep the battery full.
  • For a 12V battery, the typical float voltage is around 13.5 volts.

Different Battery Types and Float Charge

There are a few different types of float chargers; in this section, I will explain each type.

Lead-Acid Batteries

Lead-acid batteries are commonplace in cars and boats and need careful charging. Float charging helps. It maintains a voltage level. It stops overcharging, which can shorten battery life.

And sulfation can also harm lead-acid batteries. But float charging stops it in its tracks.

But what is the ideal float voltage? Glad you asked!

It is around 2.25 volts per cell for lead-acid batteries. The temperature matters, too. A change of 5°C needs -3.9 mV/°C adjustment.

Sealed, AGM and Gel Batteries

Sealed batteries, AGM (absorbent glass mat) and gel batteries are like lead-acid batteries. They have a different chemistry. For their purposes, float charging is still helpful to them. But, they need different voltage values. AGM and gel batteries have lower self-discharge rates. They are more resistant to sulfation.

Float charging will keep sealed, AGM and gel batteries at full capacity, just like our wet batteries. But, I always check the battery manufacturer’s guide to be sure. It’s possible that they may suggest different float charge settings.

Battery Charging Process

Now, shall we have a look at the charging process in a little more detail?

Well, why not?

Bulk Charge

Your battery charger will dive straight in with the bulk charge phase, delivering its full current. This helps the battery charge quickly, getting the lion’s share of the charging out of the way nice and quick. The charger works until the battery reaches a certain voltage, often 14.4 volts for a 12-volt battery. Most batteries gain around 80% charge in the bulk stage.

Absorption Charge

Next, the absorption charge kicks in. The charger slows the charging process right down, nice and gentle. The voltage stays the same, but the charging current decreases. It helps the battery reach its full capacity without damaging it. For a 24V battery, the voltage remains constant at 28.8V during this phase.

Float Charge Mode

Finally, we have the float charge mode. In this stage, the float charger maintains the battery’s voltage at a constant level. It only provides enough current to make up for the battery’s self-discharge. This keeps the battery fully charged up without overcharging.

Like we talked about before, the float charge prevents loss of battery capacity during storage. Remember, a charged and fully charged battery lasts longer.

This mode uses a trickle charge to keep the battery healthy. The amp charge supplied by the charger is low compared to the bulk and absorption phases.

For a 12V battery, the float mode maintains a voltage of around 13.2 to 13.8 volts. For a 24V battery, it’s around 26.4 to 27.6 volts.

Factors Affecting Float Charge

What can get in the way of float charging and scupper our plans?

Let’s see!

Ambient Temperature

First up, we’ve got ambient temperature. Your battery’s like Goldilocks. It doesn’t like it too hot or too cold. If it’s chilly, your battery might not perform as well.

If it’s too hot, your battery might throw a tantrum and overheat or degrade faster. To keep your battery just right, you might need to adjust the float charger’s voltage based on the temperature.

But some good news here: advanced battery chargers have a temperature compensation feature. It adjusts the float voltage based on the temperature of the battery. This is a super way to compensate and ensure your battery gets the treatment it deserves.

Battery Capacity

Secondly, the battery capacity plays a notable role in float charging. Different sizes and types of batteries require various float charging voltages.

Larger batteries, with more energy storage, often need more time to achieve a full charge. Smaller batteries should be charged at a lower voltage to avoid overcharging.

Charging Current

Thirdly, the charging current is another factor to consider. Too high a current can lead to overcharging and potential damage, while too low a current might not be sufficient to maintain the battery at full charge.

Battery Age

Don’t forget about battery age. Like fine wine, batteries age, but unlike wine, they don’t get better with time. An old battery might not hold a charge as it used to.

Internal Resistance

Lastly, the battery’s internal resistance can affect float charging. Every battery has some level of resistance. Here’s the thing: resistance can build up with age, sulfation, or poor maintenance.

Higher resistance makes it harder for the battery to charge efficiently. So, monitoring internal resistance is important to make float charging effective.

Proper charging techniques can reduce the risk of increased resistance. They can also extend the battery’s life.

Maintenance and Usage Patterns

Lastly, maintenance and usage patterns can play a big role in float charging. Regular maintenance and balanced usage can help keep the battery in good health, which in turn can enhance the effectiveness of float charging.

This includes avoiding deep discharges, overcharging, and ensuring the battery is used regularly.

Benefits and Potential Risks of Float Charge

Let’s look at some of the risks and benefits of the float charge.

Extend Battery Life

There’s no getting away from it: float charging can extend your battery’s life. It maintains a constant voltage level in the battery. This prevents damage from overcharging and helps extend its lifespan. As you might remember, when a battery is stored for long periods, a float charger can keep it at or near full charge.

Prevent Overcharging and Sulfation

Overcharging can harm your battery. Float charging helps avoid this by keeping the voltage level constant. This way, you don’t add unnecessary charge to the battery. Also, float charging can reduce sulfation. Sulfation occurs when sulfate crystals build up on battery plates, killing or diminishing your battery.

Potential Fire Hazards

However, float charging does have some risks. One risk is fire. Some float chargers may overheat if a fault or power surge could cause a fire.

During float charging, batteries can release hydrogen gas, which is highly flammable and can pose a fire hazard.

Make sure the charging area is as well-ventilated as possible, and when investing in a charger, get one from a well-known brand: don’t skimp!

Smart Battery Chargers and Float Charge

How do smart battery chargers use float charging? Let’s get into it.

Automatic Switching to Float Charge

Smart battery chargers are marvelous for charging batteries. They know when to stop charging and take away all the worry that comes with a ‘dumb’ charger. When a battery is full, the charger switches to float charge mode.

This mode keeps the battery at 100% charge. It does so by reducing the voltage and current. The voltage drops to 13.2-13.4 volts, the maximum a 12-volt battery can hold.

Battery Bank Management

Battery banks need a good float charge, too. Floating chargers manage this process. by monitoring the battery bank, keeping it at the perfect voltage.

Float chargers check each battery in the bank, ensuring all batteries are at the proper voltage. But remember the factors we mentioned earlier? These still apply, even with a smart charger.

Before You Go…

You’ve just learned what float charge means and why it’s important for your battery charger. But do you know what can drain your car battery faster than anything else? In my next article, I share the most common causes of battery drain and how to prevent them. This information is essential for any car owner. It helps them avoid getting stranded with a flat battery.

Read “What Drains Your Car Battery The Most” here. Discover how to keep your battery healthy and happy.

Frequently Asked Questions

Here’s the FAQs

What is the purpose of float charging?

Float charging is a method used to maintain a battery’s capacity at or near full charge. It helps prevent battery damage during long storage periods. It keeps the battery ready for use.

How long does float charging take?

Float charging is a continuous process. Unlike other charging methods, it runs for an indefinite time. This happens as long as the battery remains connected to the charger. It maintains the battery’s charge without overcharging.

How does float charge differ from boost charge?

A floating charge maintains a battery at total capacity. A boost charge aims to restore a depleted battery quickly. Boost charging uses higher voltages. Float charging uses lower, more consistent voltages.

What is the ideal float charging voltage?

The ideal float charging voltage varies depending on the battery type. Typically, it ranges from 0.2 to 0.6 volts above the battery’s rest state voltage when fully charged. Battery Tender explained this.

Can lithium-ion batteries be float-charged?

No, it isn’t ideal to float-charge lithium-ion batteries. They’re sensitive to overcharging, and float charging may harm them. Other battery types, like lead-acid batteries, are more suitable for float charging.

What are the effects of float charging on a battery?

Float charging helps maintain a battery’s full charge. It prevents sulfation, which can degrade battery performance. Float charging helps extend battery life by keeping it at its optimal level. This also makes sure that the battery is ready for use when needed.

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Steve Brown


Steve is a gadget enthusiast who's always been intrigued by batteries. The founder and editor of Battery Chargers Info, he's assembled a group of like-minded experts to cover every facet of portable power His aim is to help you learn more about your favorite gadgets and their batteries so you can maximize both their performance and their life. Follow him on Twitter: @batterycharge1

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