Understanding And Managing Float Voltage on Lead Acid Batteries

scanning: time:2022-04-27 classify:Technology info


You might have come across this statement that factory-filled and charged batteries need no charging and are always good to go or that any maintenance charger can charge and maintain your battery.

These are all myths; float voltage lead acid battery actually requires more care and maintenance. Float voltage has a different meaning for different types of batteries, and the key for batteries to last longer is how well you manage them. Today, we will dive deeper into the world of lead batteries and learn everything about float voltage.


What is Float Voltage?

The voltage at which a battery is maintained after it has been fully charged in order to preserve its capacity by correcting for self-discharge is known as float voltage.


To understand float voltage, it's crucial to know the following:


§  The float of a battery decreases and increases as it is charged.

§  All batteries are bound to gradually discharged, known as self-discharge

§  Battery label has a lower voltage labeled than the usual voltage battery can achieve.

§  Float voltage calculations are influenced by the temperature it operates and the battery chemistry.


The voltage could be kept constant for the entire time the cell is in operation (as in an automotive battery) or for a specific charging phase by the charger. The proper float voltage depends on the battery's chemistry and structure and the ambient temperature.


Lead Acid Batteries & Float Voltage

The lead–acid battery was developed in 1859 by French physicist Gaston Planté. It is a form of rechargeable battery and has been around since the 1880s with many improvements over the years. Having incredible manufacturing technology, high voltage, robustness, and low cost, these batteries are kept charged through float voltage to give you an excellent experience.

To fully take advantage of their functionalities, we should understand how a lead acid battery is charged and the specific requirements that help maintain lead acid battery float voltage. So keep on reading to learn more about it.


How Are Lead Acid Batteries Charged?

The constant current constant voltage (CCCV) charge method is used to charge a lead acid battery. A regulated current raises the terminal voltage until it reaches the upper charge voltage limit, at which time the current reduces due to saturation.

Lead-acid batteries are charged using the CCCV method in three stages:


1.   Constant- Current Charge

The constant-current charge (CCC) applies the majority of the charge and takes about half of the required charge time; the topping charge offers saturation at a reduced charge current, and the float charge compensates for self-discharge loss. The battery charges to around 70% in 5–8 hours with the constant-current charge, while the remaining 30% is filled with the slower topping charge, which takes another 7–10 hours.


2.   Topping Charge

The topping charge is necessary for the battery's health and might be compared to a little break after a satisfying meal. If the battery is repeatedly depleted, it will eventually lose its ability to receive a full charge, and its performance will suffer due to sulfating.


3.   Float Charge

Float charge (also known as float mode) preserves the battery by maintaining the voltage.

This is needed when the battery is fully charged and the current lowers to a certain level. At that time, float voltage is reduced, which results in self-discharge.  This happens in all the batteries, which is why they must be charged at constant intervals.

When the battery hits the predetermined voltage limit, the switch from Stage 1 to Stage 2 happens automatically. As the battery saturates, the current begins to decline, and when the current drops to 3–5% of the AH rating, the battery is fully charged. If a battery has a lot of leakage, it may never reach this low saturation current, so a plateau timer stops the charge.

Managing Float Voltage

The charge voltage limit, which should be set between 2.30V and 2.45V per cell, must be set correctly. Setting the voltage threshold is a compromise known as "dancing on the head of a pin" among battery specialists.

The battery has to be completely charged in order to achieve maximum capacity and avoid sulfating on the negative plate. However, over-saturation caused by failure to switch to float charge results in grid corrosion on the positive plate. Gassing and water loss might also be some of the many obstacles faced in this process.

The voltage changes as the temperature rises, making "dancing on the head of a pin" more difficult. A warmer environment necessitates a lower voltage threshold, while a colder environment needs a higher setting. Temperature sensors are used in chargers that are exposed to temperature fluctuations to adjust the charge voltage for maximum charge efficiency.

A lead-acid cell's charge temperature coefficient is –3mV/°C. Using 25°C (77°F) as the midpoint, the charge voltage should be lowered by 3mV per cell for every degree above and below 25°C.

After being fully charged via saturation, the battery should not be left at the topping voltage for more than 48 hours before being decreased to the float voltage level. This is especially important for sealed systems, which are less tolerant of overcharge than flooded systems. Excessive charging converts surplus energy into heat, and the battery begins to gas.

Most flooded lead acid batteries have a float voltage of 2.25V to 2.27V/cell. Large stationary batteries typically float at 2.25V/cell at 25°C (77°F). When the ambient temperature goes above 29°C (85°F), thus, manufacturers recommend decreasing the float charge.


Charging Time

Lead acid batteries are slow to charge compared to other battery types.

The charge time for a small float voltage lead acid battery is 12–16 hours, and for large batteries, it can be up to 36–48 hours. However, the charge time can be lowered to 8–10 hours without a complete topping up using higher charge currents and multi-stage charging methods.


Ending Note

The most accurate way to test lead-acid battery SOC (State of Charge) is to use a hydrometer to check the specific gravity.  Measuring the open circuit voltage (OCV) of a battery while in storage, offers a reliable indication of the battery's state of charge. A cell voltage of 2.10V at normal temperature indicates a charge of approximately 90%. This battery is in good condition and requires only a brief full charge before usage.

When the battery is fully charged, the electrolyte contains the most sulfuric acid, resulting in the highest specific gravity. Moreover, they must be constantly charged when in use.  To prevent the lead acid battery float voltage from going below 2.05V/cell and causing the battery to sulfate, a topping charge should be performed every 6 months.

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