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Flooded battery | What is it?

Updated: May 7, 2022

Batteries are one of the most essential discoveries in human history, they enable us to carry the stored power and keep us in light even when the whole power grid failed. However, it is also one of the most complicated technology and even with more than 160 years of its presence. Still, most of the general people didn’t understand that much.

The flooded lead-acid battery is invented in 1859, was the first rechargeable battery and after decades of refinement, it remains the primary choice for many applications.

Here in this blog, I cover most of the fundamentals of flooded batteries.

What is a Flooded battery?

A flooded battery is one of the types of rechargeable lead-acid batteries. It is based on the most basic and earlier designs of batteries. It has two electrodes [positive and negative] immersed in a liquid form electrolyte, which is fundamentally a solution of sulfuric acid and the cells are open for topping up with water for replenishment of electrolyte. However, its electrodes could be either flat plate type or tubular type.

The following reaction occurs in the lead-acid battery during charging and discharging:

Charged Battery

(+ve) PbO2 + (-ve) Pb + (electrolyte) 2H2SO4

Discharged Battery

(+ve) PbSO4 + (-ve) PbSO4 + (water) 2H2O

Electrodes in Flooded battery.

Fundamentally, there are two types of electrodes used in flooded batteries, that is flat plate type and tubular type. Flat-plate electrodes are the most common type of electrodes used in lead-acid batteries. Flat plate electrodes are mode by mesh grid structure of the lead alloy pasted with sponge-like lead oxide paste and pressed to form a flat plate type structure.

However, tubular electrodes are made of a frame structure consisting of a series of vertical spines connected to a common bus bar. Each spine is covered by non-conductive tubes filled with a paste of lead oxide active mass.

Furthermore, batteries with both electrodes of Flat Plate Electrodes are called Flat plate batteries and batteries with one electrode of Flat plate Electrodes and the other of tubular electrodes are called tubular batteries. [Learn more, about tubular batteries..]

We also have a detailed blog on the difference between a Flat plate battery and a tubular battery, click here to learn more.

Electrolysis in Flooded battery.

The electrolyte in a flooded lead-acid battery is a dilute solution of sulfuric acid and water. The charging current causes electrolysis of water which generates oxygen & hydrogen gases. Hence, gas vents are provided in these batteries so that these gases can escape into the environment. Also, it required time to time filling of water to recover the lost water.

In the beginning, most of the charging current is efficiently absorbed by the electrodes & there is hardly any electrolysis. Later, when the battery is 80% full & the voltage rises to around 2.3/2.35 volts/cell, charge acceptance is reduced resulting in excess energy being converted to heat & electrolysis. This point of 80% state of charge, at around 2.3/2.35 Volts/cell is known as gassing voltage.

However, the electrolysis process in AGM and Gel lead batteries is almost insignificant hence no maintenance is required. We have a detailed dedicated blog on AGM Vs Gel Vs Flooded battery.

How do you know if your battery is flooded?

The simplest method to identify a flooded battery is to look at its top. If you find numbers of valves or water level indicators [cum vlaves] on top of it, then it is fooled type. These water level indicators cum valves are used to fill up water in it as well as indicate the level of water in it.

Another method to identify a flooded battery is to take look at its sticker, most of the flooded batteries are named flooded on them. Furthermore, if the battery is named as a GEL battery, AGM battery or non-spillable battery then it can’t be flooded type.

Is a flooded battery considered a wet battery?

Flooded batteries are also known as Wet battery, due to the type of electrolyte it has. The electrolyte of a flooded battery is in liquid form and it will spill out if the battery is tilled. However, the electrolyte of other rechargeable lead-acid batteries could be in gel form or replaced by acid socked glass mat.

Is a maintenance-free battery a flooded battery?

A maintenance-free battery, can’t be a flooded type, because unlike to flooded type these batteries won’t need any water filling. Even maintenance-free batteries didn’t have any water filling valve or water level indicators on top of them.

AGM and Gel batteries are two examples of maintenance-free lead-acid batteries, that don’t require any water filling due to the type of electrolyte they have. These batteries won’t have a water filling valve, water indicators and gas vents to release gases [hydrogen & oxygen].

However, these batteries have a pressure release valve to release excess gas in case of a severe overcharge. Hence these, batteries are also called Valve Regulated Lead-Acid batteries or VRLA Batteries.

We have a detailed blog on AGM Vs Gel Vs Flooded Battery, click here to learn more.

What is an Enhanced Flooded Battery [EFB]?

Enhanced flooded batteries are advanced versions of the flooded battery, designed specifically designed for small to mid-sized vehicles [car and van] with Start-Stop, regenerative braking and other powerful fuel-saving features.

The electrodes of flooded batteries are separated by a separator called Microporous

Envelope, however polyethylene envelope is used as a separator in Enhanced flooded battery. Also, the negative electrode of the EFB is pasted with higher density active material and additives.

This increases the performance of the flooded battery and makes it suitable for small vehicles with regenerative braking. However, the price of EMF is only 20-40% higher than their conventional flooded counterparts.

Can you mix Flooded and AGM batteries?

It is not recommended to mix Flooded and AGM batteries, because it is very important that AGM batteries not be severely overcharged, as this will cause the battery to rapidly dry out. However, flooded batteries have a higher overcharge capacity due to excess water in electrolytes.

Hence AGM batteries required a lower charging voltage on account of the fact that any electrolysis or gassing caused by a higher charge voltage leads to irrecoverable loss of water from the battery. However, flooded batteries required a higher charging voltage.

The other limitation of AGM batteries is that they should not be discharged beyond 50% of their capacity as against 80% allowed in case of flood batteries since recharge becomes very difficult due to its electrolyte starved design.

What is a flooded battery used for?

The most common uses of Flooded batteries are for home inverters and for off-grid solar power plants due to their cheap price. Sometimes these batteries with thin plate construction and Enhanced versions are also used for vehicle starting.

However, flooded batteries used in home inverter/off-grid solar power plants can’t be interchanged with the vehicle starting version of it. Because vehicle starting version of flooded batteries has a low depth of discharge, that is 50% DOD.

Whereas, the home inverter version of flooded batteries has a higher depth of discharge which is 80% DOD. Hence, the home inverter version of flooded batteries is also called deep discharge batteries.

Can a flooded battery be maintenance-free?

No, a flooded battery can’t be maintenance-free because it losses water due to electrolysis during charging. Hence, it needs time to time water filling. Without water filling, flooded batteries will lose their backup capacity and get overheated.

Can a flooded battery be sealed?

No, a flooded battery can’t be sealed because produces a significant amount of gasses during charging over 80% due to electrolysis. And these gases needed to be discharged into the air. If you try to seal a flooded battery, these gases will produce a huge pressure inside it and this pressure will result in an explosion.

Do flooded batteries need to be vented?

Yes, flooded batteries need to be vented so that the gases produce inside them during the electrolysis process can be discharged into the air.



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