The lead-Acid battery is one of the most common batteries used all over the world. In this article, I’ll discuss these lead-acid battery charging methods and we’ll see which method will be the most suitable for our battery. I hope this article will be helpful to you. So let’s start selecting the right charging method.

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Before we start talking about charging methods, we need to know about different types of lead-acid batteries. Depending on the use and surrounding situation, the charging method may be different. That is why we need to know about types of batteries first to charge them properly. For more basic information about lead-acid batteries, you can read this article from the wiki. I’ll discuss the charging and maintenance of each type of battery serially.

Types of Lead-Acid battery:

Depending on battery internal configuration and the use we can device lead-acid batteries in the following types:

  • Flooded Lead Acid Batteries
  • Sealed Lead Acid Batteries
  • AGM Sealed Lead Acid Battery
  • Gel Sealed Lead Acid Types
  • Deep Cycle Lead Acid Batteries
  • Cranking or Engine Starting Lead Acid Batteries
  • Standby Lead Acid Batteries
  • Valve-Regulated Lead-Acid (VRLA) battery.

Read more about Lead-Acid Battery

You may find this helpful: A smart Battery charger circuit design guide

Flooded Lead Acid batteries:

Flooded lead-acid batteries are the most common lead-acid battery all over the world. These batteries contain an electrolyte that is free to move around in the battery encasement. When charged, the battery acid and lead plates react to store electricity. These batteries are meant to be mounted upright so that the electrolyte does not leak out of the caps on top.

This type of battery is mostly used in home inverters and other power storage systems. These batteries also are economic batteries in the market. The average lifetime of these batteries is around 5 years if properly maintained.

Regular checking of battery acid and topping of distilled water is necessary. Note that these type of batteries produces toxic hydrogen gas while charging. So a properly ventilated room should be maintained while charging the battery.

Charging flooded lead-acid batteries:

Flooded lead-acid batteries are charged with the CC/CV method. CC/CV means Constant Current/Constant Voltage. In this method, the battery is first charged keeping the charging current constant up to a level. Then a constant voltage is maintained to top up. This method is the most common for flooded lead-acid batteries.

battery charging curve

But to elongate the battery life, we must charge the batteries maintaining some other issues too.

Issues that damage batteries:

We all know how to charge the batteries, but do not care about damaging the batteries by selecting the wrong charging method. Two reasons for damaging a battery are sulfation and excessive gassing. These two reasons can be prevented in smart charging systems. But why do sulfation and excessive gassing happen?


Sulfation occurs due to undercharged or uncharged condition of the battery. If the battery is stored uncharged for a long time or used and not fully charged each time, sulfation happens. Sulfation is the process of forming lead sulfate crystals over the plates.

battery plates
battery plates

Sulfation reduces the charge storage capacity of a battery. If the process goes further time, the total capacity of the battery can be damaged at a point.

Sulfation prevension technique:

Each time the battery is discharged must be fully restored the charge and when the battery is kept unused for a long time, the battery must be charged to full storage level periodically. Otherwise, sulfation can be prevented. A smart charging system can prevent sulfation very easily. If the undercharging is prevented, sulfation can be prevented.

Excessive gassing:

Like undercharging, overcharging can damage the battery. Excessive gassing happens when the battery is overcharged. Each battery produces some gas when charged. But when overcharged, this gassing increases due to the overheat of the acid. If the battery is kept in this situation for a long time, it can be dangerous both for the battery and surrounding environment even surrounding living can be affected.

Normal charging produces some hydrogen gas. But when overheated due to overcharging, the water and acid mixture boils inside the battery. This water level reduces and once the battery plates are exposed to the air the maximum damage starts. Normally flooded lead-acid batteries have a ventilation system through which the produces air is vented. But if anyhow this ventilation is blocked, destruction is ready to hit next.

The most dangerous situation forms when the lead-acid battery is overcharged and overheated. In this situation, more hydrogen gas is produced and some oxygen gas is produced due to boiling water and the electrolysis process. When the inside gas pressure is high enough and the ventilation is not proper, a risk of explosion is formed.

How to prevent excess gassing and explosion?

First, the battery must not be overcharged in any situation and should not be overheated. Then the fluid level must not be lower than the level indicated by the manufacturer. A smart charging system can prevent this process.

flooded lead acid battery

Each flooded lead-acid battery has a marking for the internal Acid flued level. The fluid level must not exceed the upper level and should not be so dried up to cross the lower level.

If the Acid level is over the upper level, it may leak some acid water while charging which will damage surrounding materials. On the other hand, if the acid level is below the lower level, the battery plates will be out of acid and will come to contact with air. In this case, more sulfation will happen and the plate will lose its properties very soon. Also, the chance of explosion raises.

So the Acid level must be between the upper and lower level and the battery should not be overcharged or overheated to prevent producing excess gassing and to prevent an explosion.

To know why lead acid battery explodes visit this link.

So how to charge the flooded lead-acid battery for longer life?

There are two ways to select to get optimum performance and battery life from a lead-acid battery. Each method has different benefits. Let’s see these two methods.

We know that lead-acid batteries are made with multiple cells. A 12V battery will have 6 cells and each cell voltage is 2V in a normal situation. But for charging, these cell voltage rises up a little bit. Depending on cell voltage,

2.3 ~ 2.35V/Cell
2.4 ~ 2.45V/Cell
AdvantagesMaximum service life,
The battery stays cool and
no need to refill the Acid fluid frequently.
Maximum performance, Maximum capacity reading,
less sulfation
DisadvantagesCharge capacity reduces by time, a Slow charging rate, sulfation occurs without equalization.Excess gassing, Higher flued temperature, Frequent Acid refill required.
Battery charging comparisons

From the above table we can see that, if a 12V battery is charged up to 14.1V we will get longer life and if the battery is charged up to 14.7V, we can get the maximum capacity from that battery. But the first method can create sulfation and the last one can create excess gassing. Also keep in mind that, the charging current is another vital point to select. Manufacturers suggest charging the lead-acid battery at C/10 (Capacity/10) Ampere rate. That means a 100Ah battery should be charged at a 10A rate.

Advantage & disadvantage of charging amp rates:

Charging below C/10 Amp rateCharging At C/10 or higher Amp rate
AdvantageBattery stays cool, less acid refill rateQuick charging, Higher charging capacity
DisadvantageSulfation, slow chargingGassing, Frequent acid refill required
Charging current comparison

So, what we can do to get the maximum life as well as maximum capacity from the battery? The optimum method is here:

“If we charge the battery in a combined technique keeping the charging current at C/12 to C/13 and Maintain cell voltage from 2.35V to 2.4V per cell and equalize battery periodically then we can get the maximum service life as well as less sulfation as well as higher charging capacity.”

Now, another term arises, Battery equalization. Equalization is charging the battery at a higher voltage for a period of time. For a 12V battery, the equalization voltage will be 15.5V to 16V and the duration will be 6~8hrs. A three-stage charging device can charge the lead-acid battery properly with this equalization feature.

✔ The best charging method:

The best charging method for a lead-acid battery is to maintain 3 levels of charging state.

  1. Bulk charging
  2. Topping charging
  3. Float charging/Trickle charging

Modes can be explained with this graph:

Lead Acid battery charging

The Redline is the battery voltage and the Blueline is the charging current. In the first stage of charging, bulk charging mode charges the battery at a constant current (CC) at the C/12 Amp rate up to 14.25V. Then Topping(Or constant voltage, CV) charging is started. The bulk charging stage is like healing the previously discharged charge but this topping stage is the main stage of storing the battery charge capacity.

This stage may take time longer than the bulk stage depending on battery size. The topping stage going on until the current falls down to around C/200. Once the battery is not taking over C/200 amps, then the battery can be declared as a fully charged battery. Then float or trickle charging started to keep the battery voltage around 13.6V.

Here note that the battery should not be kept in the float charging stage at 13.8V or higher. Otherwise, it will heat the battery and gassing will occur.

Then another very important stage is required to get long service life from a lead-acid battery. This is Equalizing.


Once in every 20 cycles, the battery must be equalized for 6 to 8hrs. In this stage, the battery voltage is kept at 15.5V to 16V after topping. In this stage, the battery will produce some gas, so proper ventilation should be arranged.

Charging summery:

So we can summarize the charging methods to get the maximum service life from a lead-acid battery.

  1. Selecting charging current little lower (C/12 amps) than the manufacturer charging rate C/10.
  2. 3 stage charging system is used, bulk, topping, and float.
  3. Equalizing battery in every 20 cycles of charging & discharging.

These techniques will give us:

  • ✓ Longer Service life
  • ✓ Less water topping
  • ✓ Less gassing
  • ✓ Less sulfation
  • ✓ Good charging capacity.

But this is not enough:

We have learned the best method of charging a lead-acid battery to get the maximum service life. But this is not enough to get the maximum output from a battery. Proper protection in discharging is mandatory.

Discharge protection:

A battery of 12V should not be discharged lower than 60% of its charge capacity. And should not be discharged to lower than 10V in any situation. In some cases especially the thick plate flooded lead-acid batteries, the battery must not be discharged lower than 11.6V. But in deep cycle lead-acid batteries, this level should not be lower than 10.5V. And once it is discharged, reconnection should not be done more than 3 times.

Maximum service vs maximum capacity:

So, a combination of charging and discharging is very important to get the maximum service as well as the maximum capacity from a lead-acid battery.

Summarizing everything:

So, to get the maximum service from a battery we have to select the best charging method as well as proper discharging protection. A summary will help us a lot.

  • Charge at C/12 amps rather than C/10.
  • Three-stage charging, CC, CV, float.
  • Equalization once in every 20 cycles of charging
  • Do not discharge lower than 50% of capacity.

This charging technique is proven and has been used in most charging circuits. Specially inverters, solar power systems follow these techniques to get the maximum service from a lead-acid battery.

I hope this project was helpful to you. If you make one for yourself, it will be a great pleasure for me. Anywhere you need help, let me know. Please share this project and subscribe to my blog. Thank you.


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This is a personal electronics blog. The author loves to write on different topics and shares his knowledge to help others.


Josh · July 19, 2021 at 10:41 pm

Sir.. thank you.. very much informative and seems 100% practical rest oriented.. I appreciate you for this valuable
Wishes from India

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