simple 6V lead-acid battery charger circuit (how to make it at home)

Introduction of simple 6v lead-acid battery charger circuit

What is a battery? As we know battery is a type of power source consisting of one or more cells, in which chemical energy is converted into electricity. They are made of three basic components; an anode (negative pole), a cathode (positive pole), and some types of other materials that chemically reacts with the anode and cathode (electrolyte).  

So, for each electric circuit we have to have a power source. Every battery (an electric power source) can make a DC voltage so it has two terminals or poles; positive and negative. When an external load connects to it, electrons moving from negative pole to the positive pole and they create an electrical current. This electrical current can power a light bulb, a clock or watch, a motor, a computer, a cellphone, a fan, and other electronic devices or equipment.

Types of batteries

There are two main types of batteries. Primary batteries and secondary batteries. 

Primary batteries (single use or disposable) are types that just used once and discarded. They are usually cheap and must be replaced in the end of their life. Alkaline battery is a primary battery that is mostly used. 

Secondary batteries or rechargeable batteries can be discharged and recharged for more than one time (multiple times). This type can be recharged by applied electric reverse current. Lead-acid batteries (Picture 1.) and lithium-ion batteries are examples of rechargeable batteries. Lead-acid batteries are mainly used in cars and lithium-ion batteries are mainly used in portable electronics like laptops and smartphones. 

Picture 1. The image shows a lead-acid battery. lead-acid batteries are categorized as secondary (rechargeable) batteries.

Wet and dry cells

Wet cells have liquid electrolyte. Wet cells may be primary (non-rechargeable) or secondary (rechargeable) cells. Wet cells (for example lead-acid batteries) are still mainly used in automobile batteries and industries for switchgear standby or backup power, telecommunication or large UPSs (Uninterruptable Power Supplies); however, batteries with gel cells are increasingly used instead. 

Dry cells have paste electrolyte. These cells have only enough moisture to let current to flow. A dry cell contains no free liquid, so it is suitable for portable equipment and can operate in any orientation. Zinc-carbon battery and alkaline battery are common dry cell battery.  

Gel cell batteries don’t have lead-acid batteries safety and portability disadvantages. 

Capacity 

The amount of electric charge that a battery can deliver at the rated voltage is called capacity. The greater capacity needs more electrode material. Also, capacity is affected by the cell size. The capacity of battery (rated capacity) is the current (Ampere) that a new battery can continuously supply for 20 hours (while remaining above a specific terminal voltage per cell) at 20 degrees centigrade multiplied by 20 hours (A. H). So, capacity is measured by amp-hour. For example, a battery with 60 A.h can consistently deliver 3 A over 20 hours at room temperature. The relationship between capacity, discharge time and current for a lead-acid battery is approximated by Peukert’s law 

“Q_P” is the capacity when discharged at one ampere discharge rate (ampere-hours) 

“I” is the load current (actual current drawn from battery (A)). 

“t” is the amount of time (in hours) that a battery can sustain. 

“k” is a constant (Peukert constant) around 1.3.

C-rate

In order to normalize against battery capacity, which is often very different between batteries, discharge current is usually expressed as a Crate. this is a measure of the rate that a battery is fully discharged. So, it’s relative to battery maximum capacity. A 1C rate means that the entire battery capacity is discharged by discharge current in one hour. A 2C discharge rate means the battery will discharge in thirty minutes. For example, a 60 Ah battery 1C discharge rate means a discharge current of 60 A and a 2C discharge rate is 120 A.

Rechargeable batteries lifetime

The rechargeable batteries lifetime has two meanings. First it can mean the number of charge / discharge cycles before battery cells satisfactorily operate failureIt can also mean the length of time that a fully charged battery can run a device.

Self-discharge

It’s a chemical phenomenon in batteries in which internal reactions detract the stored energy (charge) of the battery without any electrode connection. Self-discharge causes batteries to initially have less than charge (when are fully charged before) compare with a full charge when actually put to use.

Some Battery Technical Specifications

Nominal voltage (v) 

Reference or reported voltage of the battery. 

Cut-off voltage 

The minimum allowable voltage of battery during discharge is called cut-off voltage. Lower voltages than cut-off voltage via deep discharge can cause irreversible damages to battery and its lifetime.  

Charge voltage 

The voltage that the battery is charged to its full capacity by charger. Charging schemes usually consist of constant current during charge until the battery voltage reaching the charge voltage, then constant voltage charging, with a very small current is continued.    

Float voltage 

The voltage at which the battery is maintained after being charge to compensate self-discharge of the battery. 

Charge current  

The ideal current under constant current charging scheme (before transitioning into constant voltage charging scheme).  

Internal resistance 

The internal resistance is the resistance within the battery. It’s generally different when the battery is charged comparing with discharge.

Most common types of rechargeable batteries

  • The lead-acid battery which is invented in 1859. It’s the oldest type of rechargeable battery. Because It is categorized as a low-cost battery, it is used widely in automobiles, home and industries.    
  • The nickel-cadmium battery which (Ni-Cd) is invented in 1899. This type is banned for most uses by European Union in 2004 because cadmium is a toxic element. 
  • The nickel-metal hydride battery (Ni-MH) has a hydrogen absorbing alloy instead of cadmium. This is now a common industrial and consumer type. 
  • The lithium-ion battery (Li-ion) has very slow loss of charge when not in use and an excellent energy density. So, it is best for consumer electronics.  
  • The lithium-ion polymer batteries (Li-Po) have higher energy density than Li-ion battery and can be made in any shape. This type is used in a wide range. 

Introducing a simple 6v lead-acid battery charger circuit and how to make it at home (100 % tested)

We’ll introduce a simple-6v-lead-acid-battery-charger-circuit. This circuit uses a LM317-n regulator (Wide Temperature Three-Pin Adjustable Regulator). This 3-pin adjustable positive voltage regulator is capable of supplying 1.5 A over a 1.25-V to 37-V output range in a wide temperature range. The LM317 offer full overload protection.  

To make a simple 6v lead-acid battery charger we need the following components: 

9-V to 40-V AC / DC adapter (we’ve used a 12-V, 2-A one) or you can make an AC / DC adapter yourself (It is introduced in a separate tutorial). 

A LM317 regulator (we’ve used a LM317T one). 

A 1000 µF capacitor (we’ve used a 50-v, 1000 µF capacitor). 

Resistors: one 100 Ω, one 1.1 KΩ, one 240 Ω. 

Picture 2. Simple 6v lead-acid battery charger circuit scheme 

R1 resistor determine charging peak current. charging peak current for this charger is 0.6-A for R1=1 Ω (We’ve used 0.5 Ω for 1.2-A charging peak current because our battery can carry maximum 1.5-A initial charging current. The battery specifications that we’ve used in this project is a 6-V, 5-AH battery). If you don’t know about the battery specifications that you want to charge, it’s extremely recommended to use a 1Ω resistor. 

A 2N2222 transistor. It’s an NPN transistor (We’ve used a BC547 transistor instead and it worked, but we recommend to use 2N2222 if it’s possible in your market). 

It’s recommended to use a heat sink for LM317, because it gets hot during charging operation. Also, we’ve used a 12-V fan for better heat reduction. 

Moreover, you can use a LED for power entry (Just for indication and could be ignored). 

simple 6v lead-acid battery charger circuit scheme with input power LED indicator

Picture 3. Simple 6v lead-acid battery charger circuit scheme with input power LED indicator

simple 6v lead-acid battery charger

We tested this charger by a 6 volt, 5 AH lead-acid battery for 3 hours charging period of time. The battery voltage before testing was 5.96V. The initial charging current was 127 mA. The charging current after one minute of charging was 125 mA. After 3 hour charging time, the voltage was 6.48 V and the charging current was 78 mA. 

It’s done! now we have a simple 6v lead-acid battery charger which is properly working. 

Simple 6V lead-acid battery charger circuit

 Picture 4. Charger that was made from simple 6v lead-acid battery charger circuit scheme

Simple 6V lead-acid battery charger circuit

 Picture 5. Charger that was made from simple 6v lead-acid battery charger circuit scheme

If you have any question or feedback about this charger you can leave a comment. Also if you want more tutorials about a subject that you interested in, please contact us. Thanks.

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