DIY nimh battery pack. Simple Guidelines for Using Secondary Batteries

Automatic NiMH battery charger circuit

This is a NiMH battery charger circuit with auto cutoff. A nickel-metal hydride battery, also known as NiMH, is a secondary cell. It is a type of rechargeable battery. They are used extensively, in cameras and other electronic devices.

Because it is cheap and very common for AA cells in many capacities as required. We can replace the generic battery right away.

Important. We can a new recharging a lot of times depends on its capacity.

  • Lower capacity, 1700 up to 2000mAh can be recharged up to 1000 times
  • 2100 to 2400 mAh batteries can give between 600 to 800 times
  • The greater power capacity, AA 2500 mAh rechargeable batteries can only be recharged approx 500 times

All rate in slow charge mode.

We will see that it saves a lot of money. But it needs a good battery charger. We can buy an instant one in a store near your house.

But we are electronics inventors. We should build it ourselves. Because it is cheap and of good quality.


The project is the NiMH battery charger circuit with automatic cutoff when fully charged. You can charge the batteries from 2-8 pcs, depending on an input voltage.

In the circuit has two LED indicators.

nimh, battery, pack, simple
  • First LED, show charging status, when the battery is full, it will be off.
  • Second LED, connect the battery correctly.

The input voltage can use the power supply of 12V, 2A. At the charged current up to 800mA.

Note: This circuit is also an automatic NiCd battery charger circuit, too. For example Nicad battery 1500mAh, 1.2V. Now I never see under 500mAh.

How it works

Updated: 22 Jul 2019Figure 1 Automatic NiMH battery charger circuit using TL072

The circuit in Figure 1, when we connect a battery to both points P3 and P4. LED2 will glow up, in case all battery connects correctly. If the battery reverse polarity, this LED2 goes out, we need to check them again.

Some voltage from the battery is compared to IC1. Which the TL072 is set in a voltage comparator circuit at between pin 2 and pin 3.

In case the voltage from the battery to charging each cell is still low. The output from pin 1 of IC1 will send to a transistor Q1.

The transistor-Q1 will switch on-off a working to transistor-Q2. So the current can flow to the battery.

While the circuit is charging, LED1 will show charging status.

When the battery is full, its the voltage is compared at the IC1 will make an output at pin 1 of IC1 go to stop working of Q1. Then also causes the Q2-transistor stop.

The end of battery charging and LED1 is off, to display that the battery is full enough.

The amount of current in the charging battery of the project is determined at 800mA. By there is R7-resistor is a current limiter as above.

The S1-switch, to start charging, in the case that the battery correctly. We also press S1 to recharge again, to test that battery is fully or not.

Here is PCB layout by Lennie Zink. Thanks for your share and suggestions.He is an excellent electronics hobbyist. I like his life. Read more: Lennie’s Projects

How to builds this project

This project has a few components so easy to build. First, make the single-sided PCB layout as Figure 2 or can also use easily the Universal PCB Board. Then assemble all parts on the PCB as Figure 3 We just recommend a short detail, because I think you can create easily yourself.

PCB layout of Automatic NiMH battery charger circuit

The Component layout for the PCB.

The Detail ComponentsIC1 = TL072—LOW NOISE J-FET DUAL OPERATIONAL AMPLIFIERS = 1 pcs.Q1 = BC327—50V 800mA PNP Transistor = 1 pcs.Q2 = MJE2955—50V 3A PNP Transistor = 1 pcsD1,D4 = 1N4148—75V 150mA Diodes = 2 pcs.D2,D3 = 1N4001—50V 1A Diodes = 2 pcs.Electrolytic capacitorsC1,C4 = 1000uF 25V—Electrolytic capacitors = 2 pcs.C5 = 1uF 50V—Electrolytic capacitors = 1 pcs.C2,C3 = 0.1uF 100V—Polyester capacitors = 2 pcs.0.25W 1% ResistorsR1 = 10K = 1 pcs.R2 = 1M = 1 pcs.R3,R9 = 1K = 1 pcs.R4 = 470 ohms = 1 pcs.R5 = 150 ohms—2 watts = 1 pcs.R6, R8 = 100K = 2 pcs.R7 = 1 ohms-2 watts = 1 pcs.

Also these NiMH battery charger circuit

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44 thoughts on “Automatic NiMH battery charger circuit”

Hi, it possible to put USB at P1 and can I replace IC1-TL072 to IC LM317. plus can I add solar panel to this circuit? Reply

No you cannot use USB port. Voltage only 5V. LM317 is a regulator – TL072 is an Opamp. Two different components altogether, so you cannot do this. Yes you can put a 12V solar panel on input. Regards, Fred Reply

Is it for 9v battery? If not than what corrections I should do in it to make it work fpr 9v rechargable battery? Reply

hi, may i know what kind of switch was u used? is’t relay ? and can this circuit charging 9v of battery ? tq =) Reply

Single Cell Applications

The single-cell configuration is the simplest battery pack; the cell does not need matching and the protection circuit on a small Li-ion cell can be kept simple. Typical examples are mobile phones and tablets with one 3.60V Li-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, wristwatches and memory backup, most of which are very low power applications.

The nominal cell voltage for a nickel-based battery is 1.2V, alkaline is 1.5V; silver-oxide is 1.6V and lead acid is 2.0V. Primary lithium batteries range between 3.0V and 3.9V. Li-ion is 3.6V; Li-phosphate is 3.2V and Li-titanate is 2.4V.

Li-manganese and other lithium-based systems often use cell voltages of 3.7V and higher. This has less to do with chemistry than promoting a higher watt-hour (Wh), which is made possible with a higher voltage. The argument goes that a low internal cell resistance keeps the voltage high under load. For operational purposes these cells go as 3.6V candidates. (See BU-303 Confusion with Voltages)

Series Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 2 shows a battery pack with four 3.6V Li-ion cells in series, also known as 4S, to produce 14.4V nominal. In comparison, a six-cell lead acid string with 2V/cell will generate 12V, and four alkaline with 1.5V/cell will give 6V.

If you need an odd voltage of, say, 9.50 volts, connect five lead acid, eight NiMH or NiCd, or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply might work in lieu of 9.50V. Most battery-operated devices can tolerate some over-voltage; the end-of-discharge voltage must be respected, however.

High voltage batteries keep the conductor size small. Cordless power tools run on 12V and 18V batteries; high-end models use 24V and 36V. Most e-bikes come with 36V Li-ion, some are 48V. The car industry wanted to increase the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing the electrical components and arcing problems on mechanical switches derailed the move.

Some mild hybrid cars run on 48V Li-ion and use DC-DC conversion to 12V for the electrical system. Starting the engine is often done by a separate 12V lead acid battery. Early hybrid cars ran on a 148V battery; electric vehicles are typically 450–500V. Such a battery needs more than 100 Li-ion cells connected in series.

nimh, battery, pack, simple

High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating at cold temperatures. With multiple cells connected in a string, the possibility of one cell failing is real and this would cause a failure. To prevent this from happening, a solid state switch in some large packs bypasses the failing cell to allow continued current flow, albeit at a lower string voltage.

Cell matching is a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of the repair, and this is why battery packs are commonly replaced as a unit.

High-voltage batteries in electric vehicles, in which a full replacement would be prohibitive, divide the pack into modules, each consisting of a specific number of cells. If one cell fails, only the affected module is replaced. A slight imbalance might occur if the new module is fitted with new cells. (See BU-910: How to Repair a Battery Pack)

Figure 3 illustrates a battery pack in which “cell 3” produces only 2.8V instead of the full nominal 3.6V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack. The voltage collapses and the device turns off with a “Low Battery” message.

Batteries in drones and remote controls for hobbyist requiring high load current often exhibit an unexpected voltage drop if one cell in a string is weak. Drawing maximum current stresses frail cells, leading to a possible crash. Reading the voltage after a charge does not identify this anomaly; examining the cell-balance or checking the capacity with a battery analyzer will.

Tapping into a Series String

There is a common practice to tap into the series string of a lead acid array to obtain a lower voltage. Heavy duty equipment running on a 24V battery bank may need a 12V supply for an auxiliary operation and this voltage is conveniently available at the half-way point.

Tapping is not recommended because it creates a cell imbalance as one side of the battery bank is loaded more than the other. Unless the disparity can be corrected by a special charger, the side effect is a shorter battery life. Here is why:

When charging an imbalanced lead acid battery bank with a regular charger, the undercharged section tends to develop sulfation as the cells never receive a full charge. The high voltage section of the battery that does not receive the extra load tends to get overcharged and this leads to corrosion and loss of water due to gassing. Please note that the charger charging the entire string looks at the average voltage and terminates the charge accordingly.

Tapping is also common on Li-ion and nickel-based batteries and the results are similar to lead acid: reduced cycle life. (See BU-803a: Cell Matching and Balancing) Newer devices use a DC-DC converter to deliver the correct voltage. Electric and hybrid vehicles, alternatively, use a separate low-voltage battery for the auxiliary system.

Charging and Handling Batteries Safely

Charge your battery at room temperature. If you recently used the battery and it still feels warm, allow it to cool down before you start charging it. Keep the charger and battery away from direct sunlight or heat sources since it can make the battery overheat and affect the capacity. Avoid letting the battery drop below 10 °C (50 °F), or else it won’t charge effectively. [10] X Research source

Unplug the charger as soon as your battery is finished charging. Avoid overcharging your battery since it will reduce the maximum capacity and it could overheat. Keep track of how long you’ve left your battery plugged in or watch the timer on the charger so you know how much longer you need to leave it plugged in. When it’s finished, disconnect the charger from power before taking your battery out. [11] X Research source

  • Many electronics stores have battery drop-offs. Simply take your battery into the store, find the drop-off box, and put your battery inside.

Community QA

It’s not a great idea unless you know what you are doing. But if it’s a functional pack you should be able to extract a functional cell. Be warned that packs may be made with cells that are not a typical consumer size, e.g. AA, and the contacts/terminals may be spot welded together. If you pierce the cells or damage a contact while disassembling, you could create a dangerous or just plain useless dead battery.

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If the light goes from red to green and is blinking, it indicates that it will be fully charged soon.

Thanks! We’re glad this was helpful. Thank you for your feedback. As a small thank you, we’d like to offer you a 30 gift card (valid at Use it to try out great new products and services nationwide without paying full price—wine, food delivery, clothing and more. Enjoy! Claim Your Gift If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even 450 helps us in our mission. Support wikiHow

Thanks! We’re glad this was helpful. Thank you for your feedback. As a small thank you, we’d like to offer you a 30 gift card (valid at Use it to try out great new products and services nationwide without paying full price—wine, food delivery, clothing and more. Enjoy! Claim Your Gift If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even 450 helps us in our mission. Support wikiHow

nimh, battery, pack, simple

How To Build This Project?

Fig 3: A simple NiMH battery charger

Components Required

  • 0-12V AC step-down transformer
  • 1N4007 X 4 or bridge rectifier module
  • TIP125 PNP transistor
  • Resistors: 47Ω, 10Ω, 2KΩ, 1KΩ
  • 2 LED lights/ resistors
  • Two 330uF capacitors
  • 9V NiMH battery

How to Connect

You need to charge NiMH batteries slowly and at a low current. For your information, overcharging or overflowing the battery causes it to heat up. A step-down transformer transforms 230V AC power into 12V AC power.

The bridge rectifier modules turn AC power into DC power, whereas capacitor C1 handles filtering duties. Furthermore, Darlington Transistor TIP125 is connected to the positive supply line for current regulation and guarding the battery against overcharging. LED1 indicates the presence of the battery at the output and supply flow.

It’s linked between the positive supply and TIP125’s base terminal, then resistor R1. LED2 indicates the output DC supply when you couple it across the output with R4 Resistor.

How To Charge a NiMH Battery

Fig 4: Panasonic Eneloop Pro rechargeable nickel metal hydride (NiMH) batteries

Charging NiMH batteries is challenging as they lack the “float charge” voltages synonymous with lithium-ion batteries. Consequently, you must consider overcharge, charging rates, trickle charging, and charging manners to prolong battery life.

The NiMH batteries have similar properties to the NiCad batteries but suffer a reduced capacity if overcharged. Additionally, you must charge them through slow charging and a constant current charging method.

Here is how to charge a NiMH battery.

Timer Charging

Timer charging is the easiest NiMH battery charging method.

Here an inbuilt electronic timer determines the end of charge by assuming that the battery was 100 percent discharged beforehand. However, this method is flawed if a battery has lost a percentage of its capacity resulting in an overcharge.

Thermal Detection

Using temperature rise to determine the end of charge is flawed thanks to difficulties in telling the accurate battery temperature.

Negative Delta Voltage Detection (NDV)

The NDV method detects a charge voltage drop that appears once the cell gets a full charge. However, you must include a noise filtering circuit in your charger to detect the tiny voltage drops in NiMH batteries.

Consequently, many modern NiMH chargers employ the three methods listed above to detect the end of charge. These types of battery chargers use NDV and temperature rise detection. They also use a timer in case they bypass the former two methods.


NiMH batteries are robust batteries that often fail due to overcharging. Therefore, it’s essential that you correctly charge them with a suitable charger and at room temperature. If you adhere to those guidelines, you can safely use them for extended durations.

Finally, feel free to contact us if you have any gray areas that need clarification or assistance with your project.

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