How to charge a cordless drill battery without the charger

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A cordless drill is of no use with a juiceless battery. Every manufacturer provides a dedicated charger for the batteries which come with the drill. In a worst-case scenario, the battery charger can die for various reasons. In this blog post, we will discuss how to charge a cordless drill battery without the charger.

In summary, all you need is to provide 3.7-4.2 volts (for Li-Ion Battery) to every individual cell or a combined rated voltage of the battery to both poles. This will automatically charge your battery. The voltage source can be provided from any source. It should be attached in the same polarity, which means a positive terminal with the positive and negative terminal with the battery’s negative.

The science behind charging a cordless battery is the same as charging a Li-ion battery with a charger. The science behind charging a Li-ion battery is the same as charging any chargeable (this is important) battery.

How to charge any battery without a charger?

The answer is simple, provide the rated voltage of the battery at both terminals. As shown in the image.

In the first image, the provided voltage is zero, so no charging.

How is a battery charged

Once the voltage goes up to 12 V, it starts charging.

Charging polarity of a battery

In reality, it is not that simple. The charging starts as soon as the potential difference rises.

This means that if a 12 V battery got discharged and went down to 6V, as soon as you connect a voltage source above 6, it will start charging. The charging rate is also directly proportional to the voltage difference.

If the battery was at 6V and connected with 12V, it will start to charge quickly, and as it approaches 12V, the charging speed will drop significantly.

This same technique is also used in Fast Charging. More than the rated voltage is applied at the beginning of the charge cycle and reduced to the rated voltage when the battery approaches full charge.

Caution: Fast charging needs a proper BMS (Battery Management System) to avoid any mishap or explosion. All batteries are not designed to stand fast charging.

Charging Li-ion battery without charger

Okay, so now we have discussed how to charge any battery, let’s move on to Li-Ion batteries.

A single lithium-ion Cell is rated at 3.7 V.

The charging cut-off voltage is 4.2 V, and the discharging cut-off voltage is 3.0 Volt. This means, at the end of the charging cycle, the cell voltage should not be more than 4.2 V, and at the end of the discharging cycle, the cell voltage should not be less than 3.0 V.

Since most cordless drills are powered by Li-ion batteries, how come they have 12-18V batteries? The answer is simple; they comprise multiple li-ion cells stacked in parallel and series, as shown in the image.

Li-Ion cell internal structure

Cells in parallel only affects the amp hour of the battery.

Standard Un-Balance Charge

Now, let’s discuss balance and unbalance charging. You will have seen multiple pins on a high-end cordless drill battery. These multiple pins are connections to each stack of the cell to balance charge it. As shown in the image above, every cell is individually accessible to charge.

A drill battery teardown

Learn more about a battery internals at this link.

In a standard charge, all we have to do is apply the rated voltage at both terminals and wait for the battery to charge.

Unbalance charging

To avoid any damage to the battery, it is recommended to charge at 12V rather than 12.6V as stated in the image above.

Balance Charge

Balance charging takes more than a standard unbalances charge. You will need special off the shelf balance chargers mostly used by RC plan enthusiast. They are cheap but great charges with a lot of parameter controls.

The chargers provided by the drill manufacturers take care of charging each cell and balances its voltage to be the same 4.1-4.2V when fully charged. This is not going to be when you plan to charge it yourself, but still, better to understand the process. You will have to charge the battery as following if you want to balance charge it.

Balance Charging a Li-ion Battery

The Hacks: How to charge a cordless drill battery without the charger

Now, let’s move on to the most interesting part. How to charge the battery without the charger at home.

We normally do not have a variable voltage source lying around. These voltage sources are expensive and hard to find. So, we need to find a hack to charge our dead battery.

Assumption: I am assuming that you will probably charge an old battery that comes with your outdated drill and you no longer want to invest in a new charger or the charger is no available in the market.

For newer batteries, we will recommend buying a proper charger to extend the battery life.

So, let’s come to the point. As discussed earlier, all you need is the same amount of voltage as the battery.

Following are a few ways you can get that desired voltage:

  • Your Car battery – A perfect 12V voltage Source
  • Laptop Charger – 12V to 18-19V Voltage Source
  • Simple AA Cells – Any amount of voltage can be built up
  • Cheap Voltage Regulators/Converters
  • Another Drill Battery
  • Solar panel

Charge a cordless drill battery with the car battery

DO IT AT YOUR OWN RISK: Risk Involved – Shorting car battery terminals, overcharging a low rated voltage battery to 12V, and exploding it.

A car battery is one of the best ways to charge your 12V cordless drill battery. You don’t need to take care of charging your car battery as that will be automatically charged by your car alternator when you drive.

All you need to do is, connect the drill battery terminals with the car battery terminals. As stated earlier, connect positively with positive and negative with negative.

Charging a battery with a car Battery

Make sure the battery rated voltage is 12V.

Charging a Cordless Drill Battery with Laptop Charger

Most laptop chargers are rated for 18-20V, which is very suitable for charging an 18V battery.

The process is simple. Understand the type of connector your laptop charger has. After that, get a hold of a compatible female connector. Solder the female connector with the internal battery connections (Positive with positive and negative with negative). Make a cut-hole for the female connector to sit in, and voila, you can charge your battery without the official charger.

Laptop charging pin types

If you don’t want to get bothered with the connector type, get an off the shelf two-pin connector, like a T-Connector, and make connections in a way positive aligns with positive and negative with negative.

An Apple Macbook charger (The old Megsafe one) is rated at 14/16/18/20V. You can easily charge an 18V battery with the 18-20 volt chargers. Do check the output voltage rating.

Again, the process is simple, connect positive with positive and negative with negative. One problem with Megsafe chargers is that it is complicated to take the connection from its pins, and to get hold of the female pin socket is very hard, so you will have to do this…

Inside apple magsafe charger cable

Note that the center’s wire will be the positive terminal, and the outer mesh will be negative. Caution: This will irreversibly damage your MagSafe charger.

Charging battery with off the shelf 1.5V AA Cells

This is an expensive and inefficient way, but it can be life-saving. In extreme cases, when you can’t find any voltage source, you can get hold of simple off-the-shelf 1.5V AA Cells and connect them in series to make up the desired voltage.

9 Cells in series for a total of 13.5V to charge 12V battery and 13 cells in series for a total of 19.5V to charge 18V battery. They may not fully charge your battery as these cells have a minimal capacity.

You can also get hold of rechargeable AA cells, and they mostly come with a charger, but that will be expensive. Still, worth sharing.

Charging battery with Cheap Voltage adapters

The process is the same as in laptop chargers, but these are cheap voltage regulators available at electronic shops and online. Get these 12V and 18V adapters from Amazon, and they will work great.

You may find these already lying around at your home, which may have come with electronic devices like Bluetooth speakers, min fans, or toys. Do a thorough search.

A 12V adapter


They may come with a female connector.


Charging battery with another drill battery

This is also a great hack. If you have another drill battery with a working charger, you can use that drill battery to charge your battery.

Again, the science is the same. Make sure the other battery is of the same voltage and fully charged. Now connect the terminals in the same polarity. Positive with positive and negative with negative. You will have to use jump wires for the connections. It will not be a solid solution but can work in an emergency.

Charging battery with solar panel

Solar Panels are a great source of DC Voltage Source. However, there is one problem. The Output voltage is not constant and varies due to the change in sun direction and temperature.

A 12V rated solar panel has an output voltage of 17 volts, which is regulated down to 13-15 volts to charge a 12V battery.

Just make sure the panel is designed for a 12V battery system, and then you can connect it directly with your battery without an MTTB controller. Make sure your battery does not become too hot to avoid any incident.

Again, these are all hacks and shall be performed with extra care. Peace.

Farooq Kamal

Farooq Kamal

Hi, I am the founder of TCT. I am a DIY enthusiast and have a passion for woodwork, building cool home improvement items, model airplanes, drones, and electric vehicles. After graduation in Electronic Engineering, I worked in an Industrial Automation Company where I had hands-on experience making electrical cabinets, installing field instruments, and programming custom large scale automation projects.

Currently, I am working on an electric conversion of an old 1960s Vespa and will keep you guys updated in my following posts. You can say hi to me on Twitter with the handle @farooqtaran. Peace.

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