Converting a screwdriver battery to lithium cells Converting a screwdriver to lithium batteries has its pros and cons. Before proceeding with the rework, you need to think through some important points.
The arguments against
- Lithium battery cells cannot be charged above 4.2 volts and discharged below 2.7 volts. In real conditions, this interval is even narrower. If you go beyond these limits, the battery can be damaged. Therefore, in addition to the lithium cans themselves, you will need to connect and install a charge-discharge controller in the screwdriver;
- The voltage of one Li─Ion cell is 3.6─3.7 volts, and for Ni─Cd and Ni─MH this value is 1.2 volts. That is, there are problems with assembling a battery for screwdrivers with a voltage rating of 12 volts. From three lithium cans connected in series, you can assemble a battery with a nominal value of 11.1 volts. Out of four ─ 14.8, out of five ─ 18.5 volts, and so on. Naturally, the voltage limits during charge-discharge will also be different. That is, there may be problems with the compatibility of the converted battery with a screwdriver;
- In most cases, 18650 standard cans are used in the role of lithium cells for conversion. They differ in size from Ni─Cd and Ni─MH cans. In addition, you will need space for a charge-discharge controller and wires. All this will need to fit in a standard battery case of a screwdriver. Otherwise, it will be extremely inconvenient for them to work;
- A cadmium battery charger may not be suitable for recharging a battery after a redesign. You may need to revise the charger or use universal chargers;
- Lithium batteries lose their performance at low temperatures. This is critical for those who use a screwdriver outdoors;
- The price of lithium batteries is higher than cadmium batteries.
Converting the battery of a screwdriver to lithium cells
Many screwdriver owners want to convert their batteries to lithium battery cells. Many articles have been written on this topic and in this material I would like to summarize information on this issue. First of all, we will consider the arguments in favor of converting a screwdriver to lithium batteries and against it. And also we will consider certain moments of the process of replacing batteries itself.
Replacing batteries in a screwdriver with lithium
What do you need to estimate before starting work?
Also, more lithium cells will give more capacity. This means more operating time of the screwdriver.
Lithium battery cells 18650
The nominal voltage of lithium cells is 3.6-3.7 volts, and the capacity in most cases is 2000-3000 mAh. If the battery case allows, you can take not 4, but 8 cells. Connect them two by two in 4 parallel assemblies, and then connect them in series. As a result, you will be able to increase the battery capacity. But not every case will be able to pack 8 cans of 18650.
And the last preparatory stage is choosing a controller. According to its characteristics, it must correspond to the rated voltage and discharge current. That is, if you decide to assemble a 14.4 volt battery, then choose a controller with this voltage. The working discharge current is usually chosen to be two times less than the maximum permissible current.
Charge-Discharge Controller Board
The arguments against
- Lithium battery cells cannot be charged above 4.2 volts and discharged below 2.7 volts. In real conditions, this interval is even narrower. If you go beyond these limits, the battery can be damaged. Therefore, in addition to the lithium cans themselves, you will need to connect and install a charge-discharge controller in the screwdriver;
- The voltage of one Li─Ion cell is 3.6─3.7 volts, and for Ni─Cd and Ni─MH this value is 1.2 volts. That is, there are problems with assembling a battery for screwdrivers with a voltage rating of 12 volts. From three lithium cans connected in series, you can assemble a battery with a nominal value of 11.1 volts. Out of four ─ 14.8, out of five ─ 18.5 volts, and so on. Naturally, the voltage limits during charge-discharge will also be different. That is, there may be problems with the compatibility of the converted battery with a screwdriver;
- In most cases, 18650 standard cans are used in the role of lithium cells for conversion. They differ in size from Ni─Cd and Ni─MH cans. In addition, you will need space for a charge-discharge controller and wires. All this will need to fit in a standard battery case of a screwdriver. Otherwise, it will be extremely inconvenient for them to work;
- A cadmium battery charger may not be suitable for recharging a battery after a redesign. You may need to revise the charger or use universal chargers;
- Lithium batteries lose their performance at low temperatures. This is critical for those who use a screwdriver outdoors;
- The price of lithium batteries is higher than cadmium batteries.
Tips for choosing a security chip
The lithium-ion battery of the screwdriver cannot function normally without the BMS protection board. Instances on sale have different parameters. BMS 3S marking assumes, for example, that the board is designed for 3 elements.
What you need to pay attention to in order to choose a suitable microcircuit:
- The presence of balancing to ensure uniformity of the charge of the cells. If it is present, the value of the balancing current should be in the description of the technical data;
- Maximum long-term operating current. On average, you need to focus on 20-30 A. But this depends on the power of the screwdriver. Low-power enough 20 A, powerful from 30 A;
- Voltage, upon reaching which the batteries are disconnected when overcharging (about 4.3 V);
- Voltage at which the screwdriver turns off. It is necessary to select this value based on the technical parameters of the battery cell (minimum voltage is about 2.6 V);
- Overload protection operation current;
- Resistance of transistor elements (minimum value is selected).
Important! The magnitude of the overload tripping current is of little importance. This value is offset from the workload current. In case of short-term overloads, even if the tool has turned off, you must release the start button, and then you can continue to work.
Connection diagram of elements with a protective board
Whether the controller has an autorun function can be determined by the presence of an Automatic recovery entry in the technical data. If there is no such function, then in order to restart the screwdriver after the protection is triggered, it will be necessary to remove the battery and connect it to the charger.
Battery assembly
Further battery replacement consists of the following steps:
- The disassembled parts of the case are well cleaned;
- Since the dimensions of the new battery cells will be smaller, they must be securely fixed: glue them to the inner wall of the case with Moment glue or sealant;
- The positive and negative wires are soldered to the old terminal block, it is placed in the same place in the case and fixed. The protective board is laid, the parts of the battery pack are connected. If they were previously glued, then Moment is used again.
Difficulties in rework
There are objective disadvantages in Li-Ion batteries, such as poor performance at low temperatures. In addition, when converting a screwdriver to lithium 18650 batteries, a number of difficulties can be encountered:
- The 18650 standard means that a single cell has a diameter of 18 mm and a length of 65 mm. These dimensions do not match the dimensions of the nickel-cadmium or nickel-metal hydride elements previously installed in the screwdriver. Replacing batteries will require placing them in a standard battery case, plus installing a protective microcircuit and connecting wires;
- The voltage at the output of the lithium cells is 3.6 V, and at the nickel-cadmium cells it is 1.2 V. Let us assume that the nominal voltage of the old battery is 12 V. Such a voltage cannot be ensured when Li-Ion cells are connected in series. The range of voltage fluctuations during the charge-discharge cycles of the ion battery also changes. Accordingly, converted batteries may not be compatible with the screwdriver;
- Ionic batteries differ in the specifics of their work. They do not withstand overcharging voltage more than 4.2 V and discharge less than 2.7 V until they fail. Therefore, when a battery is being reworked, a protective board must be installed in a screwdriver;
- The existing charger can sometimes not be used for a screwdriver with a Li-Ion battery. You will also need to redo it or purchase another.
Important! If the drill or screwdriver is cheap and not of very high quality, then it is better not to rework. This can cost more than the cost of the tool itself.
Battery selection
12 V batteries are often used for screwdrivers. Factors to consider when choosing a Li-Ion battery for a screwdriver:
- In such instruments, elements with high values of the discharge current are used;
- In many cases, the cell capacity is inversely related to the discharge current, so it cannot be selected based on capacity alone. The main indicator is the current. The value of the operating current of the screwdriver can be found in the tool passport. Usually it is from 15 to 30-40 A;
- It is not recommended to use cells with different capacities when replacing a screwdriver battery with a Li-Ion 18650;
- Sometimes there are tips to use a lithium battery from an old laptop. This is absolutely unacceptable. They are designed for much lower discharge current and have inappropriate technical characteristics;
- The number of elements is calculated based on the approximate ratio of 1 Li-Ion to 3 Ni-Cd. For a 12-volt battery, you will need to install 3 new ones to replace 10 old cans. The voltage level will be slightly reduced, but if 4 cells are installed, the overvoltage will shorten the life of the motor.
Important! Before assembly, all elements must be fully charged for equalization.
Battery assembly
Further battery replacement consists of the following steps:
- The disassembled parts of the case are well cleaned;
- Since the dimensions of the new battery cells will be smaller, they must be securely fixed: glue them to the inner wall of the case with Moment glue or sealant;
- The positive and negative wires are soldered to the old terminal block, it is placed in the same place in the case and fixed. The protective board is laid, the parts of the battery pack are connected. If they were previously glued, then Moment is used again.
The advantages of lithium-ion batteries
Nickel-cadmium batteries have a low price, withstand many charging cycles, and are not afraid of low temperatures. But the capacity of the battery will decrease if you put it on charge without waiting for a full discharge (memory effect).
Lithium-ion batteries offer the following benefits:
- High capacity, which will provide longer operating time of the screwdriver;
- Smaller size and weight;
- Keeps charge well when inoperative.
But the lithium battery for a screwdriver does not withstand a full discharge, therefore, factory tools on such batteries are equipped with additional boards that protect the battery from overheating, short-circuit, overcharge in order to avoid an explosion, a complete discharge. Placing the microcircuit directly into the battery will open the circuit if the unused battery is separated from the tool.