Replacing Ni Cd Cordless Screwdriver Batteries with 18650
Replacing an old Ni-Cd screwdriver battery with a Li-Ion cell
It happened in my case. The RYOBI CMD-1202 screwdriver was purchased by me on the occasion of repairs in the kitchen and faithfully served 3 years, after which the battery charge was enough to screw in a pair of medium-sized screws. This screwdriver was equipped with two 12 V Ni-Cd batteries with a capacity of 1.7 Ah. The cost of two new batteries was more than the cost of a new screwdriver of the same class with two batteries in the kit. The latter was purchased for household needs. The old screwdriver went on a well-deserved rest to the pantry.
Recently, in the Internet, I came across an article about replacing the nickel-cadmium elements of a screwdriver battery with lithium-ion (Li-ion) batteries of the 18650 format. Fortunately, the latter have accumulated in a decent amount, mainly from blasted laptop batteries. Having spent several hours studying the issue, it turned out that several options for implementing such a replacement are proposed.
1. A simple serial connection of the required number of 18650 cells and the use of a native charger. A superficial knowledge of the features of charging series-connected batteries is enough to exclude this option. Without going into technical terms, we can say that Li-ion batteries can not be strongly discharged and recharged, only work within 2.5. 4.2 V guarantees long and safe battery operation. Charging a battery redesigned in this way with a native charger will quickly disable Li-ion cells, and their destruction as a result of overheating is also possible.
2. Connection of elements with the addition of additional balancing wires, which are displayed in a separate socket. “Smart” chargers such as iMAX B6 and Turnigy Accucel-6 are used for charging. The charging process in this case is “correct”, however, as in the first case, the discharge of the batteries is not controlled in any way, and their excessive discharge will significantly reduce the battery life. In addition, there is a need to purchase and use a special charger, under construction conditions this is extremely inconvenient.
3. There are extremely exotic implementations of this venture. Some embed miniature voltmeters in the battery case, the readings of which determine the degree of charge. Experienced hams design and manufacture charge and balance controllers. These decisions are either difficult to repeat or do not provide the necessary control and the required reliability.
Having decided on the terms of reference: using the native charger to charge the batteries, no additional connectors and wires, the presence of charge control and balancing the elements, I began to look for options for implementation. As always, our Asian friends did not disappoint, they offer many options for charge control modules for both single and connected Li-Ion batteries. At this stage, you need to select the rated voltage for our Li-ion battery. Option 3S Li-ion batteries with a voltage range of 12.6. 7.5 V is not suitable for power equipment, since under heavy load there is a strong voltage drop even on a fully charged battery. Battery of 4S Li-Ion batteries with a voltage range of 16.8. 10 V will ensure the longest possible operation, and a slight excess of the operating voltage is not critical. The option suitable for me (bought here) has the following characteristics:
Bms 4S charge and protection controller for 4 Li-Ion 18650 batteriesCharging Voltage Limit: 4.25 VDischarge limit (cutoff): 2.5 VMaximum charging current: 3 APeak discharge current: 30 AOperating discharge current: 15 AShort circuit protectionOvercharge / Overcharge ProtectionOver current protection
Proceed directly to the alteration
We disassemble the old battery and remove the Ni-Cd cells, as in most of these batteries, the contact pad is mounted directly on one of the cells so as not to “collective farm” with contact fastening, we glue them together with this cell using hot-melt adhesive.
We assemble 4 elements of Samsung ICR18650-26C 2600 mAh in such a way as to provide the most convenient location in the case. We connect the individual elements by soldering the wires, this must be done as quickly as possible so as not to overheat the battery. We connect the resulting battery with a protection board according to the connection diagram.
Install the battery in the case. The battery is ready.
The weight of old batteries with Ni-Cd cells was 560 grams. The new Li-Ion battery “lost weight” by 180 grams. And weighs 380 gr. At the same time, the energy capacity was almost doubled from 20.4 Wh (12 V x 1.7 Ah) to 38.48 Wh (14.8 V x 2.6 Ah). A screwdriver redone in this way will serve as a faithful assistant for many more years in our service center.
Initial stresses on the elements are 3.85. 3.95 V, we charge with a native charger. To control the current consumption in the wire gap between the battery contact and the charge controller, a multimeter is connected in the current measurement mode. The voltage of the battery and a single element are controlled by two miniature digital voltmeters. Just in case, using an infrared thermometer, we control the temperature of the batteries and transistors on the charge controller board. The charging results are shown in the table and graphs.
After the process of charging the voltage on the elements of 4.15. 4.17 V, battery voltage in general. 16.63 V. During charging, the temperature of the batteries did not rise above 34.5 ° C; the charge controller board did not heat up at all.