- 18650 battery – why it is
- Disassembling the case
- How to place items
- Battery connection diagram
- What and how to charge
The rapid degradation of cordless tool batteries is a real scourge. Almost always, the life of the screwdriver itself exceeds the service life of Ni-Cd cells and you either have to buy spare batteries or say goodbye to the tool. Today we will talk about the main way to extend the life of the battery.
18650 battery – why it is
Repairing power tool batteries usually involves restoring the electrolyte of the nickel-cadmium cans or replacing them altogether. The idea of changing one type of energy element to a more perfect one is quite sensible. This eliminates a wide range of cordless tool problems, including high weight, low capacity, memory effect and low cold holding capacity..
However, why should it be exactly 18650 batteries, and not some others? The answer is simple: this is the most common type of battery, except perhaps for batteries for mobile phones or other gadgets. The latter can be used, but most of them carry a built-in charge controller on board, and this is an extra waste of money.
In addition, the batteries must be high-current, that is, capable of supporting a load of 70-100 W. Batteries for e-cigarettes made by Samsung or LG are optimal. You should not take products from an unknown manufacturer: after all, Li-ion is a rather powerful thing and the low quality of the power cell case can provoke a loss of tightness from overheating with all the consequences. And since there are half a dozen more batteries in the neighborhood, the consequences can be very dire.
You can buy batteries on Aliexpress or other Chinese online stores, where they are quite cheap (200-250 rubles apiece, cheaper in bulk). In parallel, you need to purchase a number of additional lotions, this is due to the specifics of working with lithium batteries. Well, what are these gadgets and what is the meaning of their use – we will tell you along the way of the description of the alteration.
Disassembling the case
The first step is to disassemble the battery case into two halves. The easiest way to do this is if the battery pack is tightened with 4-5 screws: just unscrew them and pull out the top.
If the battery case is glued together (Makita, AEG), then the hassle will increase significantly. We put the battery on its side and carefully tap the glue seam with a rubber mallet. The blows are accurate, not strong, and frequent. We beat the joint evenly around the perimeter and try to stretch the halves every 50–100 strokes. Even the most stubborn corps surrender in 10-15 minutes of such “execution”.
Next, we throw away the unnecessary parts of the content. The terminal block must be carefully torn off the two upper cans so that two nickel tabs remain on it. Looking ahead, we will say that usually, when reworking, a new package of batteries is welded together by contact welding in the manner of factory ones. This is a cool solution, but not everyone wants to build a welding machine. Therefore, leave the length of the strips so that the wires can be fixed to it with two small bolts, and the rest of the elements will be connected by soldering.
In any convenient part of the case, you must also make a hole for the JST-XH balancing connector. From the outside with an awl, mark a rectangle 6 mm high and 15 mm wide for a 12 V battery voltage or 20 mm for a 18 V voltage. Insert the connector into the hole made and secure with hot glue or epoxy.
How to place items
Unlike Ni-Cd or Ni-MH cells, lithium batteries have a higher capacity and voltage, so fewer of them will be included in the battery. The dimensions of the 18650 cell are 65mm height and 18mm diameter. First, check how many of them will fit into an empty case, and determine the layout, if necessary, cut off the interfering stiffeners.
If the battery pack has a protruding top, a pair of cells will fit in. Another one is convenient to put on its side right under the two vertical ones. In the remaining space, you can put another 5 to 7 batteries. If the battery has a charging slide connector, stack the cells across the case in two stacks.
The voltage of the Li-ion battery is 3.7 V, but under load there is a drawdown of about 10-12%. This means that for a 12 V screwdriver you need at least 4 batteries, and for an 18 V – at least 5 pieces, although it is better to use 6, because a lot is not a little. Do not worry that the engine will be “frightened” by the high voltage and will make you live for a long time. With a drawdown under load, the overvoltage will be minimal and well within the operational limits. You need to decide on the number of batteries before you embed a balancing connector into the case, because there should be one more contacts in it than there are elements in a serial connection.
Now about the capacity. For lithium cells, it ranges from 2.5 to 3 A / h, which is not bad in itself. To double the capacity, you will need to double the number of batteries, but it’s definitely worth it. The only thing that can stop you in this venture is the size of the battery pack. In any case, remember that the number of elements must be strictly a multiple of 4, 5 or 6, depending on the voltage.
When you put the batteries in the right order, fasten them together with electrical tape and achieve complete immobility of the elements inside the case by filling the remaining space with pieces of expanded polystyrene or polyurethane foam. There is no need to leave space for the wires; in extreme cases, during the final assembly, you will need to perform a couple of additional cuts.
Battery connection diagram
To get the cherished 12 or 18 V, the elements must be connected in series. That’s it, no tricks, just observe the polarity. The minus of each battery is connected to the plus of the next one, the outer two wires are connected to the terminal block.
If you double the capacity, not individual batteries are connected in series, but assemblies of 2 cells. In each assembly, the positive contact is connected to the positive neighbor, the same is the case with the negative.
So that in the end the jumpers between the batteries do not entwine into an indistinct web, think over the connection scheme in advance. It is most convenient to solder the batteries when they are already wound into a tight bundle, choose the minimum length of the jumpers.
For soldering, the contacts of each battery should be well tinned. First, clean them with a file or a fine sandpaper, removing the top nickel layer. Use phosphoric acid as a flux, the most common solder is POS-61 with rosin. The soldering iron must be powerful, 60 W, no less. Lithium batteries must not be overheated categorically, the contact time with the sting is no more than 2 seconds. Therefore, first we tinker, let it cool, then we solder.
Also pre-tin the jumpers from a stranded conductor of 2.5 mm2 and supplement them with balancing wires so that there is one wire per node between parallel connected batteries or groups. Length of wires – to reach the balancing connector in the case, cross-section of about 0.5 mm2.
When soldering, the tinned core of the jumper is first heated, then it is brought to the contact of the battery until the solder melts on it. During cooling, you can press down the seam with wood chips. And do not spare the tin – the connection must be very reliable. Also, do not forget to wash off the remnants of the flux, otherwise after six months or a year of operation, all efforts will go to waste. Rinse the intricate positive contact with particular care; rubbing alcohol or acetone can be used to remove it.
If you try to solder the outermost wires of the battery to the nickel contacts of the shoe, you will most likely ruin it hopelessly by overheating the plastic. It is much better to drill two holes 3-4 mm in diameter and tighten the veins to the plates with a couple of small screws. Here it is convenient to use strips with double holes, which were massively picked out of old Soviet forks..
Along with the outermost wires of the battery bundle, screw a couple more balancing wires. The resulting balancing loop must be soldered in a specific order. From the datasheet on the connector, determine its contact, numbered with one, and solder a wire from the positive terminal to it. Then follow the battery chain and solder the wires in series, one after the other, completing by connecting the last contact with a common negative core.
What and how to charge
The peculiarity of charging a bundle of Li-ion batteries is that they must be charged strictly evenly. Otherwise, one of the elements loses in the depth of charging and, due to low voltage, begins to squander the rest, discharging even more. Lithium screwdriver batteries, like laptop batteries, have special charge controllers.
Therefore, the most important and expensive purchase for you will be a universal charger. Best of all, if it is something from the SkyRC line – these devices have already repeatedly proven that they are worth the money spent. You can take a Chinese counterfeit for 300-600 rubles cheaper, but always with the function of charging Li-ion batteries with several cells. Do not worry about the high cost: such a thing should be in the arsenal of every homemade product, it will help to restore and properly charge old dead batteries, including lead-acid ones, and Ni-Cd cans recently removed from the battery pack.
To charge a converted battery, you need to redo the standard charger. The task is simple – to solder two charging wires to the main terminals, observing the polarity. The balancing connectors are connected by a male-to-male wire, the device is set to the desired mode and a fully automatic charging process is carried out. The main thing in the operation of such a battery is not to take the elements into a deep discharge, but usually the engine noticeably gives up in power long before such a significant voltage drop, so you will hardly be able to hopelessly kill new batteries.