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Model lithium battery DIY-purchase-assembly-soldering

by:dcfpower     2021-03-09
Most electric models use lithium polymer batteries as power. Due to the large capacity of batteries used in model helicopters and multi-rotor aircraft, a set of batteries will be scrapped after hundreds of charge and discharge cycles, so players need to constantly replace them. Large-capacity batteries are expensive, and in order to save money, it is necessary to find a way to save money. The author found on Taobao that the price of the battery cell is relatively cheap, and the price required to assemble the battery pack with the same capacity is only 1/2 to 1/3 of the finished battery pack. The following introduces a method and technique for assembling battery packs with batteries for reference by model friends.   One, buy batteries   There are many types of batteries on Taobao. The batteries used in model airplanes have higher requirements for discharge rate C. But the larger the C value, the larger the weight of the battery and the higher the price. Because I assemble a large-capacity battery, I don’t have to pursue a high C value, as long as it can meet the discharge current, generally 15C is enough. From the outside, the copper strips of the battery lead wires connected to the cells with high discharge rate are relatively wide, for example, the width of the battery pole pieces of 5 000mAh/25C is about 15mm. It should be noted that the negative lead of the lithium battery pack cell is a nickel-plated pure copper sheet, and the positive lead is a pure aluminum sheet. Because the aluminum sheet cannot be tinned, the manufacturer must press-weld the nickel-plated copper sheet on it to ensure welding. However, some batteries are directly sold without this treatment, so you should ask the merchant before buying to avoid failure to assemble.  Second, preparation before assembly  After the batteries are in hand, you need to use a digital voltmeter to measure the voltage of each battery cell (usually around 3.9V), and form a group of batteries with similar voltages. u003cpu003eFigure 1u003c/pu003e When the    cell leaves the factory, because the tail of the positive electrode pressure-welded copper sheet is very close to the cell’s aluminum-plastic packaging film, the manufacturer wraps a circle around the end of the battery with heat-resistant tape to make the battery The positive pole is insulated from the aluminum-plastic package. But this method is extremely unreliable. Once the positive electrode is connected to the aluminum-plastic package, when the cells are assembled together, a short circuit will occur. The improvement method is to put a piece of insulating paper under the positive electrode copper sheet, and then re-wrap the heat-resistant tape to fix the paper sheet (Figure 1) . Note that the aluminum sheet has low strength and is not resistant to bending. If it is bent multiple times, it may break and cause the battery core to be scrapped. Be careful when operating. u003cpu003eFigure 2u003c/pu003e The tinning process of the lead-out pole piece of the battery core is very necessary, otherwise it is difficult to solder well during assembly. The surface of the lead copper sheet is very smooth due to the nickel plating, so it is not easy to tin with the rosin-soluble flux in alcohol. Solution: first use a triangular knife ground with a saw blade to scrape the plating layer on the surface that needs to be tinned to reveal the red (pure copper color, Figure 2), then apply a layer of rosin alcohol flux, and then use 20W or A 30W electric soldering iron can be easily hung if tinned (Figure 3). Be careful not to use a pointed tip soldering iron, because there is too little contact between the tip and the welding surface, which is not good for heat transfer. I use a bevel soldering iron tip (Picture 4). The tin plating should not be too thick, otherwise the copper sheet is not easy to bend. The flux remaining after soldering is not corrosive and does not need to be removed. Two other points are reminded:   1. High-power soldering iron can not be used for soldering, its temperature is high, long-term soldering will damage the battery;   2. Do not use corrosive solder paste as the flux.   Now the preparatory work before assembly is completed. u003cpu003eFigure 3u003c/pu003e   三. Battery assembly   The same group of batteries are pasted together with double-sided adhesive strips and arranged neatly. Pay attention to the polarity of each cell, and two adjacent cells must be opposite to each other. After everything is correct, wrap the battery pack tightly with fiber tape (Figure 5). The empty slots formed between the pole pieces of the battery pack are filled with balsa wood strips (Figure 6).  Because the lead-out pole piece of the battery is tin-plated, it can be attached to the balsa strip for welding. The method is: first press the copper sheet with a tool to prevent it from popping up, and then use a soldering iron to heat the surface of the pole piece. The joints treated in this way are reliable in connection and will not be short-circuited. Then, select the appropriate silicone wire and the battery connector to match and weld to form the battery lead-out wire, and then solder the balanced charging wire head (Figure 7). In order to prevent short circuits from contact with foreign objects, first wrap a layer of insulating paper on the end of the battery (Figure 8), then put on a heat shrinkable film and heat it with a hair dryer to shrink. At this point, the battery assembly is complete (Figure 9). 4. Battery usage Take 850 six-axis multi-rotor aircraft (S4110 motor, KV value 400, APC 1655 propeller, Fig. 10) as an example, original 5S /8000mAh battery, take-off weight 4 800g, hovering flight for 16 minutes, termination voltage After changing to two 3S/5 000mAh batteries in series, the take-off weight is reduced to 4 600g. After 10 minutes of hovering flight, the final voltage is 3.5V. According to the discharge curves of the two sets of batteries recorded by the flight control black box, it can be seen that the actual performance of this battery is slightly better than that of a battery of a certain brand.  5. About solder   Welding quality is not only related to the correct soldering method, but also closely related to the solder quality. Soldering tin is a tin-lead alloy, and its melting point varies with the proportion of tin-lead contained, so you can tell by just using a soldering iron. When the content of tin increases, the surface of the solder is bright and silvery white; when the content of lead increases, the surface of the solder is black, the melting point increases, and the fluidity becomes worse, and the soldering quality is difficult to guarantee. Therefore, although the price is low, solder with high lead content is not suitable. When assembling by yourself, it is best to use 60% tin and 40% lead solder. The melting point is about 183℃, the fluidity is good, the welding surface is smooth, and the welding strength is high.  Assemble the battery by yourself, which not only saves money but is also more fun, and you can give it a try. u003c/pu003e
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