How to design 18650 battery pack?

Jun 25, 2019   Pageview:104

Here are the tips on how to design the 18650 battery pack describing the process you can use to create custom battery packs.


Defining your goals


Mainly help you to focus on the vital aspects of rechargeable battery pack design. And here are some of the specifications for your 18650 battery pack.


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Amperage output- this is the total current your 18650 battery pack will be able to provide.


Milliamp output-this is the total capacity of your battery. The higher the output, the longer your battery will last before to be recharged.


Voltage output-measures the electric potential energy and represents the amount of energy that can be released.


The cost-the total cost of the battery pack should be affordable.


Charge time- Keep in mind end-user – how much friction will it cause for each additional hour of charging.


Weight-18650 battery packs should be as light as possible while remaining durable.


Safety-very critical and packs are always designed to adhere to safety standards, especially those involving medicine and children more safety is required


Special requirements- individual boxes to prevent corrosion from salt spray or temperature control in very cold or hot conditions.


Problem-solving and the configuration


In this step, you deal with the following design and more technical points.


Configuration- the battery pack includes both the cell count and how they are arranged either in a series or in parallel or a combination of both. The required specifications determine the configuration types. Here are the standard battery configuration formats.


B Format –end to end

C Format- side to side

BC Format- a combination of both B and C formats

Standard –side by side interlinked with solder tags

Cluster Format- polygon shape interlinked with solder tags

Nest Format-staggered interlinked with solder tags


Deciding between 18650 battery pack formats is determined by what the general type battery pack you need and other restraints specifications set.


Cell choice- a combination of the specs listed above can determine the best cell choice for your pack.  And at this stage, our pick for lithium battery manufacturer is either LG, Samsung, or Panasonic. LG and Samsung cells typically for application which require a high maximum continuous discharge current. Much relevant to always use the same cells or match Rank A and B cells together when building a 18650 battery pack.


Battery management system- for custom battery packs is a printed circuit board that manages the battery pack to keep the policy in its safe operating area.  Also, it can be used to automate and optimize processes within the system. Most of the battery management systems are well standardized and do not need customization.


Smart charger- you put a microchip in it and creates communications between it and the battery management system.


Special requirements- at this time, come to light things like BMS or configuration format.


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Price for custom battery packs


The price to create rechargeable battery packs depends on many variables such as cell cost, unique material, quality control requirements, testing and certification, packing, shipping, and customs.


Building a sample 18650 battery pack


After knowing what it will cost, you can produce a sample.


Iterate and full production


Samples and designs are now ready, and production starts.


HOW TO CONNECT 18650 BATTERY PROPERLY


The desired operating voltage achieved by connecting several cells in series, parallel, or a combination of sets and parallel connections.


Parallel connections


One or two cells can be connected in parallel if higher currents are needed and more giant cells are not available. With identical cells, capacity, and runtime increase while the voltage stays the same. A battery that develops a high resistance is less critical in a parallel circuit. A weak cell will not affect the energy but provide a low runtime due to reduced capacity.


Series connection


Portable equipment which needs higher voltages uses battery packs with two or more cells connected in series. In comparison, a six-cell lead-acid string with 2V/cell will generate 12V, and four alkalines with 1.5V/cell will give 6V. High voltage batteries require a cell matching, majorly when drawing heavy loads or when operating at cold temperatures.


Series /parallel connection


The actual total power is the sum of voltage times current. A combination of 8 cells would produce 97.92 Wh, the allowable on an aircraft without class hazardous material.


HOW TO MAKE A 18650 BATTERY PACK


Here is how you can make a 18650 battery pack


Select the right 18650 cells for the battery pack


You will find many types of 18650 cells in the market n the price ranges from $1 to $10.


Choosing the right battery strips


To make the battery pack, you have to connect the 18650 cells using Nickel strips or thick wire. Nickel strips come with different dimensions and lengths. Choose according to the current rating.


Spot welding vs. soldering


These are the two options for connecting. Soldering is that you apply a lot of heat to the cell without dissipating very quickly. Which enhances the chemical reaction in the cell.


Spot welding


It is done to join cells together without adding much heat.


Check the cell voltage


For paralleling the cells, the energy of each cell should be near to each other. For new batteries, it is near 3.5V to 3.7V.


Battery pack capacity and voltage


You have to connect the cells in parallel to reach the desired position and nominal voltage.


Assemble the 18650 cells


You can assemble the cells to make the pack by using hot glue or using a plastic 18650 battery holder.


Spot weld the nickel strip


Spot welder have three welding choices: fixed welding head, fixed welding head with foot switch, and movable spot welding pen with foot switch.


Cut the nickel strips and lay your nickel strip on top of the five cells, ensuring that it covers all the cell terminals. Leave 10mm excess pieces for connecting it to the BMS then cut it. Series connection cut small strips. Connect the first parallel group negative terminal to the positive terminal of the second group and the negative terminal of the second group to the positive terminal of the third group. Spot welder can be used to weld the pure nickel. The welder pulse and current knob according to the thickness of the nickel strips.


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Add the battery management system


The BMS has four soldering pads B-, B1, B2, and B+.  Connect the first parallel group negative terminals bus to the B1. Similarly, the same group negative bus to the B2 and positive terminals bus to the B+. You can spot weld the nickel strips to the BMS. Then apply soldering flux to the PCB pads and then solder them together.


3D Printed enclosure


The battery pack has all around exposed nickel strips to prevent accidental shortening an enclose will be enhanced.


Wiring the components


Usually, a standard battery has only two terminal to connect the load and to charge the battery. A single wire can be used to join, and a heat shrink tubing is used to insulate.


Final assembling


Then hot glue is applied at the base of the battery compartment, then secure the battery pack to prevent loosing of wire connections. Finally, screw the top lids in place. Now the battery pack is ready to use. You can charge the battery.


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