This article assumes that you have a basic knowledge of electronics. If you don't, consult a local hobby shop for help before attempting this.

Inland Hobby Products for Kit Bashers

Building RC Battery Packs

One of the main reasons for building (or rebuilding) battery packs, other than the cost savings, is the ability to improve the connections between the cells so the current will flow more easily from one cell to the next with the least amount of resistance.  Resistance causes the cells to overwork themselves and heat up. Heat is bad for battery life and performance. The hotter the pack is, the less runtime, current, and life it will give and have. And the hotter your battery runs, the more resistance there will be.

Battery Pack Styles

A common misconception about battery packs is that they are one very large battery when they are actually made up of a number of individual batteries, called cells, that are connected together to work as a single pack. There two basic types of battery packs. Stick packs, or shot-gun, stack the batteries positive to negative ends like batteries in a flashlight. Brick, or side-by-side packs have cells laid next to each other forming a cube or “brick”. Which style you use depends on the space available and the voltage you need for your particular model. This information is generally found in the “Items Needed to Complete” section.

Cell Connection Basics

There are two ways that the cells can be connected together. “In Series” is where the positive terminal of one cell is wired to the negative terminal of another cell. With this method, the total voltage of the pack is the sum of the individual cell voltages. For example, a 6-cell NiCd or NiMH pack is made up of cells rated at a nominal 1.2 volts each. When wired in series the total nominal voltage of the pack is the number of cells in the pack (6) multiplied by the voltage of each cell (1.2V in this case) to get the total pack nominal voltage (7.2 volts for this example). This is the most common cell connection method found in the RC hobby.

The second is called “Parallel” where you connect the positive terminal of one cell to the positive terminal of another, and negative terminal of one cell to the negative of another. Wiring cells in Parallel increases the total capacity of the pack but the nominal voltage remains that of a single cell. To figure out the actual end result capacity add the mAh rating (milli-amp hour) of the cells. If you put 2 cells in parallel that are each 2100mAh (commonly referred to as “2P”), multiply 2100 by 2 for a total capacity of 4200mAh.

The number of cells you’ll need and the configuration of them in your battery pack will depend on what you are powering, how much power you need, the amount of space you have to hold that battery pack, and the weight distribution in your vehicle. In some cases your chassis space for a battery pack is not adaptable, so be aware of those constraints before you start. Check your owner’s manual for your vehicle’s specifications. It can also be very helpful to draw out the cell configuration and double check your connections and voltage requirement before you begin assembly.

What You’ll Need

Other items you may find useful are:

Putting it all together

Lay out all the parts and tools you'll be using on your heat resistant work table so you won't have to get up for anything. Make sure each cell is fully discharged to reduce the chance of an accidental short during the assembly process causing serious damage or harm. Make sure you follow all safety precautions for soldering tools, equipment, and chemical based batteries.

Cell Preparation
  1. Use the sand paper or emery cloth to lightly scuff the positive and negative contacts. Don't go overboard. You are just looking to remove dirt and tarnish leaving a nice clean surface ready to accept solder. An easy way is to wrap the sandpaper around the eraser part of a pencil and use it in a circular motion.
  2. If you are using naked cells double shrink wrap each cell following the product’s directions. If desired, you can also add an additional shrink wrap layer to cells that come covered for additional protection.
Cell Bonding
  1. Take the prepared cells and line them up as you want them in the final pack along a straight edge guide or in a battery jig. Make sure your connections will make the current flow in the correct direction for you pack. For example in a side-by-side pack, each cell in the pack will be turned in the opposite direction from the cell it is next to. So looking down the top you should see you +, -, +, -, +, - and so the bottom should be -, +, -, +, -, +.
  2. After making sure that all the cells are in the right order you can start gluing the cells together. Take one cell, apply a thin bead of adhesive along one side and press another cell against it to glue the two together. Continue, making sure they stay aligned, until all the cells are glued together. Allow the adhesive to cure according to the product directions.
Soldering the Pack
  1. Plug in your soldering iron and make sure you have a clean wet sponge handy. Wipe the hot tip on the wet sponge to clean it. Apply some solder onto the tip and then wipe it on the sponge to clean it again. Repeat this until the tip of the iron is shiny and bright with a thin coat of solder.
  2. Measure and cut your wire to the needed lengths. Remove about 3/16" to 1/4" of the wire insulation from the ends of each wire. Twist the strands of wire tight and tin all. If you are using bars, braid or strips you will need to tin the ends.
  3. Next you are going to apply a small drop of solder to the scuffed areas on both the + and - ends of all your cells. Make sure they are set level and upright with easy access to the ends. Apply the hot iron and the solder to the center of the cell at the same time. Feed the solder until you have a small drop adhered to the cell and then immediately pull back the solder and use the iron tip to move it around in a circular motion and spread it out to about ¼ inch diameter. This is a quick process and should only take a second or two to do! Repeat for all remaining cells, both ends.

CAUTION: If it takes you more than a second or two at this point you can cause damage to the battery and / or reduce its performance.

  1. Now connect your batteries together using your chosen connection type. Use pliers or tweezers to hold the connector in place. Hold the tinned connector in place and then take your iron tip and press it on top of one end of the connecter and into the solder on the cell. Once the solder melts, it will bond the connector to the cell. Apply additional solder if needed to form a strong joint. Remove the iron tip and allow the solder to solidify and cool completely before moving on to the next connection and cell. Repeat the process for all the cells making sure your connections follow your original plan.

CAUTION: it should take between 5 - 7 seconds to solder each joint. Any longer and you risk damaging or overheating the battery.

  1. Inspect all the joints carefully. You should have smooth solder seams at all your connections without lumps, beads, or little balls of solder. The ends of the connectors should appear melted into the battery. If you are unsure about the connection, or if you think it needs more solder, you can reheat the connection and apply some more solder (and flux if needed) to it.
  2. Since heat is very bad for a battery don't repeatedly rework connections or keep the iron on the battery for more than a few seconds. If the solder doesn’t seem to be melting fast enough, make sure the tip is clean and that your iron is operating properly.
Wire Connections & Finishing
  1. If you are going to hard wire the battery to your ESC, all you have to do is solder on connectors at either end of your pack. One will be the positive and the other will be the negative and then you’ll just solder the wires from your ESC to the correct connector.
  2. A more common option is to solder on some lead wires which can be soldered to a plug end or directly wired to your ESC. Follow the same guidelines as above. You will strip off about ¼" of the protective plastic sheath and then flux and tin the ends and then attach to the appropriate ends of your battery pack following the same procedure used above for connecting the cells. Then attach these leads to the plug (if being used).
  3. Wrap your cells in shrink wrap to secure them and cover the connections to finish off the project. Follow the products instructions.
  4. You need to test that the pack can safely take a charge. When charging cells for the first time, make sure to charge at a trickle charge rate! This allows the cells to equal out better than if you were to blast them with a quick charge. It is a good idea to cycle your new pack’s cells like this a couple of times prior to any quick charging.
Helpful Tips and Things to Remember

Glossary of Terms

Acid Core Solder: Solder with an acid flux in the center. It is used for soldering more difficult metals, such as galvanized iron. Soldered surfaces should be washed after each soldering to remove the corrosive effect of the acid.

Ampere: A unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 x 1018 charge carriers) moving past a specific point in one second.

AWG: American wire gauge (AWG) is a standardized wire gauge system used since 1857 predominantly in the United States for the diameters of round, solid, nonferrous (not iron based), electrically conducting wire.

Battery Bar: Strips of solid metal and some times braided wire strands that are used to connect and carry the current between cells in a battery pack.

Battery Pack: A group of any number of (preferably) identical batteries or individual battery cells configured to work together to deliver a desired voltage or power density.

Brick Pack: When cells in a battery pack are arranged side-by-side creating a cube or “brick”. Also called side-by-side packs.

Cell: A single battery in a battery pack.

Current: Essentially how fast electrons are moving in a circuit. It is measured in amperes (amps). Current (Amps) = Potential (Volts) / Resistance (Ohms)

Desoldering Braid: See soldering wick.

Desoldering Wick: See soldering wick.

ESC: Short for Electronic Speed Control. It sends power to the motor, and tells how much power to give it, in relation to trigger movement on the remote.

Flux: A chemical cleaning agent which facilitates soldering, brazing, and welding by removing oxidation from the metals being joined.

Jig: A devise used to hold the correct positional relationship between a piece of work and the tool or between parts of work during assembly.

LiPo: Short for lithium polymer, a lithium based rechargeable battery that uses a polymer case allowing it to be lighter and specifically shaped for its application and offer a very high capacity for its weight. They need to be carefully monitored during charging as overcharging or charging a physically damaged or over discharged cell can be a potential fire hazard.

Nicad, NiCd: Short for Nickel Cadmium, a type of rechargeable battery used for RC products. They are relatively inexpensive. They need to be fully discharged after each and every use. If not, they will not discharge to their full potential on subsequent discharge cycles.

NiMH, NI-MH: Short for Nickel-Metal Hydride, a type of rechargeable battery used for RC products. They have a significantly higher energy potential (capacity) in cells approximately the same size and weight of comparable NiCd cells and they don’t require complete discharge between charging. They were developed as an alternative to Nickel Cadmium cells.

Milliampere-hour: Designated as mAh. It is one-thousandth of an ampere and used to describe how much electrical charge a particular battery will hold, especially for small batteries.

Multi-Meter: An electronic measuring instrument that combines several measurement functions in one unit. A typical multi-meter may include the ability to measure voltage, current and resistance. Also known as a volt/ohm meter or VOM.

Parallel: A configuration for connecting cells in a battery pack where the positive terminal of one cell is connected to the positive terminal of another, and negative terminal of one cell to the negative of another. Polarity: Refers to the direction of electron flow. The polarity in cells is indicated by a positive (+) end (terminal) and a negative (-) end. Electrons move from negative to positive.

Resistance: Describes how easily electricity flows through a material. Where resistance is high more effort is needed. A smaller-diameter electrical wire has more resistance to electrical flow than a larger-diameter wire. It’s measured in units called ohms.

Rosin Core Solder: Solder with a rosin flux in the center. Used for soldering electrical wiring.

Series: A configuration for connecting cells in a battery pack where the positive terminal of one cell is wired to the negative terminal of the next cell, which is wired to the positive terminal of the next cell, and so forth.

Soldering Wick: Usually found as a roll of fine, braided 18 to 42 AWG wire of high conductivity electrical copper, which has been treated with a rosin solder flux. In RC it is an option used to connect cells in a pack. It is also used for removing solder from any solder joint. Also called desoldering wick or desoldering braid.

Stick Pack: When cells in a battery pack are connected in a line, positive to negative like batteries in a flashlight. Also called shotgun packs.

Tinning: The process of coating a metal with a thin layer of solder. Tinning is often done to make attaching components and wires easier and quicker.

Voltage: The rate at which energy is drawn from a source that produces a flow of electricity in a circuit; the measurement of electrical pressure.

Watts: Watts is a measure of the amount of electricity being used; a rate of electrical power consumption. The formula for determining how many watts an electrical circuit can carry or how many watts an electrical device will require is Watts = Volts x Amps.

This free how to is courtesy of


Inland Craft Products, Co.
32052 Edward Drive
Madison Heights MI 48071
(248) 583-7150
www.inlandhobby.com

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