A123 Stick Pack Assembly

By Phil Jenkins.
Article posted 9th March 2009.

Introduction

A123 or LiFE (Nanophosphate Lithium Ion) cells have been with us now for a couple of years, and are proving to be a very good power source for all types of R/C activity. Initial costs of cells are high, but their robustness and ability to take hundreds of cycles without deterioration prove them to be a very cheap alternative to Nimh or Lipo cell chemistry. A couple of things to note about them: although they look similar to Sub ‘C’ cells, they are bigger and heavier, and are almost double the weight of Lipo cell packs.

In this guide I will show you how to make a 3S1P stick pack which I will pair with an identical pack to create a 3S2P pack.

But first a word of caution. I accept no responsibility for you or your equipment. The steps that I have taken worked for me, and I would expect a degree of proficiency with a soldering iron. Please read the advice given by the cell manufacturer. And finally, you take full responsibility for your actions.

Specifications

• Nominal Voltage: 3.3V
• Nominal Capacity: 2.3Ah
• Nominal dimensions: 26mm dia x 66.5mm long
• Nominal weight: 70g

Cell Charging Parameters:

• Recommended charge current: 3A
• Recommended charge voltage: 3.6V
• Recommended float charge voltage: 3.45V
• Maximum continuous charge current: 10A
• Maximum allowable charge voltage: 4.2V MAX

Cell discharging parameters:

• Recommended discharge cut-off voltage: 2.0V
• Recommended cont. discharge current: 60A

Handling Precautions:

• Do not short circuit.
• DO NOT rotate Negative end tab (cap)
• Note that the can is the positive (+) terminal.

Before joining cells together:

• Cells are shipped at 50% state of charge (SOC).
• All cells that are being joined together within the same string must be at the same SOC.

A123 cells can be sourced from a number of different outlets, ranging in price from £6 for Dewalt battery cells to £18 from leading suppliers. The two cells shown are actually identical - one has been repackaged to be sold under the Graupner label, the other was bought in from China. Once the card sleeve is removed, they are identical cells.

A123 cells not only differ in size from sub-C cells, they also have opposing terminals from regular dry cells. What we would normally associate as the positive terminal (the button) on a normal battery is in fact the negative terminal on an A123 cell. In the photo you can see the negative terminal for an A123 cell. The metal tab is for easy connection of the balance taps.

This picture shows the positive end of the A123 cell. This in fact goes further in that the whole of the battery can is the positive terminal.

Warning!
Please when handling and connecting these cells be sure to check twice that you have clearly identified correctly Positive (+) and Negative (-) terminals.

The Card case shows the Positive (+) and Negative (-) terminal. Once this card tube is removed, the easiest way of identifying polarity is to look for the plastic washer on the Negative terminal.

One more word of caution: do not, under any circumstances, rotate the end cap or balance tap on the negative terminal. You will break the thin conductor that connects the gel inside the cell to the negative cap and render the cell useless.

To start assembly I removed the extended section of the balance tab on the negative terminal, using a Dremel with a cutting disk attached. Obviously don't cut too deeply, just scoring the tab is enough and it will fracture quite cleanly when flexed.

I also removed the sharp overhanging corners of the balance tab at this point so that it was cut back to within the circumference of the terminal cap. This will help later to keep the solder where you want it to be. I also made sure that the cut tab laid flat.

Tools you will need to attach each cell are:

• 80 watt Soldering Iron with Hammer Head.
• A good quality electrical solder.
• A soldering stand and a damp sponge.

The damp sponge is for cleaning the tip of the soldering iron and to cool recently soldered parts quickly if needed.

The balance tab on the positive end is left alone; I will be using this later to attach the balance wires.

I like to hold the cell firmly, my old side-by-side soldering jig for sub-C cells is too small, so I use the vice, just remember not to over tighten it.

Place the head of the soldering iron onto the tab and apply solder. Be careful with the amount of solder you apply, you only need a thin coating.

Leaving the balance tabs on make fors easier soldering. Aluminium is notoriously difficult to solder to; it can be done, but why create more work for yourself? The solder takes really well to the tin tab and flows well.

Repeat the process for the Negative terminal. The small plastic centre would appear to be a vent. As I understand, it isn't. The vent is on the Positive terminal. Pre-tin both ends of each cell.

At this point I removed the card case. It just adds extra weight to an already heavy cell, and if water should get to the cells then you will have real difficulty drying them out.

If you have followed the steps so far, removal of the card sleeve is easily carried out by sliding it off the cell as the balance tab on the negative terminal is no longer retaining it.

I used 50mm (when flat) cell shrink wrap which is ideal for this size of cell. Cut a length to 75mm and insert the cell so that the cell wrap overlaps each end equally, you may have to tuck it under the balance tab, then with a little light heat from a hairdryer or a hot air gun, on a low heat setting, pass the warm air over the cell until the shrink wrap contracts. If you do one end then work your way along the can of the cell this will give you a good smooth finish.

Repeat the above process until you have all your cells covered, but more importantly insulated. Remember, I am doing this for a 3s1p stick pack, which I will pair in parallel with an identical pack to create a 3s2p pack.

My inline battery jig is made from a piece of aluminium right angle section that is glued to a piece of pine. I have supported it with two pieces of triangular section balsa, all bonded together with cyano. Note that I have insulated the aluminium with a couple of lengths of masking tape.

The first cell is taped to the inline soldering jig with masking tape and then held firmly in a vice. Make sure that you have the negative terminal facing up, and the balance tab of the positive terminal facing out. When joining cells in this format I prefer to work vertically, as I find gravity helps when sliding the cells together.

Hold the next cell above the first cell with the negative terminal facing up and the positive balance tab facing out. Hold the hammer head soldering iron between both cells. Lower the top cell onto the soldering iron while lowering the soldering iron onto the first cell. You must be quick; lift the top cell slightly, pull out the soldering iron and continue the downwards movement of the second cell until it contacts the first cell.

Hold the cell until the solder has solidified.

If you find that solder has splashed out from between the cells, this would suggest that when you tinned the end caps of the cell, you applied too much solder.

Don't worry if this has happened, but you will need to clear out all the excess solder. Failure to do this could cause a short circuit. Use a thin blade to pick out the solder blobs from between the cells.

Note of caution: Inserting a metal object between the two cells could cause a short circuit so be careful.

Repeat the above two stages for the third cell.

Obviously if you want to build a longer stick pack then continue to add cells until you have the required quantity for your needs.

Once completed you should have a stick pack that looks like this.

As you can see, some solder escaped when I brought the cells together. I cannot emphasise strongly enough how important it is to remove any excess solder.

Balance Taps

Pre-tin the balance tabs. Use a normal 25watt soldering iron to do this. The balance tap can then be soldered in place. The red wire attaches to the positive terminal at the end of the pack. Then, in sequence, attach the rest of the wires. Remember to purchase a balance tap that has a connector that will fit your balancer or charger. There is no industry standard. If you look closely, you will notice that I had to extend the red and the first black wire to allow the balance connector to project past the end of the cells.

All cells should be supplied with a 50% state of charge. Before continuing, as a precaution, I balanced the pack using an inline balancer.

Power Cables

The positive and negative wires are made up from 10AWG wire. You can decide on the length that you need, but a word of caution: brushless speed controllers are very sensitive to long cables, so keep power cables as short as possible.

Pre-tin both ends of the cable, and the soldered end of the connectors. This will make soldering easier. I use a male 4mm gold connector for the positive and a female 4mm connector for the negative.

Once soldered apply heat shrink to insulate the plugs.

Solder the red cable to the positive (+) end of the stick pack, and the black cable to the negative (-) end. Cut a length of 50mm cell shrink wrap to approximately 210mm and slide this over the entire assembly - this can be a bit fiddly because of the angles of the positive and negative cables.

As a precaution, I checked the balance of the pack once more. The final job is to cycle the cell pack a few times on your charger. Make sure that you select the appropriate setting for A123 ("LiFE") cells and don't exceed the recommended charge/discharge parameters.

© Copyright Phil Jenkins and www.fastelectrics.net, 2010.

This article may not be reproduced wholly or in part without the written permission of the author and www.fastelectrics.net. If you would like to use this article or the accompanying pictures/diagrams please email articles@fastelectrics.net.

Last modified: 08th July 2010 @ 09:05