End-to-End Battery Soldering Technique

What you need:  

There’s a few key concepts to remember when doing end to end soldering. Surfaces must be clean and the iron must be hot/massive enough to transfer sufficient heat in only a few seconds to minimize damage to the cells. Wear safety goggles – the solder will splatter! Here’s what is needed to do the job:

Soldering iron: Although a larger 60-80 watt iron is often recommended, I find that I can get along fine with the Weller 40W WLC100 soldering station. This iron is great for other types of soldering that you would likely be doing and comes with the screwdriver tip in the photo below . The only drawback is that after doing several joints it’s a good idea do give the hammer head tip some time to refill its “reservoir” of heat.

  • Scotch-brite scrubbing pad
  • Kestler 60/40 rosin core electronic solder
  • Corrosion free flux (even though it apparently is a bit corrosive) is useful in tiny amounts
  • Sponge (wet) for cleaning solder tip. One is included with the WLC100
  • Masking tape – 1-1/4″ wide if to be used as insulation between cells
  • Soldering jig – one can be made from wood, but I went with a magnetic jig available through Hobby Lobby or New Creations
  • Pre-made insulators or a brass tube of the correct diameter sharpened on one end to cut out holes in the masking tape to match the diameter of the cell buttons
  • Heat shrink to fit the assembled pack , 2-5/8″
  • Heat shrink to fit assembled stick, if used, 1-7/16″
  • Hot melt glue
  • 1 pair of end caps, if used
  • Small bench vise
  • “Extra hands” stand to hold wires and connectors while soldering them. The wooden one in the top left corner is good for connectors because it doesn’t pull off the heat.
  • Large straight blade screwdriver to remove heat from cells after soldering.
  • 13 gauge silicone wire
  • battery connection braid
  • battery connector ( I use Astro Zero-Loss)
  • heat gun

Pack construction:

  1. Prepare the insulation
    • Make button insulators for the positive end (if pre-made insulators are not used)
      • Lay 4 or so strips of 1-1/4″ masking tape onto a hard cutting surface in a stack so that 4 insulators are made with each cut.
      • Cut the strip into squares with a steel rule and a knife.
      • Use a sharpened brass tube of the correct diameter to make holes to fit tight against edge of + button and drill holes in the centre of the tape squares. Make enough for more than double the number of the cells to be soldered.
    • Make 1/8″ to 3/16″ wide strips of masking tape by repeatedly scoring the roll of tape so that 4 to 6 tracks are made on the tape roll for pulling off these narrow strips. I have used a balsa stripper for this.
  2. Prepare the cells
    • Strip off both layers of heat shrink (RC2400) to give bare cells.
    • Take a small piece of Scotch-brite pad and scour the cell’s button and a similar sized area on the other end.
  3. Apply cell insulation
    • At some point get the soldering iron heating up during this stage. In fact these first steps can be intertwined with other steps so that the soldering iron can recover its heat.
    • Install the “+” button insulators. Either:
      • install pre-made insulators (usually plastic) and hold in place with a couple of small pieces of tape to be removed later.
      • apply two of the masking tape insulators made earlier – the second one being rotated 45 degrees so that there is an 8-point star wrapped around to the side of the cell. Roll on hard surface to flatten against cell.
    • Apply two wraps of 1/8″ tape on each end of each cell. In the case of the homemade button insulators above, use only one wrap over them. This insures that the tape thickness is the same on each end of the cell to keep the cells aligned exactly inline during soldering. The purpose of these wraps of tape is to provide non-meltable insulation between the cell sticks in the assembled pack in case of severe overheating – during which both the heat shrink and hot melt glue could start melting away. Remember that you don’t need these with the LOGO 10 packs and their plywood keel.
  4. Tin the cells
    • Arrange all the cells together with the same end up. I do the “+” end first.
    • (Optional) Use a toothpick to apply a little smear of flux.
    • Get the large screwdriver ready to touch to the heated area of the cell immediately after the soldering iron is removed.
    • Clean the soldering tip on the wet sponge and apply a little fresh solder to it.
    • Touch the tip to the cell end for only a couple of seconds (or until the flux is observed to melt) and touch the end of the solder to the heated area. Feed in just enough solder to give a very thin layer of solder. Swirl the solder tip around the edges of the tinned area a bit just as the tip is removed. This whole step should only take a few seconds.
    • As soon as solder is finished being fed, drop it and grab the screwdriver so that at the very instant that heat is removed one can…….
    • Press the flat screwdriver blade on the heated area so as to pull off as much heat as possible from the cell.
    • Repeat this with all the cells, stopping to allow the tip to heat up when poor soldering is noticed.
    • Flip the collection of cells over and tin the other end. Note that in the case of RC2400 cells there is a purple substance that reminds me of grape jelly in a recess at the centre of the cell. It can be ignored as it melts and flows off to the side.
  5. Solder the cell sticks together (technique particular to magnetic jig)
    • Turn off the soldering iron and allow it to cool down enough to change to the hammerhead tip and of course turn it on and give it ample time to heat up again. Hint: Visit the EZone for awhile during this operation, hehe.
    • Set up the soldering jig. In the case of the nice magnetic jig, clamp it in the vise. I add an extra measure of insulation to this jig by:
      • wrapping the piece at the end of the jig which clamps in the vise with electrical tape for those times when you must solder the sticks with the +end down (repairs).
      • putting two strips of clear tape on metal rails where the bare cell sides contact it.
    • Put the first cell on the jig with the “+” end up and put the next one just above it – slightly further apart than the height of the hammerhead tip.
    • Clean both ends of the hammer head on the wet sponge and lightly tin them.
    • Place the soldering tip between the cells in proper position for soldering in the next step.
    • Drop the end of the hammer head firmly and squarely onto the “+” button while simultaneously bearing/sliding down with the top cell onto the other end of the hammer head. I sometimes wiggle the handle of the soldering iron to get a feel for square contact between the two cells. This step should take a few seconds at the most
    • QUICKLY: Lift/slide the top cell enough to allow easy extraction of the hammer head and then smartly slide the top cell down to join the two areas of pooled solder. If you get more than a little solder splashing out then you are using too much solder. If you are not following my recommendation to remove the shrink-wrap and are soldering up a flashy pack with the labels lined up nicely (I do this sometimes), then exaggerate a bit of a lift as you are pulling out the tip to clear the edge of the shrink-wrap, so as to not melt it.
    • Lift the stick /stack high enough to place the next “victim” at the bottom and repeat. Note that since the completed portion of the battery stick is at the top, the length of the battery stick can extend well past the end of the jig.
    • You might as well pace yourself here, because after 5 or 6 joints the tip should be allowed to recover its heat for a few minutes. To kill the time, I usually take a razor knife (bonus sparks possibly), thin cardboard, or thin plastic sheet and extract any solder balls by sliding them between the cells.
    • If (when) you get a bad result, give it a deliberate, quick snap to break the joint. If the cell can is lifted a bit, I tap it flat on the bench (confession).
  6. Assemble the pack
    • Turn off the soldering iron, let the tip cool down enough to remove it, install the screwdriver tip, and turn it on again.
    • Install the shrink wrap on the battery sticks, if used.
    • Glue the sticks together (to the keel if a LOGO 10 pack) in proper orientation with a small bead of hot melt glue. I get the two sticks lined up on the bench a little ways apart, run the bead on the top of one stick, roll it so that the glue is now at “3 o’clock” and roll the other stick into contact. All this needs to be done quickly before the glue hardens.
    • Tin and install a piece of braid to connect the sticks at one end or….
    • Install the odd number cell across the end.
      • Tin and install two pieces of braid facing outwards from the pack on each cell at one end of the pack. bend them so that they will meet the ends of the “bridging” cell.
      • Place several strips of masking tape across the end of the pack to insulate this last cell where will rest and hot melt glue it in place. It must not be allowed to short out on the other cells!
      • Solder the braid to the previously tinned cell.
        • Protect the pack with cardboard or cloth and clamp it lightly to the bench.
        • Clean and tin the solder tip.
        • Apply solder while pressing the tip onto the braid while keeping it its proper place.
        • Drop the solder, grab the cooling screwdriver, and press it onto the braid to hold it in place while the solder tip is removed. Let the heat escape to the screwdriver.
        • This whole process should only take a few seconds…… Press your thumb on the heated areas. It should not hurt;)
  7. Install the wiring and connector
    • Cut the silicone wire to the proper length, strip and tin the ends, and solder it to the connector(s).
    • Protect and insulate the pack with cardboard while clamping it lightly in a vise with the unfinished pack end facing up.
    • If end caps are used, be sure to feed the wiring through the end cap.
    • Solder the red and black wires in place on the end of the pack, towards each other and slightly to one side. Make sure that the polarity is correct.
    • Heat the end caps with a heat gun to make them more pliable and reduce the chance of them tearing and then install them on the ends of the pack.
  8. Apply outer layer of heat shrink
    • Leave about 1/4″ overhang if end caps are used and the shrink will form nicely around them.
    • If end caps aren’t used leave more slack and
      • On the lead end, leave about 3/4″ extra and form it around the wires as it shrinks and is bent flat in one direction
      • On the other end leave about an inch extra, and fold the flap back against the pack.
  9. Slow charge the pack at C/10 for 14-16 hours to “equalize” it. At C/10 the batteries can dissipate the heat from overcharging while other cells “catch up” to the same voltage.

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