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External Li-Ion Pack Intermittent Connection: Dismantling the Pack

The power lead into the Li-Ion pack I’m using for the bike radio became badly intermittent on a recent ride. When I got back I swapped in a different pack and the problem Went Away, but I noticed that the coaxial power plug didn’t seem to seat all the way into the jack on the failed pack. I’d noticed that before, although I attributed it to getting two different sets of the packs; it didn’t seem to make any difference.

Given that I was going to have to either repair or replace the jack, dismantling the offending pack was next on the list. Some preliminary poking showed that there were no screws concealed under the label, so the two halves of the pack were either snapped or bonded together.

The case didn’t respond to the usual wedging and prying by revealing an opening, which suggested that it was bonded. That meant I must saw the thing apart.

I set up a 31-mil slitting saw on the Sherline and clamped the pack atop a random plastic slab atop the tooling plate. The Sherline’s limited throat depth meant I had to cut the far side of the pack. I aligned the saw to the Z-axis level of the joint along the middle of the pack by eyeballometric guesstimation.

Slitting saw setup

Slitting saw setup

Key point:

  • You absolutely do not want to saw into a lithium-ion cell, not even a little bit.

Therefore:

  • The pack must be aligned parallel to the cutter’s travel
  • The cuts must proceed in tiny increments, and
  • You must verify that each cut doesn’t reveal any surprises.

In this setup, the pack aligns against a clamp on the left side and to a parallel block (removed while cutting) along the rear edge of the tooling plate. I could then unclamp the pack, rotate it to put the next edge in place, and use the same XYZ origin with the edge parallel to X.

Here’s the view from the back of the table.

Sawing the case

Sawing the case

I ran the spindle at 5 k RPM and cut about 15 inch/secminute. I’m sure the pros do it faster, but that was enough to warm up the blade and that’s fast enough for me. [Update: typo on the units. Thanks!]

Cuts were 0.020 inch per pass, which is about 0.5 mm. I expected the case to be some hard-metric dimension and wasn’t disappointed.

After the cuts reached 0.060 inch, I manage to pry the remaining plastic in the joint apart and split the halves apart along the connectors and LEDs at the front where I couldn’t do any sawing.

Here’s a close look at the cut, just above the battery terminals. The case turned out to be 2 mm thick, about 0.080 inch, so I was just about all the way through. The cut was perfectly aligned with the case and cracked open neatly along the entire length.

Tight tolerance on the cut depth

Tight tolerance on the cut depth

An interior view, showing that the cells adhered to the left half of the case and the electronics to the right: of course. I pried the cells loose from the left side, which provided enough access to unsolder the things, as the terminals were against the case. Notice that there’s absolutely nothing between the inside of the case and the outside of the cell, so cutting just slightly too deep would be a Bad Thing™.

First look inside the case

First look inside the case

After a bit of work, here’s the entire layout…

Battery pack internal layout

Battery pack internal layout

Much to my surprise, the battery consists of two series-connected sets of three cells: 2 x 3.7 V = 7.4 V. I expected three series sets for about 3 x 3.7 = 11.1 V, with a linear regulator down to the 9.0 V output.

As it turns out, they used two switching regulators: the one between the two triplets controls the charging voltage and the one to the lower-left boosts the battery to the pack’s 9.0 V output. I had hoped for a resistor divider that I could tweak to get 9.6 V out, but it certainly wasn’t obvious.

I unsoldered the cells, dismounted the circuit board, and puzzled over it for a bit, after which the problem was obvious.

The story continues tomorrow, with a dramatic denouement…

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  1. #1 by MD on 2010-08-06 - 13:18

    15 inch/sec?? And the blade was only warm?

    • #2 by Ed on 2010-08-06 - 13:33

      Good catch…I’ll make that inch/minute.

      This trips me up every damn time. The config files use inches and seconds, which make perfect sense.

  2. #3 by John Rehwinkel on 2010-08-09 - 13:17

    I’d noticed that before

    I’d just read a treatise on troubleshooting, that makes the point that anything that you notice is likely to be relevant. Indeed.

    You absolutely do not want to saw into a lithium-ion cell, not even a little bit.

    I have actually done this. One one of those old-style lithium cells that was filled with liquid sulfur dioxide under pressure. You are quite correct, sir.

    • #4 by Ed on 2010-08-09 - 13:50

      filled with liquid sulfur dioxide under pressure

      Ewwww!

      I have a full face shield that I use for projects that could get nasty in a hurry. That sounds like such a project, fer shure…

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