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Archive for September 25th, 2009

Battery Charger Thermistor: Magnetic Attachment Thereof

Magnet and thermistor position

Magnet and thermistor position

A new fast NiMH pack charger that uses a thermistor to detect the abrupt temperature rise at full charge just arrived on my Electronics Workbench. The instructions say to tape (“Use rubberized fabric …”) the thermistor to a cell in the middle of the pack, a process which loses its charm fairly quickly.

The intent is to have the thermistor bead in intimate thermal contact with the cell, but air is a rather crappy thermal conductor. We can do better than that.

Sooo, off to the Basement Laboratory Adhesives Division we go…

NiMH cells have a steel shell, so holding the sensor in place with a magnet makes at least some sense. I used a pair of teeny rare-earth magnets (Electronic Goldmine G16913) bridged by a snippet of steel strap. One magnet points up, the other points down, the strap provides a magnetic path, and the whole assembly sticks to the cell like glue.

First epoxy setup

First epoxy setup

I trimmed the heatshrink tubing surrounding the thermistor back a bit, then applied enough epoxy to secure the magnets to the strap and smooth out the edges, leaving the thermistor sticking out in mid-air.

Although it looks risky, the epoxy doesn’t bond well to the (sacrificial, dead) cell. Doing it this way produces a nearly perfect AA-cell-shaped contour in the epoxy on the bottom of the magnets.

It’s JB-Kwik fast-curing epoxy, not quite so runny as its slower-setting and much stronger JB-Weld relative.

Epoxy covering thermistor

Epoxy covering thermistor

After the epoxy cured, I bent the thermistor down to contact the cell and dabbed epoxy over the bead. This puts the thermistor in good thermal contact with the cell. Epoxy isn’t a great thermal conductor, but it’s a lot better than air.

The alert reader will note that I wrapped a layer of masking tape around the cell for this operation. I wasn’t convinced I could pop the epoxy off the cell without cracking the thermistor leads, but that turned out not to be a problem.

Trimming the edges of the epoxy around the bead gave it a certain geeky charm.

And it works like a champ: get the assembly close to a cell and it snaps right in place. I align the thermistor more-or-less in the middle of the cell, although I suspect the temperature gradient from the middle to either end isn’t all that large.

Magnetically attached thermal sensor

Magnetically attached thermal sensor

Now, one could argue that this lump increases the thermal mass surrounding the thermistor, thus slowing the charger’s reaction time. That might be true, but the pack’s end-of-charge temperature rise seems considerably subdued now; the charger used to cook the living piss right out of the cells (with the thermistor taped down): I couldn’t hold them in my hand, so they were well over 150 °F.

Now they become just uncomfortably warm, which says they’re closer to 130 °F.

The charger’s single page instructions (two pages if you count the sheet illustrating the rubberized fabric taping thing) cautions “Stop charging when [the cell’s surface temperature] is over 70C or it feels very hot”.

Indeed!

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