Public Lab Research note


HOBO 4-Channel Logger with Larger Battery

by norbert | July 23, 2018 04:51 | 98 views | 0 comments | #16766 | 98 views | 0 comments | #16766 23 Jul 04:51

Read more: unstable.publiclab.org/n/16766


Model U12-008 is a reliable data logger for outdoor environments that can be connected to up to four temperature sensors. It only has the following disadvantages: short battery life and small memory. The power supply is a little CR-2032 coin battery (220mAh), which lasts around two years (with 1 hour logging intervals), though the manufacturer recommends replacing it every year. That is short, if the logger is in a remote field location. Moreover, to change the battery one has to open the logger, work with tiny screws and, ideally, re-lubricate the rubber O-ring, which depending on the weather, can be a hassle. I exchanged the battery holder of the U12-008 to prolong its battery life.

The CR-2477 is a 1000mAh coin cell battery with similar discharge characteristics as the CR-2032, but more than 4 times the capacity. It still fits into the logger body, although with a diameter of 24mm, instead of 20mm, it blocks one connector, so only 3 of the 4 channels remain usable. Given that the memory is so small (64kB), I figured that losing one channel is acceptable. The selection of holders available for the CR-2477 is limited. I went for the LIR2477 battery holder, which has two pins that need to be soldered.

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The photos show i) two U12-008 Hobo data loggers, one with the standard small battery and the other with the larger battery, ii) Battery holder with wire soldered to it, iii) modified data logger.

Procedure:

  1. Unsolder the old battery holder. This is easy, but do not pull on the contacts too hard, otherwise the bottom contacts will come off, which cannot be fixed with resoldering.
  2. Solder a short wire to the negative pin. This is the most difficult part, because it's not an easy connection to make. (I used an 18-gauge stranded copper cable, which had thick insulation - see photo.) Bend the contact before or after the soldering, otherwise it takes up too much vertical space.

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  1. Make sure a sensor cable is plugged into one of the connectors next to the battery, otherwise one risks blocking two rather than only one connector.
  2. Bend the positive pin of the holder and solder it straight into the board.
  3. Solder the wire to the positive contact. Done. Insert the battery.
  4. I applied acrylic conformal coating to the new contacts to protect them from humidity. I don't know whether that makes any difference, but it's better to be safe.

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Validation:

I tested the modified logger by tying several sensors together to see whether they measure the same temperature as an unmodified logger. Over a day at room temperature, the measured air temperatures remain well within 0.1 Celsius from one another, the same variation one typically finds between sensors/channels. So the accuracy of the measurements appears to be unimpaired.

Removing that battery from its holder is not easy. I practiced that, so I won't have to struggle with it in the field. Also, unlike the CR-2032, the CR-2477 is not a battery commonly found in stores, so it has to be ordered online before the day comes when even this battery needs to be replaced. I am hoping for a 5-year lifetime.


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