Hobo 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 disadvantage: a short battery life. The power supply is a puny CR-2032 coin battery (220mAh), which lasts around two years (with 1 hour logging intervals), though the manufacturer recommends replacing it every year. When the logger is in a remote field location, that is an inconveniently short time. 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 the 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 small (64kB or 43,000 measurements), I figured that losing one channel is acceptable. The selection of battery holders available for the CR-2477 is limited. I went for the LIR2477 battery holder, which has two pins that need to be soldered.
The photos show i) two U12-008 Hobo data loggers, one with the standard battery and the other with the larger battery, ii) Battery holder with wire soldered to it, iii) modified data logger.
- 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.
- Solder a short wire to the negative pin of the battery holder. This is the most difficult part, because it's not an easy connection to make. (I used an 18-gauge stranded copper cable with thick insulation - see photo.) Bend the contact before or after the soldering, otherwise it takes up too much vertical space.
- 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 channel.
- Bend the positive pin of the holder and solder it straight into the board.
- Solder the wire to the positive contact. Done. Insert the battery.
- 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.
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.