Hobo Data Logger: AA Battery Pack Hack

Hobo Battery Mod - Outside View

Hobo Battery Mod - Outside View

We have a bunch of Hobo Data Loggers recording various & sundry temperatures, humidities, and light levels around the house; as the saying goes, “If you observe something long enough, it turns into science.”

Normally the things run on single CR2032 lithium cells, which last for a good long time. However, Something Happened to the one that’s collecting groundwater temperatures at the water inlet pipe from the town supply: it started eating lithium cells like potato chips.

Hobo Battery Mod - Inside View

Hobo Battery Mod - Inside View

It was still producing good data, so I was loathe to toss it out. Instead, I figured all it needed was more battery, as a high current for a lithium cell doesn’t amount to much for an AA cell. A pair of alkaline AA cells produces just about exactly 3 V and the data logger can’t tell the difference.

So I opened the logger one last time, soldered the wires from a dual AA cell holder to the appropriate points on the circuit board, affixed the holder to the back with one of the case screws, and it’s been working fine ever since.

However, this seems like one more application where whatever plastic they thought would last doesn’t: the AA holders routinely split at the ends. Maybe the joint should be thicker, maybe it’s the wrong plastic for the job, but without the cable tie acting as a belly band one end of the holder splits off in a year or so. Bah!

Update: Maybe I got a batch of bad CR2032 cells, as the logger’s current seems to be just about right. Read the comments and then check the followup there.

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  1. #1 by Nick Lowell on 2009-08-12 - 11:05

    This is a nice little hack but is too bad that you had to do it at all. Throwing on a bigger battery will help, but it may not solve the problem. There are a few common problems. Here are some tips for debugging them if you are interested:
    1. Connect a current meter in series with the battery pack. The logger should be drawing less than 10 uA while it is between logging samples. I expect the value you measure will be more like 1 mA.
    2. While keeping one eye on the meter, poke at the capacitors on the board with tweezers. It is possible that one of the capacitors is cracked. If the cap is cracked, you should see the current on the meter flicker or spike (up or down).
    3. Try adding and removing the external pressure sensor and see if the current changes. It is always possible that the problem is the sensor or the connector.
    4. Check for any white/gray/black marks on the board that look like mildew, but is really a form of electrolysis. It can occur in high humidity environments. If you see any try scraping it off.

    Good luck.

    • #2 by Ed on 2009-08-12 - 12:38

      Throwing on a bigger battery will help, but it may not solve the problem.

      Definitely true: I’m treating the symtom, not curing the disease!
      I noticed the battery problem last year after a lightning strike blew a chunk of concrete out of the garage apron on the other side of the house. As nearly as I can tell, it also lit up the cable TV drop running parallel to the property line 50 feet away… and blew out the cable modem, a firewall router, and a network switch in the basement. The logger is three feet below the coax running along the front wall of the basement with the remote temperature sensor pretty firmly clamped to the incoming water pipe at the wall; I think it got caught between a rock and a hard place.
      I suppose it’s a miracle the thing continues to work at all; the data points before-and-after are continuous, so I figure there’s just more leakage through the circuitry than before.
      Anyhow, I did do the current measurement trick and concluded a 2500 mAh set of AA alkalines would last for slightly less than their shelf life; the current was well below 1 mA, but I don’t have it written down anywhere.
      In two weeks when I do the next data dump, I’ll tear it apart, take a look, re-measure the current, and report back…
      Thanks for the suggestions!

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