Low Temperature Limit for Puck and MDBT42Q

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  • I've got a Puck with an Automation Serice, sending E.temperature and some other builtin peripherals' data. It sits well protected on my balkony, acting as small outdoor weather station. It work well down to temperatures of about -5 °C.
    Nordic's datasheet states that the chip should work even at -25 .. -40 °C. Now I'm wondering what actually limits the working temperature to -5 °C. Would the MDBT42Q board also have this limitation?
    I'm fancying a weather station with MDBT42Q and a BME280 sensor, maybe a little display, and a battery lasting +1 year, replacing my current LCD thermomter (without BLE :)

  • I'm not an expert but likely you will hit in cold temperatures the limitation of the CR2032 cell or almost any other battery first. The chemical process within a battery is significantly more temperature dependent as any other component.

    Next to the battery your only other concern should be condensation since it may lead to shorts or corrosion.

  • Thanks for the answer. I didn't think of the batteries at all, will do some experiments with AAA batteries next. Although, the specs for CR2032 say they work down to -20 or even -40 °C.

  • The MDBT42Q module itself lists -40°C as the minimum operating temperature, so I would have thought that you'd be fine. I don't believe any other components on the Puck or MDBT42Q breakout would cause you issues (not at -5 degrees).

    I'm be pretty sure the battery would be the first thing to go - looking at the datasheet the cells supposedly are ok down to -30, but the voltage out of them drops. If the cell were already partially run down, low temperatures would definitely cause issues.

    If you output the battery voltage over Bluetooth as well then you might possibly see if the voltage was getting very low just before the Puck stopped?

  • Good idea, and here are the results:

    Puck 2018-07-25T16:35:15.258Z { temp: 25, bat: 2.998 }
    Puck 2018-07-25T17:25:02.873Z { temp: -19.75, bat: 2.668 }
    Puck 2018-07-25T17:26:02.885Z { temp: -19.75, bat: 2.674 }
    Puck 2018-07-25T17:27:02.946Z { temp: -20, bat: 2.663 }
    Puck 2018-07-25T17:28:02.996Z { temp: -20, bat: 2.665 }
    Puck 2018-07-25T17:29:03.024Z { temp: -20, bat: 2.664 }

    First conclusions, that are actually observations:

    • Puck.js works down to -20 °C
    • a fresh CR2032 (noname, cheap purchase) also works down to this temperature

    Indeed, my problem from last winter seems to be related to low cell voltage instead of low temperature!
    First of all, I changed the code from sending GATT service data to sending the data as ManufacurerData during advertising. Hoping that this change makes the battery last much longer.

    Another question (mainly for myself but input is welcome) is whether there are cells out with a slighly higher voltage (like 3.3 V - and well above 2.6 V at -20°C) that can be used through Puck's external connector, getting me close to 1+ year endurance without maintenance.

  • Yes, there are! You have:

    • 'Lithium Manganese Dioxide' - these are the same chemistry as the CR2032 so are 3v but you can get them as AA (or 1/2 AA, which is quite neat). This AA cell is 2000mAh (so about 10x the capacity of the CR2032): http://uk.farnell.com/varta/6117101301/b­attery-lithium-cr-aa/dp/8636192
    • LiFePO4 batteries are rechargable but could also be an option - the operating voltage is pretty much spot on
    • 'Lithium Thionyl Chloride' - these are the ones you get in long life smoke alarms and stuff like that. They go down to -50°C and have slightly more capacity than the Lithium Manganese Dioxide batteries. The only gotcha is that when fully charged at room temperature they're basically 3.7v.

    If you look at the nRF52832 datasheet, the recommended operating voltage maxes out at 3.6v. However the absolute maximum voltage is 3.9v.

    Nordic says:

    Maximum ratings are the extreme limits to which the chip can be exposed for a limited amount of time without permanently damaging it. Exposure to absolute maximum ratings for prolonged periods of time may affect the reliability of the device.

    So, it's on you - but if it were me I'd say that 3.68v from a Lithium Thionyl Chloride battery is probably close enough to 3.6v (given the 3.9v maximum) that it's not going to cause you any problems. Especially as at -20 the voltage does drop to 3.6v when fully charged :)

  • What a wealth of information. Thank you so much. At currently 35 °C here I'm looking forward to the next winter with my outdoor Puck ;)

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Low Temperature Limit for Puck and MDBT42Q

Posted by Avatar for Steffen @Steffen