It seems like my Eneloops doesn't like -20 Celcius - Page 1
Forum section: electronics / general

author: bingo600
author_rank: Super Contributor
timestamp: December 18, 2022, 08:23:00 pm
content: Fun observation.It seems like my Eneloops doesn't like -20 CelciusI have some Temp sensors in the summerhouse, that are fed by 2 x Pana Eneloop (AA)https://www.amazon.de/dp/B00392XX5U/I built some Arduino code, that combined with a Hope 868MHz module, keeps track of the temperatures reported from the sensors.And poll the Arduino (every 5 sec) from a Linux box. I log to CSV every minute - Both Arduino + Linux are inside the summerhouse, kept at 10C.If the Arduino don't detect a xmission from the temp sensor for 5 minutes , it reports the temperature as 0 degrees, for that sensor. In order to see a "malfunction".It's quite cold in the summerhouse, and yesterday i noticed the sensor not reporting at several periods.I actually think the Eneloops stop delivering power, when the temp goes below -19C.The purple graph is the sensor "outside" by the Grill , the red is inside the (unheated garage).I looked at the Eneloop DS , and it states they should work down to -20C , and i think i have hit the limit.Seems like they wake again, when temp reaches around -14C.I also looked at Alkaline, limit seems to be the same -20C.Is there any "payable" batteries that works down to -30C ?/Bingo

author: Gyro
author_rank: Super Contributor
timestamp: December 18, 2022, 09:43:25 pm
content: EnerSys Cyclon cylindrical SLA cells are perfectly happy at -20'C and rated down to [Edit: -65'C]. The smallest single cell capacity is 2.5Ah. They have very long life compared to normal SLAs.https://www.enersys.com/en/products/batteries/cyclon/cyclon/

author: wraper
author_rank: Unknown
timestamp: December 18, 2022, 09:56:47 pm
content: -20C is not a hard limit when they suddenly stop working. Most likely there is some problem with the device. Maybe it's not happy with lower voltage.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 18, 2022, 10:13:00 pm
content: Yeah what makes you think it's the batteries?

author: bingo600
author_rank: Super Contributor
timestamp: December 19, 2022, 02:18:14 pm
content: It could just as easy be the sensor that does'nt like the -20C, I agree.I just assumed (yes i know .....) , that an outdoor sensor was made for outdoor temp's.But maybe not Scandinavian outdoorWell no biggie

author: RoGeorge
author_rank: Super Contributor
timestamp: December 19, 2022, 03:09:01 pm
content: Low temperatures are very difficult for rechargeables. Their capacity can become 10-100 times smaller at very low temps. Same the charging rate, should be kept 10 times slower or so if charging them in very low ambient temperatures.It might help if you add thermal insulation material around the batteries (because the power source always dissipates the same amount of power as the rest of the circuit). This might keep them a few degrees warmer then the air temperature. Same, if the construction of the sensors have the extra space, add some thermal insulation fibers inside and outside of the box to keep it warmer, and expose only the sensor itself to the ambient.As a last resort, you can add a miniature light bulb and turn it on when it's too cold, or some other heating resistor controlled by the MCU.

author: wraper
author_rank: Unknown
timestamp: December 19, 2022, 03:17:19 pm
content: According to discharge curves for NiMH, capacity drops by like 30% @ -20oC. However voltage drops too. If device is not capable to work at 1.1 - 1.15V per cell, there may be a problem. Did not find such curves for Eneloop in particular.

author: wraper
author_rank: Unknown
timestamp: December 19, 2022, 03:20:44 pm
content: IMHO either try powering the thing from lab a PSU and see what minimal voltage at which it's still working. Or use fresh alkalines instead and see if it still works at -20oC.

author: tom66
author_rank: Super Contributor
timestamp: December 19, 2022, 03:41:16 pm
content: Did you repeat the test with fully charged Eneloop?The only consumer batteries that retain good capacity at -20C are primary lithium cells (e.g. Energizer Lithium, Varta Lithium, GP Lithium). Those work down to around -35C to -40C, at which point even they start to fall off at moderate loads. After that you are into esoteric industrial batteries and maybe radioactive isotope generators...

author: mjs
author_rank: Regular Contributor
timestamp: December 19, 2022, 05:23:47 pm
content: Most of the electronics is just fine at -20 °C, but many batteries start to go low voltage. The commercial spec analog domain gets interesting, as a lot of that world is designed somewhere in California - IC internal bias voltages start to go haywire, PN junction is no longer in expected range and so on.For low temperatures, you'll want lithium cells, NiMH and especially NiCd are notoriously bad there. Enegizer Ultimate AA cells, Saft LS/LSH series are my primary cells of choice there and for rechargeables my first choice is Saft MP 176065 XLR, which can be *charged* down to -30°C and discharged way lower depending on application.

author: thm_w
author_rank: Super Contributor
timestamp: December 19, 2022, 10:45:55 pm
content: Which module?Looking at RFM69HW they say: Supply voltage(1.8V-2.4V 17dBm, 2.4V-3.6V 20dBm)2.4V is quite high, although I don't know if it actually crashes below that voltage, or it just doesn't hit the full transmit power.Could be that some added capacitance, or a boost/buck-boost converter might be a solution.Quote from: mjs on December 19, 2022, 05:23:47 pmThat is cool, I just assumed all lithium would need cell heating.