0
\$\begingroup\$

I'm currently developing a solar powered data logger that sits outside in direct sunlight for a good chunk of its life. Preliminary testing has shown that the inside of the enclosure can get up to 70°C on a hot day in the Australian Sun.

My first prototype iteration used a single 5000mAh 3.7v prismatic lithium polymer cell. This met the power requirements perfectly but on moderately hot days the 50°C over-temperature cutoff would kick in and the cell would stop charging from the solar panel and the board would fail to stay on when removed from the sun. I also have concerns about the lifespan of these cells under heat-stress and using potentially flammable li-poly cells in hot conditions.

The device requires 2-3000mAh of capacity with a board Vcc of 3v, the average current draw is <100mA. I'd prefer not to vent the enclosure as the device is used in pretty dusty conditions and I'd like to keep it as a sealed unit. By better insulating the enclosure, I may be able to bring the inside down to around 60°C if that makes things easier. The battery needs to be less than 12mm thick.

For my next iteration I'm considering different chemistries to address this issue. So far the only cells rated for such temperatures I've found are the SAFT high-temperature lithium ion cells (http://www.saftbatteries.com/force_download/VL32600_125.pdf) that are rated up to 125°C, but they don't come as a flat cell and they're not readily available in small quantities.

I've also read mixed literature about what temperatures LiFePO4 cells can be used at.

What options have those of you who've needed batteries in high ambient temperatures found?

If anyone could point me in the direction of some other chemistries or specific cells that might meet my requirements I would be very appreciative.

Thanks!

\$\endgroup\$
  • \$\begingroup\$ You may be better to separate the solar panel from the data logger and keep the latter in the shade. Of course this assumes that air temperatures stay below 50C... \$\endgroup\$ – Brian Drummond Feb 19 '16 at 10:35
  • \$\begingroup\$ Two questions: Do you need to power that circuit for a long time, such as all night? and: Do you have to ability to "over size" your battery capacity and use the excess capacity to force cool (peltier or something) the battery into safe usage range? (to be clear, I mean to get a battery with either more Ah or more voltage or both and not a physically larger battery). \$\endgroup\$ – Dave Feb 19 '16 at 10:40
  • \$\begingroup\$ Have you considered using a heatsink on the outside of your sealed unit, preferably under some form of shading plate/canopy? When powering HAM repeaters from solar panels & lead-acid batteries in the West Texas desert, we usually placed the solar panels so that they shaded the battery box, but still allowed cooling airflow over the batt. With a sealed batt. box, the same could work, but you'll likely need a finned heat sink outside the sealed enclosure.. \$\endgroup\$ – Robherc KV5ROB Feb 19 '16 at 10:43
  • \$\begingroup\$ @Dave, the battery needs to have enough juice to be able to search for a WiFi network every couple of minutes until it finds one then upload its logs while packed away from sunlight, this may be a couple of days. Realistically a smaller cell would do but it's nice to have the headroom and physical size is not a huge issue. A peltier is an interesting idea, I'll keep it in mind. \$\endgroup\$ – user2036607 Feb 19 '16 at 11:07
  • \$\begingroup\$ For a similar situation I have in the past bought flat 10Ah LiFePO4 cells (and still do), though I have tested my direct factory supplier for performance at temperatures above 50 degrees and have no knowledge about the promises more readily available brands make. \$\endgroup\$ – Asmyldof Feb 19 '16 at 11:11
1
\$\begingroup\$

Shade is the first thing to try, e.g. from the solar panel. If the solar panel really is re-radiating (as you say in the comments), a sheet of space blanket on the back of it would takle care of that.

A shiny or even pale device will get a lots less hot (unshaded) than a black one. Of course if the air temperature exceeds your cutoff you'll still have a problem, though this might be solved by thermal mass (a slab of aluminium) if the nights are cold enough to lose heat.

It's a pity your thickness requirement is so low - 6V lead acids are available at 34mm thick (so are the 2V individual cells). And of course lead acids can run quite hot under car bonnets.

You could provide air-cooling without unsealing the case if you press the battery against a heatsink set into the case wall, with a fan on the outside. The fan would be exposed to dust but nothing else would, and IP55 12V fans exist (making assumptions about your solar panel).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.