Powering an Electron in subzero temperatures

Hardware
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LiPo doesn’t like severe cold.

I have a large, solar powered deployment about to kick off in North Dakota. Obviously the standard LiPo cells are not going to cut it. Primary, non-rechargeable lithium could do the trick, but I hate the idea of having to periodically replace batteries. Rechargeable lithium chemistries do exist that could supply power down to -40C, but charging well below 0 is going to be a real trick.

Space is not a constraint, so I’ve been giving some thought to good old Sealed Lead Acid. Ideally I could leverage the TI bq24195 charge controller built into the Electron and simply swap out the LiPo for a 4vdc SLA. However the TI datasheet makes no mention of charging anything other than a LiPo. Has anyone else tried this? Alternatives (I can always include another charging circuit for the SLA and bring power in thru the VIN pin I suppose)?

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@fishmastaflex How has that LTO battery charging chip worked out on your Electrons?

Figured it could be helpful here maybe.

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The PMIC on the Electron can only be used with LiPo batteries. From the bq24195 datasheet:

The bq24195L, bq24195 are highly-integrated switch- mode battery charge management and system power path management devices for single cell Li-Ion and Li-polymer battery in a wide range of power bank, tablet and other portable devices.

Also the fuel gauge can only be used with LiPo batteries:

The MAX17043/MAX17044 are ultra-compact, low-cost, host-side fuel-gauge systems for lithium-ion (Li+) batteries in handheld and portable equipment

You'll need to use a separate charger and feed the power, typically into VIN.

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This is a LiFePo4 battery charging circuit with a battery heating circuit in it to warm the battery up when temps drop below freezing.

LFP Batteries have a 400% longer cycle life compared to LiPo cells under the worst case scenario of draining the batteries to empty on every discharge which you will not be doing.

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5

Why not use something like this?

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It looks to be bigger than needed for a single cell.

A single properly sized resistor glued to the cell or something similar is the cheapest and most power efficient solution I think.

You have to be very careful heating up LiPo cells if that is what your suggessting. The LFP chemistry is much more stable.

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Agreed, that was one I found doing a quick search, and I believe it has a built in thermostat so its not constantly running. There are other options, but I would look at some kind of heater, especially if size isn’t an issue, before going to a lead acid battery.

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I would really NOT try to heat the cell with a point source. Hotspots on lithium cells tend to be dangerous.

A well insulated box - eg expanded polystyrene - with the radios and battery in it allows the normal power dissipation to keep things at comfortable temperatures - take a look at what people do with circuitry that hangs below weather balloons - and you’re also not burning any extra power to achieve it.

Have enough insulation and your only problem is likely to be cooling which is much easier to do with a fan.

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I agree the heat source should have more surface area than just a resistor on the cell. Maybe one of those kapton heating mats the size of the cell.

Tesla battery packs get excellent cycle life by heating and cooling the packs to keep the cells in range.

Adding a fan to the box would allow outside air in.
Keeping a fan inside the box with no hole to bring in outside air would just allow hot air to build quickly.

The fan is another failure point also which I would try to design out if possible.

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The fan would be to the outside, yes - not much point moving air around in a sealed box, convection usually deals with that quite effectively. I’m assuming you’d use it in direct sunlight to try to keep internal temperatures below 45C - something you’d have to do with any solution for summertime operation.

A Tesla pack, with its liquid cooling/heating system, is not quite the same design aim; from experience it can also take ~1kW to warm the pack enough to charge even in mildly cold temperatures (-5C). When you’re running from solar, you really don’t want to waste power on resistive heating.

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I was just saying your recommendation of putting the electronics into a sealed polystyrene like box to keep heat in during the winter would work if the Electron was running 24/7 or even close to that. I’m pretty sure the Electron is sleeping most of the time so there would not be any heat generated to warm up the box interior.

The polystyrene box would also slow the next day sun from warming up the battery inside of the box due to its insulation qualities.

Adding a fan to the box requires you to cut a hole in the box which allows cold air a way into the box which defeats the whole purpose of the insulated box.

The point in mentioning Tesla packs was to point out how keeping the cells in their recommended temp range can have a dramatic increase in the cells charge cycle lifespan. Last I saw owners were getting 2000+ charge cycles while maintaining close to the original total pack KWH capacity. That’s pretty impressive from cells that a usually only have a 500 cycle life span before hitting 80% of their original rated capacity.

Using the solar panel to heat up the battery makes perfect sense considering the solar power is not used at all when the battery temp is below freezing. The charging circuit I linked above directs solar panel power to the heater circuit only when the battery temp is below freezing to bring the battery temp into the recommended charging temp range then the heater turns off and solar charging continues as desired.

I have wondered how much power is required to run the Tesla battery pack heater, so thanks for that bit of info.

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I think the polystyrene box is still worth the experiment; if external conduction can be limited (eg for all the I/O wires) the heat loss may be minimal. Pretty easy to stick a box in a freezer (generally around -20C) and see how much dissipation is required to maintain +20C inside.

There’s also no reason you can’t just wake, check the temperature, and if it’s chilly, stay awake (and maybe spin busy) to warm it up a bit - no need for resistors etc. You don’t need to be transmitting to burn 100mW+. Obviously, this process burns exactly the same amount of battery power as any resistive heater.

As I’m on a plane and thermal boxes have always fascinated me, I thought I’d try and work it out… there now follows some probably incorrect calculations, assumptions, and misuse of units and/or websites, and a conclusion:

http://www.coldice.com/insulated_shipping_containers/evertemp-urethane-insulated-shipping-containers/
ET-369 has an R-value of 23.08 (I’m assuming this is imperial vs SI, hence is in sqft farenheit-hour per BTU… oh my, there’s a unit…)
Internally it’s roughly a foot cube, ie it has 6 square feet of wall area

https://rimstar.org/renewnrg/heat_transfer_loss_calculations.htm with A = -40F (-40C), B = 68F (20C), 6 square feet, R-value of 23.08 says 28.1 BTU/hour heat transfer.

1 BTU/hr is equivalent to 0.293W (ie joules per second)

So to maintain internal temperature with that 60C delta you’d need to be dissipating 8.24W. That seems quite a lot, whether you’re using a resistor to heat or anything else. Aiming for 0C/32F inside instead still needs over 5W constantly.

Ouch. Let’s try a smaller box:
ETM-309, same R-value, but 2.45sqft wall area (8x8x6ish)
Now 3.36W for 60C differential, or 2.23W for 40C.

I think it’s not going to be viable to heat through cold nights for a lithium cell unless you want to make a thick box out of aerogel (or wrap that one in aerogel). It seems much easier to just use a lead-acid battery. Of course, the calculations could be wrong - I’m a little surprised at how much power was required.

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Does anyone have the discharge and charge temp ratings of the included LiPo batteries? I haven’t been able to find them.

My assumption is that the discharge temp rating would be lower than the charge temp rating, so depending on power consumption needs and temperature fluctuations, one could conceivably simply turn off the PMIC charging when the temperature drops below a certain point, and then turn it back on when the temperature goes back above that point.

pmic.enableCharging();
pmic.disableCharging();

Would something like this work assuming the temperature doesn’t dip below discharge ratings? I seem to remember reading that most temperature related LiPo damage comes from putting a charge to them and not necessarily discharging. Anyone know if that is true?

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You only need to heat the cell during daytime charging. The batteries can discharge at lower temps than they can be recharged.

The thermal mass of the battery will hold the temp for some time so constant heating should not be necessary.

From my testing of using heating resistors for a low power way of keeping water for chickens from freezing in the winter you do not need as much wattage as your calculations say but real world testing needs to be done to figure out the minimum required setup to accomplish the goal.

I would reach out to the creator of the LFP charging circuit for some real world usage feedback on what has worked for others.

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This is the same cell that the Electron comes with I think. Specs will be similar if not.

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That thermal calculation is average - you would need to put that much energy in per hour to maintain temperature. Bursting doesn’t help over a long period, and we’re talking about a long cold night here.

It’s obviously way more complex than that to model the convection and so on (we used to do ICEPACK simulations on the iPhone to work out heat dissipation and model Li-Ion battery hotspots - that’s why the iPhone 3G has a graphite sheet between the PCBA and the battery, to prevent the 3G PAs from causing cell safety issues - the graphite conducted heat very well along the sheet, and badly across the sheet).

But yeah. Test it and let us know :slight_smile:

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I think he is more concerned with getting the battery temps high enough during daylight hours to allow charging via the solar power.

The battery can discharge just fine at lower temps compared to the higher charging temp limit, so there is no need to heat the battery at night when there is no solar input power avaliable anyways. Trying to warm the small 2ah battery from its own power reserves is not really doable considering the energy needed to heat up the resistor.

The Electrons PMIC does not have the battery temp sense pin enabled so it will discharge at any temp, even if the temp drops below the PMIC low temp setpoint. This just means the Electron will work during the night even if the battery temp is below -20 C.

Not sure if that LFP charging circuit cuts the discharge output if temps drop below -20c but it would be ideal if it did to get the most cycle life from the battery.

This is an interesting challenge indeed.

I have looked at some LTO battery cells with -40C charging and discharging temp ratings, but those cells tend to be in the 20-30ah range. There is a guy on here that is testing these LTO cells for a project that requires hundreds of Electrons deployed all over the USA where the cold temps present this cold battery charging problem.

Another benefit of LTO cells is that their cycle life ratings are in in the 5000 and some go up to 10,000 charge cycles which is perfect for long-term IOT deployments that will run on solar.

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In my personal opinion and experience, the good ole SLA battery mentioned in the Original Post will likely be the best route.

I’ve had good luck with $20 7Ah SLA’s with a common $35 12V Solar Panel & Controller.

Use a full size battery box that has lots of room for insulation and the solar panel mounts on top of the box .

Lead acid batteries work fine in North Dakota, every vehicle has one :grin:
They are hard to beat when space and weight are not a design constraint.

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WOW…!

Thank you. I am thrilled with all of the brainpower my post has garnered.

@RWB is absolutely correct that IF I stick with lipo I only really need to worry about daytime charging, not nighttime discharging. While it is true that winter temps in ND can and do drop into the -40C range, it doesn’t happen frequently. My bigger issue is that charging a lipo at 0C and below is a non-starter.

This particular application (Project Canary) involves periodic (typically every 15 minutes) air quality sampling which means that insulating the board/battery conflicts with the primary objective. Getting into dual boards where the sensor is outside an insulated enclosure is possible, but it doesn’t appeal to my KISS design philosophy.

BTW, I do have a full blown suite of meteorological sensors on board so I know what is happening to ambient conditions.

I have looked into more exotic lithium cells (LFP, LpTO, LpCO). One option is to use a primary cell like the Tadiran TLH series to power the sensor on those occassions when the lipo cannot be charged. This would mean that the sensors would have a limited lifespan, but one that might be measured in years (I will have to look into the weather data and consider adjustments to the duty cycle when extreme conditions occur.

But wait…as I was writing this post @Rftop posted what I believe is the right answer. "good ole SLA"
I have room inside the birdhouse and this old fashion battery technology has survived plenty of nasty winters in North Dakota. I’ll need to give some thought to the fuel gauge question, but it is solvable (I can probably simply monitor supply voltage…I use the fuel gauge to dynamically adjust duty cycle / reporting frequency).

So what’s wrong with lead acid that I’m not seeing?

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The only thing I dont like about lead acid batteries is their bigger size, heavier weight, and considerably shorter lifespans which equates into more frequent battery changes.

Depending on how deeply you discharge the SLA battery it could need replaced yearly which adds to your yearly total expenses.

You can always give it a shot and see how it goes.

I really like the LTO cells due to their unmatched cycle life ratings and unmatched temp operating ranges.

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