Solar boron with supercaps

I’ve followed the discussions involving @Rftop @chipmc @RWB on solar power with interest. I’m developing a carrier board for the boron to act as a data logger. I want to facilitate a variety of power options.

I see that most people seem to be directly connecting solar panels to VUSB and adjusting the input voltage limit to maximise efficiency. I presume that the main aim of this setup is to keep the lipo charged and provide continuous power via that (and using intermittent power from the solar panels to directly power the boron when available).

My use case may involve long periods at low temperatures, when lipo charging isn’t possible. For that reason I was planning to place a couple of super caps across the solar panel, with a zener diode, to provide a bit of solar charge storage. I’m using two 5.6V 15F supecaps in parallel. The zener is a BZX55C5V6. Schottky diode is a BAT43. I’m using a 2W 6V Seeed Studio solar panel. I have an additional 10 ohm resistor on the +ve before the zener.

In your experience, does that setup sound reasonable? Anything critical that I’ve missed? Should I make any adjustments to the boron input voltage limit?

Thanks for your input!


You can try it.

Usually supercaps are intended to supply relatively low currents for a relatively long time (think keeping CMOS memory contents valid, or powering a micro-amp RTC circuit) - their internal resistance (ESR) tends to make them unsuitable for powering even intermittently power-hungry devices like Boron and it’s cellular modem.

I am not optimistic this will work as well as you hope (there is no free lunch), but it might just work well enough for your application (not knowing the details) - I’m sure lots of people will be interested in the results, whichever way it goes; please report back.

I tested a few Solar Harvesting IC’s with SuperCaps…mostly for Xenons, but I found some of my notes for Borons.
For my particular setup, looks like 38 hours of storage was available assuming 20 seconds of runtime for every 2 hours of EN Pin Shutdown. Note: that was with (2) 30F in series, operating from 5V down to 3.6V. With only 1.5 days of storage (best case), that doesn’t leave any wiggle room for a Solar applications with 15 Farad.

100F (2 in series = 50F) showed promising results, but the leakage current plays a larger factor.
I believe ~4 days of storage, IIRC before brown-out conditions.
I didn’t test any 5V paralled SuperCaps, as per your post.

Give it a try. I’d love to hear about your results with 5V & 30F.

Thanks @AndyW, @Rftop.

@Rftop: Your results sound promising. The capacity doubles in a parallel config, so I have 30F@5.6V. Are you able to share more details of your configuration (solar panel, diodes etc)?

@AndyW: I have some prototype boards on the way from China, so I’ll be happy to share results when I can get a couple deployed. If this doesn’t work out, do you have any alternative suggestions for situations where lipo charging isn’t possible? I’ve struggled to find battery technologies (other than lead acid) that can cope with these situations. I’m planning to move over to the B series, so I’m not wedded to lipos.

Thanks again for your input :slight_smile:

What is wrong with LiPo for your application (fire hazard?)

There is LiFePo - can be obtained in similar format to VRLA 12V batteries - we are moving off VRLA.

Have you explored NiMH - there are PMICs for these and they don’t have the fire hazard issue. We use these for power backup.

Hi @armor. It’s that lipo can’t be charged below 0C. This could be a limiting factor for our use case. I think most (all?) of the lithium battery chemistries have this property. I’ll have to look at NiMH. Are you able to point me in the direction of a suitable PMIC?

@armor: It looks as though NiMH can’t be charged below 0C.

Battery University ( suggests that lipo charging at subzero temperatures is possible, at very low rates (0.02C). That might be a possibility.

For the 1800mAh batteries I’m currently using, that would mean a charging current of 36mA. Testing is going to be fun! I’m not sure how I’ll get solar input or a cell signal with everything in the freezer…

I should have a few posts on this forum in regards to my SuperCap testing.

The AEM10941 Solar Harvesting IC from E-peas was the best that I personally found for my use-case.

You can even have a Primary battery Pack as an automatic backup source.

I never tried the direct approach that you are considering.
You wont be able to harvest near as much energy in low-light situations verses something like the AEM, but that’s not your goal anyway. Mine was Indoor Solar Harvesting for Xenon’s w/o Li-Po’s.
I was just playing around testing SuperCaps with the Boron.
At the time (a year ago), I couldn’t find 5-6V SuperCaps that didn’t come with a high leakage current.

My notes show 0.025 mA and 0.045 mA measured leakage for 10F and 30F pairs in series.
As a reference, the Boron itself uses 0.076 mA during EN pin Shutdown.
I’ll look for my results from testing 100F and 400F SuperCaps, but they were MUCH higher leakage.

As far as the specific solar panels, I tested half a dozen different models.
IIRC, I preferred using an over-spec’d 6V panel with a reverse biased zener to clamp at 5.2V, 5.4V, 6V, etc, depending on the particular harvest chip being tested.

I like your direct approach for Sub-Zero battery-less applications when Sunlight isn’t a major design constraint.

My rough calcs say 4.1 days of storage for your case; 30F “effective” @ 5.6V down to 3.6V assuming:
0.045 mA Leakage from SuperCaps
0.076 mA Sleeping (EN Pin Shutdown)
17 mA ave current (Running @ mid-point in Voltage Range)
20 seconds Runtime vs 120 minutes Shutdown (duty cycle)

For your freezer test, use a u.fl to SMA pigtail, and an external cell antenna outside the freezer.

Lots for me to think over there @Rftop Thank you.

I had seen the AEM10941. But the datasheet says the max output is 4.2V and 80mA. Surely that’s not enough to power the boron?

Yeah you’re correct, like I said…my work was for an indoor solar xenon.
I was just playing around w/ the Boron and powering it from the series “pack” (2 * 2.7V) without using the regulator.

I like where you’re going with a direct setup.

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What about avoiding charging entirely. There are very high capacity batteries which are rechargeable but have the potential to power your device for a very long time. Once I iron out my Boron / deviceOS@1.5.1 / Enable sleep issues I plan to pick this project back up.

This has the potential to simplify your deployments as well if you can accommodate the requirement to change the batteries every once in a long while.


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Will comment later

Super caps are just fine. But I want to share that Lithium Titanate (LTO) Cells can do low temp. Many people are not aware of these cells.

-50°C ~ 65°C (Charging)
-50°C ~ 65°C (Discharging)

Also, personally I would look into a super cap charger IC, than a zener & resistor to throw off the excess as heat.

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The biggest hurdle is their lower nominal voltage which requies a LTO specific BMS chip which are rare.

LTO has superior cycle life also so it is a very promising chemistry.

Wow, you guys are awesome! So many good ideas here. My aim is to make a system that is as flexible as possible, so I may well end up implementing more than one option on the board.

@Rftop I have pcbs on the way to test the supercaps, so I am going to persevere with this approach to see if it is feasible.

@chipmc Thanks for the link to that thread. I’d missed that. I’ll muse on it for a while - there’s loads there to think about…

@seulater @RWB I had come across LTO in another context (large solar systems for RVs), but hadn’t appreciated their low temperature charging capabilities. I found this IC: which looks very promising (solar input). There seem to be a few boards around that use this IC (e.g. SD30CRMA, DD28CRTA), but they have an additional IC that I can’t identify from the images. I’ll look for ICs suitable for supercaps.

Thanks again for all of your input!