Battery question / usable range

Hardware
1

Dear All,

we’ve attached a 15Ah 3.7V lipo battery pack to our electron which is charged by a voltaic systems solar panel. I understand that we’ll never see SOC going above 85% for safety, but what is more important is the fact, that the usable range of the battery is rather limited. If I understand correctly, Electron’s UVLO is at 3.9V. Given that the Electron’s battery OVLO is at 4.1V (or 4.15?) we really have a narrow usable range for the battery of 0.2V.
Or spoken in SOC maybe a range between 70% and 85%, so we’re actually using only 15% of the battery’s capacity.

As we’re using Electron 2G which embeds a SARA-G350 which has a min VCC of 3.35V, I wonder why the minimum VIN is so restricted.

Edit: I thought it might be helpful too add wiring details. The battery is connected to LiPo connector, VIN is connected to the solar panel.

2

Where does that info come from?

This is what I'd read in the datasheet
https://docs.particle.io/datasheets/electron-(cellular)/electron-datasheet/#li-

3

Aaah, I checked the VIN specs instead of Li+

Related question: what’s the minimum Li+ then? I see the recommendation, but would 3.3V be enough to power just the Electron (it’s okay if the modem is unpowered then). That would be great as the Electron then still could take measurements and send them later when VIN >= 3.6V

Summarized:
VIN < 3.6V: Take measurements only
VIN >= 3.6V: Take measurements and send them to server

4

There are some potential “brownout” issues when going too low, most prominent the dim blue D7 issue.
This forum has some threads regarding that and one common precaution is to keep the SoC above 20% but there are also some more elaborate approaches.

@RWB has been quite involved with that kind of thing and @rickkas7 is always a source of in-depth knowledge on everything regarding Particle devices.

5

Thanks, @ScruffR
Is there something I have to do programmatically? Like configuring PMIC?

Or is Electron PMIC Settings Question a good starting point?

6

You can configure the PMIC (and that question is a good starting point) but for the start you can have some tests what you get out of your battery in the 20% to 85% SoC range and see whether you need to do any more than just monitor that and keep the from drawing any more power once it drops below 20%.

7

This post is already solved by ScruffR, but I’ll add the curve that I personally use to calculate State-Of-Charge for a single cell Li-Po (10,000 mAh) for an Electron that’s recharged by a 6 watt Voltaic Solar Panel.

image
image732×297 3.76 KB

The Rule-of-Thumb that I use is to Size the Battery and the Solar Panel with the goal to not cycle the Li-Po below 40% S.O.C during your “Design Day” conditions.
So your Energy Budget will assume that a Li-Po at ~3.8V is depleted.
Yes, this is conservative but prolongs the battery life and adds a safety margin.

Similar to what @ScruffR mentioned: For me, if Li-Po ever drops to 3.7V (20% S.O.C), it’s time to Sleep the Electron and wait for the Sun. However, if your Energy Budget is correct, you should never get near 20% S.O.C.

8

thanks for the hints, @Rftop

I think I’m only confused because the first thing you find when googling for “lipo discharge curve” is this curve by SparkFun / Prototalk:

5112a224ce395fb479000003

For my LiPo they actually state a critical voltage of 2.5V which is absurd, but still, I think I could go down to 3.3V.
But yes, I’m unsure now, because your critical voltage is really “high”. But I always thought even 3V would be okay as that is what most LiPo charger ICs use as highest available cutoff voltage (most of them us 2.5V by default)

9

Here is the code I and others have used to put the Electron into deepsleep once the battery SOC hits 20%.

The code wakes up every hour to check if the solar panel has charged the battery above 20% and starts running normally once this happens and keeps sleeping if the SOC is still 20% or below.

The SOC% sleep number is adjustable so you can change it if desired.

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10

@derchris, the SparkFun curve you posted isn’t related to the S.O.C curve I posted, at it’s discharging at 6C.
It’s not relevant to what you’re doing at all.

You shouldn’t allow your Li-Po to drop to 3.3V.
You are risking damage for no reason, reducing the expected life of the Li-Po, and it has very little available energy at that level.
I would be suprised that a 2G Electron could connect being powered only by a Li-Po @ 3.3V.
The 2G Electron demand while connecting could be higher than what’s chemically available in the Li-Po @ 3.3V.

Don’t get caught up with Critical Voltages listed in Li-Po technical specs. The Goal is to never operate anywhere near something named “Critical”.

IMHO, your CODE should start to reduce system demand (functionality) prior to reaching 20% SOC, and go to sleep if it hits 20% (see @RWB link/code). Or simply incorporate RWB’s code.

You know everything you need for a simplified Power Budget :
Watts = Amps * Volts
Idle Current ~57 mA (for 3G Electron) = 0.057 amps, not sure about 2G version.
24 hours in a day
Your Solar Panel Rating in Watts.
Assume 50% total efficiency for Solar Recharging (harvest Solar, get it into and back out of Li-Po, dirty panel, etc)
Assume the # of hours of Sunlight for your "Design Day"
Reserve 30% battery capacity, so your 15 Ah becomes 10.5 Ah for the Budget.

You check your Effective Solar Rating (watts) * # hours of expected Sunlight against your average system watt * 24 hours. If this isn’t acceptable, increase Solar Panel Size. For instance, you may decide you want 3 hours of sunlight to replace 24 hours of demand. Don’t forget that Effective Solar Watts = 1/2 of the Labeled Rating on the Panel.

Then check your 10.5 Ah Effective Battery Rating against your system Ah for 24 hours ( 0.057 amps * 24 hr example for Idle 3G only, use your total system’s real numbers) to determine how many cloudy days you can expect to operate without Solar Recharge. If this isn’t acceptable, increase battery size.

There are many other factors such as Li-Po operating temperature, Li-Po self discharge, etc, but this is a good starting point. Once you select the Solar Panel and battery size, test it in the real world to confirm your Power Budget Calculations. Use @RWB’s code for an additional safety factor…that part is free.

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11

I guess there is an extra zero in the 0.0057 amps part :wink:

I'll edit accordingly
BTW, the datasheet states this

image
image1011×438 14.8 KB

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12

That was on purpose to see who was actually reading… j/k
Those pesky little decimal places.
Good catch & thanks !

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The industry accepted average output from a solar panel in the summer is 75-77 percent of its rated output. In the colder months you can see 100 percent of the panels rated power output because the solar cells stay 75 degrees or colder.

It’s the heat that reduces the solar cells power output power assuming you have proper sun alignment.

Sizing your system for 50 percent efficiency will not hurt since it will be oversized to get the job done but sometimes the this can push you into the higher priced component category which may be important to your bottom line.

The thing about lipo batteries and draining them is that they are usually only rated for 300-500 cycles before hitting 80 percent of their rated capacity. The less deeply you discharge the battery the more cycles you can get from the battery before needing to replace it.

Publishing data less is a good way to keep power consumption down if it’s possible in your case.

I got by with publishing every 20 seconds with the 2000mah 3g Electron battery and a 3w solar panel using the battery sleep code posted above and in the winter the system basically ran 24/7.

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@RWB, I claimed 50% total efficiency for Solar Recharging to account for the additional losses:

  • Dirty panels in the future from Pollen, Dust, etc
  • Regulator Losses while Recharging the Li-Po
  • Regulator Losses when the Energy is later removed from Li-Po later for 3.3v supply

That's why I mentioned to use a "nice-round-even" 50% of the Labeled Rating for quick Power Budget Calcs.
Naturally, we would perform a full Power Budget for any production device (the reason I hinted at Self-Discharge, temperature profile, etc).

You made a Great comment about the cycle ratings. People sometimes don't understand how much longer a Li-Po will last when treated properly (don't charge to 4.24V just because you can, and don't discharge below 3.7V ). Those, and managing the enclosure temperature are easy ways to protect and prolong a Li-Po's life.

@derchris :
Rikkas7 has a great post Here that's very useful for designing Solar Powered Electrons.
I've confirmed many of his current measurements with my own equipment.

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15

wow, thanks for the many replies.

@Rftop

Maybe I wasn't clear on that one, but in my second post (and in my mind) I assumed that I don't want to power the modem below 3.6V (for example), just run measurements and power some devices with average (and max.) current of around 16mA.
So, might a possible scenario be:

  • above 50% SoC - operating normally, modem powered (standby and/or transmitting when needed)
  • between 25% and 50% - disable modem, but keep powering the Electron itself + the sensor
  • below 25% - "recovery mode" as described by @rickkas7

Would that be considered "nice to the battery"?

Regarding the solar panel:
We're currently having a really nice summer here in Germany and in the past days I had a few measurements for the 3.5W panel with > 400 mA. All-day average for the past 10 days is 50 mA.

16

50% & 25% looks like a great starting point.....you can always modify based on your field results.

A side note to Solar Panel Sizing: Personally (just my opinion), I use 2 different design scenarios for every project.

  1. Summer Design Day - mainly concerned about the maximum temperature inside the enclosure for Li-Po Protection. You'll have Plenty of Sun for the Power Budget.

  2. Winter Design Day - mainly concerned with Power Budget and minimum enclosure temperature for Li-Po Protection. Much Less Solar Energy to harvest. That's why I size the Panel to recharge 24hours worth of Energy within 2-3 hours. Battery Capacity saves you when you get a week of cloudy winter weather. Sleep Code is executed after that.

Sorry for the Rambling... :grin:

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Just for comparison of „sun conditions“: where is your device located @Rftop?

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Just re-read one of your first posts where you said that idle current is around 50mA in idle. You‘re not talking NETWORK_STANDBY, right?
I see below 10mA while sleeping with modem in standby

19

No Sir, I get <6 mA measured @ the Li-Po when Sleeping with NETWORK_STANDBY, depending on Battery Voltage.

I’m in the Southeastern USA :sunglasses:
But because of the sun & ambient temps, I’ll have enclosure temps up to 130 degrees Fahrenheit (55 C) if I don’t do things to prevent it. Lessons learned the hard way.

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As quoted in this post (see image)

50mA apply when the device is running, but cellular modem is off and no external circuitry draws power from the device.

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