Argon and LiPo charging

Is there some documentation on the limits and best practices for using a USB solar panel and a LiPo with an Argon? I’ve got a design in test right now, and what I’m seeing is very little change in voltage between day/night. The XC6802 should be directly charging my device right now based on the datasheet, but the charge LED never is lit, and the reported voltage is pretty fixed at 4.04V +/- .01V over the last 15 hours (yes from indirect sun to over night back to direct sun and voltage is not changing). It’s trickle charging for sure.

Setup is

  • 13W USB solar panel connected to Argon micro USB port
  • 2500mah LiPo battery connected to Argon battery JST
  • Feather doubler with sensor connectors on the other side
  • Sleep while not in use, wake to send data, go back to sleep
  • Uses SystemSleepMode::ULTRA_LOW_POWER
  • Voltage numbers come from float voltage = analogRead(BATT) * 0.0011224;
  • battery measurement is reported on every wakeup

I can’t test the panel output conditions right now. I have ordered a USB power meter though which will be here tomorrow.

I do have an MCP73831 paired with a photon to compare against. That charge circuit works in the panel setup (same panel, same battery, just a photon using the MCP73831 instead of the Argon). Photon runs, battery shows increasing charge in bright sun. The Argon will run (no battery plugged in) from this panel normally without drops or brownouts in bright sun.

I put the battery out intentionally at 4.1 V to test how long it would take to charge, and it’s fluctuated between 4.1 and 4.03 for the last 24 or so hours (mostly sunny yesterday, bright sun today). I’ve apparently achieved equilibrium at ~4.04 volts. The number hasn’t varied for the last 15 hours by more than +/- .01V. The voltage has never really gone up in any meaningful sense with the Argon.

Where is my design or my assumptions wrong? I can’t find anything to control charging like you can with the Boron, so I have no code to change or manage the PMIC or related.

I was facing similar. The Argon is very good at consuming very little power. I had tried a few things with the Wemos D1 mini and solar power and I think I got about a week out of it at best. With a similar set up to yours (except I used the Vusb pin), it ran for weeks before I started to notice any drop in battery life.

I think the times it was actually charging the battery for was so short, I never noticed it. If your battery is not going down it is probably a sign it is working,

Thank you. My concern was that it sits at such a low voltage for a new LiPo. I’ve been testing this battery and another I got together, and they are both less than 100% using several charging mechanisms. I’m now wondering if the batteries are good, not the circuit.

I agree, the actual usage for this scenario is too small to measure which is wonderful. It means I can checkin more often than I had in the past. I had a Photon doing something similar, and it used way more power, though the circuit design was decidedly less well thought out.

@peteb , I have not performed much testing w/ the Argon, but here’s a link to some Solar testing from a Xenon (which I “believe” uses the same charging circuitry, IIRC)

The following note might be of interest to you:
Note: the Xenon will not begin recharging the Li-Po until the voltage drops to 4.05V or below (can take days), which agrees with the Torex Datasheet

The Xenon’s Charging cycle can be terminated by a quick voltage drop from a cloud. A new Cycle wont begin until 4.05V is reached.

Again, I appoligize if this doesn’t pertain to the Argon, but I suspect it might.

Thanks. Yeah, I saw that thread. I think it does actually start to charge after 4.05V though, as plugged into USB, my other battery is charging to 4.15V.

In this case, I believe the batteries are at fault, not the circuit. I don’t see anything in the Argon charge circuit datasheet that says it won’t charge until 4.05, and even still, the weaker battery should have been charging. My confusion came from different behavior with the Photon setup, but I didn’t wait long enough to see how much it charged. That circuit does seem to goose the battery a smidge, but only a few millivolts. Overall, it didn’t change the charge capacity much, and it always seems to get back to ~4.04V. My other battery sits ~4.12V consistently. I ordered a new one, and will test with that when it shows up.

Overall, the Argon is SUPER AWESOME on battery though. I’ve got a 2500 mah battery and I expect it could go weeks checking in relatively often. This is for my parents house, and it needs to be hands off. They get good sun where they are, so now that I can get much better life on battery, it reduces my need to check in on it all the time.

Agreed on how good these are for battery life.

For my solar project, I went from late June to early October on a 2000 mah battery. In the end I think the issues were lack of sunlight and cold temperatures. I’m in the UK and my project is soil moisture sensors as part or some garden automation.

By October, I was happy to retire them for a few months. We don’t have much problem with rainfall from September onwards!