ESP8266 Community Forum

- Mon Feb 01, 2016 4:18 pm #40197

This is a subject of great interest to me too! While I think everything Adafruit do is built of awesome, a $17 charge controller (plus whatever for panel and batteries) is not my ideal solution for a $4 chip, and I think OP would share the same view. In my use case, I need to make about 10 stand alone solar powered sensors. I don't have $170 to drop on the charge controller. My sensors read from the analogue pin, connect to wifi, POST the results to a server then deep sleep for an hour.

Right now I'm considering two options for the charger, and I'm looking for any advice about which one might be best to use.

For the battery, I will probably use a 18650 as they are easily available and cheap. I'll use a solar panel for power, but I'm not sure what specs, that kind of depends on the charger.

The two chargers that I've found that might be useful are:

http://www.aliexpress.com/item/Free-Shi ... 61008.html

or

http://www.aliexpress.com/item/Free-Shi ... 58256.html

The 18650 battery is 3.7v so I can either use a boost controller to lift it to 5v, then I could use it with a NODEmcu style board, or I could step the voltage down a bit to 3.3v and use a bare ESP8266

Paying no more than a couple of $ or £ (in my case) for the charger is what I'm thinking of. Can anyone see anything at this range that might work for Solar charging a battery, capable of powering an ESP8266 for about 20secs every hour?

I also need to find out more about MPPT and how important it is to all this!

Jim

Hi Jim,

I agree with your sentiments re ideally paying no more than a couple of $ for the charger, and i've been meaning to try this so you spurred me on to give it a go. Here it is:

solar1.jpg

Thats one of those 43cent charger modules you linked to, with a 5.5V 0.66W 120mA Solar Panel, a 240mAh LIPO cell, a MAX604 voltage regulator and an ESP-12 with a BME280 sensor. The ESP is publishing the temp/pressure/humidity to Thingspeak once a minute to here, and deepSleep'ing for the rest of the time.

The 240mAh cell is way to small for serious use. Its presently charged up to 4.0v and by my calculations it might last for about 60 hours if there was no charge from the solar cell, so a couple of cloudy days and its toast.

Come back here on Thursday to see how its doing, i'll try to put it somewhere sunny...

You do not have the required permissions to view the files attached to this post.

Last edited by torntrousers on Tue Feb 02, 2016 6:12 am, edited 1 time in total.

- Mon Feb 01, 2016 5:24 pm #40203 Hi torntrousers, I like the MAX604, nice low quiescent current. Unlike the 5~10mA AMS1117 everyone else seems to use.

I'll just chip in with another suggestion here if I may. LiFePO4 cells are much more tolerant and have several characteristics that make them more suitable. In particular, they maintain a significantly steadier terminal voltage over the fully charged/discharged cycle. This voltage is also lower than for Li-ion. The cell can be charged to 3.33V and held there while the current will diminish to zero. So full capacity is attained at a voltage suitable for direct connection to ESP8266. (Anything between 2.7V and 3.5V seems to be OK with the ESP8266)

Again, in principal, LiFePO4 cells should first be charged with a constant current until the final voltage is reached. In practice constant current doesn't mean it has to be constant, just controlled within a safe upper limit. So for a solar panel incapable of 1C (the Ampere/Hour rating of the battery) a fancy charge controller isn't necessary. All that's required is a constant voltage regulator. And a blocking diode.

- Mon Feb 01, 2016 8:51 pm #40216

The ESP is publishing the temp/pressure/humidity to Thingspeak once a minute to here, and deepSleep'ing for the rest of the time.

The 240maH cell is way to small for serious use. Its presently charged up to 4.0v and by my calculations it might last for about 60 hours if there was no charge from the solar cell, so a couple of cloudy days and its toast.

Come back here on Thursday to see how its doing, i'll try to put it somewhere sunny...

Torn,

Can I ask what mode you're using for wake-up in the deepSleep() call? I tried for ages to get WAKE_RF_DISABLED to work and it seemed like there was nothing I could do to get the WiFi to reliably reconnect once the unit woke.

On linear regulators, I agree 100% with AdrianM; my go-to part for the ESP8266 is the Holtek ht7333. It comes in the familiar packages, but with a very low quiescent current. Ideal for battery-powered projects and those low-power garden LED light projects, too.

- Wed Feb 03, 2016 5:47 am #40342

Can I ask what mode you're using for wake-up in the deepSleep() call? I tried for ages to get WAKE_RF_DISABLED to work and it seemed like there was nothing I could do to get the WiFi to reliably reconnect once the unit woke.

This isn't using WAKE_RF_DISABLED, just WAKE_RF_DEFAULT. I have used WAKE_RF_DISABLED with other things though and it was working and reconnecting ok, not for a while though, i can give it a go with the latest sdk code to see if it still works for me.