Upgrade solar charge controller.

[aufdenkampe]

The Mayfly's Lipo battery solar charge controller chip (MCP73831) has a max output of 500 mA, which is about the 550 mA peak output of the Voltaic's 3.5W solar panel that we often use for our power-hungry sensors (i.e. YosemiTech turbidity with a brush) along with Adafruit's 2-cell 4400mAh or 3-cell 6600mAh Lithium Ion Battery Packs. This means that we can't increase the solar panel size beyond this for more power-hungry applications (such as several sensors with brushes).

It would be great to have a slightly higher rated and potentially more efficient charge controller.

One option might be the MCP73871 controller that is used in the Adafruit Solar Lithium Ion/Polymer charger - v2 that can be modified to 1A charging and gets high marks in this review: https://www.voltaicsystems.com/blog/lithium-ion-charge-controller/.

Not sure if there are other pros/cons. Is there value at going with a MPPT (maximum power point tracker) charge controller?

[everhamme]

Per most recommendations I've seen true MPTT is overkill and expensive. Since our incoming voltage from solar is near 6V it's not too far off from the required charge voltage. If we switch to 12v panels than Mptt might be necessary.

Also note that for max efficiency we should limit the charge current of the charger (via custom resistor) to match that as close to possible of the expected conditions in ther field or based on expected max output of panel..... so size charging rate to panel size. It's documented on the adafruit page in great detail. https://learn.adafruit.com/usb-dc-and-solar-lipoly-charger/using-the-charger

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Ed Verhamme Project Engineer 501 Avis Drive, Ann Arbor, MI 48108 Office: 734-332-1200 Direct/Cell: 734-681-0577 Email: everhamme@limno.commailto:everhamme@limno.com http://twitter.com/eddiegreatlakes LimnoTech www.limno.comhttp://www.limno.com/ Water Environment | Scientists Engineers

-------- Original message -------- From: Anthony Aufdenkampe notifications@github.com Date: 3/6/19 6:35 PM (GMT-05:00) To: EnviroDIY/EnviroDIY_Mayfly_Logger EnviroDIY_Mayfly_Logger@noreply.github.com Cc: Subscribed subscribed@noreply.github.com Subject: [EnviroDIY/EnviroDIY_Mayfly_Logger] Upgrade solar charge controller. (#23)

The Mayfly's Lipo battery solar charge controller chip (MCP73831https://www.microchip.com/wwwproducts/en/en024903#additional-features) has a max output of 500 mA, which is about the 550 mA peak output of the Voltaic's 3.5W solar panelhttps://www.voltaicsystems.com/3-5-watt-panel that we often use for our power-hungry sensors (i.e. YosemiTech turbidity with a brush) along with Adafruit's 2-cell 4400mAhhttps://www.adafruit.com/product/354 or 3-cell 6600mAhhttps://www.adafruit.com/product/353 Lithium Ion Battery Packs. This means that we can't increase the solar panel size beyond this for more power-hungry applications (such as several sensors with brushes).

It would be great to have a slightly higher rated and potentially more efficient charge controller.

One option might be the MCP73871https://www.microchip.com/wwwproducts/en/en536670 controller that is used in the Adafruit Solar Lithium Ion/Polymer charger - v2https://www.adafruit.com/product/390 that can be modified to 1A charging and gets high marks in this review: https://www.voltaicsystems.com/blog/lithium-ion-charge-controller/.

Not sure if there are other pros/cons. Is there value at going with a MPPT (maximum power point tracker) charge controller?

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[neilh10]

Great idea to move to 1A - that could mean a 8Ahr battery could be recharged in a solar day.

For an easy option to move to a higher voltage a 3pin plug-in switcher could be used. I have been thinking about could it built into the cable. Its 3-pins are centered on 0.1" - so a fairly small footprint. It would require a different polarized 2pin connector for the higher solar voltage.
(For the higher W solar panels an 18V typical can get to 30V on a cold day and no charge being taken) OKI-78SR-5/1.5-W36-C Vin=36V, Vout 5V@1.5A https://www.digikey.com/products/en?keywords=811-2196-5-ND $4.50

The MCP73871 is nice for its power-path switcher. That means that for a fully discharged LiIon (dead low voltage disconnect to processor) with weak sunshine (high impedance solar) - it will provide power to the uP before it starts charging the LiIon - the uP needs to know that it can't place a heavy demand on the battery. There would be a benefit for a direct reliable LiIon battery V instead of the current USB V. ( I'm already testing this code for myself).

The current Mayfly MCP charger trickle charge a fully discharged battery till the LiIon voltage rises enough before allowing the full charge current. That of course can be annoying to a person sitting in front of the system trying to determine if its healthy as it won't come up for some time.

There are some LiIon chargers that enable the charge current to be programmed by toggling the enable line - but I forget which ones now.

[aufdenkampe]

@everhamme and @neilh10, thanks for your encouragement and input! (also, @everhamme gets credit for pointing me to the blog post from Voltaic!).

I really like the idea of keeping future versions of the Mayfly to be flexible, for a broad range of power needs. I also like the idea of sticking within the range of Voltaic's small 6V solar panels, which include 1, 2, 3.5, 6, and 9 W sizes. And I like the idea of sticking with the same JST connector that is broadly used in the DIY electronics world.

So maybe the upgrade is to the MCP73871 controller, but with the ability to switch between 0.5A or 1.0A charging, using something like the 3-pin switching jumpers near the Mayfly's grove ports?

[neilh10]

The Mayfly Rev1 implements a BQ24074. From the manual this part of a series "BQ24072, BQ24073, BQ24074, BQ24075, BQ24079 SLUS810M –SEPTEMBER 2008–REVISED AUGUST 2019" and circuit diaram has been released in https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger/tree/master/hardware/Mayfly_v1p0

ThBQ24074. implements a power path, gives priority of available power to the "System Load" labeled V_BATT. The LiIon battery is on sperate pin LIPO_V, and can supply up to 4.5A to V_BATT. The BQ24074 checks for a short circuit protecting the LiIon battery

From my cct analysis of Mayfly_v1p0_revA3_public.pdf 10/06/2021 SJ14 can be soldered closed for taking 1A from source, (of which the battery would take up to 0.5A) and SJ15 for the battery to be charged at the full 1A SJ14 closure allows The solar input, open circuit solar voltage, can be up to 10V - though the limiting factor might be the V rating on C2 and C33, and D1 and D2. A BOM is not yet available, and the eagle files don't seem to list the part numbers. The bq24074 can use input voltages to 10.5, and is safe to an over voltage of 28V.

The solar panel connection has a 2nd socket labeled "Solar2" J4 that is different from the 2pin, Interface hasn't been released yet.