Storebought solar battery

Hi all! First time poster, long time googler of error messages. My current headache could also be filed under Science & Measurement or Power. I'm trying to put together a basic version apiary monitor, more or less a scale connected to a simple weather station.
My preferred plan was to use a solar battery/charger (this one specifically), but when I actually connected things up, I discovered that the power would flicker off and on about every 15 seconds or so, not very long but enough to reset the code. With some help from IRC I discovered that the arduino was not drawing enough power to keep the power supply supplying power. I tried bridging the arduino's 5v and ground pins with a variable resistor and found that there was only a small region ~20 Ohms which would keep the battery from falling asleep. A quarter watt seems unsustainably inefficient. I opened the charger up and looked around; the key component seems to be the HT4936S chip, but this is where my electronics knowledge breaks down.

I really wanted to avoid having to tackle a power supply. My question is:
Is there a relatively easy way to turn off the low power shutoff without damaging the rest of the battery regulation?
If not, could I keep the panel and battery for parts in a homemade one like several I've seen? Or just follow preexisting projects like this and this?
Alternatively, are there any decent, relatively inexpensive (<$100) solar power supplies which you'd recommend?

It's wild to be having too small a power draw as a problem!

How much power draw are you looking at? Are you using a standard arudio or something else?

Your link on building a solar charged arudio is overly complex if you instead use AA Nimh batteries.

Below I will cover how I build a solar charger. Very simple and works. Trying to get those fancy battery banks to work has seemed like too much work. In addition once you have hacked it into working some people complain that the voltage fluctuates and hurts their radio transmission power.

If you optimize things right you will probably be looking at less than a few ma load. If that is the case a battery box with AA batteries and a power limiter for the Arduino will work just fine for a power supply. To charge the batteries in the box just add some small solar panels with a diode. Adjust them and the number of batteries in the box to match 1/20C(some people claim 1/10C works fine for a trickle charge, I have not tested that to say, I know 1/20 works).

To be clear.

to make a 3.3 volt power system I use.
1 Battery box for 3 AA batteries.
https://www.ebay.com/itm/4-5V-3-AA-Battery-Holder-Case-Cell-with-ON-OFF-Toggle-Switch-Box-Pack-Cover-JH/113181683519?hash=item1a5a276f3f:g:Ze8AAOSwTmtaqyvb

Small solar panels. Needs to be rated for more than the fully charged voltage, plus the voltage drop of a diode. Three fully charged EBL(that is a brand that has more power in AA or AAA batteries than it normally should as they are slightly oversized) 3 EBL Nimh batteries is around 4.5V Then .5v for the diode. That means need a panel that makes at least 5v. I generally run with 5.5v or 6v so they charge a little longer. The batteries will pull the volts down so a little higher rating on the panel does not seem to hurt.

To figure the amps, need to know the draw that your project is going to have so you can scale the batteries to fit your project. I have a project that is drawing around 10ma constant with surges of 100ma. Average is less than 15ma. Figure the panel only makes the full rating for 4 hours or so.

15ma times 24 hours = 360ma needs to be replaced every day
360/4 hours = 90ma needs to be replaced into the batteries while the sun shines the brightest.

Figure in some inefficiencies and 120ma will probably be barely enough for the load. So we need at least 120ma from the panels.

Some times the sun does not shine and other things go wrong so having extra battery space is nice. Plus if you have enough battery capacity you can just trickle charge Nimh batteries at less than 1/20C.

With that in mind I tend to grab 3 EBL batteries with 2800mAh.

What is 1/20 of 2800mAh?
That is 140ma plus the 10ma that is being used. Some people claim that 1/10C works fine. I rather not push it so I shoot for the 1/20 and if it gets up to 1/15 for a short while I do not worry.

What all of that means is we are looking for a 5.5-6V panel that makes at least 150ma for 4 hours up to an extremely safe 150ma max. Testing shows the panels very rarely get anywhere close to the full rating. So what I do is grab 3 100ma panels. Point one east, one south and one west. In those conditions that greatly extends the time the panels make power so the 4 hours is a lot longer. Then with them pointing in different directions only one panel will be anywhere close to full power at a time. I normally see 50-150ma for 8 hours with the above setup.

https://www.ebay.com/itm/6V-0-6W-Solar-Power-Panel-Module-DIY-Charger-For-Battery-Phone-Toy-Portable/163044287057?epid=2247464574&hash=item25f6326251:g:uF4AAOSwxuha9o0y

Grab 3 1n5817 diodes(or most any other diode)

https://www.ebay.com/itm/1N5817-Schottky-Diode-DO-41-1A-20V-Lots-of-20-50-100/142757906413?hash=item213d08c7ed:m:m22icSZr5zECeXmqp320HGA

Oh and a voltage regulator to make sure the Arduino clone that uses 3.3 volts only ever sees 3.3 volts I use MCP1702. Very low drop out. It is in fact better than just wiring straight to the batteries in low current uses.

https://www.digikey.com/product-detail/en/microchip-technology/MCP1702-3302E-TO/MCP1702-3302E-TO-ND/1098463

Wire it all together and you have a power supply for less than $15 and most of the cost will be in the batteries which can be changed in a few years if needed.

If you need to scale it up add two boxes to try and keep close to the 1/20C or less on the batteries. If you need 5 volts instead use 5 batteries to get the voltage a bit higher and use a MCP1702-5002E for the voltage regulator.

EDIT:if going to 5V I would be looking for 8-9volt solar panels, they are on Ebay and other places.

Side note:

I read your links, a very interesting project. Sister saw me reading the links and told me to order the scale for a birthday present and to build the project for one of our hives. So I will be building one in the near future.

Have you built any part of this yet? I am wondering how a person puts a humidity sensor in a hive without the bees plugging it up.

Brandan34:
Your link on building a solar charged arudio is overly complex if you instead use AA Nimh batteries.

Below I will cover how I build a solar charger. Very simple and works. Trying to get those fancy battery banks to work has seemed like too much work. In addition once you have hacked it into working some people complain that the voltage fluctuates and hurts their radio transmission power.

Okay, that does sound a lot simpler. I'm going to have to learn about power supplies someday anyway...

I read your links, a very interesting project. Sister saw me reading the links and told me to order the scale for a birthday present and to build the project for one of our hives. So I will be building one in the near future.

Have you built any part of this yet? I am wondering how a person puts a humidity sensor in a hive without the bees plugging it up.

Thanks! I have been working on parts of it for a while; this link has some of it written up, though it is also needs updating with some of the stuff I've done with moving to a logging shield and testing out the scale. In particular I am still trying to capture a decent long-term time series to illustrate any scale drift.

I have thought about the problem of the bees building over the sensor, but haven't come up with a concrete solution yet!

There are so many solar powered Arduino projects out there! Do a search, and you'll find many examples.

Solar panel; battery charge controller; small LiPo battery. That's the typical combo. Use a 3.3V Arduino Pro Mini and you have no worries about the voltage: your battery produces anything from 3.2-4.2V which is fine. Just make sure your battery has over-discharge and over-charge protection circuits.

Measure how much current your project really draws (when active and when asleep), scale battery & solar panel accordingly.

recybery:
I have thought about the problem of the bees building over the sensor, but haven't come up with a concrete solution yet!

I have a few bottom boards that have a screened off part on the very bottom to catch mites. It would be easy to put a sensor there. The bees would not be able to mess with it. Not sure how helpful it would be as it would be kind of far from where the bees ball up so it might not tell us much.