@doobedoobedo
Thanks for your post. I had been looking at a similar scenario in the past, of which a PowerPath IC would be bet suited.
I ended up though with a simple design that included the
MCP73831 in SOT-23-5, 100mA charge, and a
MIC5252.
Here are the links on digikey
Unfortunately, I didn't end up making the board due to other things getting in my way.
So what would happen is, since the solar panel will get loaded, it will drop under 6V, so you are within its operating voltage. Now, the panel I had is a 9V version, so it made sense to use the LDO, in your case, you can omit it.
Now, in regards to this post:
I changed the Rprog for a 10k which drops the charge current down to 130mA I could easily drop it some more. I think I need to do some more calculations on how much power it's actually going to use.
The battery will power an arduino and a couple of LEDs for about 5 minutes (50mA) and a servo for about 10 seconds at around 1 amp draw each week, so even with no charging the battery should last a while. I'm probably over-thinking it as by my reckoning that's about about 200mAh/year, call it 300mAh from the battery after boosting to 5V and the battery would still last 4 years without a charge. So anything it gets is a bonus. I'll just put a higher value resistor on it so it charges over the summer.
You are making some fundamentally flawed assumptions, sorry mate.
You will ALWAYS be consuming electricity, unless you have a physical switch to turn things off. Second, you forgot the self discharge of the battery. Li-Ion batteries hate that! Please read up on the maintenance of Li-Ion batt's. Keeping the battery topped up constantly at 4P2V for a long period of time
will degrade it quickly, same as keeping it non charged. For long term storage, it needs to be held at around 3P6V.
As for using a higher value resistor, you will then be better off using a powerpath chip, leaving it at 200 mA - 500 mA. That way, you don't need to do a thing, as you can connect the bost circuit to the output, and if there is not enough current on the input, it will se it from the battery.
I found these chips suited best for a simple circuit that I was aiming for, and think they will be for you too:
LTC4099 You can use i2c to set it up and monitor it too. Great for use with Arduino, and yes, you can use it as a normal charger too.
It needs a front end though for the voltage, and a great solution would be this one:
LTC3129 or the
LTC3129-1 for a fixed voltage.
There are also solutions from T.I. and a few otehr vendors.
The question is, did you buy a ready made module, or are you able to make your own PCB's?