18650 solar powered weather station - charging circuit

I've got some NCR18650B protected panasonic cells that run my arduino and gsm set up for over a week. I'd like to add a solar panel to this so that I do not have to recharge my unit manually. So far all the information I can find has solar panels charging a single cell 18650 battery. At the moment I have two cells in series to boost the voltage to an acceptable level for the arduino.
Is it possible to create a simple charging circuit for 2 cells in series?
Is it easier seeing as the cells are protected or does that not affect the charge cycle?
Most people seem to use the TP4056 charging module for the single cell applications, has anyone adapted this to a double cell application?

Once I get my arduino 3v mini in I'll try again using a single cell and all this might go away. It's just surprising that for such a common battery there isn't more information on using multiple cells with solar.

I haven't been able to find any nice charging IC's for 2-cell applications.

Why not run a 3.3v arduino off one cell? Would probably improve battery life too... Or does the GSM need 5v? (I thought most GSM was 3.3, because they needed to run it from a single LiPo cell)

I live off-grid..so solar power is always my thinking..
for a small project -- like ur gps.. i would think u could use any small
solar panel--6v or more and not even worry about any interface to the 18650s..
for better charging efficiency u could use something like this--
dont let the 3A rating confuse things.. i wouldnt want to draw more than 250ma. from this little pup..
its one of my favorites..works good, but at low current..

DrAzzy - Yeah I think 1 cell might be the way to go. In the back of my mind I was also thinking of running two or more cells in parallel to get longer life out of them too.

backwoods jack - When you say don't worry about interface to the 18650 do you mean just run a constant currant charge through them with the step down power supply module or do you mean to power the arduino directly from the solar panel through the step down module?

if u use a small panel, 18650s are large enuff they can take 50-100ma even at full charge.. of course there will be some cost in lifetime, but i doubt if much.. they go bad anyway..

the reg power inverter will make it nicer on the battery and is adjustable..so u can find a sweet-spot trade-off
between over charging and battery life..

i failed to mention-- make sure to use a blocking diode on the output of either the dc-dc converter or even the solar panel..
so that there is no chance of discharge thru the converter or solar panel.. solar panels develop leak current at
very low light levels.. i think its called 'dark current'..
on such a small platform, u will need to protect against that..

Thanks backwoodsjack! You've been most helpful. Just waiting on shipment for that DC-DC converter. Any advice on sizing a diode?
When reading up on it I discovered this "Many older books and articles recommend using blocking diodes to prevent reverse current flow back through the panel at night ("dark current"). Many others do not (including us, mostly)... as a general rule in 12 volt systems, you will lose more power from diode losses than you will from leakage back into the panel at night."

I guess I'll have to play around with my solar panel to see if it does infact drain much currant at night vs the diode drain. On another note, how stupid would it be to have the arduino control that flow through the solar panel with a relay and a light sensor (My unit only needs to function during daylight hours so I'll potentially have a light sensor on there anyway so that it sleeps all night). I could have the switch open during the day and closed at night...

Can anyone tell me if I'm missing an obvious component.

I've got my 6V solar panel -> step down power converter to 3.7V for Lithium battery (and no return diode) -> step down power converter to arduino mini 3.3V -> My gprs runs off the voltage from the mini and so too do my sensors. Half of these parts are currently in the mail with a fairly long lead time so I'm dreading getting them and realising I just needed one more X component and having to wait another 2-3 weeks to get it.

I've got my 6V solar panel -> step down power converter to 3.7V for Lithium battery (and no return diode)

You need a proper charging circuit for a lithium battery, not a step down converter.

My gprs runs off the voltage from the mini and so too do my sensors

This won't work. GPRS takes too much current (over 1 ampere when transmitting).