OK ... this has been driving me crazy for awhile now.
Solar battery-charger products generally assume you need hundreds of watts - enough to light a house or RV or something. Dataloggers need maybe 20 mA, perhaps a brief surge to 250 mA when peripherial devices switch on for the measurements.
Dataloggers also need tightly regulated voltage OUT of the batteries - WHILE they are being charged. Ards generally want 5.05v, Pi's I think are 3.3v ... not any old thing between 3 and 7 but dead-on perfectly regulated high-conversion-efficiency voltage.
Now for the Ards, two products sold by Seeed - the "Li-Po Rider Pro" and a similar shield-incorporated setup do in fact handle both functions pretty well, for Li-ion cells anyway. Your solar can provide most anything between 3.0 - 6.5 volts and it'll charge the battery pack, and a seperate output provides rock steady 5.05v whether the sun is bright or not or you're on pure battery power.
AdaSoft also sells their version ... but it's NOT very well regulated at all - indeed it could likely roast an Arduino if the sun got really bright. They've got a good cheap outdoor solar PV panel that's decent however.
Note that said panel (1-watt, allegedly 6v), coupled with the Li-Po Rider could be a problem. In very bright sun the panel surges up to a little over 7v, and the Li-Po Rider's specs say 6.5v max. As the PV panel produces so little power, I just bridged a 6v/3w zener across the outputs to nail down any voltage peaks without wasting power during off-peak output (could probably have gone to a 6.25v, but I already had those others in the parts drawer).
Li-ion batteries do have their attractions ... but they're also nefarious for having a rather short shelf-life (three, maybe four years) and then they just die. You likely won't know when the one YOU bought was actually manufactured. If it's cheap, it's likely near its EOL.
Alternatives are, of course, Ni-Cad (charge memory effect and sudden death if crystals bridge the plates), NiMH (DC package from Recom Power (digikey 945-1690-1-nd) that can deliver a steady 5.05v up to 500 mA from a source anywhere between
9v (a tad less actually) and 36v at (claimed) 87% efficiency.The whole regulator fits onto a roughly 1/2" square tiny board with the contacts at the corners. I just put another protoboard under it and pushed some pins through at the corners so they could be easily soldered to the regulator ... instant pluggable power module. These things cost about $4 ... buy half a dozen on a tape just to have 'em around. There exist higher-output DC-->DC's in case you have really large transient loads.
Using this, I can port directly to an Arduinos +5v IN pin and bypass the wasteful onboard 5v regulator (still need the little 3.3v reg alas ... dammit !)
Now it's a pity that I can't find any commercial source for what's essentially both kinds of chips integrated into a single off-the-shelf style device. If you KNOW of one ... I'd really like to hear about it because the Powers That Be are fond of off-the-shelf solutions because they SEEM cheaper, come in a pretty box and, of course, you can blame failures on someone else.