Bob Kondner here. I have been working with small power converters and battery charging for 15+ years.
The first thing I suggest to folks is know thy load, peak and average power. Many engineers work themselves into corners not realizing temperature and peak currents significantly reduce available battery capacity. Don't go wrong here, it will bite you.
My 1.4 amp hour lead acid battery fails early.
Folks often see less than 75% of rated battery capacity and wonder why. The often blame the charger circuit. Lead acid (Flooded Plate, Gel Cell, AGM or other) are often the worst. If you look closely at the specs they are rated at a 20 hour discharge. If you try to pull 2 amps from a 1.4 amp hour battery you might only 700mahr. On a cold day or old battery even less. Try an old battery on a cold day and see how long it lasts.
I want the best battery. Is that Lithium?
Maybe yes, maybe no. If all you need is running at low current for long periods try an alkaline lantern battery. They are 6V each but about 25 Amp Hr. You will not get much current when cold or partially discharge but they sure are low cost. If that's all you need don't bother with a SunDuino if you can design a linear regulator and roll your own PCB.