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Saying the lithium cells have more energy density than coin cells is odd, as coin cells are also lithium cells. Also, another option to get 5V from a single cell is a DC-DC converter.
I’d like to see more support for other Li battery technologies from Sparkfun. I’ve been working on remote sensors a lot lately (I want to monitor my solar hot water system, and there is no mains power anywhere near the system). I see temperatures down below zero (degrees F) at times, even though the sun keeps shining. NiCd/NiMH is out because it just loses too much power to be effective at temperatures much below freezing.
I personally am very uncomfortable with using poly-envelope Li-Ion for these projects, mostly because Li-Ion WILL catch fire if charged too hard (not likely with solar as the input, but possible). I vastly prefer LiFePO4, an alternative chemistry that has many of the advantages of Li-Ion and few of the disadvantages (like, it will smoke, but won’t catch fire).
I also prefer the 18650 or 14500 battery form factors, mostly because they are more rugged than the plastic envelope flat-pack Li-Ion cells. The other advantage to LiFePO4 is that they are increasingly getting used in solar landscape lighting (mostly the 14500 form factor) so finding the cells is becoming easier and cheaper.
LiFEPO4’s biggest drawbacks are that it’s a nominal 3.2V cell, so you really need a two-cell pack (or up-conversion) for almost everything, and it takes a different charge voltage than Li-Ion (charging current is 3.6V rather than the 4.2 for Li-Ion). Sparkfun’s Solar Buddy uses a controller chip that will handle the LiFePo4, but you have to hack surface-mount resistors on the board to make that work (how about some jumpers, folks?).
At the moment, I’ve hacked together charge controllers from stuff I’ve found on eBay and elsewhere, but I’d love to see a slightly more broad approach to supporting different chemistries from Sparkfun. Maybe even an integrated solar cell-LiFePO4-MPPT charger IoT remote sensor battery pack bundle? Cobbling these together isn’t that hard, but having one I cold just buy off the shelf (with a nice waterproof enclosure, say) would make things simpler…
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