In reply to MW0WML:
I haven’t used these cells in below freezing conditions so can’t speak to their performance there.
I had a 4S pack that I bought on Ebay for my K2. Composed of cylindrical cells, not A123, but it gave a good 2 Ah and fit in the position for the SLAB that normally comes with the K2 battery kit. A second one I bought, maybe not from the exact same vendor, had a weak cell.
I have also purchased LiFePO4 4S1P packs (Zippy brand) from Hobbyking. Half of them had a bad cell, but two are still much cheaper than one from Buddipole. Even a “good” one always seems to come in at 10% to 15% under rated capacity with a discharge test - even a fairly slow discharge test. Others recently seem to be having better luck than I did a couple years ago. Usually the cost and hassle of returning a bad battery isn’t worth it to me! The Zippy prismatic batteries could use a more robust wrap to protect from drops and abrasions. I used disposable plastic water bottles - they act just like heat shrink tubing and make a nice armor when shrunk around the battery.
Finally I purchased some genuine A123 cells from Hobbyking and made up my own batteries. These have been the best so far. Heavier than prismatic, but meet their rated capacities. More expensive than Zippy, but about half what Buddipole charges for their batteries - but you have to fashion your own interconnects and pigtails w/ fuse. If you go this route be sure they have solder tabs already on them, it is difficult to solder to a bare A123 cell without damaging it.
You probably can’t go wrong with the Buddipole batteries, but you pay. Other options depend on your tolerance for making your own batteries, carrying more weight, or maybe getting some duds from Zippy in order to save some money.
I use a Cellpro Multi4 charger myself. Great little piece of kit, but there are less expensive options in balance chargers that seem to do the trick too.