Definitely charge them after each use. So basically you go to work, then you come back home. Charge them that night. Also, get a watt meter. Definitely a necessity with lithium batteries.
My suggestion would be either to buy this one:
http://www.hobbyking.com/hobbyking/s...dProduct=10080
for $24
or this one....I think this one is better but I can't vouch whether or not this is a good website to buy from. I can vouch that hobbyking is good, though because I bought from them before. I like this Trakker watt meter better because it doesn't scroll through the data. It shows data on screen at all times. The turnigy is annoying because it scrolls through the data so you have to wait about 5 seconds for it to show the AH used.
Both are basically the same price. I have a Turnigy watt meter but I wish I would have bought the one below. I think it's the same price in dollars as the Turnigy.
http://www.plus2city.com/plus2rchobb...onkj1lhafqh362
You absolutely have to charge all the batteries at the same time and charge them to full or almost full. That's very important because of how lithium works. Lithium batteries have a "knee" and when you hit that "knee" in the discharge curve, it only takes about 5 seconds for the battery to be destroyed. Since it sounds like you will only use 50 % of the battery, you don't have to worry about this as long as you charge all the batteries to full and balance them once every 5 to 10 times. If you buy 3 or 4 of those small chargers that I suggested (the 3.2v lifepo4 charger from all-battery.com), you can balance them very quickly...maybe in an hour. I own 3 of them. The good part about manually balancing them is that you don't have to rely on a circuit board (BMS) that will probably break in a year or two. Also, since you are manually balancing them, you'll know that they are all charged to full capacity rather than relying on a microchip to do the job for you. If you don't think you want the hassle of doing that, then get the balancers, though.
If you want to manually balance them like I'm doing then I would suggest buying these two chargers:
http://www.amazon.com/Black-Decker-V...pd_sbs_auto_14
That one has a 2/6/10 amp charge setting. 6 amp is the recommended current to charge. I use a 10 amp charge and it's okay. I have the same brand of charger but it's the 2/10/15 amp one.
Then buying one or two or even 4 of these. If you buy 3 or 4, then you can get them balanced much faster since you would be able to balance (fully charge) 4 at a time. When I say "balance" I mean just basically charging them to full capacity. That's called "top balancing" the cells. The idea is that when you top balance them, you prevent them from overcharging one of the cells. That's why I do it so when I use the black and decker, none of the cells are more full than any of the others, that way all of the cells reach full or near full at the same time. (so you don't ruin any of the cells). That's why it's important to charge all the cells.
http://www.all-battery.com/TenergyLi...rger01300.aspx