if you get lifepo4, just make sure you never go below 2.5v per cell or about 30v total for a 36v pack. It's the individual cell voltage that matters, though. That usually only happens once you get 95 % of the pack used, though so on a 20AH pack, that would be most likely after you've already used about 18 or 19 AH of the pack. I say "most likely" because we don't know the exact capacity of each pack. It's a good idea to buy a 8s Celllog monitor for $29 to be sure. Or, just make sure you never get close enough to doing. I only use about 50 % of my pack so I know that I'll never get close. Also, using 80 % or less of your pack, prolongs the life of the lifepo4 pack. It's just like lead acid in this regard. A watt meter where you can count the # of AH used is also an option. I have a $25 Turnigy watt meter. I would have rather bought a 8s celllog monitor, though instead and just bought one more celllog monitor.
you can get one at hobbyking. There's also a couple more models there that are 6s cell log monitors that do the same thing are a little cheaper but might not be as good of quality. Great thing about these is that they have an alarm and a programable low voltage setting so you can set the lvc to whatever you want and it will alert you when one of the cells goes that low.