I wouldn't bother with regen. I've heard it doesn't really help that much.

If you want to know how long it would take to charge your lifepo4 batteries, take the capacity and divide it by the amps being charged with.

So if you have a 12v40AH pack and you are using a Black and Decker 12v charger and you use 2 amp charge, then it would take 20 hours to charge it if the battery was totally depleted. If you have a 12v20AH pack and you only use half the pack (10AH used) and you charge with 10 amps then it would take only 1 hour to charge that pack. I use a 12v black and decker SLA charger on mine and I put it on the "Gel" setting and it works fine. I got 3 of those 12v20AH packs from elitepowersolutions.com and I charge it one 12v pack at a time and my charger can do 2/10/15 amps. Most of the time, I use 2 amp charge since I don't normally use a lot of the pack when I ride. The recommended charge rate on the 20AH packs are .3C which is 20 * .3 = 6 amps. So the recommended charger would be a 6 amp charger for the 20AH packs but using 10 amps probably wouldn't hurt it but I wouldn't go much higher than that.

You can also get a 36v 6 amp charger somewhere online and that should work as well. The first charge, though, I would monitor all the cells to make sure none of the cells go above about 3.8v per cell. So if you have a 36v lifepo4 pack with 12 cells...the total charge should not go over about 3.8 x 12 = 45.6 . I normally charge mine to 3.65v per cell which is about 43.8v hot off the charger then they settle down to about 42.9v after a while.

And if you want to monitor your cells, this is a great way of doing it...it has an alarm on it so if you go to low on your cells, it will warn you. Remember you dont' want to go below about 2.5v per cell on lifepo4 or you will ruin the cells. This is an 8 cell monitor for about $29. You'd need one more to cover all your cells.
http://hobbyking.com/hobbyking/store...itor_2-8S_Lipo