Actually you are putting both the Nimhs
and your ESC at risk, see here-
http://www.rc-monster.com/forum/showthread.php?t=17200
Do keep in mind this is straight from the president of Castle, I know you have a Tekin but this still applies.
There is also a little thing called ripple current, the simplest way I could explain would be a propeller cavitating, the ESC has very sudden gaps where it needs to bite onto the power, but it can't. This can take out your ESC in a hurry.
Not to sound like an @ss, you need to run BL with Lithium, period.
The best thing you can do with your Nimh is ziptie them together and use them for a boat anchor. Stop wasting your time, you have the lipos....
