I have a model art 4x4 monster truck comes with a 1800 ni-mah battery pack it takes 4 hours to charge and 15 min running time so can i use lipo substitute ni-mah  

Welcome to RC India

It depends on a few factors, a main one being what kind of speed controller you are using etc.

Please post more details of your electrical setup, including what voltage battery you are using now.

Also, please do not start topics with the name "help please". Reason and other rules are explained here :

http://www.rcindia.org/rc-india-forum/forum-usage-tips/ (see #12).

Like Anwar said, it depends on the electrical setup in your car.

Let's say that the controller can handle the output of a fully charged lipo, it will not have what's called 'Low Voltage Cutoff". Lipos should be discharged only to a particular extent and going further risks damaging the lipo. Modern controllers monitor voltage and cut power when the voltage sags below a predetermined level.

Until you read the specs of your ESC and it says that it can handle lipo, the answer is NO.

I'm guessing that your nihm battery is 7.2v.

Although it seems that the difference between 7.2v and 8.4v (fully charged lipo) is very slight,  some car ESCs will burn out as soon as you plug in a 2S lipo.  You can also burn out your motor from the excess heat caused by the additional voltage.





 

thanks for all your suggestion