Hey, 40V and a 7.4V battery are way out of the ball park.

Here's what you need to know.

How many cells has your battery?
It could have 4cells 4S or 2 cells 2S

What type is it?

Lithium Polymer (LiPo) ???


What current we want to charge at?

2Amps (2000mAh) will take three hours
3Amps (3000mAh) will take two hours
4Amps (4000mAh) 1.5 hours and so on.


What is the maximum voltage you can charge a Lithium based battery?
8-8.4 volts. (NOT 40V)

Note: when the voltage peaks at 8-8.4 the current (mAh) will begin to drop


It can be harmful so learn all the parameters and ensure you setup the charger correctly and use a correct charger.