Thank you for the answer, i worked it out a little bit. Actually the charging system is from a dustbuster. So there is also a voltage drop from the contacts between the charger (the adaptor) and the charging circuit. I estimated the voltage drop over this contacts as follows:
i suppose the charging current is 0.1 C so it's 130mA
there's a voltage drop of 2.9 V over the batteries, 0.7 V over the diode and 0.61 V over the resistor. So the contacts have a voltage drop of 1.289 V, it seems a little bit to much, is this possible?
For the energy calculation i made some other assumptions.
First of all, the dustbusters manual says it has to charge for 15 h to have fully loaded batteries (3.12 Wh), for the energy loss in the resistor and other components i divided the 15 hours in two, due to the working of the diode (only the positive side of the sinusoidal voltage). I also read somewhere that the batteries only store 70% of the supplied energy, the rest is lost in heat dissipation.
In this way i have an efficiency of 44% for the charging circuit. Is this a normal value for such a circuit?
The batteries are connected to a PMDC motor (i guess). The manual makes clear that you can handle the dustbuster for 10 minutes. So i have an input power of about 18 W (probably i have to take account for charge/discharge losses also??)
What i actually want to do (or at least show that it's impossible) is replacing the PMDC and the batteries by a constant torque spring motor.
Problem is i can't measure the torque of the motor because it's inside and the armature of the dustbuster influences the torque.
So i thought to handle it like this: since a PMDC has an almost constant rpm with changing torque, i can measure the rpm outside the armature.
If i know the outputpower of the motor (therefore i have to know the efficiency of the motor, something around 50% for such small motors i guess), i can calculate the working point of the dustbuster. Knowing this working point i know the required torque. Since the spring motor has a constant torque i can calculate this motor.... i hope
Maybe there is somebody who can help me with this, maybe telling me in which points i could be wrong?
thanks