Hi Leon. I am still confused, sorry
If there battery is at 11.65, how do you get a reading of 6.7V at 55rpm?
For the open circuit test, the voltage is usually quite linear with respect to rpm.
So maybe I wasn't clear before (or maybe I don't follow your data). And I just realised that since you are using a DC motor, it will try and turn as soon as you connect the battery to it (in test 2 below). So either only connect the battery once its turning, or use a diode to block current flowing into (instead of out of) the motor.
What I meant was:Test 1 (open circuit):
Connect a voltmeter across the motor
Measure voltage and rpm across a range of rpm, at least up to where you get 14V or so.Test 2 (battery as load):
Connect the motor to the (flat) battery, with an ampmeter in series with one of the leads
I.e. motor negative to battery negative; motor positive to ampmeter connection 1, ampmeter connection 2 to battery positive
Connect a voltmeter across the battery (or the motor, should get same result)
i.e. voltmeter negative to battery negative, voltmeter positive to battery positive
Measure voltage, amps and rpm across a range of rpm.
For test 2, you should get 0 amps at low rpm, then at some point (cut in speed) you will start to get amps. For this test, all the voltages should be close to 11.65V (or whatever volts the battery is at).
Finally, plot V against RPM from test one, and then seperately plot V x A (ie watts) against rpm for test 2.