Power, gears and hills

Now we are rolling along

With CV joints behaving as they should, the real testing is starting.

After dozens of runs up and down the hill outside our house, my confidence is building in Jane.  Playing it safe, I always start out heading uphill first so that if a problem occurs, gravity will get me home.

At this point the three Lead-Acid batteries easily go 2 miles.  Instrumenting the motor is a going to be key. So far there are two gauges: Motor (or Phase) current and Battery Current.   The relationship between these is dependent on motor speed and load, which has to be estimated based on the speedometer and the current gear.

I have found that there is not a single level road nearby.  This actually creates problems since any incline or decline in the street impacts the power needed to move the car.

So far, the motor current (varies dramatically based on controller settings) peaks around 420 amps (60% of the controller rating of 700A).  The Motor current meter is suspect since is jumps to 20% under the lightest load which is equivalent to 140A (20% of the controller's max amp rating).  It seems like 140A should be delivering quite a bit a power.

Battery current has only peaked at 300A which is great to see given the target battery pack is rated at 600A so this will be well below the limit, extending the overall lifetime of the batteries.

3rd Gear looks to be the best in-city gear.  During yesterday's testing, it cruised comfortably at 20 mph with the 36V battery pack.  The target pack is going to be 84V and speed scales linearly with volts, so 3rd gear should yield >40 mph.

It is a real challenge to understand how much power is actually being used.  The battery volts and amps can be multiplied to compute Watts going into the system.  The motor and controller have a combined efficiency of about 75% (conservative) so that should yield power out of the motor.  Given all of the losses in the drive train and air resistance, the predicted speed based on power should be close.  Here is one example of the power model.   From this at 30 kmh (about 19 mph) the motor should be delivering 2500 W (3.35 HP 745 Watts in a HP).  This should equate to about 3,300 W from the battery (@75% system efficiency).  At 34V, I would expect about 100A of battery current.  This comes close to what the meter reads.

Speed (km/h)	10	20	30	40	50	60
(m/s)	2.8	5.6	8.3	11.1	13.9	16.7
ForceOfAirResistance (N)	2.6	10.6	23.8	42.2	66.0	95.0
ForceOfIncline (N)	127	127	127	127	127	127
ForceOfRollingResistance (N)	115	115	115	115	115	115
TotalDrag (N)	245	253	266	284	308	337
Power to Maintain Speed (W)	756	1560	2462	3511	4755	6244

I would welcome feedback on the calculations and logic.  Next is to figure out total range based on the Watt-Hour capacity of the batteries.  The Kelly Controller has a CAN bus available and will report some useful information such as motor phase current and voltage, motor and controller temperatures, motor RPMs and battery voltage.  The current plan is to program an Arduino controller to read this data along with other system data (battery status, battery current and cell voltage).  The Arduino will act as the car's computer and display for the driver key performance data.  A bit of modern technology for a 40 year old car.