Measure what you can measure
Measurements

DC-current clamp. Click for big picture RPM-meter. Click for big picture AC current clamp for RPM meter. Click for big picture

The Toyota Prius has a battery composed of 228 cells with small capacity (6.5 Ah), but high current capability(Ik=600A).

I was curious, what currents the battery delivers in different situations. A DC-current clamp is the suitable device to get the information. It delivers 1mV/A to a digital multimeter. The clamp was put around one of the orange cables coming out of the battery box. I had no direct contact with the high voltage and nothing had to be damaged.
Another important parameter are the revolutions per minute RPM of the engine. I had a conventional car meter with an AC-clamp for the ignition cable. The Prius has no high voltage cable for the ignition, but luckily the low voltage signal to the ignition coil worked.
Then I put a thermometer with remote sensor into the tube of the battery cooling fan. But the temperature rise during the following measurements was low, only few degrees. I think the battery of the Prius is much less loaded then the one of the Honda Insight. You can see that if you compare the cooling systems. On the Insight the fans and tubes of the cooling system are bigger.

High Voltage Battery

Regenerative current @45 mph, no braking 17A 4.5 kW
Regenerative current @75 mph, no braking 20A 6 kW
Regenerative current with braking, same in N- and B-mode up to 50A 15 kW
Current in and out of the battery at constant speed +/- 2A +/- .5 kW
Battery current EV up to 30A 8 kW
Battery current creaping 2A .5 kW
Battery current accelerating up to 80A 19 kW
Calculated kinetic energy of the Prius at 60 mph 127 Wh

The electric motor with 34 kW gets about 19 kW from the battery and about 14 kW from the generator at maximum power output. I was a little bit shocked by the high currents of 80 A, that the relatively small battery had to deliver at acceleration and going uphill with full throttle. Its not a good idea, to try to load the battery even higher. We should be happy that the ECU protects the battery.

Battery charging during motor braking

speed km/h
speed mph
N-mode
B-mode
20
13
2.5 A
6 A
30
19
7 A
13 A
40
25
10 A
19 A
50
31
13 A
19 A
60
37
15 A
19 A
70
44
17 A
17 A
80
50
17 A
17 A
120
75
20 A
20 A

These measurements are not precise as the current varies. But the trend is that at low speeds the charging of the battery is higher in B-mode, at high speed the difference disappears. At high speeds in B-mode the engine brakes at high RPM. That means, it is not advisable to drive always in B-mode. The additional engine braking at high speeds reduces milage.

12 V Battery and Speedometer

Battery drain at ACC
2.3 A
Battery drain at IG
8 A
Battery drain key off
.075 A
Battery load current when fully loaded
1.3 A
Battery voltage while being loaded
14.5 V
Precision of voltmeter in diagnostic mode
add 5.5%
Precision of speedometer in diagnostic mode
~.5%
Precision of odometer
~.5%
Precision of speedometer in MD, german version
subtr 7%

EV-Testdrive

I tried to go pure electric as far as possible with battery full according to the battery symbol and with a motor current of 10 - 15 A, on an even road. The Prius drove 27 mph for 3 minuts, then the engine switched in. After this test drive the battery was reloaded for 3 minuts with about 15 A. The battery was discharged and afterwards charged by about .6 Ah, thats about 10 % of the available capacity.

Freewheeling

With the RPM-meter I could watch the "freewheeling" of the engine, which means that the engine is revolving without gas being injected. Driving downhill at 60 mph the engine showed exactly 1000 RPM, with the consumption 0 ltr/100km. At 42 mph the engine shut off. Going down with 81 mph the engine rotated even with 1900 RPM.

The reason for this is, that the generator is not allowed to spin faster than about 6000 RPM. At higher speed the torque of the generator would not allow the restart of the engine and the generator would be overloaded.

Integrator

I have built an integrator, in order to measure the charge and discharge of the battery. Here is the circuit diagram. This integrator works very good, the limit of precision is due to the drift of the Hall current clamp. The input signal comes from the Hall current probe, the output goes to a digital volt meter.

circuit diagram of integrator

The following table shows the charge and discharge at different situations.

Battery load after starting the car
.25 Ah
driving down a slope 60m height difference with braking
.14 Ah
driving up for 1 mile with max. power, starting SOC half
-.4 Ah
driving up for 1 mile with max power, starting with SOC full
-.7 Ah

After driving with max power, the battery symbol showed zero, the available power was reduced, but no turtle symbol showed up. Again its obvious: only 10% of the battery capacity is used.

Driving down the slope with 60 m, 219 Wh must be removed, 36 Wh go to the battery, i. e. 16%. That is not much, but better than nothing .

Conclusions

The 12 Volt Battery is charged if the car is in ready mode. All current neaded by the headlights, the rear window defroster, the EMPS and other electric equippment is delivered by the inverter. But in ACC mode all current is drawn from this battery of 34 AH capacity. Leave the car for 15 hours in this state and the battery is dead. The battery drain of 75 mA in the key-off mode should not be neglected. It is too high like the standby current of many modern devices. If the car is not used, the battery is empty after about 2 weeks. On the other hand, if one starts the car fast, this battery should last for a long time.

The speedometer of the German version displays 7% more than the actual speed. In the US version the speed is displayed correctly.

If the car drives with 60 mph, its kinetic energy is 127 Wh. Braking from this speed with motor only loads the battery with about 10 Wh, so only about 8% of the energy are recovered normally.

Another interesting question: how compares one "Energy star" to the energy used of gas during that 5 minuts intervall of the consumption screen? Assuming 50 mpg, 60 mph, .376 liters gas are consumed. Assuming 30% efficiency of the engine about 1000 Wh are generated by the engine with .376 liters of gas. So one energy star corresponds to 5% of the total energy used.

I think, its advisable to accelerate smoothly, as that increases the pure electric driving and battery life, reduces noise and gas consumption ...and stress.