There has been a lot of speculation about the current drain on the auxiliary 12 volt lead acid battery in the Prius during "start". Some have said that there is just a small current required to "boot" the Prius computers. I have access to high speed data acquisition equipment thanks to my job at an automotive OEM electrical systems supplier.
I used an instrumentation shunt in series with the 12 volt battery to measure the auxiliary battery current and the Prius built-in Hall effect sensor to measure the main battery pack current (Graham Davies established that 1 amp = 0.027 volts from the sensor). My acquisition system sampled each signal 30,000 times per second and logged a data sample for each channel 2,000 times per second. This technique provides excellent filtering and good transient response.
You can see in the attached PDF file two charts showing the auxiliary 12 volt battery current and voltage as well as the high voltage battery current drain during two "starts" of the Prius. One vehicle start is with the climate control set to automatic and the other with the control set to off. You can see that the drain on the 12 volt battery under either condition is a lot more than just a few amps. Time zero on the graphs is when the ignition key is turned to the "start" position. There are several transient peak discharges from the auxiliary battery of more than 35 amps with one lasting several tenths of a second beyond 20 amps. The battery drain during start can be minimized by turning the climate control to the "off" position. This stops the HVAC fan from coming on during the "boot-up" process.
Also evident in the data is the drain on the high voltage battery pack as MG1 acts as a motor to spin the engine up to speed. At about 1.7 seconds after key-on, the DC to DC converter begins to charge the 12 volt battery, which is why the terminal voltage on the lead-acid battery suddenly jumps up. The 12 volt vehicle system voltage has a lot of AC transient noise that is generated by all the 12 volt electrical loads which the 12 volt lead-acid battery does not filter out. It is very easy to spot the current draw of the ignition system once the engine spins up to speed. When the system voltage is analyzed at 2 millisecond intervals, the voltage spikes produced by the ignition coil current are easily recognized. I labeled the graph to indicate when the engine spark plugs begin to fire. This plug fire allows the calculation of engine rotation speed.
I have added a 7.5 amp hour 12 volt gas recombinant sealed lead acid battery that is mounted next to the spare tire in the trunk. I charge this battery thru a diode so that it will not discharge into the vehicle electrical system. Via a simple connector change, I can substitute it for the Prius auxiliary battery should the Prius battery every go dead. If I place a 20 amp fuse in series with this 7.2 amp-hour back-up battery, I can blow the fuse on every start if the climate control is set to "auto". If the climate control is set to "off", the fuse will not blow.