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Prius Auxiliary 12 Volt Battery Drain at Boot-Up and Engine

13600 Views 7 Replies 7 Participants Last post by  Dick Larimore
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.
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reply to voltaic...

the setup you are referring to is called "series" propulsion.. where the engine charges a battery pack and the motor powers the wheels... currently the setup of all Hybrids on the road is "parallel" propulsion where both engine and motor provide power to ground... engine also has responsibility to recharge battery pack...

reasoning = energy conversion is inefficient... I'll use simple numbers, but the logic is the point... let's say there are 100 units of energy in a gallon of gasoline, you run the engine which is inefficient you get about 30 units of energy converted to kinetic (motion)... in a standard car this power is placed straight to the ground... in a series hybrid you would charge the battery pack or directly power the motor... which means another conversion... let's say that kinetic to electrical conversion is 75% efficient, now you are down to 22 or so units of the original 100... charging a battery pack is more like 50% efficient so now we would only get 15 units of energy into the battery pack... the electric motor will be fairly efficient (compared to gas) but let's say it is 80%... so we end up with 12 units out of 100 making it from gasoline to ground through the motor, the rest ends up as heat...

next logical question is why go hybrid??
reason = the above info is for a standard engine, but some series hybrids use turbines, which are EXTREMELY efficient within a narrow RPM range, so instead of 30 units, you might get 60 units... for parallel hybrids, some power goes directly to ground, some goes to recharge, the computer maintains the engine's efficiency by charging at opportune times and taking advantage of regenerative braking
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