A supercapacitor for Xmas to jump start a 2009 prius?

A Prius doesn't have an alternator. The transmission does contain two large motor/generators, each capable of generating power (though one can only do so when the car is moving, and the other can at any time).

But neither of those needs to take over immediately when the car goes READY. There is a second battery in the back of the car, the traction battery, a large 201.6 volt affair, which is isolated from the car by a set of relays.

When you provide enough jumper power for the car's ECUs to complete their safety checklists and make the car READY, those relays close, the traction battery is put online, and immediately it is supplying the car's 12 volt power through the DC/DC converter under the hood.

The demand from a jump pack to start a Prius tends to peak around 40 to 60 amps and last a bit less than a second.

 

In my experience a jump pack comes in handy primarily to help other stranded motorists while avoiding the temptation to allow them to jump from my Prius. Which is a known disaster for the inaccessible 125a fuse if the small Prius battery experiences a reversed polarity.

Something to keep in mind is a supercap jump starter needs to pull all of its energy out of an already discharged Prius battery. This takes many minutes on a 10 volt or less battery. At that point, the Prius 12v battery has less voltage and capacity than immediately before you connected the supercap box. Plus, to take a low voltage and build it to a higher voltage, the supercap power electronics uses dc to dc conversion, inherently losing some energy to heat. You end up with less total capacity than before but with the possibility of discharging it for a very short time at a higher voltage.

Toyota recommends hooking up a jump source for five minutes prior to use, primarily to boost the Prius battery voltage up, eg a fast charge of sorts. Sometimes this is not 100% necessary - however the discharged battery can be so discharged five minutes is required to have a combined output sufficient to Ready the car. A healthy lead acid or lithium jump source can perform this function, a supercap can be dead in a minute or less. Plus a lithium jump start adds capacity rather than just shifting it from one source to another.

So the only real advantage of a supercap jump is you can forget about it and probably suck enough life out of your 12v battery if your battery is just on the edge of working. I prefer a lithium pack accessible in the passenger compartment since the hatch won't open when you need it. Plus allow it a fresh charge once a year based on a calendar reminder.

 

I can assure you Central Texas heat is no problem for well designed lithium batteries. The issue is overcharging lithium batteries which is mitigated with adequate design. I keep a Noco jump starter in the car for the last 125,000 of its 250,000 miles. I rarely see more than 120f in the car but I do have decent tint and park in the garage when at home. The charge lasts at least 18 months. I have only had to charge it three times. It started conventional cars twice and charged my phone on many occasions. The phone does not discharge it enough to warrant charging the jump pack.

 

I can assure you Central Texas heat is no problem for well designed lithium batteries. The issue is overcharging lithium which is mitigated with adequate design. I have kept a Noco jump starter in the car for the last 125,000 of its 250,000 miles. I rarely see more than 120f in the car but I do have decent tint and park in the garage when at home. I have only charged it three times. It has started conventional cars twice and revived my phone many times.

Before I bought the Noco I did have a tow truck driver jump it once. Did not work as the inverter had taken a dive stranding us. Another time prior to the Noco the hatch light discharged the battery overnight. I jumped it up front with a used 5 amp-hour alarm battery. After I bought the Noco, the brake booster failed, oil consumption rose, the egr clogged, the intake filled with oil, the timing chain cover leaked, the rear wheel bearings howled and the hybrid battery got weak. None of which required a jump. So having a jump pack will rarely insulate you from issues.