What to do after jumping?

At that age, the battery may no longer hold a full charge. Could be that it's just time for a new one.

Not sure about Gen 3s. On a Gen 2, the car only is capable of a trickle charge. it will not revive a low batt

 

Phoney baloney. That post should be reported and poster’s IPs be banned for life.

 

If you feel ambitious you could carry the battery inside and leave it overnight on some kind of smart charger. You'd lose some settings. If you have a driveway you could just leave it mounted in the car while you top it up. Leaving it siting around idling for eight hours sounds like a joke or something. Did they say anything about renting it out as an AirBnB while it's idling? Five years is what I think of as a normal life expectancy for the 12 volt battery. I think your best bet is to start looking at batteries.

 

you may be fine, but if you don't want to invest in a volt meter or trickle charger, leaving it on for 8 hours will charge it as much as it will take.

at 6 years old, the safest thing to do is replace it, but without testing, there's no way to know.

 

On what grounds? If that suggestion was in a Gen 2 forum, it's completely reasonable. Gen 2 DC/DC converter output seems to stay around 13.8 volts, and at that voltage won't produce a charge rate above about 8 amps (according to my Hall clamp) even into a well and truly discharged battery. And that rate steadily drops as the battery charge comes back up. The battery in SKS-equipped Gen 2s has a 45 amp-hour rating. You have to allow something for charging inefficiency. It sounds like whoever posted that advice knows how to divide.

Now, the picture could be different for Gen 3, where the DC/DC converter output can reach 14.7 volts or so. That can drive much larger charge currents into a severely discharged battery. AHetaFan measured 60 to 70 amps in the earliest minutes of charging.

At rates like that, a Gen 3 could take substantially less than 8 hours to bring the battery state of charge back up. (Also, at rates like that, it is much more heavily stressing the battery while recharging it. The batteries typically have labels suggesting a charge limited to 5 amps or less. A Gen 2 pushing 8 for a while isn't much of an issue, but a Gen 3 pushing 14 times the recommended rate? Ouch! However, Gen 3 does add a temperature sensor above the battery, and presumably will scale back the charge rate if the heat gets excessive.)

Because of those high charge rates, if the circumstances allow, it is gentler on the battery, instead of jumping, to plug in a nice, current-limited charger and go do something else for several hours, then come back and start the car. But the usual situation is you need to get somewhere, so you jump, and you know the heavy recharge is going to shave some off the life of the battery, and shrug and drive the car.

 

I believe what @AHetaFan's measurements showed was that the DC/DC charger in the Prius is indeed a "smart" charger (which is not surprising really) that tapers off the charge as it progresses.
The theory is very well explained here: Choosing a Battery Charger for your Boat | West Marine

 

With the age, time for a new one. Five or six years old, it's time.

If you want to get on top of it for next time, get some tools:

1. Digital Mulitimeter, for reading voltage. This gives a rudimentary idea of what the battery's up to.

2. Electronic Load Tester. These have just been on the market a few years. You input your battery type (conventional flooded, AGM, and so on), Cold Cranking Amps rating (or Cranking Amp, various other units), the tester checks it and returns a verdict: Pass, Pass but Recharge, Fail. They're around $60; Solar BA5 is one.

3. Smart Charger, 3~4 amp range. These will assess the battery (may not even start if it's toast), then run a multi-step charging regiment. They may have more than one charging regimen available. Typically they can be left on indefinitely, will act as a trickle charger once done.

4. Jump Pack. When all else fails.

 

Well, as smart as it can be, with the information available to it, which is not that much.

Remember that what's usually called a "smart charger" is a single purpose device, connected directly to a battery that is not doing anything else but being charged. The smart charger in those conditions is able to precisely observe the changes in voltage and current as the battery state of charge rises.

In contrast, the Prius DC/DC converter is charging a battery and powering a car. The battery is hanging on the end of ten feet of voltage-dropping cable, while various heavy electrical loads in the car (brake pump, steering assist, heating, etc.) come and go at unpredictable times. About the only information the Gen 3 converter has about how the battery is doing comes from a temperature sensor clipped to the hold-down bracket, measuring the air temperature a couple inches above the battery.

Gen 1 and Gen 2 had an independent, remote voltage sense circuit for the battery, a separate, higher-impedance circuit run back to the battery to determine its actual voltage, free of the voltage drop present on the power cable. If you had a rough figure for the resistance of that whole run of cable, you could even use the difference between that voltage reading and the converter's own local one to make a very rough estimate of the charging current, though I don't know that Gen 1 or 2 ever did that, as opposed to just using the sense line to maintain a steady charge voltage at the battery.

In Gen 3, apparently Toyota decided the remote V sense wasn't providing information important enough to bother with; they raised the charge voltage by a volt and added the temperature sensor. Amusingly enough, the DC/DC converter still has the remote voltage sense input, which in Gen 3 is just nutted down to a bus bar in the engine compartment fuse box, measuring the voltage about eight inches "remote" from the converter itself.

For Gen 4, they went all in on a voltage-and-current-measuring, coulomb-counting battery monitor module mounted right on the negative battery post. But Gen 4 is the first to have that.