3rd Gen Prius Battery Charger - Power/Amps?

The Prius 12-volt system doesn't have any direct control over battery charging current. The DC/DC converter is rated 120 amps, and it produces a voltage sometimes closer to 13.6 V and sometimes more like 14.7.

The car chooses those different output voltages in a kind of rudimentary scheme to control the charging of the factory-supplied 12 V battery, but of course all that really happens is that whatever current flows is just whatever that battery will draw when faced with 13.6 or 14.7 volts. The same will happen with your added battery, unless you add some extra circuitry to have more control.

 

The thing to keep in mind is that Toyota had no incentive to oversize the DC/DC converter. The 120 amp rating can be pretty much fully accounted for if you are using a lot of the electrical loads in the car (say, driving down a dark cold rainy road with the headlights and wipers and defoggers on and your seat heaters, and the engine isn't warmed up yet so the electric supplemental cabin heat is also on) and turning a tight corner at low speed with the electrical power steering.

In fact, the converter has a special signal ("IDH") wired to the HVAC module for times when the converter needs to say "I'm overloaded, please 86 the supplemental heat for now." (That supplemental heat can be something like 700 watts all by itself.)

On the other hand, there are times (a nice Sunday drive in fair weather) when plenty of the 120 amps are yours for your enjoyment. So if you're adding mods that can use a lot of power, it's partly about being aware of when you're asking for that power and when you're not.

(I've now and then suggested, for people adding extra heavy loads, the idea of tapping into that IDH circuit, even just to light an LED or something so you see when the converter is saying Uncle!.