Once the car is"Ready" the propulsion battery recharges the 12-volt battery, but it needs several hours to do so, since high charging current fries a gel cell battery. The discharge rate is such that the 12-volt battery goes dead within two weeks if the car is not driven. Apparently Toyota assumes that the Prius is to be used for daily commuting for sufficiently long to keep the 12-volt battery charged, but we are retired and only use it for infrequent short shopping trips.
This charger provides a tapered charge with a maximum of 3 amperes, which is about all the little battery will take. We use it for one day about every other week.
I have been using a trickle-charger but it takes forever at 0.4 amp. Also I desired to make a charger with accurate volts and amps readout. This design uses a 270 deg meter which I have used long before. It was made as a tachometer for a big diesel and provides 0 - 30 divisions for a frequency input of 0-720 Hz. Conveniently it uses a 12-volt power. This input is rather inconvenient but I was able to re-use a circuit board I made years ago for it which has a voltage-to-frequency converter using a XR-4151, plus convenient op-amp signal conditioners. It had enough unused space to add a PICAXE 08M2 which controls the charge current using PWM.
I also happened to have a suitable switching laptop power supply with a 16-volt, 3-amp rating I got cheap in a thrift shop. I keep the whole assembly in a old bag for easy application. The leads are long to reach the car. Since the battery is inaccessible, the Prius has a flat plate for jump-starting under a red cover in the fuse box under the hood.
There is another small board with the power MOSFET on a heat sink. There is no resistor because of not much voltage drop. I tried using a coil and free-wheel diode but it was ineffective.
The box was a parts bin I found in an industrial trash container.