Our Toyota Prius has a problem. We have no trouble with the big propulsion battery, which holds its charge well, but it also has a small 12-volt gel cell battery ("Auxiliary") hidden in the back. This battery discharges into circuits which remain live even when the car is turned off. If it dies, the car is completely dead, since it is needed to activate the propulsion battery. This has happened so often that we keep a motorcycle battery in the car to jump-start it.
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.
Here is the circuit description
|Here is the program description