Portable 12V charging system for $60.

I tried this, and it works great. If you live in an area (apartment, sketchy neighborhood) where it is impractical to connect a 110v float charger to your 12V battery, then this is for you. This is an alternative to the solar float chargers, which, to me, score low on cost/benefit.

For $30, you can buy a little charger, a Supermate DC-6, and for another $30 or so, a portable jump pack at an autoparts store. That's what I spent on mine at Pep Boys anyway. Everyone ought to have a portable jump pack, especially if you have a Prius. They are incredibly handy, and the one I have has reverse polarity protection. No chances of blowing up the inverter here.

The Supermate will act as a step-up regulator, charging the car's 12V to 14.7V at up to 3.5 amps, while draining the jump pack. The Supermate can be configured to deliver a set amount of charge in amp-hours, a set period of time, and a set low voltage cutoff on the supply. It will not over-charge the battery, as it uses a constant current/constant voltage algorithm. As long as you do not exceed the low voltage cutoff, you will know exactly how much charge was delivered to your 12V. This is the sort of thing that you might need to do once a month for the infrequently driven Prius, setting up your charger to put in 5 amp hours or so.



My spare battery was already near full charge (12.57V before charging) so the charger immediately went to the constant voltage portion of the charging algorithm. The 00023 number is the amount of charge delivered so far in milli-amp-hours.

 

These chargers are geared towards the remote-controlled vehicle market, so perhaps they have their lead acid algo with sealed lead acid/AGM batteries in mind. Also, don't forget that peak current, time duration, and amp-hours delivered are all user programmable. For those living in apartments etc., I just can't really see a better way to take care of your Prius 12V other than pulling it out of the car.