How do I charge aux battery in 2002 Prius?

Same way as you connect a jump starter will do. Do be careful with the positive and negative connections, don't reverse them. Connect positive first, then negative; disconnect in reverse order. Not sure skimming the first gen manual what the recommended max charging amperage is, but second gen says 3 amps. I would look for a charger around that range, not much higher.

With our 3rd gen Toyota recommends to disconnect the battery completely before charging, and in the first gen Owners Manual I see recommendation to disconnect just negative cable. I do no disconnection, and have our (mostly idle) car on a smart charger CTEK 4.3 pretty much constantly, never any problems.

 

2002 Prius owner here. I feel your pain. But not to worry. The 12V battery is located in the trunk, driver's side, next to the left wheel well. Remove the fiber cover by prying off the three round-headed plugs holding the fiber cover in place. With a 10mm (IIRC) socket wrench, remove the bolts holding the protective plastic cover; lift off the cover. With the battery charger unplugged, clamp the red lead to the red (+) terminal (pry off the red protective cover; it doesn't have to come all the way off, just tilt it back enough to clear the terminal). Clamp the ground (black) lead to the trunk lid latch (NOT to the battery negative post). Make sure the car is OFF (i.e., no key in ignition). Plug in the charger to a convenient outlet and let it do its job. Overnight is optimum. With battery charged and car running, drive to the nearest Horror Fright or equivalent and purchase a trickle charger; attach to car battery and leave it in place and connected while the car is not in use. You should be good to go. Or, you could drive the car more! It's such a fun car to drive. I've had 21 years of good times in mine!

 

Are you actually checking the battery voltage with a meter?

If it gets started and the battery recharges, but won't hold the charge, then it's likely that you need to replace it. If in doubt, find a locally owned, independent shop that specializes in batteries. (I have a couple of those near me). Have them test it.

You could also start with putting a full charge on it with a charger - hopefully a "smart" one that has a mode for AGM batteries. In that case, refer to the above advice.

 

Even a smart charger without an AGM setting will work. I think AGM variation is slightly higher charging voltage; at least that’s the case with mine. But regular smart charging is ok, maybe just not as efficient.

 

When it happened to me I had to replace the battery. I would take to where you bought it and check to make sure it is not a bad battery. Make sure there is nothing draining your battery. One time I tried that Progressive plug in to check your driving. It killed my battery within 48 hours. Anything plugged into your lighter like GPS will also drain it.

 

rvgrandma wrote: "Anything plugged into your lighter like GPS will also drain it."

This should not happen unless you have "hot-wired" the accessory socket ("lighter") to be always live, as I did years ago with my '02, using a jumper kit from coastaletech.com (no longer available). Otherwise, the socket is dead unless the ignition key is turned to "accessory" position. One handy thing about having the socket always live is that I can connect a 12V power source to it to boost the auxiliary 12V if it runs down to the point where it won't boot the hybrid system.