2003 Prius Aux Battery - Any way to adapt?

Read through this recent thread: Hello -- an Introduction | PriusChat

You DO NOT need to spend $350 for a 12V battery! There are several options, as discussed in the thread link. It involves a bit of DIY work, such as, e.g., conversion of the OEM battery clamps to SAE (the usual domestic size), and modification of the hold-down. If you're willing and able to do some work yourself, you can save a lot of $$. I recommend you look at a U1R lawn tractor battery as a replacement. Remember, the aux. battery does not have to turn a starter motor in our cars; it merely boots the hybrid system when the ignition key is turned, and then the HV battery starts the gas engine.

There's been a lot of back-and-forth on the forum in re: AGM batteries, vented batteries, CCA capacity, etc., etc. I prefer to keep it simple and affordable. I'm sure other members will chime in here with more opinions/advice!

 

When the goal is just to start the hybrid system, any 12V battery that is charged will do. You can start it with a battery from a screwdriver or a battery for a UPS unit.
But if you plan to drive, stop, start again, stop again, then it is better to install a good battery for a car of size 46B24.

 

I've never used an AGM battery; I figure if the car goes upside down, I'll have much more to worry about than some spilled battery acid! Nor have I ever bothered with a vented battery, since the original was replaced years ago. (Hydrogen gassing off the battery is the reason for the venting. For many years I drove a '64 VW Beetle, with the 6V battery under the rear seat!) You will get different opinions on these topics, guaranteed, but I can only share my experience as an owner for 23 years now.

I would buy a battery with more amp-hour capacity than the $30 one from Wallyworld, if starting and stopping is a concern. In that regard, I'm very satisfied with the Duracell U1R that I linked in the post cited above.

The 46B24 battery mentioned above will be a very tight fit in the Classic, if it fits at all. Note that the poster owns a 2007, which is a different critter.

 

https://www.autozone.com/searchresult?searchText=battery%20terminals

One of our members has had success bolting the existing clamps directly to the U1R spade-type terminals, which have a convenient hole already drilled.

 

I just did a deeper dive into AGM batteries: What Is an AGM Battery? AGM vs Standard Batteries | Interstate Batteries

Read it and decide if your car needs one. I say no. Remember, you aren't stopping and re-starting with a starter motor every time your Prius gas engine stops; as long as the car is in Ready mode, the hybrid system is already engaged and the HV battery starts the ICE (internal combustion engine, gas engine).

 

Just realized you have a 2009 model. My comments apply only to Classic (2001-2-3) models. Sorry!