Curious about Aux. batteries

The OEM battery is a special Absorbed Glass Mat battery since it is inside the cabin with you and not under the hood like a typical car. You would not want a regular lead acid battery in the cabin with you. It can vent gases inside the cabin and could leak acid if in an accident.

There are other batteries beside the Toyota OEM battery that are available for comparison purposes but I would stay within the OEM specs and an AGM battery.

 

As long as it is a sealed car battery and you can squeeze it in, it should be OK
That space is very limited in size so I think you will need to remove holding bracket.

 

For Gen 2 and newer, you need a battery with JIS Posts, and they should be reversed compared to normal to keep the Positive terminal away from sheet metal. For safety sake you want a battery that is AGM, and externally vented, as the battery is in the cabin with you and your passengers.

The Gen 1 battery placement is in the trunk, but it is also a AGM battery with JIS posts and externally vented.
Hybrid HV & Auxillary Battery Types | Portland Auto Index

elearnaid sells a Optima Yellowtop conversion kit.
Optima battery and installation kit for Prius 2001 to 2003

 

Can we start by finding out which old battery you have? Does it say GS or Panasonic? From the factory would have been GS. If the 2003 service bulletin for inadequate battery capacity was ever applied, it'll say Panasonic. They are different sizes and have differently sized terminals. If the service bulletin was applied, the battery tray and cable ends were changed to fit the upgraded battery. Both are externally vented to keep you safe and to avoid corroding your quarter panel. I don't have personal knowledge whether they are AGM. The Panasonic label does say not to tip it.

Both batteries can still be bought from Toyota. If you still have the GS and you drive in conditions where you've never had a capacity issue, you could stay with that or something comparable. That one originally had a 28 amp-hour capacity.

The Panasonic was a significant upgrade. You can see here that as currently sold it has 51 amp-hour capacity, something like 34% better than the often-recommended yellow top.

CCA would be pretty much the last thing to worry about for a Prius battery. That's "cold cranking amps" and a Prius aux battery is never used to crank. It needs to supply enough juice to boot some small embedded computers and pull in some relays, and then the traction battery takes over. I'm pretty sure you could do this with 8 D cells if you had to.

-Chap

 

The Prius does not have a starter like a typical car so does not require the higher cca. The Prius battery is smaller than normal because of this. The 12V battery is only used to power up the computers and pull in the relays that connect the HV battery.

The engine is started by the HV battery and one of the electric motors, MG1. So no need for the starter that requires the high cca.

So you do not need the higher cca of the batteries that you are listing. It will not hurt anything to have higher cca than needed, though, just not necessary. It is just adding additional weight to the car that is not needed.

 

There are a bunch of differences between lead-acid batteries but they're all about suiting the physical properties to the intended use. A battery built for sustained, low or moderate loads will be built differently than one for bursts of intense load like engine cranking. A cranking battery (which your Prius does not need) will go for maximum cranking amps by using lots of very thin plates to get plenty of surface area for the electrochemical reaction. The thin delicate plates will c0me at the cost of lower longevity and vibration resistance. A battery that never needs to crank does not need the thin plates and can be built for the long haul. The exact alloys used in the plates, the separator materials, and so on can also all be chosen according to how the battery will be used.

If by "a grown-up battery" you mean you're looking to find something that will be a better choice for a Prius than what Toyota's engineers chose, the more you think about it the more you'll realize they get paid to not leave you much room to do that. Of the two choices Toyota offers, the 28 amp-hour seems ok for drivers in moderate climates, without many extra electrical accessories, who don't leave the car unused for long periods or make really short trips. For more demanding conditions there's the 51 amp-hour upgrade. You won't easily find something better that fits in the space. A bunch of owners have had good things to say about some aftermarket options like the Optima yellow top, which is an upgrade to the OEM 28 Ah (but not to the 51 Ah).

There's another group of owners (you can find threads on here) who aren't interested in the safety properties of the OEM-spec sealed, outside-vented batteries, but only in finding less-expensive, electrically adequate alternatives (I think I've seen wheelchair batteries mentioned). That group, in other words, isn't trying to beat the OEM options on quality or function, but on price. Your choice reflects where your priorities are.

-Chap

 

Just my 2cents my 01 classic had a regular battery 550cca put in at used car dealer at time off delivery it lasted 6 months due to no deep cycle properties. I ordered the optima yellow installed it end of problem.