So now what?

Can't you just pick out another battery that physically fits into the battery hole? Does it even matter for the Prius what battery you get? It doesn't really even start the engine.

If you want to go with another Optima to be sure, then I assume the only problem would be the layout of the terminals (+/- switched sides), right? I'm sure it would be rather trivial to make an "extension cord" using a cheap Chinese battery terminal, some 8 gauge wire, and a lead rod (or a copper/brass bolt). Ground should be easy since it's just a wire that's bolted to the bare metal in the trunk.

 

Amazon has the D51R which is the right size and configuration for Gen 1 Prius's

or

I have this in my 02. Works fine! Sealed Lead Acid Battery 12 V 33 AMP VRLA AGM SLA (12V) (33AH) | eBay

or

a motorcycle battery that will fit in the battery pit.

or this Duracell Automotive Battery - Group Size 51R - Sam's Club

this: Exide Extreme 51R Auto Battery-51RX - The Home Depot

I probably could keep on listing sources of 51r batteries

 

What does the dealership offer? Between that and other aftermarket offerings, there's gotta be some solutions.

 

Crazy as it may sound, in a pinch, I used a 12v garden tractor battery with no issues.

 

I use the 35 Ah AGM battery from Harbor Freight. (Thunderbolt Magnum Solar - item 68680)
With a 20% off coupon it's still under $60 (assuming upto 7% tax).

The top contacts are different and reversed, so you'll have to buy two new wires to connect the battery, and connect it to the positive contact block.

 

Or you could find a similar type of battery with the terminals on the correct side and M6 threaded terminals. You can then get JIS (Pencil Post) to M6 converters. Tons of 12 V battery options with M6 threaded terminal, even put 55 mAh too.

 

I use a lot of HF tools, tomorrow I'm gonna use my 25% off coupon to get a clamp multimeter to check amperage draw in 120 v wiring. So I'm not one of those who complains about the tool quality at Harbor Freight.

That said, my experience with batteries (mostly ni cad & lithium power tools) has been consistently bad. I don't think I'd chance a cheap battery in my Prius, at least not from HF. I figger it would leave me by the side of the road There are lots of different batteries available, I'd try a tractor battery before I'd try a solar battery, road use is a fairly rough life--every pothole, expansion joint, and speed bump transmits a physical punch to a battery.

Coupons: Harbor Freight Tools Email

I'll delete after the coupons expire.