From my experience the simple answer is Yes.
* It’s OK to replace the 12volt battery with a regular acid cell car battery till you can get a better replacement.
For troubleshooting the issue you have lots of options.
* While on summer vacation last year I encountered 12v AUX battery problems on my 2010 Prius. At the time I was new to the car and was surprised to find the car will operate without too much fuss with a low to almost dead battery. My battery was way below 12v (~8v) before it started causing problems.
From a car "start" perspective, the 12v AUX battery's main goal is to power up the controls and flip the HV Relay so the big HV battery can take over and do the heavy lifting (such as crank starting the gas engine). Once you get the "Ready" light, the AUX battery is not a big factor as the 12v power is assisted by the big HV battery.
I initially used one of those little jumper / starter boxes (basically a Barbie Car battery with jumper cables) in the trunk. When the 12v battery was too low… I did a quick jump from the jumper box to the AUX battery and was on my way in no time.
When I reached a place I could troubleshoot the battery further I found the battery was very low on water. Adding water revived the battery and saved me ~$200 and the time needed to hunt down a replacement. The stock battery has 6 rubber caps covering the fill holes once you remove the top stickers and the clear plastic cover.
* If you do have a bad Aux battery, as an alternative... to get the car up and running you can use any cheap or used battery that will fit in the battery compartment hole.
On another 2010 Prius I used a retired battery from a PT Cruiser (ProStart 26R540PS) to drive home from the east cost to Texas. While not a great fit it wasn't hard to make it work.
I bolted on $5 battery posts you can buy any auto parts store (Autozone, etc). They provide a quick (and reversible) conversion from the smaller pencil posts to the standard battery posts. Once you bolt on the standard adapters you can use any battery what fits in the battery area.
Stock battery (S34B20R): L: 9.25", W: 6.75", H: 8" / 235, 172, 222mm 25lbs 46 Ah CCA: 310
Alternate (temp) battery (26R540PS):: L: 8.5", W 6.75" H 7.75" / 208, 173, 197mm 31lbs 85 Ah CCA: 675
The temp battery did not have a vent tube but the car does have a huge vent panel in the battery compartment. I used this for a few months until I found a good deal on a replacement.
Last edited by dgbear; 05-11-2015 at 02:03 AM.