Let's try replacing the 12v battery with a much smaller, lighter one

There are (cheaper) aftermarket equivalent batteries. Sometimes the cases even look identical; makes you wonder if it’s the same battery, just different badge. Or at least adhering to identical spec’s.

Up here, for 3rd gen at least, Canadian Tire has one that’s completely compatible and significantly cheaper. Still sand-pounding expensive compared to just a few years back though; all the more reason to comparison shop.

 

Ha! No, I am still functioning with an unchanged but failing 12v aux battery. Due to the desulfator, it may be improving, or it may be just continuing to survive. Overnight voltage around freezing is about 11.7-12.1v cold when the start button is pressed to orange before I engage the HV battery to charge the aux to 13.8.

I've so far gone to ogle the 35 AH U1 battery (tiny!) and I've ordered a U1 to SAE adapter/converter from the Bay of Es. Down here in the lower 48, Harborfreight sells them now for about $72 non-Loonies minus a 20 or 30% coupon, usually obtainable. My Gen II MPG seems pretty low, despite pumping the tires up, the replaced HV battery, fluids changed, so I'm looking at the new battery as my personal savior - along with an eventual new PCV, cleaned spark plugs and valve adjustment in the warmer weather -- March...

I took my HV battery apart when I bought the car (for $100) and ultimately had to replace 4 modules and recondition/balance the rest. That fixed pack got a good grade from Dr Prius, so I hope it will last me at least several years. If not, I wrote the Wiki partially to remind myself how to take it apart again. Reconditioning and balancing takes a long time, but removal-replace and balance only takes about 4 hours, especially if you've done it before, I guess.

 

If you have to have two batteries, you double the weight, and only get 1ah more then the battery that should be in it.
Seems like a waste of time and effort.... And never knowing when it will fail.
Jump packs are small and $20, more than enough for the Prius.

 

Concur with all of that, except for the word 'vacuum': it's a pressure pump for brake fluid, as no vacuum is used in this brake system.

 

I did not read it all, but as I recall, the 12V battery is also used to provide the brakes, steering, lights, etc in an emergency. The way that AGM goes from usable to dead (in my UPS use) makes them less desirable to me.

Dan

 

Thanks for following up on your research into small 12v... That booster pump is something we didn't consider in our previous discussion. And though it would probably be fine for it not to work until the car is in ready mode, it's probably bad to be sending a weak voltage signal to the booster pump for an extended period of time.

 

Thanks, folks, for the discussion above.

I just wanted to update you on my replacement for the 2012 OEM 45AH Toyota battery in my 2004 SKS Prius. In the end, I bought a Mighty Max 35 AH battery ( the capacity of the non SKS cars' OEM batteries in Gen II I believe). This was a $65ish purchase. I was careful to get a battery with left side positive terminals, but there is the issue of the car's Japanese domestic market style terminals.

I got around the battery/terminal incompatibility by first using an adapter to go from the Might Max battery U1 tab terminals to SAE ($10 on Ebay) and then I bought a replacement bolt-in SAE connector for the car-side connector (hard to find, I used Installation Kit for Optima battery in Prius). The result is that the car now has a female SAE connection and the small 35AH U1 battery has a male SAE connector.

There was much gnashing of teeth over the lack of a vent on the new battery, but other commenters on PC have said long term standard usage of similar unvented AGM batteries didn't result in any bad situations. Someday, I may carefully drill a hole in the top cap area of the new battery to allow the batteries top 6 port holes (sealed at the factory) to vent to the prius rubber vent tube.

The size of a 35AH battery is smaller in all dimensions than OEM, so to safely fasten it in place, you'll need to insert something non-metallic to hold the battery securely under the battery hold down.

So, long story short, how has it been with a 35AH AGM battery instead of the 45 AH OEM battery? Answer: It's been good to great. I've been away from the car for two weeks and found the battery could still start the car (and wasn't down to a 'shockingly' (pun) low voltage). The car's MPG went up after replacing the tired sulfated OEM battery, so I'm very glad I did this cheap replacement.

Epilogue: I didn't need to hand over my OEM tired battery, so I hooked it up to my trickle charger/desulfator (a 90s Battery Minder Plus). After a couple of months of trickle/desulfation, the sad battery now reads 80% healthy; 'good' battery on my battery load tester, a TopDon AB101 model. 45AH AGM batteries are awesome. I'm thinking what to do with my now extra, mostly healthy old 2012 battery...

One thing I won't be doing is putting the OEM battery back in the car; I'm quite happy with the 35AH substitution.

 

Good for you for buying a 35amp-hour for $65 that most people on here pay $300 for because auto parts stores and stealerships don't price 12v reasonably.

Though I must say, I'm sticking with my 55amp-hour 12v I bought for a $100 on my drive home from christmas in 2012. That 12v has yet to fail me!

 

10 years on a lead-acid battery is very laudable. Congratulations! I have only achieved that long-term ownership when cars have desulfating technology (prius?) or when I have the battery attached 24/7 to a very gentle trickle, like a small solar panel.

Comicly, my 35AH battery HAS failed me since installation. Even though I charged it up to 100%ish, a few weeks after new receipt with initial full charge up, I foolishly left the car in "on but not READY" mode and worked on the speakers for a few hours. Just leaving it like that half a day killed (down to 7v under load!) the battery. I immediately noticed it (at the end of my 5 hours work!) and charged the battery up to full capacity and it's been fine. I love AGM batteries. Not happy with being run down, but a little less angry than standard wet/plate lead acid!