my experience replacing my 12v auxiliary battery

In the interest of helping other Prius owners considering replacing their 12v battery as a DIY project, here's my experience. I previously replaced the OEM 12v on my 2006 vehicle in 2010 with a yellow top Optima from Elearnaid.

And yesterday, I replaced that battery with another yellow top Optima from Elearnaid. First, let me say that I'm a person who likes to do preventive maintenance. In both replacement decisions, I had not experienced any difficulty with the 12v and my car had been running fine. I performed some tests of the existing battery before replacing it and was unsure if it should be replaced. Despite having read up about when to replace, I didn't find it easy to decide since I had no clear evidence of a problem, but as I said, I believe in preventive maintenance, so I replaced the battery. In each case, the previous battery had been in use for about five years. I should add that I do mostly short trip drives.

So here's my experience of yesterday: I used my OEM battery, which I had kept, and hooked it up to the engine compartment post to keep my presets, and it worked out fine. I should say I first charged that OEM battery to make sure it had enough juice. Then I went to work on installing my newest battery, which had the American posts (same as my first yellow top), so I saved a few bucks on Elearnaid's conversion kit which I didn't need. It arrived in good shape as Elearnaid packs it very well. I tested its SOC: 85%, so I charged it up to 100% before installing it. The new battery had a sticker with "12/15" written on it, meaning it was manufacture in December 2015: good!

Elearnaid has a very useful set of detailed instructions for the procedure: unfortunately they had sent me the instructions for installation in a Camry hybrid! I emailed them and the same day I got an email back with an attachment containing the correct instructions for a Prius which I printed out and used. The instructions are good for an amateur like me and proved very helpful. The installation went reasonably well (took me about three hours, but I'm slow and not a mechanical person). I had just a few problems: first, when I put the vent tube into the new battery, I found that it wouldn't stay in because the hole it fit into was too big. I solved the problem by using good old duct tape to tape it to the battery and keep it inside the hole. Perhaps a new vent connector is needed that fits properly? But I received only the battery and no additional parts were provided so I had to use the vent from the previous installation. My second problem was my own fault: When I was attaching the hold down on the newly installed battery, I must have stripped the rod in back as I couldn't screw in the hold down nut more than a few turns. I solved that problem by first putting some large washers and a large nut over the rod and then screwing down the battery nut. It worked and the battery is now held down firmly. Finally, I found it difficult to screw in the end of the black air vent that attaches agains the Prius outer wall: tight space and must push back felt on sidewall with one hand and screw the screw with other hand. I was afraid of dropping the screw into the bottom of the area where the battery is located. I solved that by attaching the screw to my socket wrench (the manual kind, looks like a large screw driver) with a little scotch tape. It worked well and I just pulled out the socket wrench when the screw had tightened and it ripped off the scotch tape. Although it's not required, I found it helpful to remove the black box in front of the battery. I think it's called the brake control power assy. It's easy to remove, just unscrew three screws and then you have easier access to the battery.

My efforts were successful. I've driven my car several times since yesterday's installation and everything seems fine. I hope this narrative is useful to those of you thinking about replacing your 12volt battery.

 

I had left my OEM battery untouched on a shelf in my basement for about 4 or 5 years: a few days ago, with my Schumacher charger, I tested its charge while the battery was not in the vehicle and found it was about 50% charged. I charged it for, I think, about 5 hours at 2amps and got it to 100% charge. The next day, I tested it again , still not in my vehicle, and it was down to 75%. I don't know if that's normal. I wonder if it would hold a charge well enough now if I were to install it in my Prius? (I'm not curious enough to try this!).

Also, before buying the second Optima yellow top, I tested my first Optima yellow top while it was still installed. I performed the diagnostic tests that one can do using the Display screen on the dash. Here's what I got: 11.6volts under no load, 11.2 volts with "ig-on", 10.8 volts with "ig-on" and headlights on. When I started the vehicle, it showed 14.1 volts as charging function which told me it was charging properly.

 

This is a followup to my original post: I had said there that the vent I used from my first Optima battery (from 2010) did not fit in this new Optima I just installed because it was too small to stay in the hole. I was not happy with my makeshift solution of keeping it in the hole with duct tape and kept thinking about it. Then it hit me: could it be that this new battery was supposed to use the vent from my OE battery? I still had it and when I looked at it, I saw that indeed it was bigger than the one from my 2010 battery. So...leaving the new battery hooked up, I removed the red plastic cover over the positive terminal to give myself some room and then took off the black air duct over the battery (took a bit of effort and care, but I did it), and replaced the vent I had installed with the OE battery's vent: perfect fit (!)once I realized I needed to line up the OE vent's tit with the slit at the end of the vent hole and just push it in firmly.

I also decided to buy a new battery tray to replace the one I damaged (see my first post). I bought one from a California Toyota parts dealer for $12.60 plus $15 shipping (ouch), and sometime in the future, maybe in the spring or summer, I will install it or maybe I'll just wait years until the current battery goes bad and I need to replace it. For now, my battery is firmly held down and I don't feel the need to do anything. I'm tired of working on the battery...