How to Maintain 12V Battery?

Hello Everyone,

As I mentioned in my introduction, I've been the proud owner of a 2013 Prius C Three since the end of July 2019. At that time, I drove the car to/from work everyday, and some weekends would make longer trips to visit family (~80 miles each way). So the State Of Charge (SOC) of the 12V Battery never crossed my mind.

In fact, I never even disconnected the battery or checked its voltage after buying the car, worried about losing security system programming and re-programming my keys at the dealer for an expensive price. As far as I know, this is the original battery (will take pictures later and post to confirm).

However, the pandemic has changed everything, and now I probably drive to/from work once every week or two, and manually maintaining the 12V Battery's SOC is essential. I visit family maybe once a month, and last weekend I went down, I didn't even take my Prius C.

I am using a (750mA max) battery tender to charge/maintain the battery, but I'm a little unsure of the best way to set things up.

The main question is "should I disconnect the black (negative chassis ground) terminal before I connect the charger, and why?" The Prius C owners' manual, clearly states in §4.3 (Do-it-yourself maintenance):



In contrast, the battery tender manual, states:



So Deltran wants you to leave the negative terminal connected to the battery, and connect the negative charger clip to the chassis away from the battery.

After recent terrible experience getting another vehicle in our family (a 1998 Plymouth Grand Voyager) to pass emissions testing, I have learned that I should do everything in my power to avoid disconnecting the 12V battery in a modern vehicle for any significant length of time. We had to waste a lot of time/fuel trying to get all the I/M Readiness tests complete before we could have it tested, after I had let its 12V battery get too low and the ECM lost all its memory.

Coming back to the Prius C, I think the best option is to leave the negative terminal connected, and connect the charger clips directly to the battery posts. I don't see any other obvious place under the rear seat where the battery is located, that I could connect the negative charger clip. Please let me know if anyone is aware of such.

For full disclosure, last month, when I first charged the Prius C's 12V battery, I did in fact disconnect the negative terminal per OM. I lost a lot of saved information (e.g trip logs) and the date/time setting, etc. I now also fear I have lost all of the I/M readiness information, and I'd need to build that back up through regular driving. Luckily, I am not due for an emissions inspection until next year, but who knows how much I will be driving until then.

Sorry for the wall of text, but I appreciate anyone taking the time to read, and offering any advice!

 

Sorry, I had intended to link to the charger in my original post while editing, but forgot to .
Here it is: Deltran Battery Tender Junior 021-0123. (I guess I don't have enough posts yet to provide the link, but you can find it on Google easily).

Mine is a slightly older model than the one pictured, but the features are the same.

Yeah, I'll keep that in mind. The previous owner was driving the car fairly heavily (long highway commute daily), so I believe the battery was kept charged by the Prius's inverter system automatically, with no manual intervention needed. For now I just want to keep it alive to avoid losing any critical information (like I/M readiness and smart key/security system programming). And in decent enough condition to be able to drive the car around when needed.

 

The I/M readiness monitors are the self-check routines for the various systems that control emissions (e.g. evap, O2 sensor, cat converter, etc.). They require certain specific driving conditions to complete, and if they are incomplete, you will fail the CA SMOG test, even if the engine is otherwise in perfect condition. We recently went through a lot of grief because of this very reason on another vehicle (1998 Plymouth Grand Voyager), which I had let the battery get too low (could no longer start the engine). I hope I haven't already lost this information when I disconnected the Prius's battery, but I don't have my Dad's OBD-II reader with me at the moment to confirm...

 

Thanks Sam... I know it sounds like I'm making a big deal out of it, but it is just a lesson learned for me given the recent experience. Anyway, I currently have the battery tender connected to the battery posts, with the ground cable connected to the battery. So far, no issues.

I'm seeing an "idle load" voltage (i.e. battery tender disconnected overnight, power switch off, security system disarmed) of about 12.4V. I didn't want to disconnect the battery to measure the passive current draw. With the battery tender connected (and in maintenance mode i.e. solid green light) I see around 13.1V.

I will not disconnect the negative terminal anymore, until I have to replace the battery.