Question from a New 2021 Prius Prime Owner

I tried to find a page in the owner's manual that mentioned or recommended that I remove the charging cord once the traction battery has stopped charging so I wouldn't discharge the 12V battery. I typically plug the car in when I return from errands in the afternoon and I may unplug it later that evening when it is charged, or I may leave it for a day or two until the next time I go to the garage. I realize there will always be some parasitic draw on that battery, so if I am gone more than a month I use a battery tender. But why would the charging cable draw current from the 12V battery when not charging?

 

When the charging cable is plugged in, the computer inside of the car always tries to communicate with the battery, inverrter, EVSE, charge port door, charge cable lock, etc. I don't think it is a constant draw, but the periodical check still uses some current. I have checked the resting voltage on the 12v battery with or without the charging cable attached after the traction battery charging has completed. Keeping the charging cable plugged caused ~0.1v drop in the resting voltage of the 12v battery in 24 hour period after completion of charging traction battery compared to removing the charging cord immediately after the completion.

 

Thanks, that was the information that I was looking for. With a tenth of a volt drop in 24 hours, it tells me that it does cause a voltage drop, but I can confidently unplug it the next morning rather than worrying about doing it after exactly five hours and ten minutes from plugging it in.

 

For long-term storage (longer than a few weeks), keep the traction battery as low as it can get (depleting the EV range) no charging cable and either remove the 12v battery or connect it to the battery tender would be the best.

 

The choice of roof rack is up to you. The factory rack is OK. Yakima or Thule racks can be wider if that's what you need.

About your new Prime...the 12v battery does not start the engine--the engine has neither starter nor alternator. The 12v battery powers the electronics that let the system start for either electric or gasoline operation. The 12v battery is recharged from the traction battery under certain conditions. For long storage (have we agreed on how long is long?) a 12v automatic battery maintainer is a very good idea. Battery Tender® is a trademark brand that is one of the good ones; there are others. A nearby 120 volt household-type receptacle is needed to power the battery maintainer.

 

As others mentioned, leaving the battery at full charge can reduce longevity, so the ideal process would be to charge the battery up just before driving. Not always practical of course, and charging early sometimes is not a problem, just try to avoid it when possible. If the car is going to be parked for several days, best to leave the charge at ~50% or less.

This has been a consideration for us during COVID, since my wife and I have been working from home and barely leave the house. So we only charge our cars (PiP & Leaf) a couple times a week, just before driving, when possible.

Incidentally, pre-COVID when we commuted daily, our routine was to charge the PiP with the L2 at 7pm (energy prices drop), then before turning in for the night I would swap and plug in the Leaf overnight. My 2014 PiP with 183k KM (~114k mi) now gets about 15-16km on a charge, down from about 17-18, so about a 10-15% reduction in capacity. The Leaf is a 2016 with 65k KM, and we lost the first capacity bar about 2-3 years ago, expecting another to drop off soon, but it does have slowed since shifting our charge behaviour. Anyway, that’s my anecdote.

As a side note, wouldn’t it be nice if the Prime models had a “vacation mode” feature where you could just leave the car (with or without the HVSE plugged in) and specify the return date while you are away and the computer would automatically maintain the battery at the most ideal SOC for longevity? Seems to me all the tools are there to automatically maintain the battery per ideal health (start up / charge / discharge automatically a small amount for both HV and 12v), the car would just need to know when it is necessary to do this. You listening Toyota?? Where is that suggestion box....

 

It's strange that the manual doesn't mention that batteries degrade faster in a discharged state like they do in a fully charged state. Ideal storage for the lithium chemistry is around the nominal voltage, which is around 3.7V per cell (351.5V for the traction battery). Does anyone know what SOC that corresponds to on the dash? In absolute terms it should be about 30%, but the PP's 0-100% range falls between something like 14% - 85%.