2018 Prius C 12V Battery

I am not sure what kind of auxiliary battery a 2018 has. I have a 2012. It has an AGM battery. It has been parked since May of last year. For every 3 weeks, I just turn on the car for 20 - 30 minutes. The original battery still works fine.

The capacity of the auxiliary battery on the 2012 is pretty small, 35 AH. So it is probably a good idea to keep the car on when you use the battery to do other things.

If you don't run the car much, it is a good idea to keep the battery charged. From what I have read, it is better to use a smart charger than using a trickle charger. AGM batteries are more sensitive to over charging than flooded lead acid batteries.

I would expect the new battery be factory charged. It should be ready to use when arrived. But you may still want to get it fully charged since either the factory may have not fully charged it, or it may have lost some charge on shelf or during shipping. SOC maybe is not near 100%.

Besides the auxiliary battery, pay attention to the hybrid battery. If its SOC gets too low, you may have a much bigger problem.

 

Yep it's a vented AGM. Not much reserve capacity. We've never had a problem with ours despite a similar sharp drop in use- but my tire inflator runs on detachable bricks it shares with the lawnmower. I had it in my head that I'd be (preventatively) replacing our 12v in the spring or summer of 2022. Were I to severely discharge it during current cold weather I'd replace it straight off.

The ones sold by Toyota are fully prepped, load-ready. Can't speak for any others.

Sounds like you've got both short term and long term plans set to go, just waiting for the mailman.

 

That's a pretty short life for an OEM12V IMHO though you don't drive much to keep it charged apparently. Running a compressor for 20 min in colder temps pushed it too low to power the computers but the tender will recover it from a discharge like that. It's getting time for most 2015s to replace theirs. I will be this summer.

Driving so little I'd be more worried about the traction battery. The Prius is not meant to be driven little. If you know you won't be driving for a few weeks then make sure you're at least half on the gauge when you power down. A newer one like yours should have no trouble holding charge. If you leave it set for weeks on its normal low charge state (i.e., just before engine would kick in to charge it) it is possible of not having enough power to start the engine, especially if it is cold.

 

You should NOT count on using the original battery for actual driving again......ever. It likely will leave you stranded.

Your new battery MUST be fully charged before being used. That can be done in or out of the car.
Your BT Jr. should handle that fine but it might take a LONG time.......like 12 hours.

The "pig tail" for the charger can be left attached as long as it won't get pinched or otherwise shorted out.
If the vehicle is parked where there is power available, just leave the tender connected when not in use.
That is how it is made to be used.

AND, I just have to ask: Why in the world would you have a battery shipped to you instead of getting one at a local shop ?
There are several potential problems with "mail order", including what happens if your new battery ends up being bad ??

 

For the battery's sake, I don't think there is a significant difference between your charging choices. If you choose to do it by leaving the car in READY mode, I'd wager that you would hit your 12v state-of-charge goal sooner than if you hooked up the tender.

I don't think you'll see any difference between the car sitting in READY mode in the driveway and actually being driven around. The 12V charging system is not dependent on engine RPM.

If I understand you correctly, you want to install the terminals permanently on the battery and then periodically connect the tender by way of a quick-disconnect connector further up that wire?

If so, the only problem I can see is an electrical safety hazard on the + wire itself- it should have a fuse in the line as close to the battery terminal as possible. Even though you aren't planning on using the cable in transit, you don't want to risk fire from that hot wire being pinched up against a (grounded) chassis part in a crash.

So if there isn't an inline fuse close to the battery end, add one and you should be set.

 

I'd love to recommend the $20 analog cen-tech model from harbor freight because it's cheap, reliable, and easy to use. Unfortunately it isn't very accurate when used on a battery as small as what's in the Prius- the default load it presents is a bit too big, and the meter scale isn't well calibrated for the lower range.

AGM batteries have a very low internal resistance that can lead to false results with digital battery analyzers unless they have a compensation applied. Look for a tester that specifically says it can do AGM.

I imagine you would do best with a motorcycle battery analyzer because they're going to be in the same capacity range- I just don't have any direct experience with any.

 

A voltmeter really is good enough if you use it right.
IF it is sitting unused and totally OFF, and you see the voltage continues to slowly drop over a short period like maybe 7 days until it gets down to 12 or less........then the battery has failed. And likely won't "start" the car again.

There is little else that you need to know.
"Watching" it with a different kind of tester accomplishes little to nothing useful.

 

A fun and easy way to monitor your 12v system voltage: use a "cigarette lighter LED voltmeter" Amazon has them for $10-15

 

With most modern cars, that only allows you to easily monitor the CHARGING voltage, since that power socket is disconnected when the car of OFF. In that case, it tells you pretty much nothing about the battery itself.

 

True.

But... the Accesory mode energizes the power outlet w/o the charging voltage from the DC/DC inverter.


From the owners manual:
...The “POWER” switch is in ACCESSORY or ON mode. ■ To prevent 12-volt battery discharge Do not use the power outlet longer than necessary when the hybrid system is off.

 

Of course there IS an aftermarket replacement; several probably depending on your mechanical skills and ingenuity in making it "fit".
There might not be an "exact fit" but it doesn't need to be exact.

You don't HAVE to have a vent if it is a sealed AGM and the capacity ratings won't be a problem as long as they are close.
They CAN'T be that much more if the physical size is close.