How occasionally monitor 12v battery strength?

the best way to monitor battery health is a free load test at an auto parts store, anything short of that won't tell the whole story.
or you can buy a load tester for under a hundred dollars.

alternatively, spend that money on a lithium jump pack, and just drive it until it dies. and you'll have the jump pack for many years and multiple vehicles.

 

I posted instructions in 2017—but I don’t think I’ve accessed that screen at all since I wrote them.

I use a multimeter (set to DC volts, with the test leads connected accordingly, of course) to check the voltage across the + and – terminals of the auxiliary battery when the car is off.

In the Repair Manual (more info) procedure for on-vehicle inspection of the charging system, Toyota instructs, before doing this test, to “Turn the power switch off and turn on the high beam headlights for 30 seconds. This will remove the surface charge from the auxiliary battery.” If the voltage is 12.0 V or higher (at 68 °F), the battery is OK; if not, it needs recharging. It might also need replacement, or there could be some other problem with the electrical system, such as high parasitic load (dark current) from an accessory.

As @bisco kindly noted, a simple test such as this isn’t necessarily a reliable indicator of the battery’s state of health. Toyota dealers in the U.S., when submitting warranty claims for 12-volt batteries, are required to include a report from a Midtronics battery tester.

 

Get something like Solar BA6? It'll test the battery's viability, which is more than just the voltage.

For volts alone, just get a multimeter; checking voltage via the dash displays is misleading.

 

I use an Optimate 5 battery tester and maintainer that I hook up once a month. My current 12V is only 2 years old, so still in good shape. I have it on the maintainer right now in fact, and I tested the voltage before hooking up at 12.23 volts at 42 degrees F. The device will run a quick charge and discharge test and report a green or red status light. It will then go into maintainer mode which is for extended battery maintenance. It has a de-sulfate cycle if it detects excessive discharge during a test, but it won't run that if the battery is still in-circuit (installed in the car). The Gen IV batteries are a little bigger than previous generations, but Toyota still recommends a slow charge at 5 amps or less. This unit charges at 2.8 amps. They also recommend disconnecting the ground clamp from the battery which will cause loss of various electrical presets and ECU initializations. Not a problem. I usually just leave everything connected, run a quick charge and test, and test again in another month.

 

Since I maintain all 7 of my families vehicles, I have a TopDon BT100 tester. (They are around $60 on Amazon) It tests battery condition, starter, and alternator. My wife's OEM in her 2017 Prius is around 80% so keeping and eye on it. From past experience, 12-volts seem to give up the ghost shortly after hitting around 60% of battery life left and sudden cold weather seems to do them in when they are on the verge.

 

i think you do

 

I haven't had to purchase a Gen IV 12V battery yet, but I would likely go with the Toyota dealer Truestart brand if that is what they are selling. I put one in my Gen III Prius. Kind of $$ but good warranty and reasonable pro-rated replacement, which should be true for any dealer supplied battery replacement. My Gen IV has a factory installed Furukawa battery that I know absolutely nothing about. It does have access to the cells so I can maintain electrolyte levels with distilled water. I don't think the Gen IV model has a "chicken dance" to access a hidden screen that will list the battery voltage. As already mentioned, an economy digital volt/ohm meter (DVOM) can tell you the voltage. Generally a battery will last at least 4 years and will lose capacity steadily after that. There are stories in PriusChat about batteries lasting 6-9 years or longer before replacement. The best way to find out if a replacement is needed is to take the car to probably any auto parts store where they can load test the battery. And if it's bad they might even have a replacement and put it in for you.

 

None of my comments are directive in nature, which could be said for the entirety of this forum. We chat with the understanding that there is a healthy dose of "To each his own" and "Your mileage may vary".