Probably New Auxiliary Battery Time

Perhaps it's a reminder to replace the battery. The current battery was changed in July 2016 at 72,000km. Car now has 129,000km.


Is it necessary to go with this sort of specialised batteries?[/QUOTE]

A bit late to see your original post but as I am in Australia as well I would put my 2 cents worth. After using 2 genuine Toyota 12V batteries in my gen2 which only lasted 2-3 years each ( around $400 each from memory) I found suppliers of Optima Yellow top D51-T1 (Prius).
This was recommended highly by US users at the time. First one $259 lasted 5 years from 2017.
April 2022 replaced for $ 321

 

I came back from a two week vacation not having disconnected the aux batt and found I couldn't start the vehicle (a 2015 model, not sure of the battery vintage). The battery was at 6.5V when disconnected, so I jumped it with another vehicle batt which allowed the car to start. I've left it charging (from the jump vehicle) but am thinking the low original voltage is an unambiguous indication that the aux batt is dead and won't hold sufficient charge to allow restart of the vehicle by itself . If I leave it charging long enough to resuscitate the battery's ability to start the vehicle on its own (assuming that is possible), am I risking stranding myself?

 

Update - after about 2 hrs of charging , I started the vehicle then disconnected the charging batt. The ICE comes on as usual after about 30 sec and every now and then subsequently (the vehicle being parked) and the aux batt maintains about 14V even when the ice is off but vehicle is on (and thus under load of computer and various dash lights). Perhaps I don't need an aux batt replacement

 

You're just reading the volts supplied by the hybrid battery, when the car's fully on (in "Ready" mode), regardless if the engine is running or not.

Considering your initial 6.5 volt reading, I'd go directly to battery replacement.

 

More than likely your battery is irreparably damaged by it getting so slow and more so the longer it was at this low level.

Two hours is nowhere near enough time to charge up from this low voltage. Think more like 18 to 20 hours.

As pointed out by Mendel the 12 V battery is not maintaining about 14 volts, it is the charging voltage (and I'd expect it to be more like 14.4 V) supplied by the DC/DC converter trying its hardest to charge up that battery.

 

Ok - I'll make sure to disconnect the batt when trying to determine its actual voltage (and/or turn the vehicle off).
I took it into a car shop yesterday to replace the aux batt and today the guy calls to report the battery is fine, so possibly he was also checking the voltage with the car on. Somewhat surprising since one of the other shop owners has a prius , will see what the story is tomorrow.

 

My auxiliary battery has a capacity of around 12.5 Ah, not the 46 Ah it should. Yet it passes a conventional battery test with flying colors. That is because those tests are for the wrong thing: they yank a ton of current out for a short period of time and then check that the voltage is never below 10V (or something like that, depends on temperature). It isn't a test of battery capacity at all, and loss of capacity is how these AGM batteries die. I posted somewhere on this site a way to measure the capacity using just the car and an AGM SOC table. Let me see if I can find it... Here:

Parasitic drain - is 55 milliamps ok? | PriusChat

 

Ok this is great info . The guys at firestone used a somewhat better battery tester than just hi-load , it gives both a SOC (state of charge) and SOH (state of health) - the SOC was indeed low (25%) when I brought it in , subsequent driving has prob. improved that, while SOH (which is the remaining capacity, how they determine this without charging the batt is an interesting question, your post mentions a method that basically finds the slope of voltage vs. charge (AH) if I've understood right, which would make sense) was passable , something like 20AH remaining out of 40AH expected for a new one.
Bottom line for my case is despite hitting 6.5V the battery appears to have some legs left, surprising me - my recall from the 'EVDL' list back in the day left me with the impression that a battery hitting that state was clearly kaput.

 

This HF battery tester also has a "SOH"

12V Digital Battery and System Tester

What does SOH mean on this device? Well, the manual says nothing, so I wrote their technical support and they said:

"Battery Health" is actual battery "condition" after testing.

If battery health is 100% battery is good, no issues.
If battery health is anything but 1000% tool will indicate what issue is.

Well, it was 100% on mine, with only 1/4 of the original capacity and an internal resistance over .008 Ohm. Never mind 1000% (10X better) than it should have been!

As far as I can tell SOH has a very mushy definition, even if it is from a device a bit more respectable than the HF tester:

https://en.wikipedia.org/wiki/State_of_health

 

The bold bit above is all you need to do. Still, the battery health is more than just voltage. Doubly so when you've freshly charged it, it'll read high voltage, but that won't last. To see for yourself where your battery's at, test with something like this:

He's using a Solar BA5, which I got, after watching the video. Works great. Current iteration is BA9 I think.

 

The SOH from the firestone guys was given in terms of capacity (which is one of the things wikipedia says ppl mean by SOH) , again apparently derived from dV/dC and not V