12V dies every two years

Agreed, but I would put a smart charger on a Prius, and have done so

 

Lead acid batteries off gas even during regular use. The process is actually electrolysis splitting water molecules. Saying water was boiled off isn't technically correct, but the end result is the same. Water, and just water, is lost from the solution. This is why regular maintenance for unsealed batteries includes watering to replenish the lost water.

When discharging, the acid is becoming chemically bound to the lead plates. This lowers the solution's density and volume. Aside: measuring that density might be more accurate than voltage for state of charge. Adding water then can lead to adding too much water, that then dilutes the electrolyte, and maybe a mess with the battery is fully charged.

Watering a battery needs to be done to a fully charged one.

Going back to boiling off. If the battery is charged too fast, or too long, the heat generation can also lead to the water actually boiling. At the very least, more evaporation on top of the off gassing. Though the off gassing then is a bigger worry, as you are mixing hydrogen and oxygen gas with heat.

Overcharging the battery isn't good for it, but what kills most car starter batteries is deep discharging. Lead acid has the longest life when spending the most time at 100% SOC, and the lower that SOC drops, the more life that is lost. That's why batteries that have 'died' previously need replacing sooner.

A car starter battery ends up stored at under 100%, because there is some drain going on with the car shut off, and/or because the car is only driven short trips that don't allow enough time to charge the battery back up after the starting event. The last can even happen with the Primes super awesome charging system as it isn't going to overheat the battery by charging too fast.

 

Putting 15.5 volts into a 12 volt lead acid battery is termed equalizing. Periodically it can be done for a short time. In use the acid tends to stratify with more dense acid settling to the bottom of the cells. A short equalization period will correct this. If the 15.5 voltage is put in too long the battery is likely to overheat and be ruined.

Common charging voltage for the absorption phase is 14.4 volts (cold: 14.8, hot 14.0), and for the float phase 13.5 (cold: 13.9, hot: 13.1). Some battery makers and some charger makers use slightly different voltages.

 

For a battery in motion, does the acid stratify as much?

 

Put a Deltran BatteryTender Plus on it overnight once a week.

 

I don't think so. Even the best auto chargers I've seen don't have the equalize function. I've only seen that in stationary applications. Anyway, that's what 15.5 volts is properly used for on 12v batteries. And, I've seen batteries bulged and ruined from getting the equalizing charge for too long a time.

 

A few days ago, my 2017 EU Prime 12V battery was discharged to the point that the HSD was unable to get to the READY state, as stated by the annunciator panel on the dashboard (“12 V battery discharged”). It had been my fault, because I had left the car parked for more than one hour with both parking lights and blinker ON.
Anyway, once I got the car home, I recharged the 12V battery with my CTEK MSX 5.0 smart charger, selecting the RECOND function.

Now, at the completion of the charge, with the battery SOC supposed to be at 100%, its voltage read with a multimeter was 12,4 V only, i.e. around 80% according tto the table published in the above post #27.

Shall I need a new 12V battery? Probably I will have the answer next Friday, because on last Monday I left the car sitting at the airport parking lot.

 

Did you disconnect the battery to recharge it? The values on that chart are based on absolutely no loads on the battery, and there are going to be some even with a car shut off.
Then was the battery allowed to rest before measuring? The value will be off if taken right after the battery was charging or discharging. It needs to sit, unconnected, before measuring. The battery manufacturers recommend 24hrs for lead acid.
BU-903: How to Measure State-of-charge - Battery University
The temperature of the battery is also going to effect what the voltage vs SOC is.

Assuming the battery is the original, it is probably time for a new one. 5 to 6 years is nearing the end of their dependable life. A full discharge will just bring the end on faster.

 

Probably because letting the car sit with lights on for one hour wouldn't drain a healthy battery.

 

We’re lucky to put on 3k kms a year, current battery, an Optima Yellow Top, was installed September of 2015. It’s on a CTEK 4.3 pretty all the time the car’s idle. Still passing assessment with a Solar BA5 tester, CCA about 100 more than spec, and volts read around 12.75 (after a few days driving, not fresh off the charger).

 

No, I did not disconnect the battery from the car while charging it, and I did not let it rest after charge completion before measuring the voltage. I was in a rush…

Anyway, I am reluctant to replace the Prius 12V battery before its time come.. maybe it’s different for wet lead batteries, but my 13 years old Lexus RX450 that I use only during weekends has still its OEM 12V AGM battery, which I periodically refresh with my Ctek smart charger.
Furthermore, my EU Prius Prime has the solar roof fitted: according to the car User manual, in addition to slighly increase the EV range when the car is left parked outside, the solar roof should also maintain the 12V SOC.

However, there is al element that I haven’t cosidered so far: I have not checked the electrolyte level… for sure I will do it when I am back home.

BTW it’s strange that Toyota did not installed a sealed 12V battery… and I wonder if the dealer really checked the electrolyte level during the yearly maintenance inspections, the majority of cars nowaday have sealed 12V batteries fitted… mmh 10 caps to be unplugged, plus check and eventual refill with distilled H2O… time consuming operation.

 

AGM tends to last longer than a wet cell, and you are taking good care of it. If the solar roof is putting energy to range, how much is left for the 12V? I'm guessing it prioritizes range, and it only charges the 12V once the solar battery is fully charged.

I should be checking the liquid level more often in my batteries. The watering should be done with the battery at full charge. When below full SOC, it is possible to add too much water and dilute the electrolyte. If there is access to the electrolyte, measuring the specific gravity is a more accurate way of measuring charge level.

 

I really don’t think that matters. I know Toyota says to do that when charging, in Owners Manual, maybe at their lawyers request? Way too onerous. FWIW I’ve got our 2010 pretty much constantly on a charger, in the car.

also yeah: measuring voltage directly after charging it’ll be way high, takes a day or two driving to stabilize, to a more “honest” reading.

 

Leaving it connected for a voltage reading could mean it being too low from any vampire loads.

 

get a jump pack, and keep the battery until it dies. could be years

 

That is what I have done, plus I fitted this on the 12V battery (mine has a direct battery connector) it came with the Ctek but I did not care to install it… so far.