12 volt battery charging voltage when fully charged?

Your resting voltage is consistent with what I see on my 2018 Prime, with a voltmeter hooked directly to the battery terminals.

 

i think you posted a while ago, your battery was dead. you recharged it and it has been fine since. i would run on this battery until it dies again, hopefully a few years
then see how the new battery behaves. there are a few of us actively monitoring voltage the same way.
my first guess is that your plug volt meter is off, but that cant be it, as you get the 14.5 reading initially, i see that briefly as well. yours settles on the low side for my comfort, but it is probably fine.
try the same test with everything shut off. radio lights hvac OFF. see if your steady voltage picks up to 12.9-13.1
maybe its a weak battery so with stuff on its showing weakness, but with everything off you will see what charge the car is really providing ?
i find it hard to believe the car with a few things turned on allows the battery to sit at 12.6. i also dont think the charging system is broken. i think your battery is just badly weakened from the time it died and the lack of mileage ( as i recall you dont put many miles on...)
once a battery has been damaged by discharge it may never really recover. it will work ok in the prius for years anyway, but we are seeing low voltage due to the damage.

 

i agree the resting voltage is ok, but consider it is right after a quick charge. may not be correct.
the voltage while driving around at 12.6 is not normal. what is causing that is the question. likely the load of the accessories on a weak battery ? the car actively charging should easily overcome the load. but this points to the possibility that when in operation the car is so stingy with volts and amps, preserving every watt for driving range, that the battery is starved even with regular operation of accessories.
what else causes a battery to read 12.6 during operation, when in theory its got 100 amps of charging capability at 13.9 to 14.5 volts ?
its designed to taper the charge to near death levels at all times , not wasting energy on preserving the not warrantied,replaceable 12v.

 

Why not? If the battery is fully charged at this voltage, and the DC converter is carrying all the 12V electrical loads, with no load on the 12VDC battery, why would it need to be charged more?

The amount of energy that goes back into the battery to charge it from sitting for even a week is quite small. The 12VDC battery holds 45Ah on my Prime. Assuming full discharge with no losses, etc, that's 540 watt hours or 0.54 KWh. If the battery were discharged by even 20% in a week, that would be 0.11 Kwh. Not a large amount.

During operation (i.e. READY mode), the battery is not carrying the accessories. The DC converter is powering these loads, and charging the battery at the same time.

 

This "discussion" is getting kind of old.
It is the nominal resting voltage for the TYPE of battery he has.
Conventional wet cell lead acid: 12.6
AGM: 12.8-12.9

Normal float voltage for both types: about 13.4

Like I said before, while it is possible that the Pruis "voltage regulator" function turns the voltage down below proper float on purpose, it is highly unlikely. I have never seen that in about 55 years of working on all kinds of vehicles.

 

What the OP is seeing is what others of us with Primes are also seeing. That indicates that the behavior is normal for this model. I have not had problems with dead 12VDC batteries, the car charges consistently at similar voltages. If the resting voltage of the 12VDC in the morning is 12.6-ish, then the battery got a full charge and all is well, regardless of how the car achieved it.

 

Because you don't have a good grasp of the way it works.

ALL wet cell types of batteries VERY slowly self discharge if a voltage slightly higher than nominal is not applied: Commonly referred to as a "Float voltage. That is just how it works.

If you have an AGM battery, the 12.6 while the vehicle is running will NOT even keep the battery from supplying part of the current needed to supply the loads.

Will this immediately damage the battery: NO. But over time it will cause a loss of some of it's useful life by not being fully charged all the time.

I quit. And you should too.
If you really want to know more about how this works.............LOOK IT UP.

 

My response to the OP is the same - I don't think there is a problem with your car, it is functioning normally.

 

NOT true.
There is at least one other Prime owner who has posted that he actually sees a proper float voltage while his car is running.

 

And there are at least two other Prime owners who see what the OP sees.

 

Happy Fathers Day to all the dads out there I

Especially the ones that have a working 12v battery....



Rob43

 

And I think you are absolutely wrong.
That is based on 50+ years as an Electrical Engineering Technician.

IF Toyota really designed the car to work that way, it is a deviation from about 100 years of charging system design and likely is a mistake.

 

i'm not an EE , but i have never seen any vehicle show less than 13.0 V when running, usually see more like 13.6-14.1.
anything less than 13.0 i would think , something is wrong.
could be the meter isnt accurate, or test point isnt seeing the true system charging, or some other heavy load, or the system isnt charging...
if the EVSE charges while its plugged in, could that be masking a fault in the cars charging system??
like i can run my ICE car for a few hours without an alternator, but the battery will get weaker and weaker until it stops firing.
short trips with a slightly recharged battery would work indefinitely ???
maybe if OP drove for 5 hours, the 12v would drop and drop and drop, exposing a faulty charging system?

 

I agree with you on any vehicle that has an alternator. But I think the way the Prius charges the battery is quite different. I’m gonna run a test on my prime advance tomorrow and see what it shows. Happy Father’s Day to all the fathers on this forum..

 

What's being observed here is common even on non-hybrid vehicles.

My last full ICE behaved quite similarly. At startup, the voltage would command to 14.0-14.5, settle in the mid 13s, and at cruising, would drop to as low as 12.7 if the battery's state of charge > 80%. The idea was to back off the alternator's duty cycle to reduce drag and improve fuel economy. (Yes, I know, no alternator here...)

The fun part was coast down--it would spike to 14 at times if it had allowed itself to drop into the 12s in the first place. It generally only hit this mode if ambient temperatures were on the warmer side. Otherwise, the voltage regulation strategy had been more traditional in practice, staying firmly in the 13s or 14s if truly bitterly cold.

There was a stiff debate on the charging wisdom of this particular strategy. But in the end, it was by design--not defect.

 

Exactly. It's behaving as designed, whether or not anyone agrees with the wisdom of the design. And one tenth of a volt on a 12V system is nit picking.

 

No, that is not an advantage with a lead-acid battery. The higher the charge the better.

So.....after giving this some thought overnight.....
If this really is "normal", I think this is what might be happening:
The Prime went back to a wet-cell battery with some kind of caps to check the electrolyte level, right ??

That likely was done mostly to save money in manufacturing; guessing at least $50 and maybe $100.
So then......the engineers said: "The public is now OUT of the habit of checking the water level in the batteries (for those who ever were)
so now we have a problem. The "ideal" float voltage, over a long period of time, will "gas" out some of the water and the batteries are likely to fail within a year or so. We can't have that.

In order to mostly solve that problem, they re-designed the charging algorithm so that it really does NOT charge (float) the battery all the time but only periodically when it falls below a certain point.

This will greatly reduce the gassing........so that the battery wears out slower due to insufficient charging and a slower loss of fluid and may last two or three years before it goes dry instead of one.

Alas, this sad because an AGM with proper float voltage should last 4-6 years.

How many of you with Primes actually check the water level in your 12 V batteries.......assuming that they really are back to the conventional type ??

 

Not if it is the difference between charging and discharging it isn't.

 

If it's staying at 12.6 volts, it's not discharging. If it was discharging, it would not stay there; it would get lower and lower. Not full is not the same as charging or discharging. If it stays 0.1 volts below full fussing about that IS nitpicking.