12 Volt battery operation range?

Use your MFD in maintenance mode to confirm the reading. See my description for using your MFD in:

Checking 12v Battery Health - Hybrid Car Forums

It should not drop below 13.8 when the charging system is operating correctly.

JeffD

 

A "12 volt" battery will in fact show ~12.6-12.7 vdc at rest,,, neither under load nor being charged. While under charge,, it is fine if it goes to ~14.2-14.6vdc. Under load,, with no charge happening,, it can read into the low 11 range. This is NORMAL. If you see voltages above 14.6 or below say 12.4 while the car is running,,,then have it checked.. other wise it is fine.

Icaru

 

Icarus,

The OP is not talking about any "12v battery" he is discussing a Prius AGM 12v battery. Our Prii use a "constant voltage" charging system. 14 volts are supplied by a DC-DC converter (from the 200v battery) to maintain the charge on the AGM battery anytime the car is in "Ready" or "Accesory" mode.

The accesory voltage will drop to about 12v when the charging system is off, but if it drops much below 12v there is a problem. Either you have depleted the charge, or the battery is due for replacement.

JeffD

 

You are correct about the AGM having it's charge voltage limited,, due largely to the battery not being able to accept water if it is overcharged like a conventional lead/acid battery. The reality is,, while the float/absorb/charge volts are a bit less with an AGM,, the at rest/under voltages are essentially the same as any other lead acid battery,,, as the basic battery chemistry is the same.

To the OP point,, at 13 vdc +- he is fine. If he is sub 12 under charge,,, he MAY have a problem.

Icarus

 

The key point is to make certain that the charging system is not on while assessing the battery. Use the built-in self test, or use a voltmeter at the charging point under the hood with the car OFF. If it is less than 12.0V with no load the battery is near death.

 

The old joke,,,,

What's the voltage of a 12 volt battery???? 12.6 volts!

12 volts is almost dead!

Deep Cycle Battery FAQ

Icarus

 

On the occasions where I have measured voltage on the 12V bus while the Prius is READY, I've measured 13.8V +/- 0.1V. However I've seen some posters assert that the voltage may decline into the low 13V range depending upon ambient temps and 12V battery charge.

Meanwhile, the repair manual says that 13V to 15V is an acceptable range when the car is READY and all electrical accessories are on (from page 05-486-6, 2005 Prius Repair Manual). Therefore the OP should not be concerned with a measurement of 13.3V.

13 CHECK OPERATION OF W/ CONVERTER INVERTER ASSY
(a) Connect the C5 and C6 converter inverter connectors.
(b) Set the electrical tester with the AC/DC 400 A probe to the
wire harness of the C6 connector.
(c) Connect the I14 and I15 connectors (color: orange) to the
inverter.
(d) Install the inverter cover.
(e) Install the service plug grip.
(f) Turn the power switch ON (READY).
(g) Wait 5 minutes.
(h) Operate the electrical devices as listed below:​

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Set the lamp control switch to High beam.​

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Set the heater blower fun to HI.​

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Turn on the defogger.​

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Depress the brake pedal​

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Press all the switches of the window regulator master​

switch at the same time.
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Turn the steering wheel to right or left.​

(i) Measure the amperage and voltage.

​

​

Standard:
Item Specified Condition
Output current 40 to 100 A​

Battery voltage 13 to 15 V​

​

​

 

So, would you say 9.6V is dead? That's what mine is at before 'Ready'. However the dealer has tested it and says that the tester indicates that the battery needs to be charged, not replaced. I may have an argument on my hands here. Once 'Ready' I get 13.8V or 13.9V.

 

Well, it should be easy enough to charge your battery and see what happens. Either charge it using a charger set to 2A overnight; or else make the car READY and leave it in that condition for several hours so that the car has sufficient time to charge the battery...

 

There is dead,,, and there is DEAD!

In this context,, a battery discharged (in need of a recharge) to 9.6 volts is certainly "dead". Dead in the sense that it can't support the loads intended. If however it can be recharged and will hold the charge it is not Dead,, but maybe dying!

It is important to understand how batteries work. Batteries don't store electricity, they MAKE electricity. They are in essence, small chemical plants, that turn chemical (potential) energy into electrical energy. People also have the misconception that when you draw down a battery (especially too far) that all you have to do is "pour" more electricity back in. In fact by discharging a battery,, below about 12.4 vdc you risk permanently damaging it such that it's long term performance will suffer. Also leaving a battery discharged is a sure recipe for destroying a battery quite quickly.

What is most important,, if you are trying to determine the health of a lead acid battery,, is to measure the voltage in an at rest state, neither being charged nor discharged, preferably after sitting several hours at rest. This is quite difficult to do in the real world,, as most batteries are subject to small phantom loads all the time,,, radio, clock, smart key, key relay, alarm system etc.. The real key to testing a battery health is to put a known controlled load on a battery for a given amount of time,,, measure the voltage under load,, and measure the natural recovery. Only then will you get a good sense of battery health. The 9.6 might be a concern,, depending on what loads are on as the car boots. I would have to have a better idea of the test circumstances.

The reality is,,, there are discharged batteries and there are DEAD batteries. Continually discharging a battery below ~80% state of charge, or leaving it below 80% for more than a day or two will turn that discharged battery into a DEAD battery.

More battery stuff than anyone really needs to know!

Deep Cycle Battery FAQ
Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and
Car and Deep Cycle Battery FAQ

 

9.6V is with the car switched on but not yet in 'Ready'. Switching headlights on drops that to around 8.6V. The dealer has had one attempt at recharging the battery and it's bad again.

I think I need to take it to them again and leave it long enough for it to fall to 9.6V and see what their tester makes of it. I do understand their dilemma, if the tester says recharge then they cannot replace under warranty.

At 9.6V the MFD works, at 8.6V it does not - how's that for Prius trivia?

 

Sounds like it is going south!

Icarus

 

Charge it if you want, but you will be buying a new one soon.

 

I swapped out the Panasonic 12 volt battery a year ago with a Optima battery. The factory batter was losing it's charge, after three years. The Optima should last the life of the car. I bought the battery from a book publisher in SoCal, of all places. It came with the mod kit for the larger terminals, and cost about what the dealer wanted for a battery ($125).

 

Mine does the same thing I drive it back and fourth to work. Sometimes the battery seems like its not charged but jumps up to 13.9 just b4 the engine starts up.

 

Do you mean the same thing as the OP? Sometimes only around 13.3 to 13.5 volts and other times up around 13.8 to 14.0 volts? I have also documented this and determined that it is normal behavior for the Prius charging system. It's actually a crude two stage charging system, but it doesn't seem to be all that well implemented and certain conditions inhibit it's operation. Because of this many people never see it (the lower voltage step) happen.

It may be inhibited in cold weather and it is definitely inhibited by running any type of A/C, even just the fan. With a battery in good condition if you drive for some time (without AC or fan) then after a while it reduces the battery charging voltage by an approx 0.5 volt step. So for example it will abruptly drop from 13.9 to 13.4 for example. Several other users have confirmed this is a "feature" and not a fault.