Tested my battery.

triumph1,

It very much sounds like your 12V battery is on it's way out.

I don't know what you mean by "unloaded," but a healthy battery
should be reading in the mid-high 12s after sitting unused overnight.
When you are in the "Ready" condition a voltage reading reflects the
output from the DC-DC converter to the 12V bus, not the battery itself.

There are any number of small continuous draws on the 12 V battery;
the security system, the SKS transmitter/receiver, etc.

Typical service life of the 12V battery is 4 years. But there are many
factors that shorten it, these come immediately to mind; service in
very hot or cold climes, even one complete discharge due leaving
lights on overnight, and incomplete initial charging when the car was
prepped for sale by the dealer.

I'd say it's time to get a new battery, before the cold winds and snows
arrive.

 

This sounds weird. Unloaded battery voltage must be greater than voltage under load, not the opposite. What do you mean by "loaded"? If you started your car and the voltage on the battery increased, it is normal behaviour, because the car is actually charging the battery, not draining it.

Anyway, if your (charged) battery voltage reads 11.6V under *no load*, it is on its way out.

If by "load" you mean *charge*, your post would make more sense - a fully charged battery reading 13.8-14V (and discharged 11.6V) seems perfectly healthy, but this is not a sufficient test. Voltage drop under some meaningful load would tell you more, because you could determine the internal resistance of the battery - or you might want to discharge the battery under controlled conditions, determining its capacity.

Anyway, do you have any suspicion that your battery might be bad? What are the symptoms?

 

You went too far when trying to test 'loaded', all the way into READY mode. Press the power button without pressing the brake pedal, to go into IG-ON mode. The LED on the power button will glow orange in this mode, all the various lights will appear on the dash.

Once you're in READY mode, the car starts charging the 12V battery from the high-voltage system, via the DC/DC converter. This happens even when the engine isn't started: compared to the HV battery, the 12V is really low capacity and charging it normally represents very little load on the HV battery. However, if the battery has become damaged by deep cycling, it can become almost a short circuit.

There is always a small amount of power used by the car to allow software bootup (monitoring the power button), receiving signals from the remote keyfob and, if fitted, Smart Key and running the alarm system. It's not a great deal of power drain but the battery does remain connected. In addition, batteries all have a certain amount of self-discharge - lead-acid's is quite low, but still about 5% per month at room temperature.

None of this is significantly different from any other car's 12V battery, except that because the car doesn't actually turn over the engine using a 12V starter motor, and the computers generally run quite happily from significantly less than 12V (traditional 'transistor-transistor logic', TTL, was 5V; classic CMOS was 3.3V; frequently lower voltages are now used), you can still boot the car from quite a low voltage. However, various actuators need a lot more voltage - the Skid Control ECU, which contains the solenoids that operate the brake lines and the accumulator pump, wants at least 9V. As a result, you can boot a Prius to READY with quite a weak battery but you can then get a bunch of errors if the 12V system is being pulled down by excess load.

Because that's all the battery has to do, Toyota specified quite a small battery. That means that if you leave the car on with all the electronics going - in IG-ON mode - it doesn't last very long. Some people have reported only about an hour. In ACC mode (green light on the power button) it's longer. Finally, even when it's off you can drain the battery quite quickly by leaving headlights on, or a door ajar (which leaves the courtesy lights in the doors, and the dome lights in the roof headlining, on).

 

Sounds like things are working as they should and you do need a new battery.

 

I have a problem with my 2004 prius. It is READY but the trans stuck in Park.

 

I found that the voltage displayed in Vehicle Signal Check was actually 0.7V lower than when I checked the battery with a voltmeter. This was true when I first turned the system on and then again after I put the car in Ready mode and the battery charger kicked in. So, I would suggest reading the battery voltage with a voltmeter and determine the error between this and the Vehicle Signal Check.

 

Thank you for pointing this out! I have long been suspecting this discrepancy between the displayed and actual voltages although I haven't got around to measure it myself. Last time when I looked after the car had been garaged for two weeks the ScanGauge read 12.0 in ACC mode, 11.7 in ON mode, and 11.4 with headlights on - not very healthy if just taking its face value. However the car runs fine and fuel efficiency is great. So I suspect the displayed voltage (either via MFD or ScanGauge) can be misleading.

 

Re: Displayed voltage versus direct battery measurement with digital voltmeter (DVM).

Yes there are two issues here. One is the absolute accuracy of the voltage displayed in the diagnostic mode the other is the exact point in the circuit at which the voltage is measured. If the displayed voltage is not measured directly at the battery but somewhere else on the 12 volt bus then you would expect some small differences depending on the current level and direction of flow (charging/discharging).

About a week ago I made the following measurements on my 2004 pruis (which I think still has the original 12V battery).

Car sitting for about 4 hrs since being driven.

1. Car completely off.
- Voltage directly at battery (DVM) = 12.58 volts
- MFD displayed = couldn't measure (car completely off)

2. Power button once without brake pedal and then into diagnostic mode on the MDF.
- Voltage at the battery (DVM) = 12.38 Volts
- MFD displayed = 12.4 volts


3. Power button one more time to put some load on the battery.
- Voltage at the battery (DVM) = 12.21 Volts.
- MFD displayed = 12.1 Volts.

4. Brake pedal and power on to go to ready mode.
- Voltage at battery (DVM) = didn't measure.
- Voltage on the MFD = 14.0 volts (charging)

 

I finally got around to make some measurements comparing 1) the MFD vehicle signal check, 2) the ScanGauge, and 3) the dedicated jump start point read by two multimeters. Essentially the three measurements are consistent within about 0.1 V. The ScanGauge and MFD are sometimes about 0.1 V lower than the multimeters.

 

I would assume that the scangauge and mfd are getting the reading from the same internal meter, so they should agree. Not sure why they're lower, but 0.1V isn't likely enough to give a very different diagnosis of anything.