Charging 12V step change - 14.0 to 13.5 volts.

I made a recording of of the voltage directly across my battery on a short drive. Data samples were taken every 2 seconds, so the graph shows the initial car off voltage and then about a 20 minute ride and then a short time while I sat in the car and then exited and closed the door. During the ride the voltage cycles between 13.25 and 13.8 volts. Some of the voltage changes occurred when I cycled the air conditioning on and off. With the blower on the voltage stayed at 13.8 volts. Additionally when I first started the car you can see that the voltage went immediately to 13.8 volts for a couple of minutes. Based on my testing I agree charge voltage does vary depending on demand and 12 volt battery state of charge.

 

Yes some time ago someone reported this phenomenon of charging voltage dropping which I could not reproduce and attributed to a possibly weak battery. Glad to see two more independent confirmations here.

 

If Pearl's battery is a bit low (short trips and battery drained a bit from headlamps being on with car not in "ready" such as at shutdown - lights stay on until door opened) then she will run at 13.9 or 14.0 for a short time. After a while she drops to and stays at 13.8. Is your battery good? I monitor my 12V system voltage with the Scangauge.

 

I posted about this exact same condition in April of last year. See: http://priuschat.com/forums/gen-ii-...eshooting/78982-dc-buss-drops-13-4-volts.html where I detailed my observations.

The only thing I can add is that the weather was usually in the high fifties and sixties. During the Summer I didn't see it but I'm big on comfort and had the ac on. During the fall season, I'd see it again. As I stated in my original post, it's just a sometimes kind of thing. If I shut the car down and make it ready again, it usually will not drop to 13.4 volts again. I think it's an intermittant bug. I haven't seen it happen this winter with temps in the 20's and 30's even if I don't use the fan or auto ac.

Oh...and my battery is on the bannana peel: 12 volts at rest (date of manufacture: Oct 08.

 

Regarding any voltage differences, since I used a somewhat generic/uncalibrated ebay purchased digital voltmeter with an RS232 output to the computer for recording, the calibrated value of the voltages shown could easily differ by a couple of tenths of a volt.

 

Wow this is the most complete investigation I have seen and I wonder how I missed it before!

uart and direstraits71 could you please test your cars to see if you could reproduce koolingit's observations? If so then I have to agree that the large drop is a bug - giving me hope that my battery is not that bad after all.

 

I don't think we're seeing a problem here at all. The car senses the battery state of charge and knows what electrical demands are being put on the system and decides to vary the charge voltage by around half a volt. Precisely how it makes that decision could be determined only by knowing what's programmed in the car's computers. It appears to me that generally if the demand on the 12 volt system is high or the battery is in need of charging the system applies the higher voltage. If demand is low and the battery is seen as adequately charged, the system lowers the charging voltage by about half a volt. It also appears that when you first run the car the system selects the higher voltage for some period of time and then enters the decision mode. This may have been done to replace any charge that is lost while the car has been off. A smaller 12 volt battery is more easily overcharged than a conventional sized battery, hence the smart charging system.

 

Yes this had been what I was thinking all along as well until I saw this:

How does park or neutral require more 12V power than drive or reverse?
And this:

With such a heavy load from the rear defogger why doesn't the smart charging go back up to 14V and instead let it sag down further?

 

Someone else could run the same tests as these on another car and confirm or deny identical behavior, but if all the cars do this it would probably take Toyota Software Engineering's knowledge of how the car's charging algorithm is programmed to say why this is. I wasn't trying to say that I knew how the charging algorithm worked, only observing that in my test the A/C consistently returned the charging voltage to the higher level. The car has way too much software and there are way too many variables to figure out why it does what it does. My previous comments were my speculation on how the charging might work based on my test results. If the car keeps a nominal battery charged to a level that always starts the car I think that's all that matters. My testing confirmed to my satisfaction that the charge voltage variations that I saw on scanguage are normal and I was able to find at least one cause for the voltage changing.

 

I found one more thread that addresses this issue: http://priuschat.com/forums/gen-ii-...ssion/54193-13-4-volts-scanguage-running.html

In that one, Patrick Wong says: "I suggest that you check the battery cables to ensure that all connections are tight. When READY the voltage on the DC bus should hold constant at around 13.8VDC. When IG-OFF, a fully-charged 12V battery should produce 12.6V or better.

Therefore it is disconcerting to see your report that the voltage is fluctuating when the car is READY. If you don't find a loose connection, the implication is that the DC to DC converter is becoming flaky or that your measuring device is not working properly."

I've had the battery out twice (to recharge it) and I've checked the voltage with two fluke meters, the scanguage and the signal check. When the two flukes are reading 14.01, the scanguage reads 13.9 or 14; the signal check reads 14.1 or 14.2

I wish Patrick would weigh in on this again since he's just put a new battery in his 07. Maybe he can shed some new light on this.

I don't know how the DC to DC converter regulates itself but I wonder how the AC fan affects it. When this 13.4 volt bug is present: the car is in drive or reverse; turning the ac fan (cabin air fan) on will bring the voltage back up to 14 -- this is just the fan -- not the compressor! I can turn the fan on and off and watch the voltage go back and forth. That's as far as I can go. Now my head is starting to hurt.

 

Check your battery post connectors. They may have corrosion or oxide under them. Also check the -ve connection to the car body. There have been reports of that connection corroding.
If the voltage drops much when you turn on a load it points to poor regulation, and resistance between the battery and the cars system will result in poor regulation.

 

My 2004 has 115K miles on the odometer and the 12V battery is one year old, purchased from the local Toyota dealer. I had fully-charged the battery earlier in the week. This morning we went out to breakfast and drove around a total of ~15 miles. Upon return, I left the Prius READY and hooked up my Fluke 87 to the dedicated positive jumpstart terminal and one of the mounting studs of the nearby strut.

READY, no electrical accessories turned on: 13.85V

READY, AC fan at full speed, AC compressor on, rear hatch defroster on, HID headlights on: 13.78V

As I turned on each accessory, I could see the voltage decline slightly, yielding a total of 0.07V drop when all accessories were on.

The voltage dropped 0.01V when the AC compressor was turned on. This was interesting to see, since another portion of the inverter feeds the compressor motor (not the DC/DC converter). I assume that the traction battery voltage sags slightly as the AC compressor is turned on, which has an effect on the DC/DC converter.

I do not know why the OP is seeing a voltage reduction to 13.4V - 13.5V on the 12V bus. Since there is a SENSE line to the 12V battery, if the battery is at a very high SOC the 12V bus voltage may decrease to compensate. However, I personally haven't noticed this voltage drop to 13.5.

koolingit's initial post indicates that his 12V battery is marginal. That may be part of the problem, which results in further loading down of the DC/DC converter.

 

Patrick, I suspect the drops you see are just losses in the wires. They're about the right order for that.
Those larger drops indicate to me a resistance in the system, and the most common is in the battery connections, as I posted above.

 

Thanks Patrick for looking at this. However, I don't think there's any loading going on when the voltage drops to 13.4

Here's a video I made last summer when this bug popped up

I hope it's plain to see the voltage went back to 14 when the AC was turned on.

 

I see that koolingit noticed the voltage drop on his car only when the gear selector was in D or R. Hence I tried measuring voltages again, with the gear selector in P, then D and R. No noticeable change as the gear selector is moved around. In each gear, I turned the electrical accessories on and saw the same 0.07V drop.

I also did the same measurements on my 2007 and got similar results in the three gears, except that the starting voltage when the car was READY and with all accessories off was 14.05V (or 0.2V greater than the 2004.) The 12V battery measured 12.7V when the car was IG-OFF, prior to conducting the measurements.

 

Hi Patrick,

Thanks once again. You really go above and beyond..

I guess I can't do any more until I get a new battery. Whether that fixes the issue or not, I do need the new battery.

Thankyou all