Charging 12V step change - 14.0 to 13.5 volts.

This is getting "interesting". Here are the theories I gathered so far:

1. The charging voltage drops when the (good) 12V battery is fully charged
2. It's a bug
3. The charging voltage drops due to excessive load from a bad battery
4. Bad connection causing poor regulation

But none of them explains all observations:

1. Why doesn't Patrick see the drop in his two cars with good batteries? Why does koolingit see it with his marginal battery?
2. It's just hard to imagine Toyota engineers would miss this for so many years.
3. Why do some people only see the drop after changing to a new 12V battery?
4. Why would it depend on the shift position?

It's a good puzzle. I am leaning toward #4 now but not 100% sure. Once koolingit have his battery changed hopefully we will get some more data. Still pondering...

 

Just read the posts since my last post. When I ran my test I connected my voltmeter directly across the battery and ran the RS232 output of the voltmeter to an RS232/USB converter then a USB cable to the netbook where I recorded the voltage sampled every two seconds. My graph was done in Excel from the recorded data. There is definitely a step voltage change when the AC is cycled on and off, and the same value step occurs at other times. In my graph where the voltage is changing levels rapidly is where I was turning the AC on and off. I didn't try to figure out any other causes. I performed my test as a curiosity since I had seen the same two distinct levels on my scanguage.

Uart you make a good observation vis a vis model years. Mine is an 08 and yours and others have 09's while Patrick has an 07. Currently I have about 25K on the car and the battery has never failed to start or been disconnected since the car was new. And I live in a super mild climate and the car is always garaged.

 

Sorry my error on your car's year. As you point out, yours is a non North American car so that might make a difference. I was curious at what scanguage was showing so I made sure I didn't rely on anything the car was telling me about the voltage. By using a completely isolated set of test instrumentation and my setup was totally powered by the DMM battery and the netbook battery not connected to the car except two wires directly from the battery. This assured me that the voltage across the battery itself was truly varying as I had seen on the Scanguage II.

 

All I can add is that on a 70 mile drive yesterday SGII was showing 13.8-13.9VLT the whole time. It was so boring I changed it to LOD.

- D

 

This is interesting, they have apparently implemented a 2-stage charging algorithm. Strange that they went to that trouble, but neglected to temperature-compensate it. In the winter the 12v battery never gets a full charge for this reason.

 

I have seen the low 13.5 voltage on my 04 with the original battery when driving on the second trip of the day (which is about 1 hour from the first), so perhaps starting the car with the battery being fully or at least recently charged has some relevance. The other conditions uart mentioned, -no other electrical loads- also apply, the second I turn on the fan the voltage is around 13.9, 14.0. .

 

For the record: I haven't seen the voltage drop to 13.4 in the cold months. It was 50 degrees today and the scanguage stayed at 14 volts all the time.

 

Hi and hello to all from downunder.

First post but I have been sandbagging on the side for quite a while.

I have an Australian white Prius built April 2008 with 130,000 Km on the clock. We are lucky in Auz as our cars come with no bladder or thermos and we get EV and rear discs along with adjustable driver’s seat as standard.

After following this thread I decided to reproduce uart's configuration. It has bothered me that Toyota would leave the auxiliary battery continually sitting around 13.8-14.00v. I personally find this too high for extended periods and thought that if Toyota could put such a stellar effort into maintaining the SOC of the HV battery then looking after the little battery would be a piece of cake. The amount of software for controlling this last stage of charge would be minimal.

I first off placed the car in accessory mode to read the initial state of charge by going into maintenance mode; it was 12.5V this was after sitting for four days. I then switched to IGG ON and tuned the AC off and the fan off. I then went back to Accessory mode and placed the display in maintenance mode again, by this time the voltage was reading 12.3V. As soon as the car was made ready the voltage rose to 13.9V for a few seconds then stabilized on 13.8V.

I live in the northern part of New South Wales and my trip was to the local supermarket which is about half an hours’ drive. Leaving the maintenance screen up and all accessories off the voltage stayed constantly on 13.8V while driving with the occasional blip to 13.7V. At around the 25th minute into the trip the voltage immediately dropped to 13.2V and just sat there. After some time I turned the AC on by using the steering wheel switch, this caused the voltage to rise back to 13.8V until I shut down. On the way home I repeated the test and at around the 22nd minute it did the same.

This is not a bug, it is a feature and I am glad my battery is being looked after properly. I think that these results will be confirmed by all who repeat this test as detailed providing your battery is in a good state of charge (and condition) at the start of the test.

I hope that what I have witnessed will be helpful to others

Cheers GranDad.

 

If you look at my graph in post #2 I recorded the same behavior upon startup. Looks like there's a feature that requires the full charge voltage for 20 or so minutes after start along with forcing the full charge voltage with the fan being turned on. I agree its a feature not a bug!

I don't think we've heard from any one with a North American version 2004-2006 that has seen this? Anyone with a scanguage can observe if there are two distinct voltages and can try observing as the fan is turned on and off.

 

I really don’t think it is a function of the fan or the air conditioner. I think it is just because a load is placed on the system and the battery is being taken off float. Next time I will just turn the lights on and see what happens.

Cheers GranDad.

 

Hi GranDad,

Thanks for weighing in with your observations but I don't agree it's a feature.

If it is a feature, it's kind of lame. In order not to overcharge the 12 volt battery, you have to keep the AC turned off (not even the fan is allowed). If the AC is off and you turn on other loads, the voltage will sag further.

Most people will have the AC on in the hot weather. A lot of us will have just the fan on in spring and fall just to keep fresh air moving through the cabin. So, most times this 'feature' doesn't work!

 

I haven't seen it mentioned here but if you're reluctant to put the MFD in maintainance mode because you have to shut down the car to get out of it -- IT'S NOT NECESSARY TO SHUT DOWN THE CAR.

To get out of maintainance mode just depress the display button for more than 5 seconds and the regular display will come on and the radio will come back on (if you had it on before going into maintainance mode). Five seconds is kind of a long time so just hold the display button down till the screen changes.

Maybe this will encourage others to check their voltage.

 

Hi koolingit

I think you have to look at this from the other way round.

It’s not that you have to keep the AC or fan off to keep from overcharging the battery it is rather the fact that if you operate the car with light electrical loads for extended periods and you don’t want the electrolyte in your battery to boil the car is smart enough to sense this and bring the charge voltage down to an acceptable float level.

By applying an electrical load due to the turning on of headlights, AC, fan, defogger etc. you are compensating for the higher (13.8-14.0V) charge, and the battery is happy and wont overcharge while there is a load on it. The Prius is smart enough to sense any change in electrical load and what to do.

The moderate climate levels here in Australia mean for weeks on end I don’t need air conditioning or fan (I like the window down) or any electrical loads while I drive. If the car did not have this feature my auxiliary battery would be damaged by overcharge.


Thank you for the tip on how to restore the display it is very useful

Cheers GranDad.

 

Thanks for the tip! Mine is '05 and on original 12v batt and I always get 14.2V on the Display menu when have it on Ig-Ready. I'm gonna to video the car's Display Menu when I get a chance tomorrow. Let's see if there's any Voltage change like some you guys have seen.
Curious why you need to buy a new battery since you've a '09. Maybe for the sake of Science.
Wondering which battery you ordered. I'm going to order mine from Champion ToyotaWorld. It's $115 Shipped.

 

I believe that you will find the dealer will not ship the 12V battery (on the basis that it is a hazardous item) despite the fact that you can find it in the parts catalog.

 

Well the lowest found is $125 before tax for within 40 miles in NorCal. Thanks.

 

I'm retired and don't drive the car on any regular schedule. When I first got the car in Dec of 08, I didn't know it was a no no to have the radio on while not being in 'ready mode.' The first few months, while I was in the gee wiz phase of ownership, I'd sit in the car for hours going through the manual finding things out (it took me three days to figure out how to remove the cover from the fuse box under the hood.) So I was probably in a low state of charge those first few months.

I've never ran the battery down so low that the car wouldn't boot up but I don't know how close I came before getting the scanguage in May of 09. I guess, by that time, the damage was done.

I'm not ready to get a new battery yet, but when I do, it will be an Optima.

I left out the most important part: With my driving habits, this 'feature' doesn't work for me.