Question re: 12-volt battery, please?

Yes, I placed the red probe (positive) on the jump post in the fuse box, and I placed the black probe (negative) on the nut above the strut.

You should note that when you use the multimeter, if you check the voltage with everything powered down, a fully charged battery should read 12.6 volts or higher. If you are in accessory mode, the meter should read around 12.4 or 12.5 or better, and when in ignition on mode, the meter should read 12.2 or 12.3 or better. If you are getting less than those figures, I don't believe your battery is 100% charged. Voltage will drop as you apply a load to the battery, but the voltage will return to normal figures unless the battery is defective or discharged. A fully charged Prius battery should have ample reserve for these tests and still return to the figures I am stating. I am basing this on what I am seeing with actual hands on testing of our Prius. Your readings should be better than mine, since you are working with a new battery and mine is 3+ years old. It takes a few minutes for the voltage to return to full voltage after a load is placed on it, but it does happen to a certain extent. You can actually hook the meter up and watch the numbers change after you power down the car. It has to do with the chemistry of the 12 volt battery and the way batteries work.

Also, you may notice a slight difference in the MFD readings versus your multimeter. This has been documented here on Prius Chat before. I would trust the multimeter over the MFD for testing purposes of your battery.

 

ok, while playing around in the garage waiting for the new battery to arrive I checked the voltage of the original battery I pulled out, it now shows 12.1 volts after just sitting for a few hours after checking it the first time when I got a reading below 10 volts. Is that crazy or what can that happen it seems like it charged it self up just being out of the car and just sitting in the hot garage? I am miffed

 

New battery arrived, I was going to put it on the charger overnight but it shows 13 volts and my smart charger shows it at 100% and says it is charged so I guess I am good I have it installed now and going to go on a 38 mile trip and back

the old battery is slowy gaining more volts? I still wonder why and how? but It was under 10 volts this morning and that was afer a 5 mile ride to take the wife to the train.

 

went for a 30 mile ride and I know now what to look for in a dying 12 volt battery, our mpg went down from 48-49 mpg down to 39 mpg we just figured it was the heat and having the ac on all the time, nope it was a dying sulfated battery, bought a new one went for a 30 mile drive and now getting 48.8 mpg again, I had to re-set the radio chnls and re-learn the auto up/down window, but that was easy, operate the window full down and up 3 times like magic it works again. lol, I also connected the old battery to my smart charger, it showed 12.2 volts and 42% charge then went into a desulfation mode, so that battery is full of crap.

 

Since your battery charger is also a desulfator, are you considering trying to desulfate it?

 

Hi gone2green, this is the difference between a loaded and an unloaded battery. For a battery in good condition the voltage drop (for small loads) is not that big, but for a near dead battery (like your old one) the voltage drop can be massive, even when a small load is applied.

When a battery is under load the electrolyte nearest the plates depletes first, causing the battery voltage to fall. In a good battery where the plates aren't sulfated and the electrolyte is not dried out, fresh reactants will quickly diffuse to the plate to keep the reaction going and the voltage stable. In an older battery this process doesn't work so well, hence you get more voltage drop. Also in an old near dead battery, it may not simply recover this voltage the instant you disconnect the load, though it may gradually recover voltage over time as the reagents in the electrolyte slowly diffuse back. This is not really like charging the battery so much, think of it more as "charge" that was already in there (but couldn't be used because it was too far from the plates) now diffusing to where it can be used.

BTW. All batteries have recovery transients like this, both on being removed from a charger (where the voltage falls) and on being removed from a load (where the voltage rises). It's just that these are much more pronounced on an old near dead battery.

 

First of all, I apologise for repeating this in every 12V battery thread I see: The MFD voltage test is a very good, convenient indication, but to be 100% sure, the voltage must be measured directly at the battery posts with a voltage meter that has an accuracy of +-.1 Volt or better.

Hi Julia! In this respect, the Prius is not different from other cars. Long periods of exclusively short trips lead to less than optimal 12V battery state of charge, which leads to a somewhat shorter 12V battery life. Typical 12V battery life for most cars is around four to eight years, depending on conditions. (climate, trip length, audio system use when parked, etc.) I have the impression that the 12V battery gets a lot of attention in Prius forums because there are relatively few other problems.

Some of us car guys and gals like to fiddle around with our cars, and prefer to do things to make our 12V batteries last a bit longer. Some of us prefer to wait until the 12V battery is really bad, and risk having to do a jump start, before replacing it. Some of us prefer to replace the 12V battery every five or six years or at the first sign of weakness, to be on the safe side.

What one chooses to do is a matter of personal preference, lifestyle, and other circumstances, for example garage or street parking.


I hope no one minds if I take this opportunity to go on a slight tangent. (new thread if desired)
Does anyone else besides me do a capacity (Ah) test of the 12V battery each year just before winter?

 

Hi Fred,

I do not; and am wondering what is your process to perform this test?

 

Yes, yes I am going to try and revive it, and if anything I will use it on my small solar panel set up I use in the garage for 12 volt lighting.

 

what is the date on top of your battery Julia? mine is 8/12

 

thanks Uart,
I was getting miffed as why the voltage increased , the more it sat.
thanks for the explaation.

 

It's a long thread, but somewhere a few pages back she said it was 6/12. So probably sitting for about 10 to 12 weeks.

According to this site (Generic battery technology comparison) a lead acid battery might self discharge at around 20% per month (might be less for some "advanced" batteries). So I guess it's not too unrealistic that Julia received her's at about 65% SOC.

 

you might notice if you read my posts in other battery threads that I never saw MPG drop but I did have unable to start the car issues and My battery got down to 9.x at one point even after charging. Yes I charge with a desulfanating charger repeatedly but the OEM battery just got worse and worse. I literally used the charger on an extension cord in place of a working 12v battery a couple of times while waiting for the new battery to show up by fedex/ups/whoever they shipped it by.

Symptom not seen : MPG drop

Symptoms seen as it got worse: MFD warning about transaxle status because it wasn't put in Park correctly or somesuch
MFD not turning on all the way (dark or stuck with partial display unusable)
MFD coming on but car not entering ready mode
Car doing nothing at all (not enough power to display anything, no beep, no nothing)

The transmission warning came on when the battery was stronger but borderline, the other symptoms were when the battery was too weak for the computer to display that error.

 

At that point I'd call it good and stop checking the voltage for a year or two unless you plan on going out in a blizzard or some other possibly life threatening event.

You charged the battery and the battery is higher in AH than the OEM so even if it isn't 100% it's still more than the car needs. It should last years to come assuming you don't abuse it with prolonged undercharge situations (like parking the car for two years without ever turning it on or charging the battery).

As to the 13.x volts reported by the Optima rep I'm assuming he got that from the mindset of designing a charger cutoff limit, where as I check the voltage after letting the battery sit disconnected (the voltage drops every hour it's disconnected). So when I got 12.95 it might have been over 13 during the charging process but I didn't care, I just wanted to know the resting voltage.

 

At this rate the thread is going to be a sticky

 

Hi Patrick, my capacity test process is to simply measure the time that the battery delivers power under a defined load before the voltage drops too much. My personal test procedure is similar to the one, for example, here(click) at batteryfaq.org , but tailored to my personal circumstances and preferences:

Switch on low beam headlights.
Measure voltage every few minutes; and plot voltage, or calculate and log the rate of change
Turn off lights after about an hour, or when voltage drops below about 11V, or when rate of change exceeds about 2V/hour or increases suddenly, whichever comes first.
Recharge the battery immediately.
Calculate Amp hours, including all the little lights inside and out which were on.

This procedure gives only an approximate measurement, especially since I calculate the approximate amperage based upon simply adding up the nominal wattage of the light bulbs.

My personal comfort level/rule of thumb: When calculated Ah is below about 1/4 of rated capacity, or if the voltage drops very suddenly, then I start thinking about getting a new battery before the weather gets extremely cold.

I am not certain that the capacity test actually gives me more or better information than the normal voltage test, which is why I am wondering if anyone else does it. But it does give me a little added confidence, because I am not quite ready to put all my confidence in just a voltage test.

 

Voltage is only part of the story. Take for example a AA battery. If you grabe a duracel or energizer alkaline it will have a higher voltage than a rechargable NiMH battery. It doesn't mean the NiMH is worse just because of the lower voltage it's just different.

The catch is you have to do a lot of work to truly know the health of a battery, no simple non destructive test will tell you the true state of a battery.

Do you have a battery as good as mine? No one knows. Is it likely that it is OK, and I'm guessing so. You definately don't have to take my word for it but don't assume that a single battery voltage means a bad battery. You'll have to do much more work to prove that to anyone (thought they may just swap the battery on principle and sell it to someone else).