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Weak 12V Battery?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by mycnam, Apr 15, 2014.

  1. mycnam

    mycnam Member

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    Today I checked the voltage of the 12V battery. The battery is the original 4-year old.

    I measured directly at the battery terminal. 12.5V right after switched off, but after I pressed the power button twice to IG-ON without pressing the brake and turned on the headlight, it dropped to 11.75V and dropping in 0.01V steadily. I tested the power window which was slightly slower in IG-ON comparing to READY. After switched off the car again, it read 12.1V.

    The car has a 6-mile round trip everyday, and occasional long trip during the weekends. May be it's just not charged enough. But since the battery is more than 4 years old, and with the recent Optima free charger deal, I may get a new 12V battery.
     
  2. macman408

    macman408 Electron Guidance Counselor

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    That's actually not too far off from what my battery measured when it was almost new, 4 years ago; 12.50V with everything off (no doors open, etc.). Your loaded IG-ON is a bit lower - mine was 12.05V, but yours isn't THAT much lower. Mine was 12.3V after turning it off; again, a little higher, but not too much.

    I'm amazed at all the people that want to replace their 12V battery before it shows any real signs of weakness. I get that having a dead 12V is generally a huge inconvenience... but still, I plan on running with mine until I really notice it doing something bad.
     
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  3. dorunron

    dorunron Senior Member

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    Going by the numbers.

    A "fully charged" AGM (Battery found in Prius) should measure 12.8 volts OR higher when measured at the post's with NO load. Remember the battery is loaded at all times while the battery is connected to the Prius even in "Power Off" mode. Most user's will report approximately 12.6 to 12.7 volts if measuring the voltage at the battery or jump post after the Prius has been powered down for at least a couple of hours. Remember even opening a door will activate certain ECU's even though the Prius is still in "Power Off" condition. Voltage will vary depending on how much load is placed on the battery.

    Now looking at the OP's circumstances he/she did the following. "Pushed the power button twice AND turned on the headlights." He/she then measured the voltage while these circuits were active and found 11.75 volts with a .01 volt drop (steadily?). Again going by the numbers and various load factors IMO the measurement of 11.75 volts is really NOT bad when you consider how many amps are being pulled at that time to power up all of the devices, ECU's and the headlights at that time. Sure, it would not hurt to be a couple of "tenths" (.1 or .2 volts) higher, but IMO is still okay.

    When checking voltage as the OP did it is important to understand that any load applied will cause the battery voltage to drop. The more load you place on the battery the lower the voltage will read.

    If you truly want to test a battery to see if it is good or bad, a "load test" should be run on it. The average person might or might not have that tool, but usually will have a fairly decent "digital" multimeter.

    Furthermore, it is important to understand that IF you are only using the multimeter to test voltage as the OP is doing, consider the fact that the voltage should return to close to the level the battery read before it was loaded up as he did by turning on the lights and ignition. HOWEVER the "voltage return" will NOT be instantaneous. It generally takes a minute or so for resistance values to settle down and everything else in the system to equalize before the voltage returns to the normal value UNLESS the "auxiliary" battery (12 volt) has been drained substantially while doing a test such as turning on various items as the OP did.

    To sum it up, at this time I believe the OP's purpose of changing the battery will really only help IF the OP is concerned with longevity of said 12 volt battery. IMO the voltage is measuring fine for the conditions of the test the OP ran. IMO, I would wait until I saw the voltage level of 12.5 which the OP measured drop down to at least 12.2 or lower when testing with NO load applied including testing at the jump post or battery post while the Prius has been off for a while.

    Please refer to the following chart for a better understanding of what the voltage levels mean and how they relate to the percentage of charge also known as "State of Charge" (SOC). One other factor concerning voltage. It is NORMAL for voltage measurement to be LOWER if the ambient temperature is "cold". By cold I am referring to temperatures similar to 35 degrees Fahrenheit or lower. More information on temperature and voltage variation can be found by doing a Google search on that topic.

    Ron (dorunron)

    Battery SOC Chart.jpg
     
  4. GrumpyCabbie

    GrumpyCabbie Senior Member

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    For the folks out there reading this, can I ask on what you plan to notice? In a traditional car the car will crank over slower and slower over about a week giving ample notice the 12v is failing. How you going to tell in a Prius? It will work, work, work, and one day it won't. No clue, no slow cranking. One day there will just be enough power to boot the computers and one day there won't be.
     
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  5. dorunron

    dorunron Senior Member

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    On the Gen II, the red triangle would light up momentarily when the 12 volt is failing. That is generally caused if the voltage falls below 11.6 volts when powering up the Prius.

    The Gen III though is a little different in my opinion. Not much of warning signs that I am aware of. Good point raised GrumpyCabbie | PriusChat

    Ron
     
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  6. macman408

    macman408 Electron Guidance Counselor

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    Yeah, that's one good sign I'll keep an eye out for. ;) If I have to jump it two or three days in a row, I'll start really considering if the battery has another year left in it or not. :D

    A few people have reported computer problems, like odd display behavior or poor mileage, or a car that doesn't quite start all the way when the power button is pressed; and that these have resolved when replacing the 12V battery.

    An astute observer might also be able to draw some conclusions based on the sound of the brake booster pump that runs when you open the door; its speed seems to be fairly voltage-dependent.

    Also, I'll probably check it periodically with my multimeter - Toyota specs the battery as discharged when measured at 11.5-11.9V (20 minutes after turning the car and all lights off, and at 68°F/20°C).
     
  7. mycnam

    mycnam Member

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    I left the car in READY for 2 hours today. Now it reads 12.7V with everything off. I think I might not need a new battery now. :)
     
  8. dorunron

    dorunron Senior Member

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    Check it in the morning and see if it still reads 12.7 If it does, invest in a smart battery charger. If it is weak again, think about a battery sometime in the near future. FWIW, quite a few of Gen II users have been complaining about the quality of the Yellow Tops recently. Now the hot ticket battery seems to come in as the one Toyota sells. Just wanted to mention that to you.

    Battery SOC Chart.jpg
     
  9. mycnam

    mycnam Member

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    Thanks dorunron, So according to this chart 12.7V is still not yet 100%. I drove and charged in READY for at least 3 hours today. May be that's still not long enough. Doesn't people here usually get 3-5 years life from the original battery?
     
  10. dorunron

    dorunron Senior Member

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    Read the lengthy first post I wrote above this in this string. 12.7 volts is not bad IF the battery is hooked up in the Prius. Even when the Prius is powered down, it is still drawing on the battery, hence the voltage will read lower than a fully charged AGM battery will measure with "NO" load. A fully charged AGM battery WILL measure 12.8 volts or higher if the battery is operating to factory specifications (IE: NEW).

    So, 12.7 is not bad IF the battery will HOLD that charge overnight.

    4 years is a average lifetime of a 12 volt battery. Some last longer than others, some don't last that long at all. It depends on how the battery is used and maintained. I have seen folks get 6 years and longer out of them, but they took good care of them and did not abuse them.

    FWIW, 12.6 is about what you will see when you measure at the jump post with a good fully charged AGM battery in the Prius. That would be considered normal. Remember even when the Prius is off, there is still a parasitic drain at all times.

    Ron
     
  11. mycnam

    mycnam Member

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    The battery was charged to 12.7V two days ago but today it measures 12.5V again.
     
  12. dorunron

    dorunron Senior Member

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    Sounds good to me.

    .2 volts drop in two days is fine. If you see stuff like 12.7 one night, then the next day 11.9 or lower then you have a problem.

    Keep on keeping on.

    Ron
     
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  13. The Critic

    The Critic Resident Critic

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    Ok, so I tested the original 12v battery on my 2011. I have about 87k on mine. The voltage checked out okay, but my digital HF tester showed 6.61 mOhm of resistance. I was told that the value is unusually high. Any ideas? Thanks.
     
  14. macman408

    macman408 Electron Guidance Counselor

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    Are they used to checking Prius batteries? It's possible that it has a higher resistance, which wouldn't matter as much since the current demand on it is much lower than on a standard vehicle, which has to crank the engine. A long time ago, I think somebody hooked up an ammeter to their battery while powering it on, and found a couple of 40-amp peaks for a few seconds (I think mainly for the brake booster pump); if the DC resistance were 6.61 mOhms, that would give a 0.26 V drop, which is pretty insignificant. A 250-amp starter motor would see a 1.6V drop, which might be significant.
     
  15. Welshdog

    Welshdog Member

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    Well here's the birds eye lowdown on my battery caper. Wifey is out the door this morning on her way to work. About a minute later she's back in the front door saying "Prius won't start - nothing." After a bit of running around she takes my car and I start troubleshooting the problem. After reading the manual I find the battery, disconnect the ground and put a slow charger on it. After an hour I check the voltage and it is 12.9 and then after connecting the ground it is 12.7 - so far so good. I get in and try to start - no joy. All I see is the door open indicator on the display and hear a sad moaning sound coming from some component under the hood. I go back to check the voltage and it is 5.8 volts. I am concluding worn out battery. This is on a 2010 Prius purchased in July of 2009 so it is a 5 year old battery. I'll probably go to the dealer to morrow and buy a battery from them and install myself.

    Oh - we are in Austin, Texas and it is very hot here. In fact during the life of this car we have suffered many record high temperatures year after year. I feel good about the 5 year life of the battery.
     
  16. JC91006

    JC91006 Senior Member

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    If that's an original 2010 battery, most likely it's dead. If you can fully charge the battery in 1 hour, then it's most definitely bad. It normally takes about 8 hours or more at 4 amps to charge that battery up.
     
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  17. Welshdog

    Welshdog Member

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    Good to know - the meter on the charger showed a very low level of current when I connected it. In other words it was not charging very heavily. When I have used it on other larger dead batteries the meter shows a very heavy current draw as it is charging. In that one hour, the meter went from reading about 2 amps to zero amps of charge. I suppose I can try leaving it on longer, but I doubt it will do much.
     
  18. JC91006

    JC91006 Senior Member

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    I don't suggest you continue trying to charge that battery. It's past the sell by date, needs to be replaced.
     
  19. Welshdog

    Welshdog Member

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    So the dealer wants $240 for the battery alone - no installation. Going to try the other Toyota dealer. Anyone have luck with other brands that fit and have the vent tube? Seems like I saw a post that O'Reilly had one that worked well.
     
  20. Roland1555

    Roland1555 Senior Member

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    I put the Yellow Top Optima in myself. It has the vent hole and is an exact fit. Many people have had great results with them over the years, myself, it was only installed in the spring so I'll have to get back at you in another 5-10 years to see if I can add to their reports.

    Other people seem happy with the Exide replacement battery that also is an exact fit again, with vent hole and the small reversed Japanese battery posts. Just be sure to get a battery that has a very recent date on it in terms of when it was manufactured. Don't want to buy something that was sitting on the shelf for a year and not maintained.

    Roland
     
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