Request for Advice: 3.5yrs 12V Battery Replacement Time?

It seems a little early, but if it bothers you, get the peace of mind of a new battery. Fearing your car for months on end is not good.

 

especially if you're going to keep it another 4-5 years or more, you'll need to replace it at some point anyway.

 

11.9 is circling the bowl. True test is do that mfd test again first thing in the am before starting the car. We are interested in the first 2. I bet it will 11.6 which means replace now.

And for gods sake don't use your other Prius to jump. Thats doubling your chances for a $**** tragedy.

 

Gotcha. Its unhappy. When you start the car does it now throw up a red exclamation point for a second on the dash?

You may get another year or 2 out of it by doing some battery maintenance. Buy a battery charger with anti-sulphation mode and charge it as often as you can. Only takes a few minutes. May come around. Really handy to have a battery charger. There very cheap.

The best thing about having a battery charger and charging all your car's batteries once a month like alot of us here do is you can track battery health. Just watch how long it takes to fully charge the battery in 2 amp mode. If all of a sudden it takes twice as long that means its getting ready to leave you. The stock battery agm when it begins sulphating with no remediation seems to fail quickly for no apparent reason.

 

Your issue seem to be a fully drained battery for some reason. It takes a bit of a toll on the charge retaining capacity but does not ruin the battery.

Battery life has a lot more to do with how & where you drive (cold weather, long trips vs short trips, headlight on vs off etc.) The aux battery in my company car (2006 Prius) is 6.5 years old now and the car has 70K miles on it. The car gets driven infrequently (may be once a week at most) but when it does, it is usually > 100 miles at one time. I tested it about a year ago and it was showing 10.5 V. I tested it last week and it was down to 10.0V. I do think it is in the last stages of its life but the car still starts ok even after a few days standing to the elements with an overall mpg of ~46 (remember there are probably 20 different people who drive it).

In my opinion, 11.9V cold loaded is NOT a low battery as people seem to imply. There is definitely a lot of life (at least a few years) left in your battery if you maintain it using a good battery charger.

 

I agree with you 2006_HK. I know there is a lot of advice on this sight about replacing the 12V. I also know that it is well intentioned and people with good intentions tend to err on the side of not being left out in the cold. I think most DIYer's can deal with that possibility better and tend to downplay early replacement of the 12V. I would be comfortable with a 12V that shows 11.6 cold loaded but that doesn't mean everyone can deal with it like us. I guess it just comes down to a personal choice.

 

Just to add a piece of data: my 12V battery hasn't measured above 11.5V with headlights on for over a year now. It seems still working fine but YMMV.

 

And...it will probably last a lot longer too.

 

One other thing to point out here is the battery was completely discharged and you the drove the car for half an hour.

This is no where near enough to charge the battery. The Prius does not have an alternator pushing out 60 to 80 amps, it charges at 4 amps so you only put in a 2 amp charge or 5% of the batteries capacity.

Give it a charge for at least 20 hours at 2 amps then let it rest and do the battery test again and report the results.

 

Totally agree Brit. You the man

 

I understand the concerns people have about getting stranded but it is almost always due to a complete drain of a battery rather than a battery gradually losing charge carrying capacity. In other cases, the battery does not get fully charged since the car is not driven much and in that case a new battery is not going to help.

I hope people know that lead-acid battery recycling is quite harmful for the environment, especially because it is done in third world countries without many implemented pollution controls. Replacing fully functioning batteries early probably offsets the relative benefits you have achieved in using your high mpg car. The battery is damn expensive anyway so why not get the full use out of it. There is no reason to replace a 3-4 year old battery unless you have/detect a problem.

 

I will just add my own observation ... I replaced the 12V battery that was registering below 12V and immediately saw an improvement in MPG. Speculation is that the charging circuit that constantly loads the Prius can rob your MPG.

 

I have read this before and it is very interesting. However, I don't understand it. Would appreciate if somebody can explain about how/why this happens. Did your old battery get the opportunity to fully charge before you compared your mpg?

This is pure speculation on my part so please be kind if something is grossly incorrect: I can see the car charging a battery until the battery's capacity is full. This should be true for any battery, old or new. It may just be that your old battery is quite discharged so the circuit is charging it the whole time you drive the car. When you replace your old battery with a fully charged new battery, the car does not need to charge it as much so you see some mpg gains (how many?) But those gains should disappear with time as your new battery discharges and does not get fully charged with the amount of driving you do.

FYI, the battery on my 2006 measures 11.3V under load. I don't know its age since we bought the car used. We drive very little but I get 50 - 55 mpg with conservative driving. Wife gets 45 - 50. However, I have been using a battery tender once a week for the last 1 year to keep the battery fully charged.

 

I tried to top off the old battery with an external charger before I replaced it. It's an 07 with 100,000 miles so it gets driven quite a bit. I noticed that it had dropped about 5 mpg sometime over the summer so I went looking for what the problem might be and arrived at the 12V battery.

 

Some, not all, failure modes for a 12 volt lead acid battery involve one cell 'dieing'. The battery cannot get to 12 volts as it is only 5/6 working. In this case, the Inverter will try to charge the 12 volt battery forever.

Other failure modes effect capacity, not peak voltage, and have no impact on MPG.

 

Aha! Is this particular failure mode of one cell dying time dependent? I mean does it make sense to say that if your battery gets low on voltage (how much?) early in its useful life, it is more indicative of this particular failure mode than another battery that slowly loses its voltage over the years?

Additionally, do we have any data around what the voltage loss should look like assuming the Prius Gen 2 OEM battery is charged & maintained properly?

New - 12.8 V
1 Year - 12.x V
2 Year - 12.x V
3 Year - 12.x V
4 Year - 11.x V

 

A shorted cell is a random event, and is dependent on sulfation level of the battery. A small bit of sulfation can cause warping of the plates, and they will touch at some point. If caught early, with, for instance, a pulsing type of desulfator/charger, then that fault can be cleared and the battery will live on. If it is bad enough, then no amount of pulsing will clear it, and it's time to replace. The car's charger, even though it only draws 4A X 12V
~50W while trying to charge a shorted cell battery, seems eventually to create trouble for the inverter cooling, such that the ECU responds by keeping the engine running more. At least that is what others have reported enough times that it makes sense to me.

 

What a mess. My 2005 still has it's original battery which measures 11.6V on step 2 of the MFD test. All summer I have been getting crappy mileage, however, the A/C has been on quite a bit. I'm on the fence as far as replacing it goes. Part of me wants to see how long it will be until I get stranded and the other wants to test out that Exide battery. This whole argument about MPGs makes me want to see what would happen if I replace it, however, the weather is getting colder and I think that will effect my results.