Auxiliary battery keeps dying

That's WAY too low.
The other main connection depends on what model you have; it's the other end of the red positive cable leaving the battery.
But with the reading you reported above, you might as well start shopping for a new battery.......assuming that the charging voltage when running is above 13.

 

Here's the latest. I ran some voltage tests. 12.25V using a meter under the hood, car off. Then the MFD checks: 11.9V vehicle signal check mode, 10.7V with headlights, rear defrost, and fan, 14.3V IG-on and charging. When I reported all of this to the guy at eLearnAid this morning, he said that those numbers don't indicate battery failure. He recommends charging it and checking for loose connections, then trying to track down something in the car that may be draining it. So some agreement and some disagreement with the advice here.

 

Well I'm sorry but he is just plain wrong.
IF.....the car is driven much, the 14+ charging voltage is plenty to keep it charged.
Significant parasitic drains are not impossible but VERY rare.

So.....to prove what the problem is.......charge the battery good with a small charger.
Disconnect one of the posts. That insures that there is no external parasitic drain.
IF the battery voltage drops below 12.6 volts after sitting.....then it is bad.
No further tests or discussion is needed.

But I really don't think all of that is necessary. The readings you found are EXACTLY what happens when a battery gets old and fails to hold a charge.

What is eLearnAid anyway ??
Not a good source if information in this case.....probably.

 

The car isn't driven much, which takes us back to the original P. Wong suggestion: Charge it overnight occasionally.
I will try this test of charging it and then checking it, but how long do I leave it off the charger before testing?
eLearnAid is the company who sells the Optima yellow top at the best price with the quickest shipping. I only know about them because that's where all the Prius Chat links go when one needs the battery replaced. Their website has pages and pages about the Prius battery. They have a three year warranty on the Optima, which is why I called them. This is why I need to prove one way or the other if the battery is dying.
So I'll charge it tonight, but again, how long do I wait before testing the voltage after charging?

 

Several hours resting time to bleed off the "surface charge", The voltage should then hold at about 12.9 in a new AGM battery and as was said, above 12.5 for several days.

I am surprised that eLeanaid gave you incorrect advise. They are usually a good outfit.

JeffD

 

toyota says that 30 seconds with the headlights on will do it as well. i'm just not sure how turning the car on to turn the headlights on affects it.

Merged

maybe the elearnaid guy (who knows way more than me) is saying that the active numbers are good, and wouldn't be so if the battery was defective, and it simply needs a charge to bring up the resting voltage, which the prius doesn't do well.

you may need to put a smart charger on it every now and again, but a good battery should hold for 5 or 6 weeks in decent weather, and fyi, the reason people buy the optima is just so it will hold a charge longer than the oem and they can leave it at the airport and not worry.

 

Not needed. Opening the drivers door with the FOB present should fire off enough activity to take care of that.
Or run the headlights for 30 seconds.

NOTE: It is important to NOT charge the battery too long with a high capacity charger.
An automatic one around 2 amps is best and it may need to be connected up to 12 hours.

 

The ongoing tests seem to indicate the battery is dying. I've been unable to get it to anywhere near 13V, either by external charging (admittedly, using a Battery Tender Jr., which provides .75 A, but was connected for about 16 hours) or by internal (driving for a few short trips, then leaving the car parked, on, with all accessories and display turned off for three hours). The former came to 12.71, but faded to 12.41 in a few minutes, without the car even on. The latter only got me to 12.41 measured at the battery and 12.1 when I used the MFD Vehicle Signal Check, and that was without any deliberate waiting or bleeding off.
I plan to call eLearnAid again tomorrow and present these results. If the battery can't be brought to 12.9, then it's not doing its job.

As a side question, someone told me he fought this kind of problem in his Prius by using a jumpstarter-type battery pack, but one that could charge through its own cables. He permanently connected it to the battery and kept it in the trunk well. The car would charge the aux battery and keep this topped off, and then if the aux battery began to fail, it would automatically be buffed by the extra one. Is this crazy or genius?

 

crazy.

 

Crazy. One day the bad battery will use all the power from extra battery while car is sitting. Then he will just have two empty batteries.

 

The top no load voltage means virtually nothing.

If it is not an AGM, the top voltage may very well never exceed 12.7 when not actively charging.
The voltage readings after a rest and under a small load are MUCH more telling........and they say that you need a new battery.

It is neither crazy nor genius. It simply increases the battery capacity......which could be done with a bigger battery.
But as pointed out, in many circumstances they BOTH would end up dead at the same time.
It is much better to keep your "emergency" battery charged but not continuously connected.

Why are you so stuck on eLearnAid ? Did you get the old battery from them ??

 

I'm stuck on them because they sold me the battery. If it's failed, it should be covered under their three year warranty.
I've ordered an Optima brand charger. I'll charge the battery fully (to whatever voltage the charger deems full, which I don't seem to be able to do using the Prius itself) and then resume our normal driving patterns. I should be able to determine a good schedule for topping it off on the charger. But if it plummets after a day, I'll present that to eLearnAid.

 

a good question might be to ask them how long a healthy battery should be able to sit, unused, and still start the car. my 3 year old oem sits for 5 weeks and starts just fine.

 

That's not a practical question. I can't leave the car untouched for a few weeks to see if it starts. And then if it does, try it again and wait even longer to see how long it can go before it doesn't start.

 

sorry, i was thinking of another poster who left his car for 2 months. too many bad 12v threads.

 

I got the Optima Digital 400 charger/Maintainer. I'll use it to fully charge the battery and then check it every night to see how many days it takes before needing another top off. I should be able to determine a regular schedule, and hopefully, this restores some health to the battery. I'll post results.

 

You are just postponing the inevitable......probably until the battery is out of warranty.

If the car's charging system is working right, and yours seems to be, there is no reason that the battery should not be near to a full charge all the time.......if the car is driven regularly.

What you propose to do won't hurt and if it finally convinces you of what is going on then it will be a good thing.
There is an outside chance that the "new" battery never was fully charged and sometimes the car won't be able to get it full.

Please DO keep us posted with your results.

 

I recently got a reminder for some service in the car, which gave me an idea of my annual mileage. We drove 7,500 miles in about 48 weeks, which will average to about 8,125 miles annually, mostly short trips. By all accounts, this isn't enough to keep the battery charged. If I find it needs charging every few days, I'll call again about warranty service. But if I only have to top it off every week or two, and long trips keep it full, then I think it is performing as expected. We'll see.

 

OK, it depends on your definition of "short trips".

Having it on and running for 15 to 20 minutes.......regardless of the distance covered........should keep it fully charged, or nearly so.......IF it was fully charged to begin with.

 

i only drive 9000 a year and have never had a problem. if you start with a fully charged battery, and never drain it down, you should be fine.