Aux Batteries keep dying

Just a thought, did you check if someone left a dime light on those couple of times the 12v batt was dead?

 

That is one reason I change dome lights to low-current LEDs. While this doesn't save the 12V battery, it does allow far more time to find and remedy this user error before the battery fails or drains enough to not start the car. With the right LEDs, overnight errors shouldn't be a problem, it may even survive full weekends.

 

So is that a meter on a battery charger?

Here is another one (on a load tester).


If you look at your chart, it is grossly more optimistic (as in the lower margins and ranges are lower) than this battery load tester.

A couple points, none of these data points show the correct top voltage for AGM/Gel. This is not a problem, per se, as I believe they are hedging their bets to provide a simplistic one-size-fits-all view. I don't have a problem with this, it is very pragmatic, but I don't think this forms the basis of fact which you can base your assumptions. I'd still like to see a more scholarly basis, myself.

In any case, the load tester will find a bad battery, I just think they would consign a redeemable battery to fail status more often. Any professional using this generally will, or at least will be inclined, to not take the time to mess around trying to save the battery, especially if the meter provides justification to sell a new battery. I'm not saying this is a bad thing, again, just pragmatic.

While I agree with the sentiment, I still think a load test has benefits. If the battery is truly bad a load test will show it.

As generalist advice, this may be true, but for a Gen 2 Prius, (before fully quiesced) the figure might be more like 167 mA (0.167 A) – Note, the Gen 2 Prius progresses toward total quiescence in three tranches. In the first tranche the draw is still above 1 A, maybe 1.67 A (but don't quote me). In the second tranche, it reduces to around 50 mA, and then finally when quiesced at ~21 mA (+/- 5 mA) with periodic spikes to 50 mA. This takes about 30-40 min. So this is when you should test.

For a Gen 2, this would be more like 21-25 mA, but the lower the better. (Also factor in the accuracy of the voltmeter you are using.)

I don't know how I skipped over this, so I apologize for not mentioning this sooner, but as has been commented at this point it may be more productive to do a rudimentary load test. This procedure can be done without having to buy specialist equipment you might not use again (and as you have the time, why not?)

• The car should be off for at least 1 hour.
• Measure 12 V battery at battery terminals
  • Write this voltage down.
• With car still off, turn the headlights on to high beam and leave on for 5 mins.
  • Notice the voltage drops quickly, but should settle maybe at 0.5 V lower than the start voltage. The actual amount is only significant as a guide to how worn the battery is.
  • CAUTION If the voltage goes into freefall and continues past 10 V. Stop the test. Your battery needs to be replaced.
• At the end of 5 minutes with headlights still on, measure the voltage
  • Again, write this value down.
• Turn the headlights off.
• Wait 5-10 min.
  • Observe that the voltage starts to go back up at a reasonably quick rate.
• After the 5-10 mins, the voltage should have settled, if not wait another 5 mins.
  • NOTE If necessary repeat this step until the voltage holds stable.
• Measure the stable voltage.
  • Write this final voltage down.
  • NOTE this finishing voltage should be very close to the starting voltage (within 0.1-0.2 V), if not the same. Again, the significance of the difference is an indicator of how worn the 12 V battery is.

If you can get through this test, I am willing to say that, on the chance of probability, your battery is more likely to be serviceable than not. It could still make sense to be prepared to replace the battery if it strays into the low 11 V range while under that (relatively light) load.

If you do get through this test, you can move on and check to see what the quiescent draw is. If it is significantly above 50 mA (which it should be) proceed with a current draw test. If it is less 60 mA, you may decide not to bother, but then something does not add up if you get to this point.

I hope that helps, all the best.

 

Good test. But I think you missed a (somewhat self explanatory) line:

Turn off the lights.

 

Hah hah. Good catch!

Thanks so much, I really laugh at myself. Glad somebody picked that up in time so I could fix it. I found several other errors/typos as well.

 

No it isn't.
It might give a false "good" indication, but it it FAILS you can be pretty sure that it is indeed BAD.

 

A video on load testing:

 

Two things about that:
A flooded conventional battery that reads over 13 volts at rest IS suspect.
A single load test proves pretty much nothing, which is why so MANY shops pronounce that a bad battery is good when it isn't.
A longer test is needed......or several tests in a row.

Note: that SOLAR tester in the video......it seems like I remember someone on here recommending that adnauseum.
Wonder who that was ??

 

My god, will you EVER stop providing completely false and misleading information? About time to educate yourself, don't you think? Then again, I need to stop feeding trolls; at least that is WIP.

 

Given the credentials that you list in your signature, this just goes to prove that most mechanics.....even highly trained ones......have no clue about how electrical stuff works.
There is no trolling here.
What I said is absolutely true.
A battery that significantly fails a load test.......IS BAD.

 

Have my 07 for 13 years bought new. Its had 3 battery's. A healthy battery in the car will live at 12.6 volts with car off. Its normal charging voltage when car is on is 13.8 to 14.1 depending on the load on it. All 3 battery's were the same. Been that way since new.

I have had two 4 amp chargers and to get to its rated 13.6 volts takes a long time as the battery becomes full it has less draw so it takes hours and hours to go from say 13.1 volts to its 13.6. On my newest charger the Ctek MUS 4.3 it takes about 20 hours to get from 12.6 to 13.6. It also has recondition mode.

Biggest suspect on a parasitic draw is the back up power box for the brake system. it is full of caps that become esr'ed over the years and then basically become a resistor. Especially if the car lives outside. That box is connected directly to the 12 volt battery.

But I would say really the biggest parasitic draw is the owner. People that keep having dead 12 volt battery's all have one thing in common. They never set the alarm every single time they get out of the car even in the garage.
If they did it would alert them to a light left on or door ajar or hatch ajar.

Best load test to do is easy. Just need a good voltmeter.

With car off measure the battery at the front jump point. Dont ever bother to jump or measure at the battery itself that is useless. And dangerous.
Always measure at the front jump point because thats what the fuse box see's. After going through the fuses on the battery itself and a long power lead in the car. Lots of things to go through before that voltage even gets to the fuse box. When you measure at the jump point you are testing that whole circuit.

Car off measure the battery at the front jump point. Write that down. Now with car still off turn on the headlights on high beam. leave them on for exactly 5 minutes. Then shut off the lights and measure the battery voltage again. My 2 year old Optima does not drop 1 volt.
Yours shouldn't either. I bet the OP's battery will fail this test.