Time to replace the aux battery?

I've heard that a bunch of times, but it still doesn't make sense. When the vehicle is running, and the charging system is maintaining system voltage, it really doesn't matter about the battery. In fact, most cars can run with the battery removed.

I think it's a common experience for a lot of people; the car dies, it won't start, so someone goes and buys a new battery, puts it in, and voila! "problem solved. It's got to be just like a flashlight with dead batteries, right? It needs new ones when they run down."

It's not really like that; what often happens, is a battery terminal was corroded, and the car was running fine, but when it was shut off, it wouldn't start. The old battery was discarded, even though it may have been fine.

The other most-likely scenario is that the alternator had failed, or is very weak. As the person drives, system voltage gets lower and lower; people passing by may notice the headlights very dim, but the driver is unaware of a problem until the car dies. Then someone puts in a new battery, and presto! the car seems to be fixed. It may work for a few days, and the same problem re-occurs. Damned batteries! Eventually the car gets a new alternator or charging system component. Damn cars nowadays, nothing lasts!

Even though the battery is a convenient whipping-boy, it's really not the cause of a lot of the problems it gets blamed for, but its a good money-maker for those who sell them, so they don't argue with you. Once the car is started and running, provided everything else is OK, it's really done it's job. After that, it's just a buffer for the electrical system. A good mechanic and a load-test is the best way to diagnose a battery before throwing it out.

 

In this case, your conclusion is mostly invalid for several reasons:

1) Faults in the computer system(s) may occur at a time when the battery is the ONLY source of voltage, like when the key is first turned ON.

2) A battery can fail in several different ways but the most common is the internal resistance going up such that it won't take or give up a charge. When that happens, the charging system is "fooled" into thinking the battery is charged when it isn't. As the charging system cuts off and the voltage drops suddenly, the spikes can cause problems in the computers too.

3) If a cell shorts out, the charging system can't keep up. That should be obvious with a voltage test but might make you think that the converter is bad when it is really the battery.

4) An intermittent open inside the battery can cause wild spikes in the charging system too; it wasn't designed to operate without a battery.

You indicated that you might be open to learning something but instead you seem to just want to argue.

Just because something doesn't make sense TO YOU doesn't mean that it is invalid.

 

if the aux battery is weak, the ice will run more in an attempt to charge it. that does affect mpg.

 

A common seat-of-pants test is running the windows up/down with engine off. A borderline battery will struggle. Still:

5.5 year old battery, displaying symptoms? Replace it. Don't want surprises? Go OEM.

 

Regular car has battery that has CCA of about 500A and Prius battery has CCA of maybe 250A. On cold start regular car draws about 200…400A. On start Prius will draw about 30A. So Prius battery can be degraded way more than regular cars battery can and still get it to start. Regular car will work fine with (almost any) battery that can just get it to start but Prius battery that can just get it to start can be in just a bad shape that it will cause other problems.

 

Easy Rider 2, 69shovlhed and valde3 have covered off all the salient points I would have mentioned so no need to rehash here.

The only tenet of this post I'd agree with though is this: it is important to correctly diagnose the fault causing your problem. It could be any of those things you mentioned. I too despair at the thought of people throwing out a perfectly good battery that just may need a good charge up, so it is important to get the diagnosis right.

 

Myths are stronger than facts, as we've seen here over and over.

 

Just because you can't understand something does not make it a myth.

People have offered you some explainations to help you understand.

At the end of the day you can believe what you wish.

As for myself, I have replaced two 12V's in two different Prii and seen the same improvement. I may have imagined it, but I find keeping detailed records helps mitigate placebo effect, so I'm confident in my findings. As I said, you can believe what you wish.

 

This is a common misconception. Many people believe that truth is decided by by consensus. Nothing could be farther from the truth. I'm even tempted many times to replace a battery when it's not necessary; we're conditioned to think that way by our consumerist, throwaway society. While it's true that a new battery "fixes" most electrical system problems, it's not necessarily the right solution.

I hate to say it, but most repair people have more to gain by selling you a new battery than cleaning your terminals, therefore they don't tell you any more than they have to. There are honest mechanics of course who will genuinely give good advice, but they aren't the ones that go on to start their own chains. The ones who tell you whatever makes you spend the most money are usually the ones that do. We need to support the honest mechanics, so that they are the ones who have their own chains.

A "denier" is someone who ignores evidence to make their own points, and conclusions. Someone who doesn't take things at face value, and wants to get the issue sorted out is the very opposite of a denier. Those who question the myths or the consensus, aren't automatically right, but chances are that someone who has spent more time with the evidence is closer to the truth.

 

When I say I replaced the 12V's, I mean after testing and concluding the batteries had failed, I replace them.

There was none of this, no salesmen or mechanics as I did my own investigation and diagnosis, but I have no interest in starting a battery repair chain.

The pot calling the kettle black, methinks.

 

Things are taking a sour turn here. If I disagree, it doesn't mean I don't understand what you're saying. If you don't understand my rationale, ask for clarification, don't cite others who have the same misconception. It just adds to the confusion.

I agree, it can be frustrating, but try to focus on the subject matter, without getting personal.

 

What misconception would that be exactly?

 

The claim that the "condition" of the battery somehow affects performance requires evidence, or at least good rationale, not just hearsay. It goes against what I know from many years of experience.

 

When that condition is "failed" as in will not hold a charge, how can you even believe that it will not have a negative effect on a Prius' performance in terms of fuel consumption?

The basis for my conjecture is what I have been able to determine myself, so it not hearsay. Hearsay is repeating what you heard someone else say but did not witness it yourself. The fact I can corroborate what others have observed, adds credibility in my book.

What experiments, or otherwise, have you conducted on a Prius to prove your belief?

Let me be specific here that I am talking about Toyota Prii and not general motor vehicles, perhaps the experience to which you refer is in general motor vehicles, so let me be specific here that I am talking about Toyota Prii and not general motor vehicles.

To come back to the OP's question, is it time to replace the aux battery? My answer is: if you have tested the aux battery and found it has failed, then yes it is time. Simple as that.

A battery that has failed is using resources and, as it is on continuous charge, is going to cost you fuel. Let me also say this, it is not a lot, maybe 0.3-0.5 l/100km, so the OP's hope that a bad 12V battery would be the reason for his poor economy (42.3 MPG, --> low-30s) is a vain hope in my opinion. There are obviously others things going on with his vehicle.

I hope this makes my position clearer.

 

I'd be very interested to see evidence of that claim. In over 40 years, I haven't seen it.

When any vehicle is running, it's not drawing power from the battery for most things. The charging system is matching generated power to the load consumed, most of the time. Exceptions might be when you have the blower on full, headlights on, brake-lights on, and crank the electric steering box. In such a case, the battery reserve might temporarily draw-down a little because the charging circuit can't quite compensate, but if that happens at all, it's temporary.

The battery is for starting the car, and using accessories when the engine is stopped. That's about it. When the car is running, it doesn't care about the battery. Any voltage fluctuations while running are due to the charging circuit, not the battery.

 

So if the 12V battery is low, does the car care enough to charge it?

You've been working on Prius for 40 years?

 

Generators, alternators, regulators, converters, rectifiers, inverters, batteries, you name it; on nearly any machine you could imagine, I've troubleshooted, repaired or modified it. I think I know my way around this stuff.

 

Remember when the prius is "running" the ice often isn't-- the prius does not have a typical alternator that is charging and supplying voltage to everything in the car the entire time it is "running". if the batteries' voltage is low, the ice will kick on to charge them even if your not moving, so you use more fuel. how much more? fiik, but you will see a drop in mpg.

think about it: if you're driving at night with the radio, lights and a/c on, where they getting power from when you're sitting at a stop light in heavy traffic? ... but wait-- the aux battery is weak; now where's the power coming from?

 

What part of my message that was 100% true do you think was only sale tactic? I’m not for replacing battery every time there is problem that can have something to do with battery. When I bought my Prius it was probably started with jumper cables. First day it even had couple of odd behavers at startup. But after long drive it has been ok and battery voltage tests are ok so battery was just empty.

If the 12V battery would only have high internal resistance it wouldn’t actually cause any other than startup problems but battery can have many other problems as well.

 

It has nothing to do with the ICE. The DC-DC converter takes the place of a conventional charging system.