Time to replace the aux battery?

Your knowledge consists of multiple assumptions based on incomplete observations........or can you tell us where you got your training in Electronics Engineering ??

Your experience obviously was with conventional engines and electrical systems, as proved by your own comment about failing to start.
In older systems, the 12 V was the starting battery.
Your OLD experience is virtually worthless in the new systems where everything is computer controlled and the starting is done by the HV system.

At one point, you claimed to be open to learning something new.......but then stubbornly refuse to listen to new information.

Your participation in these electrical discussions is inept, useless and disruptive.
If you can't bring yourself to LISTEN once in a while, I think you should just take a vacation from posting for a while.

 

I started out with a bad battery from the dealership; it had been in their showroom for months, regularly going dead and being boosted, until when I took delivery it couldn't hold a charge for more than 10 minutes while in acc-mode. I didn't notice any change in fuel economy at all, before or after I had a new battery installed.

Interestingly, over this past weekend I noticed a significant change in the fuel economy readout. During the weeks previous, I hadn't been plugging in, and I was averaging 4.8-5.0 L/100km on my daily commute. This is down from the typical 4.0-4.4 I was seeing in the summer months. I put this down to the colder weather. But over the weekend, I gave it a plug-in charge, and drove it with about 14-18 kms showing as EV range, but with the EV mode turned off. My mileage on my commute this morning was 4.1. This is back on par with the summer, even though it's as cold here now as it's been for the last few weeks. I'm wondering if the stored EV charge is somehow influencing the fuel economy. It doesn't make sense that it should, but I will continue to test this phenomenon.


Spirited might not be the word I'd use, because the points of dispute were minuscule. I believe the real intent was trolling to insult and try to embarrass someone for the sake of starting a fight. The challenge in such a situation is to maintain objectivity and not to fall to the level of retaliating to the personal insults. As usual, once the facts were clarified, the troll headed back under the bridge, waiting for the next chance to pounce.

 

A freshly charged battery will read deceptively high, is said to have a "surface charge"*. What I'll do is just drive the car for a day or two, then check. Basically, you need to apply a bit of load to the battery, to remove this surface charge, before the check. Another option is to turn the headlights for a couple of minutes, with the car off.

* Not sure of the science, just work with it.

 

The load of the headlights for about 15 seconds is more than enough........as is just letting it sit for a few hours.
Usually the load of just opening a door with the dome lights and computers is enough too.

 

When I google "how to remove surface charge from a 12 volt battery" the first 3 hits suggest headlights for 3 minutes.

 

How far do you drive the car on one trip?
What is the cabin temperature set at? If you have the temp set high, the gasoline engine will need to run to keep the cabin heater core hot.

A new, fully-charged AGM battery should measure 13.0VDC after surface charge has dissipated. To the extent your batteries measure lower than that, this gives you an indication of their state of charge.

 

That’s just one example. But main thing about this is not that that short is using power as Prius is driven but that it has to charge battery up every time it is driven since battery has discharged itself.

 

Well, that's a fair point, milliamp-hours turn into amp-hours eventually, if it sits long enough. If you take short trips, your system is charging the battery a lot of the time.

 

It's strange, I've heard similar numbers as this, but for some reason our OEM battery, and I assume it's AGM, it's the stock battery, displays voltages similar to any lead-acid batteries we've had over the years, ie: full charged over 12.6, moderately charged around 12.5, getting iffy around 12.4.

 

A 3.5 mile commute taking 15 minutes is going to result in poor fuel economy no matter what your ambient air temp is. The Prius mpg in the first five minutes is going to be really bad because the gasoline engine runs to heat up the catalytic converter.

I wouldn't worry about your mpg at this time. When you have a chance to take a long road trip in more pleasant ambient temps, then see what it is.

Regarding ECU learning, I would not expect that to materially impact your actual mpg. It might impact the mpg reported per the MFD.

 

Well, how do you know your 12V battery is fully-charged at 12.6V? More likely, your battery charger is stopping its charging process before it should. This is why modern battery chargers have the ability to switch between an AGM mode vs. a flooded lead acid mode.

 

Trading interior heat for a tiny bit of fuel-efficiency? That comes close to a definition of insanity.

 

I could think of a few words and spirited is not one of them.

Once again an incorrect assessment of the available facts. I do not engage in trolling. As I have previously stated, my intent and motivation was education, mine specifically. If someone says, I'm wrong, I like discuss and flesh out the reasons why that might be so and then learn from it. Most people oblige and are forth coming to explain their objection to what I have said. I have great trouble accepting " because I said so"from strangers, so strangers have to provide me with a reason I should listen to them.

Says the man who didn't like something someone did a few posts back and then intimated they were insane. There have been so many occasions where I could have gone that route, but it is not what I do, so I just bit my tongue. However, if someone's behaviour is inappropriate, I have no qualms about pulling them up on it. It's not the same thing as personal attack.

If you believe you have clarified "the facts", you are indeed deluded. I merely extracted (like a dentist!) the information (eventually) I needed to evaluate the information provided. As I'm not a troll, there is no bridge dwelling for me, no pouncing either. If I'm no longer engaging with you, there are other reasons. From here on in, do not take no response as agreement, I'm just choosing not to respond.

 

One thing at a time. I think it's obvious where the trolling occurred and I don't think it was you. Most of the conflict came from some someone making a great deal of hay over the meaning of a word, and the interpretation of rationale supporting an argument.

No need to crap in the nest here. This thread is already crappy enough.

 

HEY!!! ITS COMFY UNDER MY BRIDGE!!!

 

Hi Greg. I haven't read though this whole thread, but have been involved with similar discussions many times. My personal opinion is that as long as the battery doesn't have shorted cells *and* as long as the battery doesn't allow the instantaneous voltage to dip below about 10 volts during boot up, then a weak 12V battery has minimal impact on fuel economy.

The majority of weak and ageing AGM batteries simply have a somewhat high internal resistance or somewhat low capacity, not shorted cells. This can easily be tested by placing the battery on a small (say 2A) charger. If the voltage under charge rises to around 13.8 volts or higher in a reasonable time then this battery won't be draining any significant power from the DC-DC converter, and wont have any impact on fuel efficiency.

This was precisely what happened when my OEM battery finally took a dive after about 8 years of service. The battery was not taking any excessive charge, it was not heating up, it was not impacting on fuel economy. It was simple case of high resistance / low capacity and started indicating lower voltages each time I tested it in "ignition on" mode. I had been noticing slightly low voltages at ignition on mode (about 11.7) for over a month, but then at the start of winter it took a dive and that reading fell from 11.7 to about 10.5 over just a few days. I didn't even get glitch in fuel economy, I just popped in a new battery and went on my way.

Now this isn't to say that people aren't going to get problems if the battery is so poor that it lets the voltage dip too much on start up, when all the ECU's are booting. The prius doesn't need anywhere near the cranking current that a regular car does, but it does still take a brief spike of current as everything boots up. If the voltage dips too low at start up then it's possible that some ECU's wont initial properly, and then people start getting strange anomalies. You definitely want to replace a weak battery before any of these type of problems start to arise.

 

Hi Uart,

I can offer a counter-example. Earlier this year, the two-year-old 12V Exide AGM battery on my daughter's 2007 was weak. I put it on a lab power supply set to 14.4V and 3A. The battery continued to draw 3A for an extended period so it was clear it had a shorted cell and I returned the battery to the store for a warranty replacement.

While that defective battery was installed in the Prius, its current draw once the car was made READY would be limited by the resistance in the battery cables. That is probably why the negative cable gauge is relatively narrow and the cable is secured only by a little bolt to the inner fender.

Even if the defective battery was drawing 30A current, that would only be 400W of power and it is not likely that incremental power loss would be noticeable regarding a change in mpg.

Suppose the driving cycle is such that the Prius requires 25 hp average. 25 hp = 18,650 watts. 400 watts is a 2% change on that base. A 2% change reflects a 1 mpg difference on an 50 mpg average. Who can measure their fuel economy that precisely?

I agree that a low starting voltage can cause problems with the ECUs logging spurious fault codes so a diligent owner will keep the 12V battery in good condition and be alert for any signs of a problem such as dim cabin lighting. Or, the owner should not be surprised when the car experiences a no-start issue.