“Not charging to its full capacity” is not quite 100% accurate. Alternators have indeed been downsized for fuel economy, since there’s no use to produce excess current that in turn must be converted into heat by the voltage regulator, but they are still totally capable of charging a healthy battery back to full capacity given enough time provided the battery has not been deeply discharged. So: How deeply has the battery been discharged? How much time is the vehicle running for the recharge cycle?

For example, suppose someone has a short drive to and from work every morning with several stop signs and red lights along the way. Winter rolls around; they go to work in the dark and come home in the dark, with the headlights on and the heater cranked full blast. The battery won’t get recharged, since at idle the alternator doesn’t produce much juice and the current draw from the headlights and heater can exceed the alternator output, so the power drawn from cranking the engine will soon deplete the battery. However, take that same battery and vehicle on a daytime drive for a couple of hours and the battery can get recharged fully, provided it hasn’t gotten too discharged. Then the battery must be put on an external charger to get it back up to snuff—but if it stays too far gone for too long, it could sulfate and then it’s time for a new battery. That wasn’t the car’s fault or the battery’s fault, just the usage conditions were not favorable to the battery.

 

As stated previously in my prior post - do what works for you.

I am not here to persuade you to do otherwise.

The more you read on the internet, the bewildered you are. This is what happens when you are curious, however, you need to put effort to learn new things that you are not used to. I will put it that way.

If you still think a modern alternator nowadays can fully charge a battery you can try it yourself and even prove it to yourself. Have the battery discharged to a certain extent. Drive it and use a battery tester to see the State of Charge health. That will be the evidence.

 

Well, I am an engineer with extensive experience in electronics, and I did a brief stint in the warranty side of the engineering department of a major automotive manufacturer (and that department dealt with customer complaints on “dead batteries” on brand new vehicles) …so I don't depend on the Internet for my knowledge. The battery requires voltage and current to recharge. Specifically, is the current that’s available enough to overcome the internal resistance of the battery and the demands of converting the electrolyte? Voltage will be sufficient…if the alternator and voltage regulator can supply enough current. Undersized alternators can still provide enough voltage (that’s easy) for a light current draw but will sag significantly under serious load.

So I have no idea why you are under the illusion that a modern alternator cannot fully recharge a healthy battery. If the battery is damaged (sulfated) or extremely discharged, that’s a different kettle of fish, but a healthy battery is easily topped off.

 

RX has a 150 amp alternator, not sure what you mean by the above? Alternator can output between basically zero and it's rated current it doesn't blast full power all the time.

 

The voltage regulator still must work (and consume some energy) to control the voltage output, and it’s more efficient to manage a smaller output alternator, that’s all I meant. And a less-powerful alternator is physically smaller, lighter and cheaper, I remember the old days when we could go into the auto parts store and get giant massive alternators (which had external voltage regulators), those days have long gone.

With mileage and emissions standards, if the manufacturers can save a tenth of a horsepower here and there, that’s significant to them. If a smaller alternator can be more efficient and save 100 watts, that’s all of 1/8 of a horsepower! Woo-hoo!

 

There isn't much difference in wasted electrical energy between a small output and large output alternator. Greater rotational mass will take more energy, how much not sure. I had a Corolla with an external voltage regulator is was 100% analog consisted of basically a giant relay that vibrated right at 14 volts to control output.
Problem is modern cars are loaded with electronics requiring large output alternators. Some alternators have decoupler pulleys (I hate them) to try and increase efficiency.

 

I decided to check the 3.5-year-old AGM battery on my 2017 RX350 with a Foxwell BT705 tester. I've had no battery problems whatsoever since new, but thought it was time to consider a new battery as preventative medicine.

The previous night I had completed a 25-mile highway trip. I checked the battery cold the next morning. The results were 12.30 volts; measured CCA 611 against 710 rated: "State of Health "87%; "State of Charge" 63% and resistance 4.92 MR. The unit also said good battery and recharge.

So I put my Battery Tender on the battery and after about 6 hours (the charger said fully charged then) the test results were: 12.73 volts, 713 CCA, SOH 100%, SOC 100%, 4.23 MR and "Good Battery".

My question is: what is the expected range for "State of Charge" during everyday use? Should we expect the battery to be at nearly 100% after a longish drive, or is the electrical system designed to keep at a lower optimal range - and what range would be acceptable?

 

First, there is currently a raging discussion on AGM battery charging in the Battery Replacement - DIY or Dealer? - ClubLexus - Lexus Forum Discussion thread. Some of the latest posts are related to your question. (Warning, there's lots of passion on the topic...)

Second, did you test the battery immediately after taking it off the tender, or after several hours? I couldn't tell from the wording. I ask because I recently had a dead battery that tested fairly well and healthy after a long drive, post jump start. The next morning, though, it tested at 15% life and REPLACE.

 

Thanks. I've been on that thread. I think this specific sub-topic, which relates to both AGM and conventional lead-acid batteries, requires a separate thread.

PS: yes, I did test it soon after removing the battery tender. Since the battery tested as good even in it's weaken state, I don't think it would have lost charge if I waited to test it. But I will try that sometime soon.

 

Due to all respect - how do you determine a battery is 100% full charged? Do you determine that by looking at the OCV or SOC? The subject would be on a known good battery that is not damaged for the purpose of this conversation. Obviously, an alternator won't be able to revive a bad battery, as a real charger can only help to a certain extent.

 

Since I got the current AGM discussion started with a seemingly innocuous comment about my battery replacement, I'll add a little more information. Those of you with good knowledge of the subject may find this interesting and be able to describe its relevance to the discussion.

I have a USB car charger with a voltage display that is usually kept in my wife's Honda Pilot. The Pilot has an AGM battery as original equipment due to its engine start/stop feature. The voltage when driving is usually 14.1-14.0V according to the USB charger. Sometimes, though, I see it at only 12.6V.

I moved the USB charger to my RX350 that now has an AGM battery. Over a few 30+ minute drives I have only seen it read in the 13.7-13.6V range. I don't have enough drives to know if it ever drops below this value.

Other notes:
* Nothing was plugged into the USB charger during any of the readings.
* Both vehicles have relatively new AGM batteries. The Pilot's was replaced in April 2024 and the RX's in Oct 2024. The batteries, though, are not identical.
* Lower trim lines of the Pilot do not have engine stop/start and those have a FLA battery installed from the factory. I have verified that the alternator for both FLA and AGM Pilot versions is the same part number. That's not to say that other electrical components and software are the same; I have no knowledge of that.

 

Aren't the voltages you are seeing the output voltage of the alternator rather than the battery itself?

I'm still interested in my earlier post about what is the usual state of charge of a battery (with the engine off) that has been charged by only extended driving, rather than brought up to 100% by a trickle charger. Should it also be 100%? Or in a lower range?

 

The charging strategy is dependent on the PCM as it controls the voltage regulator. It is the one that sends the pulse width modulated signal to the alternator how much voltage to be produced. So if the vehicle does not charge and you think the alternator bad, you change the alternator and the problem still persists then the culprit is the PCM not the alternator. Each manufacturer is different, so this does not apply to all.

 

You are correct. The only way to fully charge the battery is by using an external battery charger of some sort. The SOC is what determines the health capacity of the battery not the voltage you are seeing on the voltmeter.

 

Thanks. So, under normal operating conditions, if the SOC is not 100%, do you happen to know what is usually the normal range of the SOC? Just wondering how a good battery would test if not charged to 100% by an outside source.