Why do you think every vehicle manufacturer picked 13.8V and as a consequence is undercharging batteries?

13.8V is the float charge for wet cell lead acid batteries. Manufacturers chose to work with the technology of the time. 13.8 stuck around for quite a while. Then came AGM's, something I am far less familiar with. I have since learned that their float charge is 14.4V. When placed in a car whose OEM battery is 13.8V, there becomes a need to UP the voltage to suit the AGM type of battery.
Today you can get (or make) Lithium Ion batteries. Certainly their charging profile is profoundly different from wet cell technology. Switching to a Li-Ion battery would require modifications to the charging system so as to not produce an under-the-hood bomb.
This is why it's good to ask before you jump. I didn't ask, or learn enough about AGM before purchasing a $175 battery. Actually, two. One for my car and one for my wife's. Fortunately for her car a simple 10 amp diode will produce the desired change. Unfortunately for my car the only way to modify the alternators output is to have the computer flashed to suit use of an AGM battery. Something that IS doable.

 

Then came AGM's, something I am far less familiar with. I have since learned that their float charge is 14.4V.

What if I told you that 14.4V is wildly inaccurate and way too high?

There is only about 0.1V to 0.2V difference between AGM and flooded batteries, and AGM float is the higher number, so they are basically backwards compatible with flooded battery charging systems.

13.8V is not the float voltage for either AGM or flooded either. Float voltage is when your current flow from the charger to the battery is practically 0. It's just offsetting self discharge. And that would be under 13V.

 

I would be interested to see your supporting evidence for this. The numbers I used for wet cell lead acid are numbers I've known (or believed) to be as stated. If you know different I'd like to see your support for such.

 

I would be interested to see your supporting evidence for this.

What do you consider a credible source?

 

https://www.batteryuniversity.com/article/bu-403-charging-lead-acid/

 

Interesting reading. Thanks.

What if I told you that 14.4V is wildly inaccurate and way too high?

Here's something I've found regarding AGM's:


Based on that last sentence, long drives are something I rarely do anymore.
2.25 to 2.3V/cell = 13.5 to 13.8 volts. 2.40V/cell = 14.4 volts. Since (second sentence) they can tolerate that and higher charges, I don't see any issues unless I'm taking a long drive, something like a cross country trip.

As I've read; AGM's take a charge more quickly. With all the short hops here in town that may be a more attractive feature. One thing I will have to consider is if we take a trip we take the wife's car since changing it from 13.8 to 14.4 is easily done by removing a diode or adding a switch to bypass the diode. As for my Rav4 - that doesn't get the same gas mileage so a long trip could become problematic - based on what I'm reading.

Source: https://www.batteryuniversity.com/article/advancements-in-lead-acid/

 

With all the short hops here in town that may be a more attractive feature.

Why not get yourself a solar panel that will trickle charge your battery when it's parked?

All the numbers you posted are for are for 25°C.

The charge temperature coefficient of a lead acid cell is –3mV/°C

On a hot summer day it's easily 80°C under the hood. 80-25=55*0.003*6=0.99V reduction. You plan to go the other way. But none of this matters anyway since you won't get any noticeable faster charging at 14.4V than at 13.8V. Lead acid battery health comes from keeping it cool, not charging fast, and keeping it topped off at 100% all the time. Your mod won't help with any of that. A solar panel will help with one.

 

For the Venza (with Alt-S fuse):


Figure A shows current alternator sensing . Figure B shows the modified version.
SW1 can bypass D1 (Figure B) making the system run on 13.8V. Opening the switch causes a 0.6V drop in the sensed voltage causing the alternator to push out 14.4V.

With regards the Rav4, I may still want to have the computer flashed, at least to a setting of 14.2V. In case I go for a long drive this shouldn't be too much of a problem for the AGM battery.

 

Why not get yourself a solar panel that will trickle charge your battery when it's parked?

Kept parked in the garage except for when I'm out and about. The Rav4 spends more time inside a garage. Solar would hardly be practical.

All the numbers you posted are for are for 25°C.

With global warming - that might become a concern. But there will still be cool nights. And given that the vehicle is parked inside a garage I don't see this as being much of a concern

On a hot summer day it's easily 80°C under the hood. 80-25=55*0.003*6=0.99V reduction. You plan to go the other way. But none of this matters anyway since you won't get any noticeable faster charging at 14.4V than at 13.8V.

Same would be true, relatively speaking, of a wet cell battery. A fact of life we all must live with. You can't change physics.

 

I may still want to have the computer flashed

You are trying to mess with your cars for no reason at all, and for no benefit at all. And you clearly are not making good informed decision. But it's your car, and you are not my elderly relative. So this is the end of my effort to dissuade you.

 

You are trying to mess with your cars for no reason at all, and for no benefit at all. And you clearly are not making good informed decision. But it's your car.

Fair enough. But I kind of got the impression others here are not in agreement with you. Perhaps it's my perception but I've been warned in both directions. Do it - Don't do it. Whose advice do I take? And since I've already installed the batteries - it's kind of like going down a slide. Once on the way it's real hard to change your mind successfully. And I can't take the batteries back. So I can only go forward.

 

The MBR1045G diode arrives tomorrow. The data sheet claims a 0.55 to 0.57Vf at 125˚C (257˚F).

So at higher temperatures the volt charge will be lower by a few tenths of a volt.
Source: https://www.onsemi.com/pdf/datasheet/mbr1035-d.pdf

 

Well, MikeA thank you for your input. It's worth something.

 

But I kind of got the impression others here are not in agreement with you.

I'm not in favor of modifying the charging system. From what I've read, you can substitute AGM for flooded cell in cars designed for flooded cell, but not the other way around, so modification shouldn't be required. Overcharging would be more of an issue with a sealed battery.

 

With the figure in post 28, Figure B (no switch) Installed the diode this morning.
Before starting, the battery resting voltage was 12.4, which I know is low for a lead acid battery.
Started the car and the battery voltage went up to 14.1 volts.
Shut the car off, removed the Alt-S fuse (7.5A) and installed the diode along with the original fuse in series.
Started the car and the battery voltage was now 14.37 volts.
The wife's car is now modified. And if ever need be the case I can remove the fuse/diode and just run the fuse alone and it will be ready for a wet cell battery.

Still trying to find someone to flash my car. If I'm having it flashed then I'm also going to go with a sport tune with improved transmission shift points. But that part of the mod's is going to have to wait until I can find someone.