AGM battery and ISG system explained by the OEM battery manufacturer

[Duckman7]

Sebang Global Battery is the OEM for Genesis AGM batteries.
They posted a Youtube video explaining how to replace and properly maintain AGM batteries in vehicles with ISG system.

This video is in Korean, so I will briefly explain what they are saying in this video.

1) Smart generator control system
When the battery's SOC (state of charge) reaches 84% (~12.65V), the car stops charging and let the battery discharge to 12.5V (~70%).
Then the cycle repeats.
(Nominal alternator charging voltage is 14.3V, this should not be confused with resting voltage that SOC refers to.)
The battery is not charged when accelerating to save on gas and also not to burden the engine, and charged when decelerating.

2) ISG (Idling Stop and Go) system has integrated regenerative braking system.
ISG system is designed to save gas when you briefly stop at a light by stopping the engine and restart when you put your foot off the brake.
This obviously takes some juice from the battery each and every time.
To help with this, the ISG system integrates regenerative braking system.
Regenerative braking is a common feature for a hybrid or all-electric system, but I'm surprised to find out all-gasoline vehicles with ISG also have a version of regenerative braking system albeit probably much smaller in capacity.
Nevertheless, this gives me some peace of mind when using ISG that every time I slow down, the AGM battery is receiving some boosted charge from the brakes (as opposed to relaying on alternator and gas only) and use that harvested energy to help start the engine the next time.

3) They recommend their AGM batteries to be maintained with minimum of 12.60V (~76%).


What this tells me is that after driving the vehicle for a while, we should see battery level anywhere between 70% to 84%, provided you drove long enough to allow for sufficient time to charge/discharge.
Then the question is, why max 84% only and not 100%.
All other sources I've managed to find on the internet says the lead-acid batteries are best kept at full charge.
If left at a lower level for a long time, the sulfation crystals harden and the battery can no longer be charged beyond that level.
I suspect the reason the car is only designed to charge up to 84% is because that's the max level the bulk charging process can achieve.
To receive the remaining 16%, the battery must be kept in absorption charge phase and that takes relatively long time to receive the remaining 16%.
During the absorption charge phase, if there is a stop and go event, the amount of charge the battery lost cannot be recovered quickly because the battery cannot be charged quickly with high current like in the bulk charge phase.

Although this manufacturer does not say in the video (for obvious reasons), I think it is still a good idea to charge the battery to 100% using an external charger like once a month or as often as you can if you intend to keep the battery for a long time like 6+ years according to several owners who did just that.

 

[joegr]

...
Nevertheless, this gives me some peace of mind when using ISG that every time I slow down, the AGM battery is receiving some boosted charge from the brakes (as opposed to relaying on alternator and gas only) and use that harvested energy to help start the engine the next time.
...

This "regenerative braking" is not what you think. It just means that the alternator is put in charging mode while decelerating, as long as the battery is less than 100%.

 

[Duckman7]

This "regenerative braking" is not what you think. It just means that the alternator is put in charging mode while decelerating, as long as the battery is less than 100%.

Thanks for your input.
The video called it 'regenerative braking system' and the graphical illustration showed as such, and so I assumed it's coming from the brakes and not from the alternator.
But you are probably right.
Even if it's coming from the alternator, this energy is still harvested from the kinetic energy of the vehicle when decelerating (if what they are saying is at all true).
Regenerated power in this case would be much smaller compared to the ones found in a hybrid or all-electric because the AGM battery is much smaller in capacity and charge rate in comparison.
I wonder how much deceleration it would take to support one engine start.
This should be a relatively straight forward math if we know how much energy it takes to start the engine.

 

[joegr]

I don't know how much you get back this way, but it is enough that several car makers are doing it to get ever so slightly better epa gas mileage figures.
BMW was the first that I was aware of doing this, but they may not have actually been the first...

 

[NVQB10]

I don't know how much you get back this way, but it is enough that several car makers are doing it to get ever so slightly better epa gas mileage figures.
BMW was the first that I was aware of doing this, but they may not have actually been the first...

You probably already know this but the auto-start-stop features on nearly all new US-sold ICE cars was mandated less for MPG and more for EPA emission reductions, particularly in urban environments. I am pretty sure the A-S-S feature was introduced first in Europe. I remember driving a friend's Mazda 3 in Switzerland in the late 80s or early 90s that had that feature. I was shocked the first time I came to a stop light and the car "died" only to come back to life a few seconds later.