battery understanding question

[Ersch]

my battery is charged, 12.7 V.
I start, idle with joke, the multimeter shows 14.4 V at 2000 rpm. I'm driving off ...

all good so far. i'm driving through a city, brake blinking, idle. the battery is now being discharged.

now i'm driving overland again. the voltage at the poles is now 13.5 V at 3500 rpm. it takes a while before it shows 14.4 V at 3500 again. Is that correct?

when i attach an empty battery to the charger, it also reaches 14.4 V when it is full and not from the start.

i still have an agm maintenance-free battery, if i fully charge it and install it i will not get over 13.5 V after the same procedure ... not even at 5000 rpm.

[Volker_P]

Welcome here Erik!

Your voltages for the first battery seem reasonable. 12.7V without load might be a bit weak but as long as it is able to turn the engine when it is hot there is no need to worry.

The end-of-charge voltage (Ladeschlußspannung) is given by the cell chemistry. Even for lead batteries it may differ a little bit due to different alloys.
At non-lead batteries often charging instructions and the end-of-charge voltage are provided on the label on the battery.

Your case seems suspicious as maintenance-free batteries typically have higher end-of-charge voltages than the 14.4V of classic lead batteries.
So what is your criterion for your agm battery to be "fully charged" before you insert it to the bike?

Another option is that the charging voltage is not completely smooth (though full-wave rectifying three phases at 3000rpm more should be already quite good) and the battery types (capacity or again chemistry) show different impact on the peak voltages. But I am no expert on electrical engineering.

[Ersch]

what I'm getting at and what irritated me is the fact that I don't always measure 14.xx V at the battery poles.
As soon as the battery is loaded, the charging current increases and the voltage drops.

Is my assumption correct?

[Volker_P]

I am not sure if I understand this question properly.

14.x Volts is the end-of-charge voltage. At this level the regulator makes sure that you have no charging current any more (or charging is compensated more or less exactly by consumption if you want).
So the battery voltage is kind of a measure of the charging state of your battery. If it drops below 14.4V, the regulator will try to charge it with a current from the alternator that however depends on rpm. So during riding with lights on at different rpm the voltage may rise and drop somewhat depending on actual alternator power and actual electric power consumption of the bike.
Therefore e.g. at dirtbikes that have no battery the headlight brightness varies visibly on rpm or when you switch turn signals on and off.

At zero current/unconnected battery the 14.4V typically drop slowly maybe to 13.7V when the battery is still quite fine.

Hope this helps.