I'll vouch for anonymous1 !! (We have differences, but have more in common. [Sorry anon - not meant offensively...] And he states and understands some (IMO) very important basics & principles that (IMO) others may fail to grasp....)
Paraphrasing bigger alternators or bigger (or 2) batteries (without internal fusion)....
A loaded battery discharges.
If not recharged, it eventually goes "flat".
The alternator recharges the battery.
The alternator supplies the vehicle's load (engine, audio, charging batteries, etc).
If the alternator can't keep up with the load, the battery discharges.
Simple enough?
If the battery goes flat enough, the vehicle stops.
Or it may get too flat to crank & start the vehicle.
Solutions:
- an alternator that is big enough;
- get home and recharge the battery (eg from a wall socket) before the "flattery" kills the car....;
- reduce loads before stalling and hence recharge the battery.
A bigger battery merely takes longer to become a flattery, but maybe it will last long enough to get you home, or to a charger, or long enough to last the date. Maybe.
That might be ok. But when your date becomes a honeymoon and your battery flattens on that longer trip; just short of before Las Vegas (or Disneyland, wherever....)
Or maybe you turn down the audio to let the battery re-charge.... well, let's just say your honeymoon may well be over before it even began!
So how do we have our date and eat it too? A big enough alternator.
Or - how do we try to guarantee cranking and staring power? A second battery.
Or what if on the first date at the Drive-In, the audio flattened the battery? A second battery (that was isolated from the main battery).
Whilst there are a few benefits to a second battery (specifically an AGM close to the amplifier), it is not a proper substitute for an undersized alternator.
[Exceptions - short periods that the alternator can't handle - eg: whilst the engine is idling, or very loud (audio) output power/volume (eg during musical peaks, or peaks of dates (shhhh - LOL) - where the battery supplies the shortfall and the alternator replaces the charge later - ie, recharges).]
Another issue is that an alternator usually keeps the system voltage between 13.8V and 14.4V. (That's an average long-term and it varies with temperature...)
When the alternator can't keep up, that voltage drops towards the battery voltage and maybe lower - eg, 12.7V to under 10V. (That's a bit complicated, but 12.7V might be a typical "full" battery and (say) 11.5V is flat. But during cranking, a good & full battery might dip to 10V or less....).
In other words, with an adequate alternator, your vehicle's system voltage should be (say) 13.8V to 14.4V.
When inadequate, that can drop to below 12.8V etc.
[Again, complicated maybe... But that means dimmer lights (unless HIDs etc); maybe weaker ignition; maybe less audio output - else higher currents into "constant power loads" (like HIDs, some audios, etc).]
But I'll pause here... (my mate wants to hog this PC...)
I can explain more (oh no no.... please not!) or describe "simple" and cheap systems... and the "no need to
match batteries" etc etc.
But the BIG preference is for a BIG enough alternator.
And of course "the BIG 3" upgrades.
I was going write "How do I know when/if I'm under charging?".. or "if I have a flat battery"...
That's called a voltmeter...
But in respect of my recently landed mate from afar.....
(Ask if more is wanted... I have written the options elsewhere...)