Every once in a while I throw on a 12v battery charger and they are usually fully charged within a few minutes. As far as the batteries go I have never ran out of Juice, so it works good for me.
Not running out of juice is a good thing. How old are the batteries? If they did not have enough juice to power everything you want, the next time you use it, would their lifespan have been acceptable?
A 'smart' battery charger quickly shutting off and flashing the green light fully charged light is a bad sign, often. It means that very few amps were required to bring the battery(s) upto high voltage.
This happens when a battery is indeed near fully charged, or when it is badly sulfated/worn out, and the charger does not know the difference.
It can also occur when one shuts off the engine, and relatively quickly hooks up their 'smart' charger. The battery surface voltage, since the alternator was just charging it, will likely still be above 12.8v. When the smart charger sees 12.8v+ on initial hook up, it assumes thebattery is fully charged, flashes the green light and just seeks a 'float voltage of 13.2 volts or so, or 13.6v if it has an AGM setting. Since it takes quite a long time to get from 80% charged to 100% charged, a smart charger quitting at 80% charged is doing an extreme disservice to the 80% charged battery, pissing on the owner and saying tis raining.
One really needs to be smarter than their 'smart' charger.
I found it infuriating trying to actually achieve a true full charge using one on a flooded battery, determined by using a hydrometer. The dang thing just kept shutting off and flashing the green light and dropping to float voltage. Specific gravity when full should have been at 1.275, yet it was still in the 1.235 range and the dang charger would not hold higher voltages!!!. To get it to restart, I would hav to load the battery with something to drop the voltage to below 12.8v, then restart the charger, over and over and over again. This got old quick and I got an adjustable voltage power supply instead which holds the voltage I choose for as long as I choose to hold it, how long being determined by a hydrometer or an ammeter.
An AGM battery can only be determined to be fully charged when it can accept 0.5% of its capacity or less, at absorption voltage( 14.2 to 14.8v). In other words, when a group 27 or 31 AGM battery, which are right around 100 amp hours in capacity when new, when they can only accept 0.5 amps or less, at Absorption voltage, only then can they be considered to be fully charged.
Full charge of an AGM battery cannot be determined at lesser voltages!! This is simple fact, not open for debate.
Since No garage 'smart' charger asks for the capacity of the battery, they cannot actually sense when the battery is fully charged by the amount of amperage it is accepting at higher voltages. They just have a general 'good enough to not overcharge' algorithm, and lots of marketing claiming how great they are, written by people without a clue as to how a lead acid battery charges, or what it needs to reach full charge.
Any charging source is better than no charging source applied to a depleted lead acid battery, but ideally the lead acid battery always wants to be returned to a true full charge after ANY level of depletion, and the 'smart' products out there are very bad at bringing a deeply cycled battery to full charge, especially one with many cycles on it when it has been cycled from a partial state of charge.
The rating of the alternator, is also not what most people assume. If there were a 45 amp alternator, whose voltage regulator were always seeking and holding 14.8v, it would more quickly and effectively charge a 50% depleted battery than a 300 amp alternator that was seeking only 13.8v. Unless of course one was running 45 amps of loads when the engine was running.
An alternator only makes as much juice as is required to maintain the voltage its voltage regulator is asking for. The voltage regulator alters the field currrent to the alternator in order to change the amount of amperage required to maintain that voltage. The battery itself decides hom much amperage it wants to accept at the voltage reaching the battery terminals. If the alternator cannot make enough amperage to meet a certain system voltage becase the battery it gobbling up all of its capacity, then the voltage does not rise as high until the battery state of carge increases and it requires less amperage.
An alternators rating is only achieved when it is still cold, spinning quickly, whe nthere are loads capacble of asking for its full rating and cabling capable of passing it. Idle speed amperage is usually 1/3 or less of the actual alternator's rating, and at Idle speeed, a maxed out alternator gets very hot, very quickly. I have a temp sensor on mine and speeds below 25mph maxing ot the alternator the thing quickly approaches the 220f danger zone. At 65mph it will not be much more than half of that, maxed out, due to underhood airflow and 2000 engine rpm spinning the laternator fan faster, sucking cooler air through it.
A depleted healthy battery can accept a lot of amperage at high voltage. It accepts much less amperage at lower voltages(electrical pressure).
A nearly fully charged battery cannot accept much amperage at high or low voltages but wil acept 2 to 3 times more at higher voltages.
An unhealthy battery will accept slightly higher amperages when nearly fully charged, than a healthy battery at high or low voltages. The myth that a dying battery will take out an alternator should be shot and killed and buried in the old wives tale burial ground. The slight increase a dying battery requires over a healthy one is like saying 'unplug that smart phone, it is going to kill my alternator!!'' If a alternator fails soon after a battery dies, it was likely the failing alternator which hammered a nail into the unhealthy battery, not the other way around.
An unhealthy depleted battery will accept much much less amperage than a healthy depleted battery at high or low voltages.
So basically of one does not know the voltage at the battery terminals, and how much amperage is flowing at that voltage, they cannot know how charged the battery is, nor can they guestimate the battery's state of health.
When one can see the voltage at the battery terminals, and see how much amperage is flowing into the battery at charging voltages, and also see how much voltage the battery can maintain when powering loads/discharging of various amperages, then with time, noticing these trends and tendencies, they can develop a good idea of how charged, and how healthy and capable their battery is at that point in time.
If one cannot be bothered to do these things, well that is fine too. Their charging system might take Ok to good(enough) care of the batteries. They might last an acceptable amount of time, or they might not. Failed batteries can simply be replaced. The cost of doing this more regularly for some, is perfectly acceptable, to them.
But the person wanting to get the maximum possible lifespan from their batteries, or even somewhat close to the maximum possible, needs to only not overdischarge them, but insure the charging sources can return them to fully charged with minimal overcharge.
'Good enough' is subjective, opinion. working 'Just fine', often means has simply not yet failed.
If one is trying to ineed fully charge them, the alternator is a very poor tool for doing so, because of the voltage regulation, and The amount of time that actually required at higher battery charging voltages to achieve this ideal 100% state of charge. The same is true of 'smart' chargers, as they are basically designed to not overcharge and as such almost always stop well short of 100%.
To reach true full charge the battery needs to be held at higher voltages for as long as is requred to either maximize specific gravity on a flooded battery, checked with a hydrometer by a human with one and the temperment to employ it, or with an AGM battery one needs to hold it at absorption voltages and wait for amps to taper to the prescribed level for that battery. Amperage tapering is also an effective indicator on flooded batteries, but generally instead of 0.5% of capacity they might be full when amps taper to 1 to 3.5% of capacity, it depends on the battery itself, and its health.
One should also note that a unhealthy overdscharged battery, when hooked to a charging source will take very few amps at first, well under half an amp. Often a smart charger shuts down entirely. if one is able to keep charging currents available, then the not entirely sulfated battery will slowly wake up and start accepting current and the voltage will actually drop as the amperage increases to the maximum the charger allows. this amperage then remains steady until voltage then rises to the maximum allowed by the charger. once that voltage is reached then the amperage required to maintain that voltage begins to decline. When it declines to a certain level, it is fully charged. this process is not going to take less than 5.5 hours, and likely much much longer.
Draw a battery to 50% charged, and it CANNOT be truly fully charged in less than 5.5 hours, and that minimum possible time assumes a high amp charging source seeking then holding mid to high 14 volts, AND a still healthy battery.
Lesser voltage increases the time required to reach full charge, substantially, and the less healthy the battery is, the more time it needs at higher voltages as well. So an unhealthy battery fed mid 13 volts is guaranted to remain less than fully charged, even if one is driving cross country.
This is not debateable. Confidence in product choices, or the product's marketing mumbo jumbo does not negate facts. It does not matter if one has a 300 amp recently polished chrome alternator and 1/0 cable. If its voltage cannot be controlled to hold mid to high 14's, then charging a 50% charged lead acid battery to truly full in less than 6.5 hours is impossible.
The battery which never reaches 100% full charge degrades much faster.
The battery which does get to 100% full charge often can live an exceptional lifespan.
100% true full charge is not indicated by a charging source's green light.
Anybody wanting good battery life should do what they can with what they own, to get the battery to as high a state of charge as possible, and never let self discharge or parasitic drains slowly discharge the battery to the point it cannot start, or can just barely start the engine.
It does not actually requie much of a battery to start an modern fuel injected engine in mild or warm ambient temperatures. I have ad one 7 year old wally world group 27 marine battery easily start my engine if fully charged, but if I tried fully charging my old flip phone from it then starting the engine, it could not.
Many many starting batteries right now, could not fully charge a smart phone, and then still start the engine. Many of these owners still assume the battery 'is still going strong'.
if they had a voltmeter that sampled several ties a second, and watched it every time they started their motor, they would have noticed the voltage the battery could maintain starting the motor falling lower and lower as the battery ages. Once it starts falling into teh mid 8 volt range, it will still start, and start quite easily in most instances, but complete failure is not far off.
Wth a hghi sample rate voltmeter, each engine start can be considered a load test. No need to take it out to a auto parts store. Their load testers can be quite poor, and some other testers which measure battery resistance and deliver a CCA figure and % oif health remaining can also be inaccurtate and quite misleading. Also do not forget the many employees in AP stores using them are not exactly being held out of Mensa because they ran out of room.
They are trying to sell a battery, or deny warranty coverage.