A lead acid battery cannot be recharged from 80%, to 100% state of charge in less than 3.5 hours, although a high amperage charging source seeking a higher voltage can bring the battery to 80% charged fairly quickly.
Those 3.5 hours for 80% to 100% assume the charger is capable of enough amperage to bring the battery to, and hold the battery in the mid 14.5 volt range.
A 30% charged battery can easily start a modern fuel injected engine, especially an already warm one.
Very few voltage regulators, whether on a grid powered 'smart' charger or an Automotive voltage regulator which controls the alternator's amperage output, will allow mid 14 volts to be held for very long.
A lead acid battery ideally, fr maximum longevity an maximum capacity retention and cranking amps throughout its lifespan, always wants to be 100% charged, and kept cool.
A 1.5 amp maintainer, attached to a well depleted battery, has pretty much Zero chance of bringing the battery to full charge overnight.
A 12 amp charger has almost zero chance of fully charging a depleted lead acif battery 'in a few hours', as 3.5 hours from 80% to 100% is unassailable fact, and almost every 'smart' charge out there is not designed to fully charge a battery, they are designed to NOT overcharge it.
Reaching a true 100% state of charge can only truly be verified with a temperature compensated hydrometer on a flooded battery
Those who believe the green light on a charger indicates a full charge are delusional, misinformed/ignorant, or some combination of all three.
The green light indicates ONLY that the charger has decided it has held the battery at an absorption voltage ( Mid 14's) for long enough. A good temperature compensating hydrometer, like the OTC4619, will easily prove this time and again to anybody who doubts it, but ignorance is bliss, proveable time and again, which is a shame.
The time in which a battery needs to be held at absorption voltages, in order to reach full charge, varies with every battery, and changes as the battery ages. It changes with battery temperature, it changes with the state of charge at which the charger was initially hooked to the battery, it varies with the amount of initial amperage the charging source was initially able to supply. It changes with the length of time the battery was left at lesser states of charge, and the depth of those discharges.
An AGM battery can only be determined to be fully charge when it can only accept 0.5% of its capacity, measured at the 20 hour rate, so a group 31 AGm battery with 100 amp hours can only be determined to be fully charged, when it is held at 14.5 ish volts( at ~77f degrees) until amperage tapers to 0.5 amps or less.
No ammeter? guess what, you have NO IDEA how charger the battery is or not. Not able to hold the battery in the mid 14's?????, well one cannot determine full charge at lesser voltages.
Almost every battery, in every vehicle on the planet as you read this, are not 100% charged, but 99.9% of their owners will beleive the opposite.
The average state of charge, and the average temperature the battery is, determine the lifespan of the Lead acid battery.
The 'trickle charge it overnight' mentality in terms of battery recovery, as pretty much asinine in the extreme.
Lets assume the 1.5 amp charger is capable of seeking 14.5 volts( most are not).
Lets say the battery was depleted below 10.5v( which is considered 100% discharged)
Lets say the battery was 100% healthy when it was inadvertently depleted below 10.5v
Lets say the battery had, when new, 100 amp hours of capacity. This means it can , when new and fully charged, supply 5 amps for 20 hours before voltage falls below 10.5v)
A 1.5 amp charger ( seeking mid 14.5v) applied to such a battery for 12 hours would be able to return only a small fraction of the battery's capacity, 18 of the battery's 100AH capacity.
But this will likely be enough to allow the vehicle to start, and the vehicle owner will think the battery is indeed fully charged, when a hydrometer would laugh at this incredibly false assumption.
In the case of a 12v battery drained to sub 10.5v, most every modern 'smart' charger will not attempt to start charging the battery, and one can, like the OP, hook up another battery in parallel, for a couple dozen seconds, and many amps will flow into teh depleted battery, raising its voltage to the point a 'smart' charger will recognize it and start charging. To assume this 'recovers' the battery is unwise.
Batteries slowly drained to well below 10.5v have been damaged, capacity compromised. How much capacity they will be able to recover is dependent on just how long they have been left below 10.5v, and their condition before that level of depletion was allowed to occur, and of course the chargings sources amperage and the time in which it is allowed to hold higher batter charging voltages.
Such a battery is badly sulfated, the plates occluded with hardened sulfate. the best chance of recovery to dissolve that sulfate back into the electrolyte is NOT with a slow low amperage 'trickle' charger, but by a blast of higher amperage which will heat the plates, but not to over 100F, and 120F is the absolute limit.
Such a battery will likely require little amperage to have voltage be initially brought up into the mid 14's, but then, as the battery starts accepting this current and 'wakes up', the voltage will drop and the amperage it can accept will rise, assuming the charging source can provide higher amperage and is still seeking to bring the battery to and hold it at the mid 14 volt range.
This theoretical group31 battery( 13longx7widex9.5 inches tall is pretty much the largest automotive battery) should then be be allowed to accept 20 amps until voltage again rises to 14.5ish volts. How quickly 20 amps can get the battery to 14.5 amps is indicative of its remaining capacity and health.
Once the battery gets to the maximum voltage the charger allows, the amperage required to maintain that voltage decrreases, starts tapering in this constant voltage phase, also called absorption. If the amperage does not taper then the voltage keeps rising (constant current), and one should not allow a 77F battery to exceed 15 volts. Manual buzz boxes with high amperage available can easily push a battery past this. It is Effective but much more abusive than it need be, and dangerous, in terms of gassing hydrogen and oxygen and taking that sulfuric acid mist along with it.
Truly recovering a sulfated battery to its maximum remining capacity is not something any garage charge is capable of, no matter how much faith one has in its marketing.
After a sulfated battery is held at 14.5 volts until specific graity of the electrolyte stops rising, which can be anywhere from 8 to 24 hours, if the specific gravity is not in the 1.275 range, then ONLY higher voltages can possibly raise that specific gravity further. Such a charge is a Forced overcharge, called an Equalization, a term often misused. The battery can be brought to 16.2 volts, and will bubble vigorously and heat relatively rapidly, but the equipment which will allow this is few and far between. the battery must be monitored during an equalization and the charge stopped once the battery heats much beyond 100F as it will rise in temperature much quicker after this. Once allowed to cool, assuing the specific gravity readings are still below 1.275ish, the 16.2v can be initiated again. they hydrometer should be dipped into the lowest reading cell often and the charging stopped once the specific gravity no longer rises, accounting for the rising electrolyte temperature. Eye protection mandatory! No sparks or open flame nearby!
The Pulse desulfating chargers have been largely dismissed by the Marine experts who have performed many tests attempting to recover sulfated batteries. Any capacity recovery is the same as if the battery had been exposed to the same charging voltages for the same amount of time with a NON 'pulse charger'. Belive the marketing and in your purchase if you have to.
'Smart' charger by and large are not very smart. Any charging is of course better than No charging, but getting a battery to 95% charged, is only half as good as achieving 100% charged, in terms of maintaining battery capacity and performance.
Achieving 100% state of charge can be difficult if not impossible without a charging source which is capable of not only holding mid 14 volts for much longer than smart chargers allow, but also allowing Equalizing voltages upto 16.2 volts, a process which can be dangerous to the user and dangerous to the company who made such a product. As such most are emasculated and are not capable of truly fully charging a battery, but are intended to stop charging at a much safer 92 to 95% charged.
Getting the 95% charged battrey in the middle of its expected life to 100% charge, can require 4 more hours held at 14.5 volts.
I use an adjustble voltage power supply, modified with mroe ventilation heatsinking and better adjustment dial. i am oftern brougth batteries to 'recover', and watch voltmeters and ammeters and amp hour counters, and dip hydrometers often to see the results and gauge the health of the battery. So many of these batteries were placed on smart chargers before they were brough to me and their specific gravity was deep in the red and their smart charger would refuse to charge it for any longer. many of these batteries were unrecoverable, many accepted a lot of amp hours and returned to electrolyte levels in teh green and are still going a few years later.
I would recommend those with 'smart' chargers who want to get their batteries close as they can to 100% state of charge, apply a relatively load to the battery to drop the voltage to below 12.6( like headlights), then reattach the alligator clamps, plug in and restart the charger on the next lowest amperage setting, then remove the load. Lather rinse repeat. This might be effective, or it might not.
The truly 100% charged Lead acid battery, is a happy battery, and if regularly returned to 100% charged and living its life up in that range, will live a good long life, even in Southern Texas.
Living at lower states of charge is highly detrimental to Lead acid battery longevity and higher average temperatures of the battery exaserbate the capacity decline. The lower the average state of charge and the higher the average temperature the faster the battery deteriorates.
Do NOT assume your vehicle is trying to, or even capable of returning teh battery to 100% charged, even if one drives for 8 hours.
Anytime a jumpstart is required the battery should be brought to, the mid 14 volt range, and held there for several hours. it might take 12+ hours to bring the battery to the mid 14v range. that depends on teh amperage the chrging source is capable of delivering, and the voltage is is seeking to attain then hold.
Do not asusme a 'trickle' charge is seeking mid 14 volts. Do not assume a trickle charger even has enough amperage available to bring a battery to 14.5v.
A healthy battery when fully charged might require only 0.4 amps to be held at 14.5v, but a sulfated older battery with 1.5 amps applied constantly might never exceed 13.9v, no matter how long it is applied. I have one marine battery on its last legs where 5 amps cannot get it above 13.87v no matter how long it is applied. 1.5 amps cannot get it over 13.22.
The higher the voltage the more amperage is required to hold the battery there.
The less healthy battery will require more amperage to maintain the same voltage when near fully charged, compared to a more healthy one.
The less healthy depleted battery will rise in voltage faster with lesser amperage applied, than thehealthier battery.
The healthy depleted battery will accept more amperage and its voltage will rise slower, compared to one less healthy.
The vehicles voltage regulator controls how much amperage an alternator makes. This voltage varies greatly among different vehicles, and on the same vehicle depending on how long the engine has been running.
The battery accepts as much amperage as it wants at the voltage reaching its battery terminals.
How much amperage the battery 'wants' at a charging voltage, depends on its size and health and temperature and state of charge.
Every battery is different and will exhibit different behavior, as it ages.
"good enough' is Subjective. Opinion.
But a fact is the healthy lead acid battery canot be recharged from 80% to 100% in less than 3.5 hours, assuming a charging source is capable of holding mid 14 volt range, and the lead acid battery, ideally, always wants to be 100% charged, and kept cool.