Here is a graph I made charging my old flooded battery with a 40 amp adjustable voltage power supply. The battery had about 350 deep cycles on it at the time, and wound up lasting nearly 500 deep cycles before I removed it from service in my Van. It remaining capacity is Now about 55AH total, and I still cycle it in my workshop, and it works 'just fine' powering my TV fans and lights for 35 amp hours worth of total consumption, and it would also work 'just fine', in my van, as long as i do not take more than 35Ah from it. But considering the battery claimed to have 130AH when new, 55Ah remaining cannot be considered healthy. I am surprised it has not shorted a cell yet.
In this test, The "130AH" battery was 44AH from Full at the beginning of the test. So it was well over 50% charged
It took 40 amps constantly for 35 minutes before battery voltage hit 14.7v.
From 0 to 35 minutes the charger was in constant current mode outputting everything it could.
A higher amp charging source, such as a powerful well wired alternator spinning fast enough, would have reduced the length of this 35 minutes' flat line'. It would not affect recharge rate at all once absorption voltage is reached. Once absorption voltage is reached, the battery dictates how many amps are required to maintain absorption voltage, and these continuously taper downward as the battery climbs above 80% charged.
Once the battery reaches absorption voltage, the amperage required to maintain absorption voltage, decreases, thus the downslope on the remaining 2.5 hours on the graph.
At the 3 hour mark , my battery monitor said the battery was 2Ah from full, but this particular battery was very inefficient in these ranges
Amps at 14.7 had tapered to 3.7 amps after 3 hours. I ended the test here as it was 3 in the morning, but another hour or 2 would have been required before Specific gravity would have maxed out as amperage continued to taper downward, but going no lower than 1.5 amps on this particular battery. Much of that 1.5 amps was turned to heat, as this flooded battery was not healthy at this point.
So basically the battery cannot be charged any faster once it reaches absorption voltage. That Amp taper to 100% full will always be in the 3+ hour range no matter what, even if one just got done polishing their chromed alternator while whispering lovingly to it.
On An abused , more sulfated battery this amp taper would be be steeper initially, and the flat line constant current stage much shorter. The taper to 2 to 3% of capacity would take significantly longer too
Since a vehicle's voltage regulator would Likely NOT HOLD 14.7v if it even goes that high in the first place, the time to full if 13.6v were the maximum voltage allowed, then 100% full would take 12 hours or more of running the engine.
I am in the process of installing an adjustable voltage regulator on my 89 Dodge whose original voltage regulator is in the engine computer, as It chooses 14.9 or 13.7 with no nod toward logic at all. 14.9v is too high for my AGM battery when it is depleted and can suck high amperages still, and 13.7v max allowed when the battery is depleted significantly slows recharging as it is less electrical pressure between source( alternator) and load( battery).
14.9v held when the battery is already full is abusive.
Any lead acid battery will behave similarly to the battery in this test.
The battery can only be recharged so quickly, that flat line of constant current is the only area where reduction in charge times can be accomplished.
Had I a 50 amp charging source, that flat line of constant current would have been about 5 to 10 minutes shorter. The amp taper, nearly exactly the same. Hours to a full recharge.
A flooded deep cycle battery will always require 'some' current to be held at absorption voltage. A 100Ah battery will likely require 0.8 amps or more to be held at 14.7v even if bursting full with 1.275+ specific gravity measured across all cells.
If an Ammeter reads zero on such a battery, the ammeter is inaccurate.
A healthy battery requires less amperage to be held at absorption voltage when fully charged, compared to a older battery which is sulfated.
Note that AGM batteries can taper to sub 0.05amps to maintain absorption voltage when fully charged.
Maintenance free flooded batteries also exhibit this behavior of needing little amperage to be held at absorption voltage when fully charged. These are starting batteries with more calcium in the plates and should not be utilized in deep cycle service. If they are cycled deeply, absorption voltage should be 15 volts at 77f and Absorption voltage should be held until amps taper to nearly nothing. These maintenance free flooded batteries do much poorer than 'regular' starting batteries when inadvertently deeply cycled. Both want to be returned to 100% ASAP, but the maintenance free flooded battery needs higher voltage held longer for 100% to be achieved. If 100% is not achieved then the doomsday clock starts ticking much faster for it, so it is way more important to NOT RELY on the alternator to top off these maintenance free flooded batteries, as it takes too much time and requires too high a voltage.
The best way to top it off is with a plug in charging source capable of 15 volts, or a solar controller whose output can be raised to 15 volts and held there until amps into the battery taper to nearly nothing.
Now, If the battery were a LiFepo4 depleted the same amount and of the same overall capacity, then the flat line of constant current would be much longer, and the tapering stage once absorption/max voltage is attained, would be very steep, right at the end of the significantly shorter charge cycle.
It seems most of the general public believes a Lead acid battery can be recharged as quickly as LifePo4, but it is not to be.
The 80 to 100% charged range is Always going to take hours, no matter what, on a Lead acid battery A higher amp charging source can only decrease the'flat line' times of constant/bulk current.
If the Lead acid battery is regularly cycled deeper than 80% state of charge does not regularly get to 100% charged, the faster its capacity will degrade. the deeper the discharge cycle, the more important it is that a true 100% recharge is attained, ASAP, if one cares to get excellent service from their battery.
But poor fair good and excellent are subjective to the user, and internet claims as to MPG or battery longevity with no verifiable Data with accurate tools to back up such claims, are completely worthless and serve only to confuse a Newbie hoping to learn, Well that and self stroke one's likely overinflated undeserved ego.
