Poor AGM Battery Service Life

Can parts stores even accurately test batteries?

My battery is a bit of a bear to remove.
They usually have a load tester that puts a load on the battery. You could probably buy your own. You don't have to remove the battery to load test it. The other way would be to just fully charge the battery and once you disconnect it, see what the voltage is after it's fully charged and after it's been sitting for a while after you take it off the charger. There are several state of charge charts out there. Mine wouldn't go above 12.5 and 12.7 was what it should be at a no load condition when it's 70 degrees. Still when it's in the car, it starts fine and my voltage gauge will show anywhere from 11.8-12 volts depending on the temperature. When it gets really cold, it can go down to 11.5. Been that way for years. You get battery warning messages when the voltage drops below 11.
 
They usually have a load tester that puts a load on the battery. You could probably buy your own. You don't have to remove the battery to load test it. The other way would be to just fully charge the battery and once you disconnect it, see what the voltage is after it's fully charged and after it's been sitting for a while after you take it off the charger. There are several state of charge charts out there. Mine wouldn't go above 12.5 and 12.7 was what it should be at a no load condition when it's 70 degrees. Still when it's in the car, it starts fine and my voltage gauge will show anywhere from 11.8-12 volts depending on the temperature. When it gets really cold, it can go down to 11.5. Been that way for years. You get battery warning messages when the voltage drops below 11.
I ordered one of the resistive style testers today.
 
They usually have a load tester that puts a load on the battery. You could probably buy your own. You don't have to remove the battery to load test it. The other way would be to just fully charge the battery and once you disconnect it, see what the voltage is after it's fully charged and after it's been sitting for a while after you take it off the charger. There are several state of charge charts out there. Mine wouldn't go above 12.5 and 12.7 was what it should be at a no load condition when it's 70 degrees. Still when it's in the car, it starts fine and my voltage gauge will show anywhere from 11.8-12 volts depending on the temperature. When it gets really cold, it can go down to 11.5. Been that way for years. You get battery warning messages when the voltage drops below 11.
My friend's 212 has start/stop and the feature stopped working.

The main battery tested "fine" using most testers - whether load or electronic. The aux battery failed testing. So I replaced the Aux Battery. No change. I even fully charged the main battery - no change. Tried a few other things- disconnect cable, reset, etc.

Several months later I had the car for an unrelated reason. I replaced the main battery with the correct AGM from Costco. Within a few miles of driving, the start/stop worked again. I talked to my MB contact and he advised me that the onboard computers tend to have a stricter criteria of acceptable battery condition than most testers. Supposedly for warranty, they do not have to use the testers anymore under some circumstances.
 
My friend's 212 has start/stop and the feature stopped working.

The main battery tested "fine" using most testers - whether load or electronic. The aux battery failed testing. So I replaced the Aux Battery. No change. I even fully charged the main battery - no change. Tried a few other things- disconnect cable, reset, etc.

Several months later I had the car for an unrelated reason. I replaced the main battery with the correct AGM from Costco. Within a few miles of driving, the start/stop worked again. I talked to my MB contact and he advised me that the onboard computers tend to have a stricter criteria of acceptable battery condition than most testers. Supposedly for warranty, they do not have to use the testers anymore under some circumstances.
Most people are happy about the start/stop feature not working. And yes they mostly replace the aux battery. Glad I don't have it, but then I just have 268hp instead of 302hp in the newer engines with start/stop. I think there's some bypass you can buy to plug into the OBD port but I think it was $200+ and probably not really worth it. To be fair, some people hate working on German cars because they can be so fickle. I replaced a temperature sensor on mine once as it had no outside temperature. It also used that for the AC so no AC. AC still wouldn't work after replacing it. Got weird temperature numbers at first. Then the temperature display worked, but still no AC. Went through several start stop cycles before it finally started working. Of course the outside temperature display only fails on a hot summer day. Oh and to be fair, after I noticed the temperature display wasn't working the AC still worked, waited a few cycles before it decided not to work.
 
FWIW, I have a Deka AGM approaching the 4 year mark in a Prius - which is an “easy” application for an AGM, it uses a 12V step-down transformer in the inverter in place of an alternator. The Optima I had barely crossed the 3 year mark.

Unless there’s some kind of adaptive charging, throwing a AGM onto an conventional charging system may be asking for trouble IMO.
 
Those cycling batteries in the marine world, have come to realize that AGMS are more finicky than flooded. They need to be returned to true full charge more often than flooded, and partial state of charge deep cycling of them is far more detrimental to them then flooded deep cycle batteries.
PSOC cycling means starting the next discharge cycke before having been returned to 100% state of charge.
PSOC cycling causes the battery to have less and less total available capacity, and it also increases the time it takes to return the battery to truly full. 10 deep cycles in a row only getting back to 85% charged and the battery bank is punch drunk

Agms are still desirable as deep cycling batteries for those who have concerns of offgassing, terminal corrosion, or have difficulty accessing/ remembering to access the flooded batteries to water them before the plate tops get exposed. As Flooded batteries age, they require more time at absorption voltages to reach full charge, and thus use more water, and require checking of their levels far more often.

Few batteries perform well after plate exposure, that exposed portion basically becomes useless in most cases, even once refilled.

AGMS can, at the same voltage, charge faster. Especially when depleted, they will be able to get to 85% charged faster than their flooded counterpart, ( if the charging source can feed them the higher amperage they can accept) but the 85% to 100% range always takes hours and AGMS have no significant advantage in this range.

Many cheaper AGMS say no more than 30 amps charge current per 100 amp hours of capacity
The much revered and often relabelled bitog favorite, the Deka Intimidator, is one such battery.


High $$ AGM Batteries such as Lifeline, Odyssey state specifically, in deep cycle usage, a minimum charge rate, with Lifeline stating no less than 20 amps per 100Ah of capacity, and Odyssey stating no less than 40!

how does this relate to an AGM used only as a starting battery, never intentionally deep cycled.
good question.

Lab coats, expensive tools and lots of time and testing and experiments would be required to answer it accurately.

My take on AGMs is they need the occasional workout. Drain them to 12 volts with a 15 amp load and immediately apply 20+ amps to them, but then once their voltage rises to 14.4 to 14.7v, hold it until it tapers to 0.5 or less, then float at 13.6v until amps taper to zero.

I spoke with an Odyssey technical assistant and he was adamant that if one could not float their battery at 13.6v, to not float it at all, just remove the loads and let it sit, as AGMS , especially high $$ AGMS have such low self discharge.

Odyssey states 13.6v float voltage @ 77f, same with Northstar, Lifeline is a bit lower ar 13.2 o 13.4.
most flooded batteries say float at 13.2v or near there

My AGMS get lazy when I keep them at their prescribed float voltage for days on end, then cycle them.
Their voltage retention, powering high loads like the starter motor, or lesser loads like a refrigerator after days floating, is not as good as hoped for.
When I get annoyed at the voltage retention, considering the amp hours used and the size of the existing load, I will intentionally take the battery down to 50% charged or less, then hit them with high amperage. I have upto 140 amps of plug in adjustable voltage charging sources and two 120 amp alternators, whose voltage I manually control.

Every time I high amp recharge my AGMS, even the Dekas that say no more than 30 amps per 100Ah of capacity, their performance returns, they can maintain higher voltage when powering loads like the starter, or an inverter powering a Skilsaw, or a dc refrigerator overnight.
My 190 amp hours of 5 year old Dekas can accept 116 amps when depleted to 50%
My 103Ah of TPPL Northstar AGM, just turned 3 years old, can still accept 140+

AGms if deeply cycled and fed ONLY low and slow charging currents will be tickled to death.
If they are never discharged much, % wise, they will never be able to accept high charge currents.
Perhaps this is a contributing factor in their less than expected service life even if they are kept at relatively high states of charge.


I'm looking into getting a more powerful charger, this is on my short list:



17 amps in fast charge mode, would that be enough to give the AGM batteries a workout? They're 70 Ah batteries, made by Varta/Bosch
 
Voltage = electrical pressure.
higher pressure, more amperage flow into a depleted battery, in most instances.

The battery itself, determines how much amperage it will accept at the voltage the charging source is able to deliver to its battery terminals.

The higher the state of charge, or the less healthy it is, the less amperage will be required to bring it to 'absorption voltage', generally 14.2 to 14.8v.
The bigger the battery, and the more depleted it is, and the more healthy it is, the more amperage it can accept, for longer, especially when the charging source is not only seeking to bring the battery to mid 14 volts, but hold it there for a few hours.

So one can put a 100 amp charger on a 98% charged battery and it will still only require 2 to 3 amps after 5 seconds to hold the battery at 14.5v, and the amperage required to hold the battery at 14.5v will keep declining, and at some point stop declining.

Lifeline batteries, basically top dog deep cycle AGM batteries, say when a 100 amp hour battery can accept no more than 0.5 amps when held at 14.4v, it can be considered fully charged.

Lifeline also says that in deep cycle duty, when discharging the battery to the 50% charged range, that the charging source needs to be able to deliver no less than 20 amps per 100 amp hours of capacity.

There is reports all over the web, on Rv and boating forums, where they use primarily solar to recharge their batteries, their hyper expensive top of the line Lifeline AGMS, only to have them obviously capacity compromised in far too short a period. They whine and scream and complain, not realizing that their solar is not only well below a 20% charge rate, even at noon, but that slow ramp up in potential wattage, as the sun rising towards solar noon, is inadequate to meet the requirement that the battery be hit with higher amperage at its most depleted state.


Odyssey AGM states no less than 40 amps per 100Ah of capacity when their batteries are deeply cycled.

The AGMS marketed to the automotive starting battery world are no anywhere near the quality of Lifeline or Odyssey, and their price reflects that. Their CCA and reserve capacity is often poorer than their flooded counterparts.

Most lower$$ AGMs will recommend limiting amperage to 27 to 33 amps per 100Ah of capacity. Somewhere in the 30% range.
Note that a healthy AGM unintentionally depleted and jumpstarted, will easily accept more than this from an alternator, yet no mushroom clouds.

The AGM battery never discharged much will never approach 30% acceptance for more than a second or 5 after engine start up.

Since I deep cycle both some Deka intimidators, and Northstar AGMS, and have amp hour counters, and ammeters that not only display charger output, but also how much of that amperage output is powering loads vs what the battery is accepting, and ALL my charging sources have adjustable voltage, a voltage I choose, I get to experience the performance of batteries on a deep level.

It is easy for me to see when a battery is performing less well.
Its voltage retention, for amp hours removed, under X amount of amperage load, is less than I am used to seeing, means the battery is less than happy.
At 28 amp hours from full, under a 10 amp load, I might see 12.34v instead of 12.55v. Removing all loads and voltage might rebound to 12.55 instead of 12.72, as expected.

This poorer voltage retention, always occurs when the deeply cycling battery is either not cycled at all, just had been held at float voltage by my adjustable voltage power supplies, or when deeply cycled and it is only returned to full by low and slow solar, many cycles in a row, and or when deep cycling and there is not enough time, at absorption voltage, to get the battery beyond 96% charged.

The biggest detractor in performance, is when the battery is cycled and only returned to 95% charged, time after time. When this happens that 10 amp load, even when 95% charged, will have voltage drop instantly to 12.2v, and just in general act like a much physically smaller battery.
At this point, simply plugging in and holding the battery at 14.5v until amps taper to very low levels does little to nothing but reset my battery monitor to full, but does nothing for voltage retention during load. Its kind of like expecting to run 11's and only getting 13.5's. Very obvious, to me.

In order to return the expected/ hoped for performance, I need to discharge the battery to ~ 1/2 of its amp hour rating, and then apply my high amperage charging sources.
In some instances, usually when My Northstar TPPL AGM discharged below 50%, 40 amps quickly raises voltage to 14.4v, but than at a constant 40 amps, the maximum output of my one power supply, the voltage will drop to 13.9v, and then again start climbing towards 14.4v over the next 25 to 35 minutes.
I then hold that 14.4v to 14.8v, for as long as it takes for amperage to taper to the 0.4 range, at which point I lower voltage.

With my Deka Intimidator AGMS, I've noticed similar, but once they get back upto 14.4v at 40 amps, lowering the voltage to 14.0, does not lower the amperage required to hold them at 14.0v much if any, where the Northstar will require ~ 25% less amperage to be held at 14.0 vs 14.4, and so there is little benefit to holding these particular Deka AGMS at 14.4v, vs 14.1v, but it is important to get them to 14.4v before lowering the pressure.

So in Deep cycle duty, one should have a high amperage charging source, to high amp recharge from the battery's most depleted state. If applies a high amp potential charger to a battery at a high state of charge, the battery will not be able to accept the higher amperage, even if the charging source is seeking and holding 14.8v. Higher voltage will allow only slightly more amperage to flow, and if the charging source is constant amperage with out voltage limitation occuring then the voltage will be pushed well higher than 14.8v, at higher states of charge.

The automotive starting AGM is far different than an AGM in deep cycle duty, and the SLI( starting lighting ignition) AGM marketed to the automotive crowd, is built far different than a Lifeline or Odyssey TPPL AGM.

Beware of marketing, as many AGMS and Marine flooded batteries claim to be deep cycle batteries, but really cant handle lots of deep cycling. The Deka intimidator AGM is not rated by Deka themselves, for may 'lab' cycles, which never occur in real life, yet the bitog reader might be led to believe this battery can walk on water.

I do feel that an AGM battery, kept in the 80% to 95% charged range, as the voltage regulation of many modern vehicles intentionally seek, will benefit from an intentional deeper discharge, drawn to the 11.8v range, in a few hours or less, then immediately hit with no less than 20 amps per 100Ah of capacity and then held at 14.4 to 14.8v until amperage tapers to very low levels. It might benefit from doing this twice intentionally. It will not benefit from this if done daily.

This low level of amperage threshold, when brought to and held at absorption voltage from the 50% charged range, will take no less than 5.5 hours, and might take 24 hours, or more. The charge logic many chargers apply, is largely just a one size fits all. They might hold absorption voltage for X amount of time, or they might hold absorption voltage until amperage required to maintain that voltage falls to x.x amount, or they might hold absorption voltage until the amount of amperage required to hold absorption voltage only declines x amount of X amount of time.


When one hooks a battery to a charger and the charger says 80%, this is just voltage based. One can unplug the charger 5 minutes after starting it, from AC and DC, then re apply it it will likely show 100% and instantly switch to float voltage. One must be far smarter than their smart charger.

Most chargers marketed to today's automotive consumer, know their customers are not going to monitor the charge profile, and really only care about the green light. The green light means the battery is charged enough to start the vehicle, but it does not mean the battery is truly fully charged. In most cases the green light only means the charger has switched from absorption voltage, to float voltage.

Many chargers get totally confused easily. The simple act of opening the vehicles doors triggering dome lights and other DC loads, when it is absorption stage, will have the amperage required to hold absorption voltage rise, but then fall once the doors are closed. This amperage fall triggers float voltage, and the charger owner sees the green light and assumes the battery is full when it is only actually 89% charged.

HOw any specific 'smart' charger responds to loads placed on the dc system, then removed, is going to vary wildly.

Ideal is not subjective, but 'good enough' certainly is.

Ideal is a battery returned to truly fully charged, and kept cool.
With AGMS and arguably, with flooded, in deep cycle duty, then the charge rate also plays a part in achieving ideal.

'trickle' charging a deeply cycling AGM will tickle it to a premature demise, despite how many times one reads abut how awesome trickle charging is, despite the term 'trickle' having no set value of ampergae, much less what amperage at what pressure. Its as if the word 'trickle' releases serotonin to the person quoting grandpa.
One can put a huge amperage charger on a sulfated AGM or one at relatively high states of charge, and it will never accept large amperages, even if the charging source allows 17 volts, which it never should.

I am not advocating for high amp recharging each and every deep cycle, nor am I advocating that the average auto enthusiast regularly and frequently deeply discharge their AGMS and high amp recharge them.
I am however advocating that the automotive AGM that is kept in the 85% to 95% charged range, will benefit from the owner draining it relatively quickly to the 50% charged range, then immediately applying no less than 20 amps per 100Ah of battery capacity, and then once the charger has gotten it to the 14.4 to 14.8v range, hold it there until amps stop tapering, or taper ot 0.5 per 100Ah of capacity, and only then lowering it to the usual AGm float voltage of 13.6v.

MOst smart chargers will not do this, most smart chargers do not have enough amperage to meet 20%, much less Odyssey's 40% recommendation, and most consumers will never ever be able to tell the difference in performance, whether their battery maintains 11.6v starting their overnight cold engine, or 10.2v.

Most will only know when it starts cranking slower on that cold morning start, likely only when it is falling below 9 volts, and only then start putting a plug in charging source on the battery. But by then it is far too late to return performance with an intentional deep discharge followed by a high amp recharge and then holding absorption voltage for as long as it takes for amperage to taper to low levels or stop tapering, at absorption voltage.
It might help, it might not, but it should buy some time.

Both of My NOrthstar AGMS, would not hold a 13.0v+ full charge resting voltage, until I cycled them and recharged them with 25+ amps from that well depleted state. The voltage they could hold when cranking my overnight cold engine also improved after the deep cycle high amp recharge. I've not tested other AGMS in this regard, but feel a good lung stretching is beneficial, just dont discharge much below 50% and leave them there, dont slowly drain them to below 50% over days, and dont think trickle charging from this low state of charge is beneficial to them.

Most readers here, their largest potential amperage source is their alternator, but the alternators amperage output is determined by the field curent sent to its rotor by the voltage regulator.
Vehicles voltage regulators are as imperfect as smart chargers, in achieving ideal lead acid battery recharging, though by reading comments here some seem to think that they are.

Those who want to be able to accurately gauge the performance of their battery, and/or their charger, need to see how much amperage is flowing to or from the battery, at the electrical pressure measured at the battery terminals.

For most, the only thing that matters is whether the vehicle starts, and for how long.
That so many batteries last for as long as they do is not a testament to the vehicles charging system, but more that modern fuel injected engines start so easily, the battery can be severely degraded, and still easily start the cold engine, in most of the lower 48 anyway.

Cold reduces available capacity and cranking amps of a battery, as well as increasing the amperage it must provide to the starter, but cold is not the battery killer. Heat and chronic undercharging/ living at well less than 100% charged is.
With AGMS, deep discharges followed by low amperage rate recharging, is also detrimental, more so than their cheaper flooded counterparts.

Unfortunately most people seem to believe AGMS are super batteries, immune to abuse and far superior, when in reality they are finicky princesses apt to pack up all their shoes and stomp off home, if not treated like a princess.
 
Ther charger just arrived. I only had a few minutes with it so far and the wife wants me to take her shopping but can already say this:

In the fast charge mode I've seen the voltage rise to 15.6V (AGM/gel mode) before switching off. My other chargers won't go over 14.8V

In start-mode, the charger attempts to sens a lower current into the battery until you try to start. It appears this mode doesn't turn off immediately if the battery is reluctant to take current, seen it go up to 14.8V on a suspected sulfated AGM, though voltage has dropped slightly now to 14.6V. Both my other chargers would turn off in seconds (one to float voltage, the other cycling between float and off) on this battery.

I think I need something to measure DC current as the documentation doesn't reveal anything about what is going on. But I like that I can now force a sensible voltage and presumeably a charging current when I want to.

what do you guys think about this? They have a cheaper one limited to 100A but this one should suffice for measuring starting currents aswell.

 
The voltage is fluctuating slowly between 14.4 and 14.8V not sure what is going on but I used this battery as a testbed as I was going to junk it anyway.

I'm not comfortable leaving the charger connected while we are out although the battery is not getting warm at all at this point. I do think something is happening though and it's accepting a charge.
 
The rising of voltage into the 15s might indicate it just sends out a constant amperage for X amount of time, before trying to limit the max voltage.

My one not so smart sc2500a schumacher, if put on a nearly fully charged battery, even on teh 2 amp setting, will blow right past 15 volts.
If put on a 85% charged battery on the 25 amp setting it will do the same
If put on a 50% charged battery it will take far longer for 12 or 25 amps to being voltage up in the 14's and it might, or might not blow past 14.8v at a constant 13.3 or 26.2 amps, on the 12 or 25 amp setting.

Your linked clampmeter looks nice. I have an older craftsman clampmeter whose layout appears very similar, in terms of buttons and the dial and display location. I am surprised at how accurate it reads down to 0.05 amps.
Do note that these should be placed next to the one wire to be measured, and zeroed, then clamped over one wire. Mine will drift a bit the longer It is left over this wire. Sometimes the zeroing process takes multiple attempts to get it to read zero, before clamping over a single wire.

One can read slightly different readings whether clamped over + or - wire, and can get slightly different readings depending on where on the length of wire, it is clamped. The charger itself can make a good amount of interference, and the battery itself is like a bigger filter, so I usually try to take readings closer to the battery than right next to charging source.

There's something to be said for shunted ammeters.
Basically all my ammeters, I check their accuracy by comparing to multiple other ammeters, clampon or permanent hall effect sensors. The accuracy is desired at low amperage levels, below say 0.7 amps. I want to know when my batteries are accepting less than 0.5 amps when held at 14.4 or 14.7v, and most are good enough for this. whether one reads 39.4 amps and the other reads 40.1 and a third says 40.7 is not a concern.

A quick browsing of Ammeters on Ebay reveals one I have not seen before:
https://www.ebay.com/itm/393698244845?hash=item5baa3e78ed:g:SqYAAOSwRKthjnEo
When they can count the total amount of current passed, in watt hours and amp hours, one gets a much better idea of the state of charge when they first attached the charger, and then can better infer state of health.

I can't find the thread, but someone asked about loading a battery in order to charge it at higher rates for longer.
in order for a battery to accept higher amperage from a higher amperage charger, for a while,and perhaps be exercised into performing better afterwards, it needs to be discharged a fair amount. Headlights and blower motor are pretty good loads, but hours on bulbs is a consideration. I generally try to drain my battery to the 50% range over 5 to 6 hours, then immediately apply my 40 amp or 100 amp plug in adjustable voltage power supplies, and sometimes both together, in parallel, though this Maxing out a 15 amp outlet and will easily reveal less than ideal Ac connections.

To do it quicker, I have a small 200 watt lasko heater. On my 800 watt MSW inverter this is approximately 19 amps of load measured by shunted ammeter. An inverter is an excellent way to put a decent sized load on a battery and drain it quickly to the 50% range. Too easily in many cases.

When loading a battery and watching voltage drop with that load, knowing how much load is being applied and then seeing the voltage the battery was able to maintain powering that load, then seeing how high it rebounds when the load is removed, can give one a great idea of how low they brought the battery.
a 15 amp load might take an hour to drop a fully charged, healthy, 100 amp hour battery to 12.3v, but remove that load and voltage will probably rebound to 12.5x nearly instantly and over 15 minutes rise back to 12.6x.
Generally, 12.2v 15 minutes after the load is removed, is in the 50% charged range, but before load removal the voltage might read as low as 11.8. The bigger the load the lower the voltage the mattery maintains powering it, and the more that battery voltage will rebound once that load is removed.

One does not really want to drain a battery to low states of charge, and let it sit, if one does not have to. The worst thing for a battery is to slowly be brought to 50% or less over 3 weeks, then only recharged to 75%. Likewise a carbon pile load tester loading the battery at 250 amps for greater than 15 seconds is also not great for it.

A 100 amp hour battery can maintain a 5 amp load for 20 hours before voltage falls to 10.5v, which is considered 100% discharged, though some AGMS will say 11.2 is 100% discharged.
This same 100 amp hour battery cannot power a 20 amp load for 5 hours, due to the Peukert effect.

The deeper the discharge, the more important it not only is to recharge quickly, but recharge to truly full.
So if intending to exercise an AGM, which is acting a bit lazy, I would recommend draining it to ~11.8v under load, with a load large enough that can do it in 3 to 6 hours, then immediately apply their higher amperage charging source.
with the ammeter and the voltmeter, see how long it takes that amperage to get battery voltage upto the 14.4v to14.8v range
Then once the charger is holding that absorption voltage, see how long it takes for amperage to taper to low levels.
A new healthy battery will take about 3.5 hours for amps to taper to 0.5% of the amp hour capacity.

An older sulfated battery will take longer, and perhaps much longer for amps to taper to low levels, and most smart chargers will never hold absorption voltage for this long, and judging state of charge by amperage acceptance at lesser voltages can be a lesson in futility, especially without previous experience doing it before on this same battery with other methods that can confirm the estimates.

As most smart chargers will not hold absorption voltage for long enough, for amperage to taper to low levels, the user desiring to hold absorption voltage will likely have to apply a load to the battery to drop its voltage to the 12.7v or less range, then restart the charger. One might have to disconnect both the AD and DC in order to get the charger to restart, opr drop the battery voltage lower than 12.69v, via the loads, and sometimes it is easiest to restart the charger while the load is still on the battery, then remove the load.
When doing this, with a charger that has different amperage outputs, use the lower settings, as 5 amps at this point can easily push battery voltage well above 15, quickly, and it might show an error code.

The reason I use adjustable voltage power supplies, as it is far easier to hold absorption voltage for as long as it takes amperage to taper to 0.5 amps or less, as opposed to trying to trick a smart charger, over and over, into holding absorption voltage for long enough to recharge a deeply cycled battery back to a true full state of charge.
Since it takes so much longer the more aged and cycled a battery becomes, and that 100% charge is so important to maintain capacity and cranking amps/ general performance, on a deep cycling battery, the power supply saves me a lot of time and effort.

The issue with an adjustable voltage power supply is it needs to be turned off, or turned down to float voltage once the battery is fully charged. An 8 hour spring wound timer set to 6.5 hours is going to get a healthy newish battery in the 98% charged range. as it ages the full 8 hours will be needed, and neared end of life or after a long time being undercharged that 8 hour timer will be cranked up to 8 2 and 3 times.

An adjustable voltage power supply also does not know if the battery developed a shorted cell or if it has gone into thermal runaway.
Batteries nearer end of life, held at constant absorption voltage, of 14.2 to 14.8v, will have their amperage taper to a point, and then start rising, At this point there all charging should be terminated. My smaller Ub12180 agm will sometimes bounce off an amperage level, still above the 0.09 amps at 14.5 to 14.9v then start rising as the battery heats, and amperage will rise as battery heats. If I stop charging, let battery sit for 6+ hours, then return it to absorption voltage, amps will taper to that 0.09 level at which point I can consider it fully charged.
My UB12220 which is a year newer, will taper to zero at 14.8v, even though it can be considered fully charged when accepting 0.11 amps at 14.5 to 14.9v.

The ammeter is a wonderful tool to get an idea of the batteries ability to accept and store energy. If it can count amperage accepted over X amount of time, it is even more useful. When teh charging source can hold the battery at absorption voltage for that whole time it is even more revealing, as the amount accepted at float voltage is not revealing at all.

I get why it is desirable for a 'smart charger' to just flash a green light in the 'good enough' zone, the problem only becomes apparent, when that battery suddenly and obviously is performing badly, at which point returning performance is highly unlikely, especially without a charging source capable of holding the higher voltage for longer, which would have prevented the premature capacity loss in the first place.

In a starting battery application this might be 4 years vs 2, or 5 years vs 3, but in a deep cycle application it can mean 5 years vs 4 months, or 8 years vs 6 months.
 
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