Voltage is electrical pressure.
A 12.8v battery hooked to a charging source, which only seeks 12.8volts, there will be no current flow. No amps will flow from charging source to battery as there is NO difference in electrical pressure between source( charger) and load( battery).
The charging source needs to raise the pressure in order for amperage to flow from the charger to the battery.
The higher the difference in pressure between the source/ charger, and load/ battery, the more amperage will flow.
a 25 amp charger has 25 amps potential current to flow from itself, into the battery.
The battery itself determines how much amperage flows into it from a charging source, at the pressure/voltage reaching its battery terminals. A healthy depleted battery can accept a LOT of amperage, when the chrging source is seeking high pressure. much less amperage flows when it is seeking lower pressure.
An unhealthy depleted battery cannot accept nearly as much amperage as a healthy de[pleted battery can at either high or low charging pressures.
A healthy battery at high states of charge will not accept as much amperage as an unhealthy battery will at float or absorption voltage, but unless the battery has a shorted cell, this difference will be about 0.5 amps more at 13.6v and perhaps as much as 1.5 amps at 14.5v
Charging an android phone places about a 1 amp extra load on teh alternator.
Don't charge your smart phone, it will kill your alternator!!!!!.
The whole 'old weak battery killed my alternator claim', one reads over and over, here, and everywhere, is much more likely that the failing alternator killed the old end of life battery by not charging it properly for a long while. Then the battery was replaced first, then it became obvious the alternator was the cause of the failed battery. Chicken or the egg. It is the chicken, not the egg, most likely as 1 extra amp required to feed an unhealthy battery at higher states of charge, is not going to fry an alternator. The healthy battery that can drink up 55 amps for a half hour before tapering, is going to stress teh alternator far more than the unhealthy battery which can only accept 35 amps for 10 minutes before tapering.
large banks of deeply cycled TPPL Agms are known to fry an alternator that was perfectly fine charging the same amp hour bank of flooded, as healthy TPPL agms are so thirsty when well depleted, and will max out an alternator, overheasting the stator, frying the varnish on the windings, shorting them out
A 40 amp power supply set to 14.7v, and hooked to an ~ 50% depleted battery which measures 12.2v, open circuit voltage(OCV) is likely going to instantly provide 40 amps, and then the voltage. measured at the battery terminals will climb from 12.2v towards the maximum setpoint the 'smart' charger allows, which is anywhere in the 14.2 to 14.9 vollt range, depending on teh charger, and potentially its temperature too.
The adjustable voltage power supply, one chooses the maximum voltage, and just lets the battery feed on as much amperage as it wants.
When well depleted the 40 amp charger will provide 40 amps until the battery raises to the maximum allowed voltage, in my case I choose 14.7v. Other chargers will have decided that 14.4 is fine and dandy, andother one will say agms need more voltage, and choose 14.8v as their maximum.
A 14.8v maximum charger, will have those 40 amps flow for longer, than will the 14.4v charger.
Once the battery has reached 14.4 to 14.8 volts, then the amperage required to maintain this voltage tapers. The longer this voltage is held, as the battery charges, the less amperage is required to maintain that voltage.
A higher maximum voltage, will charge the battery faster. It will allow the battery to feed on the charging sources maximum output for a bit longer, before amperage tapering begins.
This does not mean, that a 14.4v cannot ever fully charge a healthy battery, it means the 14.4v will simply take longer to fully charge it.
Old sulfated batteries might indeed require higher voltages tpo overcome the batterys resistance ind indeed reach true full charge.
By and large the whole ' agms need higher voltage to fully charge' one reads over and over is BS.
What a deeply cycled AGM needs is for it to be hooked not to a 4 amp charger which will take ~ 12 hours before it can raise that voltage to 14.6v, it much prefers that a 25 amp or even higher amperage charging source can get the battery to 14.6v in less than an hour.
This higher amperage flow forces the migration of electrolyte through the glass matting and into the plates better than the traditional low and slow 'trickle charge it for days thing, that one reads so much about, concerning flooded batteries.
By and large, in my opinion, the ultimate maximum voltage the charger allows, does not matter all that much, as long as the charging source is actually going to hold that maximum voltage for long enough for the battery to drink all it wants for as long as it needs to become truly fully charged.
A 25 amp charging source with a higher maximum allowed voltage of 14.8v, will be able to get a discharged battery to higher state of charge, faster than a 25 amp charger which will not bring teh battery above 14.4v.
If the charger is going to quit early, as most do, then the higher voltage means the battery is likely at a higher state of charge when the charger says enough absorption voltage for you.
AGMS like the high amp recharge. They can( especially TPPL agms) have significantly less resistance than a flooded battery, and they can recharge faster from below 85% state of charge, to 85% state of charge faster than a flooded battery. From 85% to 100%, they can charge slightly faster.
When a flooded battery gets charged to teh 14.2v range on its way to 14.6v,, or 14.8v it starts gassing bubbling. electrolysis. The higher the state of charge it gets, the more if bubbles and offgasses and basically this is using water. Often the flooded battery voltage will be recommended to be lower, simply to reduce the amount of water usage.
AGMS will basically not use any water, unless overcharged, and they will likely only offgas a tiny fraction the amount of a flooded battery, if they not only are overcharged, but got really hot on their way from undercharged to overcharged. Mostly the gasses recombine within the cells unless they get too hot and create too much pressure and the relief valves open and let some escape.
In my opinion the AGMS 'need higher voltage' claim is merely because they are unlikely to ever offgass, and a higher voltage will charger them to a higher state of charge when teh charging source is actually on, and the higher the average state of charge, without going to overcharge, the better.
Lifeline AGM states 14.2 to 14.4v absorption voltage, and their conditioning charge voltage is 15.5v, to be applied only after significant time was spent at 14.2 to 14.4. Lifeline says float at 13.2 to 13.4 at 77f.
Here are some secs from memory, and in the last 10 years some of these manufacturer recommendations have changed slightly as they are likely double guessing what/ how the batteries will be used, and recharged, and what recommended values will likely yield the least warranty return rate, therefore, max profit
Deka intimidator AGM says 14.1 to 14.4v absorption, and 14.6 is the 'equalization voltage' and float is 13.2 to 13.5v, at 77f
Trojan flooded deep cycle says absorption voltage should be 12.82v , and float 13.3v at 77f, and equaization at 16.2v.
Northstar AGM says absorption voltage should be 14.4 to 14.8v, and float at 13.6v
Odyssey I believe says 14.7vABS and 13.6v float, @ 77f
Don't get hung up on absorption voltage, when one is not trying to race the clock, a lower than spec'd absorption voltage on a healthy battery just will have it take 7.2 hours instead of 6.8 to get truly fully charged.
The specs above show that there is huge overlap on manufacturer recommended charging voltages for both flooded AND AGM, so Ignore those who shout this needs that and those need this so sayith the manual which cameth with my alpha smart charger.
If a person really wants to be a voltage stickler, then by all means, goto the manufacturer's recommendations, but dont freak out if the charger does not match absorption voltage exactly, and if float voltage is not within spec, well just don't float the battery for more than a day or 2.
Northstar Says 14.4 to 14.8v absorption. I generally choose 14.7 @ 77f and give it a bit more when cool out, and take some off when it is hot, or I expect significant battery heating from high amp recharging from a low state of charge.
Also, An alternator can and will certainly fully charge a battery, even a well depleted one, but it is not going to defy the 6.5 hours minimum from 50% charge to 100% charged. It will only do the 50 to 100% thing, if it is seeking and then holding an absorption voltage in the 14.4 to 14.8v range. Before teh whole MPG/ regulation became a thing, the voltage regulator would likely allow 14.4 for a few minutes after engine start then lower to 13.6 ish volts. If the battery is well depleted to around 50% , it will take 6.5 hours to fully charge it at 14.4v, but at a lower pressure of 13.6v, it might take 15 or 20 hours.
The danger to an alternator is heat. A well depleted battery can easily max out the alternator, and if idling parked it will be bathed in radiator heat. and generating tons of its own heat, especially if its voltage regulator is commanding 14.4v the entire time.
My Northstar, depleted to about 75% state of charge, if put on my 40 amp power supply set to 13.6v, after about 10 minutes the 40 amps will taper to about 6 amps. At this point if I crank the voltage/pressure from 13.6 ,to 14.8v, the battery is feeding on 26 to 32 amps. same thing with my alternators, but they have 120 amps available with 2000+ engine rpm. I've modified my charging system with adjustable external voltage regulators and I manually control voltage with a dial on my dashboard. So I am very used to spinniing the pressure dial and watching the amperage increase or decrease with the different pressure I command.
Float voltage is more important, if the battery is to be floated long term. Too low a float voltage is to be avoided, as it will allow the battery to slowly and slightly discharge and sulfate, albeit slowly, too high a float will ( eventually) overcharge and cause positive plate degradation.
The Odyssey engineer could not shout this loud enough, do not float a Odyssey long term, unless the float voltage is 13.6 at 77f and is adjusted higher for cold temps and lower for higher temps. And I have certainly experienced this with my Northstars. if I do not adjust that 13.6v higher at 40 degrees for 12+ hours then the battery obviously is not full anymore, but I am usually running other loads making this issue worse than would a slight parasitic draw of modern vehicle.
AGMS are not a panacea, they have their advantages, they(especially TPPL Agms), can vastly outperform their flooded brethern, but they are likely less tolerant of improper charging than their much flooded brethern. Their sulfation forms easier and faster when chronically undercharged and takes more effort to redissolve if the battery owner happens to notice performance loss and tried to correct it.
But usually, this comes far to late and a regular smart charger has far too little to correct it.
To everyday Joe driver, who only uses their battery for engine starting, the ONLY advantages to AGM are no offgassing, nearly eliminating the usual battery terminal corrosion, and the AGM is likely to maintain higher CCA when the parasitic load has drained either flooded or AGm to the same level of depletion. The 35% charged AGM will likely still easily start the engine, the 35% charged flooded battery, all other factors being equal, will likely struggle. When super cold, the same thing, the AGM will likely stand a better chance of starting at the same depleted level at same temperature as the flooded.
AGMS do have lower self discharge, but in modern vehicles with significant parasitic loads, the lower self discharge might not make any difference.
Also note that the least $$ AGMS do not really have much better self discharge than than flooded batts, nor is their CCA much if any higher, and if they are rated in amp hours( actualenergy storage capacity), the flooded battery might have 5% more am hours even though it is lighter.
The TPPL agm can take as much amperage as most any charging source can deliver. They can be charger from below 85% state of charge, to 85 % state of charge very quickly, if that high amp charging source is seeking( and holding) 14.7v. but agm charge only marginally faster than flooded, from 85% to 100%, and 80% to 100% is pretty much never less than 3.5 hours and only that low when the battery is still new and healthy.
The AGM charged only to 97% and kept at an average 94% state of charge, compared to the same exact size flooded battery kept 97% averaging 94%, will likely lose capacity faster than the flooded. If both are put on a charging source capable of holding 14.7v plus for a good while, the flooded battery likely has a better chance of having come capacity and Cranking amps returned.
Overall unless one absolutely needs a no offgassing battery and it is life or death cold outside with a long sitting unused vehicle The AGM is a waste of money. They can spin the starter faster, when new and healthy, but that 'wow' factor might disappear in a month or 3, if the battery is not kept healthy with lung expanding regular true full charges.
I use northstars, as I deep cycle regularly, and often only have a limited distance to drive to recharge as much as possible in as short a time as possible, or a minimum time to plug in and use a high amperage adjustable voltage power supply set to 14.7 or 14.8v. These TPPL agms laugh off huge charging currents and pretty much require them when they are deep cycled.
I've also noted that my Northstars would NOT hold their 13.06 full charge resting voltages, when new, no matter how long I held absrption voltage, until I deep cycled them, then high amp recharged them until amps tapered to low levels. All new batteries behave a bit weirdly that first charging. I've noticed their amps do not really taper as low as expected, until they are cycled then recharged. I've noticed the scary fast engine cranking of my Northstars did not happen until after I cycled it and high amp recharged it to full.
I think slapping a TPPL agm into a vehicle that is never going to cycle it deeper , will neuter its potential ability, and perhaps lifespan too..
A 12.8v battery hooked to a charging source, which only seeks 12.8volts, there will be no current flow. No amps will flow from charging source to battery as there is NO difference in electrical pressure between source( charger) and load( battery).
The charging source needs to raise the pressure in order for amperage to flow from the charger to the battery.
The higher the difference in pressure between the source/ charger, and load/ battery, the more amperage will flow.
a 25 amp charger has 25 amps potential current to flow from itself, into the battery.
The battery itself determines how much amperage flows into it from a charging source, at the pressure/voltage reaching its battery terminals. A healthy depleted battery can accept a LOT of amperage, when the chrging source is seeking high pressure. much less amperage flows when it is seeking lower pressure.
An unhealthy depleted battery cannot accept nearly as much amperage as a healthy de[pleted battery can at either high or low charging pressures.
A healthy battery at high states of charge will not accept as much amperage as an unhealthy battery will at float or absorption voltage, but unless the battery has a shorted cell, this difference will be about 0.5 amps more at 13.6v and perhaps as much as 1.5 amps at 14.5v
Charging an android phone places about a 1 amp extra load on teh alternator.
Don't charge your smart phone, it will kill your alternator!!!!!.
The whole 'old weak battery killed my alternator claim', one reads over and over, here, and everywhere, is much more likely that the failing alternator killed the old end of life battery by not charging it properly for a long while. Then the battery was replaced first, then it became obvious the alternator was the cause of the failed battery. Chicken or the egg. It is the chicken, not the egg, most likely as 1 extra amp required to feed an unhealthy battery at higher states of charge, is not going to fry an alternator. The healthy battery that can drink up 55 amps for a half hour before tapering, is going to stress teh alternator far more than the unhealthy battery which can only accept 35 amps for 10 minutes before tapering.
large banks of deeply cycled TPPL Agms are known to fry an alternator that was perfectly fine charging the same amp hour bank of flooded, as healthy TPPL agms are so thirsty when well depleted, and will max out an alternator, overheasting the stator, frying the varnish on the windings, shorting them out
A 40 amp power supply set to 14.7v, and hooked to an ~ 50% depleted battery which measures 12.2v, open circuit voltage(OCV) is likely going to instantly provide 40 amps, and then the voltage. measured at the battery terminals will climb from 12.2v towards the maximum setpoint the 'smart' charger allows, which is anywhere in the 14.2 to 14.9 vollt range, depending on teh charger, and potentially its temperature too.
The adjustable voltage power supply, one chooses the maximum voltage, and just lets the battery feed on as much amperage as it wants.
When well depleted the 40 amp charger will provide 40 amps until the battery raises to the maximum allowed voltage, in my case I choose 14.7v. Other chargers will have decided that 14.4 is fine and dandy, andother one will say agms need more voltage, and choose 14.8v as their maximum.
A 14.8v maximum charger, will have those 40 amps flow for longer, than will the 14.4v charger.
Once the battery has reached 14.4 to 14.8 volts, then the amperage required to maintain this voltage tapers. The longer this voltage is held, as the battery charges, the less amperage is required to maintain that voltage.
A higher maximum voltage, will charge the battery faster. It will allow the battery to feed on the charging sources maximum output for a bit longer, before amperage tapering begins.
This does not mean, that a 14.4v cannot ever fully charge a healthy battery, it means the 14.4v will simply take longer to fully charge it.
Old sulfated batteries might indeed require higher voltages tpo overcome the batterys resistance ind indeed reach true full charge.
By and large the whole ' agms need higher voltage to fully charge' one reads over and over is BS.
What a deeply cycled AGM needs is for it to be hooked not to a 4 amp charger which will take ~ 12 hours before it can raise that voltage to 14.6v, it much prefers that a 25 amp or even higher amperage charging source can get the battery to 14.6v in less than an hour.
This higher amperage flow forces the migration of electrolyte through the glass matting and into the plates better than the traditional low and slow 'trickle charge it for days thing, that one reads so much about, concerning flooded batteries.
By and large, in my opinion, the ultimate maximum voltage the charger allows, does not matter all that much, as long as the charging source is actually going to hold that maximum voltage for long enough for the battery to drink all it wants for as long as it needs to become truly fully charged.
A 25 amp charging source with a higher maximum allowed voltage of 14.8v, will be able to get a discharged battery to higher state of charge, faster than a 25 amp charger which will not bring teh battery above 14.4v.
If the charger is going to quit early, as most do, then the higher voltage means the battery is likely at a higher state of charge when the charger says enough absorption voltage for you.
AGMS like the high amp recharge. They can( especially TPPL agms) have significantly less resistance than a flooded battery, and they can recharge faster from below 85% state of charge, to 85% state of charge faster than a flooded battery. From 85% to 100%, they can charge slightly faster.
When a flooded battery gets charged to teh 14.2v range on its way to 14.6v,, or 14.8v it starts gassing bubbling. electrolysis. The higher the state of charge it gets, the more if bubbles and offgasses and basically this is using water. Often the flooded battery voltage will be recommended to be lower, simply to reduce the amount of water usage.
AGMS will basically not use any water, unless overcharged, and they will likely only offgas a tiny fraction the amount of a flooded battery, if they not only are overcharged, but got really hot on their way from undercharged to overcharged. Mostly the gasses recombine within the cells unless they get too hot and create too much pressure and the relief valves open and let some escape.
In my opinion the AGMS 'need higher voltage' claim is merely because they are unlikely to ever offgass, and a higher voltage will charger them to a higher state of charge when teh charging source is actually on, and the higher the average state of charge, without going to overcharge, the better.
Lifeline AGM states 14.2 to 14.4v absorption voltage, and their conditioning charge voltage is 15.5v, to be applied only after significant time was spent at 14.2 to 14.4. Lifeline says float at 13.2 to 13.4 at 77f.
Here are some secs from memory, and in the last 10 years some of these manufacturer recommendations have changed slightly as they are likely double guessing what/ how the batteries will be used, and recharged, and what recommended values will likely yield the least warranty return rate, therefore, max profit
Deka intimidator AGM says 14.1 to 14.4v absorption, and 14.6 is the 'equalization voltage' and float is 13.2 to 13.5v, at 77f
Trojan flooded deep cycle says absorption voltage should be 12.82v , and float 13.3v at 77f, and equaization at 16.2v.
Northstar AGM says absorption voltage should be 14.4 to 14.8v, and float at 13.6v
Odyssey I believe says 14.7vABS and 13.6v float, @ 77f
Don't get hung up on absorption voltage, when one is not trying to race the clock, a lower than spec'd absorption voltage on a healthy battery just will have it take 7.2 hours instead of 6.8 to get truly fully charged.
The specs above show that there is huge overlap on manufacturer recommended charging voltages for both flooded AND AGM, so Ignore those who shout this needs that and those need this so sayith the manual which cameth with my alpha smart charger.
If a person really wants to be a voltage stickler, then by all means, goto the manufacturer's recommendations, but dont freak out if the charger does not match absorption voltage exactly, and if float voltage is not within spec, well just don't float the battery for more than a day or 2.
Northstar Says 14.4 to 14.8v absorption. I generally choose 14.7 @ 77f and give it a bit more when cool out, and take some off when it is hot, or I expect significant battery heating from high amp recharging from a low state of charge.
Also, An alternator can and will certainly fully charge a battery, even a well depleted one, but it is not going to defy the 6.5 hours minimum from 50% charge to 100% charged. It will only do the 50 to 100% thing, if it is seeking and then holding an absorption voltage in the 14.4 to 14.8v range. Before teh whole MPG/ regulation became a thing, the voltage regulator would likely allow 14.4 for a few minutes after engine start then lower to 13.6 ish volts. If the battery is well depleted to around 50% , it will take 6.5 hours to fully charge it at 14.4v, but at a lower pressure of 13.6v, it might take 15 or 20 hours.
The danger to an alternator is heat. A well depleted battery can easily max out the alternator, and if idling parked it will be bathed in radiator heat. and generating tons of its own heat, especially if its voltage regulator is commanding 14.4v the entire time.
My Northstar, depleted to about 75% state of charge, if put on my 40 amp power supply set to 13.6v, after about 10 minutes the 40 amps will taper to about 6 amps. At this point if I crank the voltage/pressure from 13.6 ,to 14.8v, the battery is feeding on 26 to 32 amps. same thing with my alternators, but they have 120 amps available with 2000+ engine rpm. I've modified my charging system with adjustable external voltage regulators and I manually control voltage with a dial on my dashboard. So I am very used to spinniing the pressure dial and watching the amperage increase or decrease with the different pressure I command.
Float voltage is more important, if the battery is to be floated long term. Too low a float voltage is to be avoided, as it will allow the battery to slowly and slightly discharge and sulfate, albeit slowly, too high a float will ( eventually) overcharge and cause positive plate degradation.
The Odyssey engineer could not shout this loud enough, do not float a Odyssey long term, unless the float voltage is 13.6 at 77f and is adjusted higher for cold temps and lower for higher temps. And I have certainly experienced this with my Northstars. if I do not adjust that 13.6v higher at 40 degrees for 12+ hours then the battery obviously is not full anymore, but I am usually running other loads making this issue worse than would a slight parasitic draw of modern vehicle.
AGMS are not a panacea, they have their advantages, they(especially TPPL Agms), can vastly outperform their flooded brethern, but they are likely less tolerant of improper charging than their much flooded brethern. Their sulfation forms easier and faster when chronically undercharged and takes more effort to redissolve if the battery owner happens to notice performance loss and tried to correct it.
But usually, this comes far to late and a regular smart charger has far too little to correct it.
To everyday Joe driver, who only uses their battery for engine starting, the ONLY advantages to AGM are no offgassing, nearly eliminating the usual battery terminal corrosion, and the AGM is likely to maintain higher CCA when the parasitic load has drained either flooded or AGm to the same level of depletion. The 35% charged AGM will likely still easily start the engine, the 35% charged flooded battery, all other factors being equal, will likely struggle. When super cold, the same thing, the AGM will likely stand a better chance of starting at the same depleted level at same temperature as the flooded.
AGMS do have lower self discharge, but in modern vehicles with significant parasitic loads, the lower self discharge might not make any difference.
Also note that the least $$ AGMS do not really have much better self discharge than than flooded batts, nor is their CCA much if any higher, and if they are rated in amp hours( actualenergy storage capacity), the flooded battery might have 5% more am hours even though it is lighter.
The TPPL agm can take as much amperage as most any charging source can deliver. They can be charger from below 85% state of charge, to 85 % state of charge very quickly, if that high amp charging source is seeking( and holding) 14.7v. but agm charge only marginally faster than flooded, from 85% to 100%, and 80% to 100% is pretty much never less than 3.5 hours and only that low when the battery is still new and healthy.
The AGM charged only to 97% and kept at an average 94% state of charge, compared to the same exact size flooded battery kept 97% averaging 94%, will likely lose capacity faster than the flooded. If both are put on a charging source capable of holding 14.7v plus for a good while, the flooded battery likely has a better chance of having come capacity and Cranking amps returned.
Overall unless one absolutely needs a no offgassing battery and it is life or death cold outside with a long sitting unused vehicle The AGM is a waste of money. They can spin the starter faster, when new and healthy, but that 'wow' factor might disappear in a month or 3, if the battery is not kept healthy with lung expanding regular true full charges.
I use northstars, as I deep cycle regularly, and often only have a limited distance to drive to recharge as much as possible in as short a time as possible, or a minimum time to plug in and use a high amperage adjustable voltage power supply set to 14.7 or 14.8v. These TPPL agms laugh off huge charging currents and pretty much require them when they are deep cycled.
I've also noted that my Northstars would NOT hold their 13.06 full charge resting voltages, when new, no matter how long I held absrption voltage, until I deep cycled them, then high amp recharged them until amps tapered to low levels. All new batteries behave a bit weirdly that first charging. I've noticed their amps do not really taper as low as expected, until they are cycled then recharged. I've noticed the scary fast engine cranking of my Northstars did not happen until after I cycled it and high amp recharged it to full.
I think slapping a TPPL agm into a vehicle that is never going to cycle it deeper , will neuter its potential ability, and perhaps lifespan too..
