Do AGM's charge quicker?

jharrell wrote:
To use a constant voltage power supply as a charger it must have over current protection, preferably constant current protection.

Not necessarily. Suppose that the constant voltage power supply has current capacity sufficient to supply "all" that the battery can absorb at the constant voltage that you set the power supply at?

For an extreme example to get at a clear understanding - say a 500 amp power supply that you set to 14.7 volts in order to charge a single 12V Lifeline Group 27 battery. The power supply's maximum current probably cannot be reached before - regardless of what Lifeline may claim - the battery would go into thermal runaway.

What this means is that in order to do true constant voltage charging, the charger must have a current capacity that exceeds what the battery will ever absorb at that voltage.

I prefer to go the other way - use an RV battery charger that is under-rated with respect to what our AGM batteries can take during recharge. What this brings about is a longer time to charge, a money saving by not having to buy a more powerfull charger, and (maybe) causing the batteries to lose capacity slowly over time because they might not be seeing high enough initial charging voltages over their lifetime.

However we're not full-timers who stay put drycamping for long periods. Our motorhome's alternator dumps large amounts of current into our AGM batteries between campsites and at campsites via short main engine idling runs.

I guess one has to pick their poison.

To use a constant voltage power supply as a charger it must have over current protection, preferably constant current protection. The constant voltage is a setting, when attached to a discharged large lead acid battery it will attempt to supply set constant voltage, immediately hit over current protection and go into constant current mode with voltage dropping accordingly. This is bulk charging. As battery charges the voltage to maintain max power supply output will rise until hitting power supply constant voltage setting, at which point current will be lowered to maintain set voltage. This is absorb charging. Congratulations you have a two stage charger, just be sure to turn it off after a while or lower it’s constant voltage setting to float voltage for a manual third stage.

The battery voltage rises till the end when it is finally the same as the charger's voltage and no amps flow.

A sulphated battery has lower capacity and SG, but also has a slightly lower resting voltage. 12.6 instead of 12.7

But you have to use the voltage per SOC for your own battery specs. My AGMs are 13.0 full and 12.4 at 50%. Trojan T-105-AGMs are more like Flooded ones for voltage/SOC. I don't know why.

BFL13 wrote:
The battery's voltage continues to climb all the way through both stages.

That's the head-scratching tricky part ... what is the "battery's voltage"?

With a high quality and powerful enough battery charger - during charging at either the constant current stage or constant voltage stage (... and of course using a charger with no voltage sag due to it's current delivery capacity) - the "battery's voltage" is the voltage on the battery's terminals that has been forced by the charger ... and hench is that which is showing on the charger's meter and matching what you set the charger at.

To me however ... the "battery's voltage" is what it reads several minutes after you have removed it from the charger (to get rid of surface charge).

So I guess what has been talked mostly around so much in these forums is: If a battery's at-rest terminal voltage after somehow/someway charging it long enough over the years with "whatever" charging voltages (like I have) is still showing at it's original as-new design value - that does NOT MEAN that it's full capacity is still present.

IAW, a battery's terminal voltage reading may indicate full charge at present capacity, but not full charge at it's original capacity when new.

So ... I've been slowly ruining my RV batteries by fully charging them (per at-rest terminal voltage readings) with too-low charging voltages time and time again over the years. :(

As long as you have picked better terminology my way of charging AGM batteries is to apply constant 28.80 volts from beginning to end.

I constant volt charge my single cells but issue a time limit for the 14.8 volts which must then be reduced. Stage 2 is via a cute International Controls timer cube 1-6 hours on the big charger. DPDT enclosed 100 amp 240vac relay. Defaults to 28.0 volts.

But when public power comes on, the Trace inverter is prioritized and it does it's lame Trace charging cycle.

Phil the charger's voltage during the constant current stage (Bulk Stage) is the same as the charger's voltage during the constant voltage stage.(Absorption Stage) in the normal usage of those terms.

The battery's voltage continues to climb all the way through both stages.

Where language goes off the rails is with those "three- stage converter" guys. Eg, Iota does 14.8v at first, but then drops to 14.2v for its "absorption voltage". The three "stages" being just three different voltages. You could do Bulk and Absorption at either of two of their voltages 14.4 or 13.6. But they don't talk that way. Can't be helped. :(

It might be helpful if we quit using the phrases "bulk charging" and "absorption charging" of batteries.

From an electrical point of view, it's more clear to say "constant current charging" (instead of bulk charging), and "constant voltage charging" (instead of absorption charging).

The constant current charging phase means supplying a high constant current for awhile holding whatever (high) voltage it takes to do that, while the constant voltage charging phase means holding a lower constant voltage while supplying a continuously tapering amount of current into the battery until the battery's intrinsic voltage equals whatever voltage that the charger was holding.

For me at least, the above is a lot clearer way to be discussing battery type/size, versus charger mode/capacity, versus charge time issues. I notice that a lot of the technical papers on battery charging also use the terms constatnt current charging and constant voltage charging.

Not clear at all! Different meanings of "bulk" and "absorption" from what I have been using. I think we are in agreement though if my translation is correct :)