Thanks for all that. OK, so.... Correct me if I'm wrong. These chargers don't "push" current, like a standard lead acid battery charger does. Instead the device plugged into it only allows enough current that it knows it can accept?
Sorry to sidetrack, but lead acid charging is something I've been dealing a lot with lately, albeit in small applications.
There are actually two "methods" to charging lead acid, and ideally you use both of them. When I charge free-standing batteries I use a bench power supply that can function either in constant current or constant voltage mode.
Lead acid loves being charged "low and slow" as the saying goes, and either constant voltage or constant current can work but which one is "ideal" depends on the current state of charge of the battery.
For small sealed batteries, generally the desired constant voltage charging point is 2.4V/cell(I ALWAYS go by the manufacturer's spec sheet when I have it), while flooded batteries often are best at more like 2.3V/cell. Of course a "12V" automotive battery is 6 cells, so when I give these voltages multiply them by the number of cells. I deal with single cell, 3 cell, and 4 cell batteries quite often.
BTW, the ideal voltage is a balancing act and is also temperature dependent. Charging at too low of a voltage can lead to sulfation, while too high causes "gassing"(electrolysis of the electrolyte). Sealed cells typically have some ability to contain pressure although of course also have safety valves to "pop." When I'm trying to ressurect one from the dead(and I can sometimes do it...) I may need to use stupid voltages to get it there and will "unseal" a sealed battery where I can to better handle this.
When I set up a bench charger, before connecting, I will set the voltage to 2.4V/cell, and then connect a resistor across it(since the one I have won't let you see the current set point unless it's under load) to set the current to .1C, or 1/10 the Ah rating.
When one is low, it will charge at the set current. The power supply will only put out the voltage needed to maintain that current, and that is dictated by the internal resistance of the cell. For a really flat/dead battery that might start out at say 1.8V/cell. As the state of charge increases, the battery resistance increases so the voltage rises. Eventually the voltage will reach your desired set point, at which point current will decrease as charge state and resistance continues to rise. On a really good, fresh single cell of relatively low capacity I might see current drop to zero, although the rule of thumb I use for full charge is .01C or less. Many will also tell you not to stay at 2.4V/cell(or whatever) for more than a certain amount of time(say 12 hours) although that's not a hard and fast rule as long as long as you're not beating the battery up too badly or gassing too much.
Of course once you reach full charge(again using .01C as my rule of thumb) it's a good idea to "float" for 12-24 hours, which will be in the range of 2.2-2.3V/cell. Current draw should be low-if it's even .01C I'd be suspicious of the battery condition-but in theory a lead acid cell can sit at float voltage indefinitely. I've seen including a float as part of your charge regimen called a "rest period" after the full charge, though, and many references I've read recommend floating for some period of time after any charge.
Nickel battery chemistry(NiCd and NiMH) are a different beast, as are lithium chemistries, but with charging the same rules still apply, namely that at a given charge voltage the battery will draw no more current than the internal resistance will allow. I'll also mention too that I've recharged nickel based batteries from a bench PSU(it can be touchy, especially with NiMH, and the signs of full charge can be subtle), but lithium scares me
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Sorry for the diversion, but I thought it might be interesting to show the charging set-up I use for small lead acid batteries. This is a Cyclon cell, which is a small, self contained single AGM-type battery. This particular one is a D sized cell, so physically the same size as an alkaline D cell you can find anywhere. This is the smallest they make-there's also an E size cell, DT(same diameter is this, roughly twice the height), an X sized cell, and maybe a few others. These Cyclon cells are nifty little units that let you easily build portable lead acid packs of any size/shape/configuration you need. The D sized is rated at 2.5Ah, hence my setting it to 250mA. Note that this has just started charging, so the voltage is well below 2.4V.
