Series 12v Chargers ?

BFL13 wrote:
"You would in theory max out at 55 amps or so. The power involved would, of course, be the current multiplied by the total voltage, ignoring conversion losses. The 100A converter would not be operating at full power output."

If they were both 75 ampers at double the voltage then at the same full power, output would be 75 amps--37.5 amps each?

So now we have 100 and 55, but ISTR in series you get the lower amps if they are not the same. But wouldn't that be 55/2 = 27.5?

...

I really am confused.

It would not be gentlemanly for me to disagree with you. ;)

I think you're confused partly because you aren't quite sure what volts and amps and watts are measuring. It's certainly understandable, in that we can't really see or sense any of them directly. Electricity isn't like, say, hydraulics or pneumatics where the physical parts are pretty easy to detect on their own.

An ampere is a measure of how quickly an electric current is flowing. (An electric current is basically just a steady stream of electrons going somewhere--down a wire or whatever. It is possible for there to be other charge carriers than electrons in some rather esoteric circumstances, but that detail is not worth worrying about.) It's possible to define an ampere in terms of some (very large) number of electrons per second, though the actual formal definition is quite different and involves measuring the physical force between two parallel wires of infinite length. Since current is a rate of flow, it's always measured through a point or through a device.

A volt is a measure of "electromotive force," kind of like a pressure for the electrons. A higher voltage pushes them "harder" so that they flow faster through a given conductor or load or whatever. If they cannot flow at all, of course, the voltage may vary but the current would stay at zero. Voltage is always measured between two different points in a circuit. Conventionally one of those points is often ground, some location taken as a baseline for other voltage measurements; in RV DC electrical systems (and automotive 12V systems), ground is conventionally tied to the chassis and is the most negative part of the circuit so voltages are positive with respect to ground.

A watt is a measure of power, or the rate of doing some useful work. For electricity, you can get more power either by having more current or by having a greater voltage, and indeed a watt is derived from voltage and current (one amp flowing, with a voltage of one volt, is one watt.)

A watt-hour is a measure of energy, or the amount of useful work that can be done.

An amp-hour is a measure of charge, basically a quantity of electrons. In RVs an amp-hour is often used as a stand in for energy, the voltage being assumed to be constant and typically 12V nominally. Thus, an amp-hour would be equivalent to 12 watt-hours under those assumptions.

An analogy that's often used is that of hydraulics. Voltage is equivalent to pressure in this analogy, current to flow rate, and charge to volume. If you have a pump that produces a flow of 100 gpm at 14 psi, and you hook it in line with another identical pump, you can get 100 gpm at 28 psi. If you hook them in parallel with each other, you get 200 gpm at 14 psi.

The actual voltage and current in a circuit depend on the supply and the load. A converter is, at least ideally, a fairly simple supply to model: its output is at a fixed voltage for any current from zero through its maximum, at which point it operates at a fixed current (the voltage dropping as needed). Real converters may be rather far from this idealized version, but it's still adequate for the sorts of things we're talking about.

So a 100A converter, at 14.4V connected in series with a 55A converter, also at 14.4V, will supply 28.8V at anywhere from 0 to 55A, and if the load is greater than that the voltage will drop from 28.8 volts until the current is 55A. (Bear in mind that, for a given load, the lower voltage will pretty much always result in lower current, though the relationship may not be linear.) For charging a battery (of sufficient size) in bulk, the system would basically be operating in a current limited manner, at 55A.

When comparing 24V systems with 12V systems, as the voltage is doubled, for a given power the current involved is half. That doesn't mean that an ampere is half as big; rather, that you need half as many to accomplish the same work since the voltage is greater.