parallax power supply 7345ru parts PWM

Salvo wrote:
You're measuring somewhere outside the converter. Unless you have a voltage sense line, ...

Sal,

I watch what my 7345 converter is doing - voltage-wise on it's output terminals - when charging my AGM batteries via a four-place digital meter plugged into a regular 12V receptacle located quite a ways from the 7345 elsewhere in the RV. However, since this volt meter only presents a very high impedance ("infinite") load, relative to the battery bank's load, to the 7345 when making this measurement via a receptacle ... what's on the voltmeter just about perfectly represents the actual voltage on the 7345's terminals.

I have no idea where the 7345's negative feedback loop sense point is relative to it's physical terminals. I assume that the sense point on the printed circuit board is very close, physically and electrically, to the 7345's metal output lugs ... as it should be and better be.

What I see on this voltmeter is that it is NEVER at or near 13.8 volts when it begins charging the 200 AH battery bank. I interpret this as meaning that the 7345's output, as measured at it's terminals, is for some reason sagging from it's advertised "nominal output value" of 13.8 volts. However as the battery bank charges up, it's terminal output (as indicated via the infinite input impedance voltmeter plugged into a 12V receptacle) eventually rises up close to 13.8 volts.

Here's another proof of sagging 7345 output: I've installed all LED lighting in our RV ... and for each LED we turn ON, the voltmeter I use (that I mentioned above) to show 7345 output lowers 0.01 (one one-hundreth) of a volt. i.e. One light ON drops voltage 0.01 volt, two lights ON drop voltage 0.02 volts, etc..

Here then, IMHO are some questions/issues:

1) I don't think that Pulse Width Modulation power supply design technology automatically requires that negative feedback around it's output stage(s) be employed for voltage requlation. I expect that PWM power supplies can be designed, and do in fact exist, with only open ended output stages. Output on these of course would then sag with increasing output currents.

2) Do the Parallax 73XX line of converters perhaps employ only very sloppy negative feedback to regulate it's output voltage such that a couple of tenths or more volts of sag can, and does, occur under load?

3) Two-three years ago as an experiment I think I recall that you did in fact move a 7345's sense point out to the battery terminals to force full 7345 voltage (plus a little more, as I remember) at the battery independent of any voltage drop in the cables between the 7345 and the battery. One of my responses to this was that this would be "dangerous" because too-thin wiring between the converter and battery could then heat up from too-heavy current flow because voltage drop in the cabling would then NOT reduce the current acceptance rate of the battery.