How Does a Converter Hold its Voltage?

[BFL13]

A solar controller uses PWM but how does a typical RV converter do it? How much play is there up and down to hold it there?

I have a 12v mystery draw I am chasing down, but in the middle of this, I see the Trimetric often has negative amps showing (up to 2 amps it varies a lot over time), while the battery voltage is held ok at the setting (13.6 or whatever else)

If I raise the converter's adjustable voltage a tiny bit, the Tri shows charging again, but soon falls back to negative amps. Weird?

Is this an AGM thing? I always had a small positive amps with the Flooded batts and the Trimetric lamp was always showing a charge. AGMs have little self-discharge for the converter to overcome.

I am wondering if the converter is "cycling" a little so there can be times when no amps are going to the battery and you see some negative amps. (I do have a 12v draw of about 2.7 amps with the converter unplugged--But I think the converter should be covering that even if the batts don't want any amps. Another mystery.)

I have split the battery bank to see if it is a "bad" battery issue, but no difference.

Anyway, I am wondering about the converter's operating procedure just to know more about that, and AFAIK the mystery draw is a separate issue. Thanks

[MEXICOWANDERER]

Missing millivolts can be a frustration. The only temporary connection I really trust is something with a latch, gold plated hardware treated with CAIG protector. Good enough for the F18A Super Hornet maintenance -- good enough for me 🙂

[BFL13]

MEXICOWANDERER wrote:
This is why the MV wires on my shunts are soldered directly to the block. Wires are twisted and shielded and meters remain dead accurate for years.

I did it to myself, but learned something anyway. I had previously split the battery bank and had one solar set to each part, but with one Trimetric display, so I arranged two shunts and wire sets but for the one display. (Taking turns)

I rigged up a trailer 4-pin from each shunt to one on the display wires, so the display would show from whichever 4-pin it was plugged into. Later, I went to one big battery bank, paralleled the two solars on the one bank, and only used the one shunt with its 4-pin.

My crime this time, was that I did not squeeze the two 4-pins together tight enough. (Hard to tell--when the display lights up, you think you are good to go. Nope! ) So the lesson for me is squeeze the 4-pins really tight! (Being my own worst enemy as usual)

[MEXICOWANDERER]

This is why the MV wires on my shunts are soldered directly to the block. Wires are twisted and shielded and meters remain dead accurate for years.

[wa8yxm]

I would be intrested in knowing the DURATION of negative amps on ther Tri-Metric.. IE> one minute. One hour. one second?

My Theory is this: HOw does the converter know when to go into BOOST (BULK) mode and the answer is by reading the battery voltage. To do this it has to back off or else all it is reading is the converter output voltage.

[BFL13]

I think I found it! The (at least one) Trimetric wires for the shunt were a bit loose in the connection I have between shunt and display, so the amps were not reading right, but the red positive wire was reading voltage correctly. Sorry about that! 😞

[theoldwizard1]

duplicate

[theoldwizard1]

BFL13 wrote:
A solar controller uses PWM but how does a typical RV converter do it? How much play is there up and down to hold it there?

Voltage regulation and ripple very from manufacturer to manufacturer. In general, "you get what you pay for".

In general, there are two types, linear and switching (PWM is basically switching). Most every DC power supply built in the past 20-30 years uses switching technology because it is more efficient and at least in the last 10 years or so, probably cheaper.

Switching does not guarantee better regulation and less ripple, but it is much more likely.

[Sam_Spade]

BFL13 wrote:

Perhaps my question is: Once the two voltages are equal except for wiring loss, and there is no amps to the battery, just to the load, how far down would the battery voltage get before it started to get amps from the converter again?

During that time, would you see negative amps on the Trimetric, even though the battery voltage is still as set (to one decimal place) ?

Your senerio should never happen, because the battery acts like just another resistive load to the charger/converter.....UNTIL the total load gets so high that the converter alone can't supply enough current and the battery helps out and is no longer charging.

The charger itself should NEVER actually experience "negative amps" because that would mean current going INTO the charger instead of out of it.

Are you sure that the control panel is not trying to show that a little power is coming OUT of the battery ? That might happen if it is designed to cut the charging completely when a certain charge voltage is reached.....instead of shifting down to "float".

[BFL13]

I don't know if it the converter or Trimetric or just what the heck, but I don't understand how the Tri can be showing 13.6v (battery)steady, while the amps are showing minus 1.5 (it varies a little from time to time) and since this morning the AH counter says I am down 5.4AH

How can AH be going down and voltage remain steady? --I do know you can lose capacity so that "full" is the same good voltage, but your 100% is just 100% of less than before. I cannot see how that can apply here.

Before the Gods kill you, first they drive you mad! 😞

[pianotuna]

BFL13,

As the agm bank grows older (and weaker) I'm rarely seeing them get to 1/2 of 1% of capacity.

It is 0 C here (32 f) and they are floating at 13.9 volts. Last night I saw 15 volts. Solar is doing nada and Magnum is at zero amps. Shore power is 30 amps and I'm drawing 2500 watts to keep warm, on average. RV has been plugged in for 22 hours. I did end up using load support during supper time. That kicked the Magnum to Absorb @ 23 amps.

[pianotuna]

Hi DrewE,

Make that 15 minutes of 14.4 every 22 hours. It is equalization only in the sense that it "stirs" the electrolyte.

DrewE wrote:
My PD will cycle between three different setpoint voltages, including several minutes of higher voltage (14.4V) every several hours when it's in float mode (13.2V), which they stretch the truth to call equalization.

[MEXICOWANDERER]

From too much experience, I can safety state that no large cheap power supply I have seen will maintain rock solid voltage regulation from no load to full load.

The best of my bunch is a Lamda which within range varies an astounding .02 within a wide input and output range. It's cost induces sticker-shock.

Meanwell and Megawatt all droop .2 to .3 volts open/circuit vs a 2 amp load. My Chinese bench 0-60vdc 10 amp power supply droops to the hundredths of of volt.

[BFL13]

Thanks. So my converter voltage is steady as a rock based on observation when it is not connected to a battery. The battery has a draw on it.

So connect, the battery's loaded voltage rises to meet the converter's voltage so now the converter is carrying the load. Except for the wiring loss (not much at low amps, but some)--I could have that wrong as to what really happens there...

Perhaps my question is: Once the two voltages are equal except for wiring loss, and there is no amps to the battery, just to the load, how far down would the battery voltage get before it started to get amps from the converter again?

During that time, would you see negative amps on the Trimetric, even though the battery voltage is still as set (to one decimal place) ?

[DrewE]

I think most converters these days are basically voltage regulated switching power supplies. The regulation may not be absolutely perfect with changing loading, but it should be pretty close until you start to run into current limiting or power limiting and the voltage sags. Line to load regulation should be very good, a feature inherent with switching power supplies in general. The regulation would be of the voltage at the converter output, not at the battery, unless you have some setup with remote voltage sense lines and I haven't run across a converter that supports that.

This is, of course, separate from any higher-level charge algorithm that's changing the voltage setpoint. My PD will cycle between three different setpoint voltages, including several minutes of higher voltage (14.4V) every several hours when it's in float mode (13.2V), which they stretch the truth to call equalization. Other makers have different algorithms, but will cycle between stages according to some program.

If you have some wiring impedance, it's possible that the battery floating at the converter's float voltage could discharge a bit and later charge a bit as the overall 12V load changes, due to the relative impedance between the load and the two voltage supplies. That's not something to worry about in itself IMHO (well, aside from wanting to have good low-impedance connections).