Voltage Drop at Charge Stages - goes to MPPT v PWM

[DLTooley]

I have observed a significant decrease in voltage drop from my panels on a time frame consistent with typical absorption phase settings. 

Does this voltage drop change indicate the batteries actual switch from absorption to bulk as opposed to the charger estimated switch?

I currently have a cheapo controller. I don’t have exact numbers but typically I’ll see voltages jump from 15 is to 18 or 19, in consistent full sun. 

When my alternator went out deep I was able to use that change to drive out at 5-10 miles a pop. It seemed to work.

 

[John61CT]

I'm having a hard time deciphering your phrasing.

If you could be more specific about exactly what equipment you have, ideally with links to their specs, and

what volts and amps you are measuring

via what tools

at what wiring points and

how things change going from 50% depleted to Full,

that would help.

If panel voltage gets too close to battery voltage, there's not enough V difference for many chargers to keep charging.

Which is why "nominal 12V" panels are often rated over 20V.

 

[frater secessus]

I have observed a significant decrease in voltage drop from my panels on a time frame consistent with typical absorption phase settings. 

Does this voltage drop change indicate the batteries actual switch from absorption to bulk as opposed to the charger estimated switch?

Are you talking about panel voltage going into an MPPT charger?  The MPPT will run the panels at whatever voltage it needs to extract a given amount of power needed at that moment.  It tracks power points and uses them as needed.

In an MPPT context panel voltage is uncoupled from charging voltage and can move around quite a bit.  Here are some examples.

 

[DLTooley]

Excuse the error, I meant when switching from the absorption phase to float.

I am using a white ammeter placed between the panels and the controller.

Recall that a load causes voltage drop. I believe the 20 volt design of a panel is engineered to account for that drop and provide an approximate charging voltage during absorption.

This the highest rate of charge phase. As such the benefits of mppt are biggest when needed least. That’s my theory I’m putting out.

A note on the Bayite. At under $20 it is a great product however it looks like it is susceptible to damage if power is connected before the load (battery), just like a controller.

Fwiw I do have a Victron on order.

 

[John61CT]

In general measuring between panel and SC is not required.

For practical purposes the panel + SC can just be considered a "black box", measure its charging output volts and amps to test and make comparisons.

 

[DLTooley]

I used the same device to measure my big loads. I now use it to decide when to run the off battery uses, my daytime only 12v fridge and charging my laptop. So far, as an application of the above, it works well. No problems unless I don’t turn it off early enough. I do like to keep a small charge ‘tail’ as long as possible, but that is not quantified.

 

[frater secessus]

Excuse the error, I meant when switching from the absorption phase to float.

Generally speaking an MPPT controller will increase Vpanel, sometimes quite close to Voc in order to reduce current when transitioning from Absorption to Float.  Or any other low-load period when the sun is out.

But as John points out it can be thought of as a black box.

I am using a  white ammeter placed between the panels and the controller.

To measure voltage drop?

If you are really talking about amps dropping, yes Amps will drop throughout Absorption and into Float where it should be stable.   All else being equal.

Recall that a load causes voltage drop. I believe the 20 volt design of a panel is engineered to account for that drop and provide an approximate charging voltage during absorption.

The 20v Vmp is designed to give headroom for voltage depression due cell temperatures higher than lab conditions (almost 0F ambient).  

This the highest rate of charge phase.

/squinting

It's the highest voltage and slowest charging phase.  Highest rate of charge occurs in Bulk.

As such the benefits of mppt are biggest when needed least. That’s my theory I’m putting out.

The theory cannot bear scrutiny.

Benefits of MPPT exists ~anytime:

1. the system needs more power than the panel can deliver at Vbatt.
2. (Vpanel x .95) > Vbatt.    The .95 figure accounts for buck conversion losses.

This happens most often in Bulk because Vbatt is quite low then.  But it can also happen in other stages when substantial loads are put on the system.  Now whether or not it's worth the capital investment is another matter altogether.

 

[John61CT]

> I now use it to decide when to run the off battery uses, my daytime only 12v fridge and charging my laptop.

I interpret that to mean "I want to run those loads off 'excess' solar output after the amps accepted by the mostly-charged bank has dropped.

Did I get it right?

A fridge should be run 24*7, just running a standard unit a few hours a day would be both inefficient and very ineffective.

> No problems unless I don’t turn it off early enough.

Turn what off?

 

[DLTooley]

I observe a pronounced reduction in voltage drop mid morning, also at other times when there has been an unusually big load.

My definition of bulk is everything until full absorb voltage is reached. I do recall a different definition from a marine source cited here, but I don’t recall specifics.

 

[John61CT]

> I observe a pronounced reduction in voltage drop mid morning, also at other times when there has been an unusually big load.

Well yes, heavy loads drop voltage down, how much depends on bank size and chemistry.

LFP drops much less than lead, therefore more suitable for aircon, or say say a large inverter running appliances designed for home use on mains.

This has nothing to do with the charging cycle. To test that precisely, you need to turn off all loads.

Of course if you had a very high-current 12V source available, it would not drop as much.

Same with doubling or tripling bank size, or replacing an old worn bank with new batts.

Is the V drop actually causing a problem? What is the lower range of what you've seen?


> My definition of bulk is everything until full absorb voltage is reached.

That is the standard definition, in line with my post above


> I do recall a different definition from a marine source cited here, but I don’t recall specifics.

Maine Sail is an excellent resource for DC electrics, his new URL is http://marinehowto.com

His use of those terms is also the standard meanings.

Other terms like "boost" are used very confusingly in some manufacturer documents, often ones from more than a few years ago.

 

[DLTooley]

Charging is, in this context, a load.

Typically I will see panel to sc voltage climb to around 15 volts hold in that area than jump to 18 or so the. Slowly climb to 21 over the remainder of the day.

The condition of my battery bank might be a factor.

At this level of detail LFP isn’t relevant to lead acid tech, save when considering an upgrade path.

Let me try a restatement of my theory, based on my observations and what I l have learned on this forum.

The load effect on voltage during higher acceptance charging states will cause a voltage drop in panel voltage to near absorption levels. As such the relative benefits of MPPT over pwm or ‘cheapo’ charging is less relevant for lead acid charging as that efficiency benefit occurs when the added power can’t be applied to the bank in the float phase.

 

[DLTooley]

I’ve also observed this same effect on an AGM battery, starter, when recharged by solar post an alternator failure. That battery was also slightly degraded due previous intermittent alternator problems.

 

[John61CT]

If you are discussing voltages between the panel and the SC, that IMO is of little practical interest.

The **charging output** is what needs accurate monitoring and observation.

The overall greater power output of MPPT is not disputable, treat it as a given.

It may in some circumstances not be worth spending more money on, but that depends on so many variables really comes down to a personal decision.

Your setup your choice.

 

[DLTooley]

Let me try one last time to explain the theory I am putting out.

The drop in panel voltage is due the charge ‘load’ as such the relative advantage of mppt over pwm during the maximum charging absorption phase is reduced. The benefit of mppt is greatest when the battery is able to use it the least.

From the main pwm v mppt thread it seems pwm May also have other absorption phase benefits.

I do appreciate the previous help you’ve given me, but in the future don’t bother. Again, again, thanks.

 

[frater secessus]

I won't bother, either. Your recent posts have been both confusing/confused and resistant to correction.

 

[tripper]

Let me try one last time to explain the theory I am putting out.

The drop in panel voltage is due the charge ‘load’ as such the relative advantage of mppt over pwm during the maximum charging absorption phase is reduced.   The benefit of mppt is greatest when the battery is able to use it the least.

From the main pwm v mppt thread it seems pwm May also have other absorption phase benefits.

I do appreciate the previous help you’ve given me, but in the future don’t bother. Again, again, thanks.

The major benefit of MPPT (there are several) is MPPT puts out considerably more power when your batteries are very low, say 50% discharged.

As you can see from this graph, the conventional controller (PWM) puts out less watts the lower the battery voltage, when your battery voltage is really low is exactly the time you want the most power output, not less. The MPPT outperforms the PWM across the entire range, but really shines the lower the battery voltage.
PWM is just an older technology that is cheaper to produce, MPPT is technologically superior and the hardware and software is advancing rapidly and getting less expensive.

The newest MPPT controllers know the actual current being put in during the absorb phase and taper off until it reaches a point where the battery no longer is accepting much current.

 

[DLTooley]

My definition of absorb was wrong based on ‘eggtimer’ based documentation of the phase.

Your graph goes directly to my theory. My understanding of mppt is that the benefit is from converting excess voltage in to extra amps.

I am observing a panel voltage drop in the earlier stages of ‘absorb’ on my cheapo controller premubably due the acceptance rate ‘load’. My interpretation is that the drop in voltage reduces the benefit of mppt so would contradict that graph.

Curiously there is a pretty discrete jump in voltage at around ‘eggtimer’ time frames, more or less a couple of hours into full sun. This was the basis for my faulty interpretation of absorb. I have used that discrete voltage jump for short term battery state management including the application of non charge loads and when charging a starter battery in an alternator failure situation over a dozen shortened cycles.

 

[DLTooley]

I have a Victron mppt enroute. I’ll see how the voltage drop pattern compares.