MPPT: Why/How alter voltage settings?

[dave777]

Other than selecting LeadAcid, Gel, or AGM battery types, what advantage is there to manually specifying charging voltages?

Is it merely to compensate for any voltage drop between the MPPT controller and the battery?

[smac999]

it's for programing for batteries. most battery models give you charging voltages. not all AGM's are the same.

[dave777]

Originally Posted by smac999

it's for programing for batteries. most battery models give you charging voltages. not all AGM's are the same.

Didn't know that.

Are all Lead Acid batteries the same as far as charging voltages?

[denverd0n]

Check different manufacturers and they will have slightly different numbers for recommended charging voltages. That's because there is no one, single, charging regimen--right down to the tenth of a volt--that everyone in the world agrees is the only right way to do it. Opinions vary, and all the evidence indicates that there are a number of different "right" ways to do it. That is, different ways that are all equally effective and just as good for the batteries.

So, many charge controllers will allow you to set the charging regimen that you want. They will allow some variability.

[Maine Sail]

Originally Posted by dave777

Other than selecting LeadAcid, Gel, or AGM battery types, what advantage is there to manually specifying charging voltages?

Is it merely to compensate for any voltage drop between the MPPT controller and the battery?

Every battery maker has slightly different charging voltage recommendations. I was on a boat last week with a very expensive inverter charger that physically can't be set up to charge the customers Trojan batteries correctly. He's bemoaning that his batteries are basically toast in 4.5 years. The reality is they are only toast because he chose a charger that is a poor fit for his batteries without considering what Trojan actually recommends.. Simply selecting a dip switch is more often than not incorrect despite the label they gave the charge profile.

Unless an MPPT controller can be ordered with the correct voltages for your batteries, like the small Genasun controllers can, I would strongly urge one that can be custom programmed. Morningstar, Victron, Midnite, Outback, BlueSky etc. are all examples of products that allow custom programming though some require additional parts in order to do that. Also if the controller is too large to mount close to the batteries it should have a remote battery temp sensor because the charge voltage guidance for most batteries is only at 77-80F. As temps drop or increase voltage needs to be adjusted.

I had this exact question last week for AGM batteries and the controller set point choice was fixed at 14.4V & 13.8V... There is not a single AGM on this list where that profile works as the battery maker wants it to.

For example:

Lifeline AGM's = 14.4V & 13.4V
Odyssey TPPL AGM's = 14.7V & 13.6V
Firefly AGM =14.4V & 13.2V
Mastervolt AGM = 14.4V & 13.2V
Full River AGM = 14.7V & 13.7V
Rolls AGM = 14.7V & 13.7V
East Penn/Deka = 14.6V & 13.6V
US Battery AGM = 14.4V & 13.4V
Trojan AGM = 14.4V & 13.5V

How about Trojan Flooded batteries?

Trojan Flooded = 14.8V & 13.2V

Battery makers provide charge voltage guidance for the longest life.Any charge product that can't be programmed or custom ordered to specifically to match your batteries should ideally be considered second tier on your choice list.

[btrayfors]

I take slight exception to that rule, RC. In my experience, recommended charging/float voltages are very conservative and not necessarily to be followed exactly.

For example, I have floated my T-105 Trojans, and now my CR235 Crowns, at 13.8VDC. That works very well. I also charge them at 14.8 - 15.0VDC in the absorption phase, and from the float level kick them up to the absorption level periodically -- in my case, every other day for a half-hour.

I have found this to be beneficial to the batteries.

In my discussions with Concorde engineers, I was told that higher float voltages for their AGMs are a good thing.

What I can tell you is that the higher float voltages work well, and do not result in extra bubbling or water loss. And, hopefully, they result in less sulfating because batteries will continue to sulfate if kept at the lower float voltages.

Bill

[Maine Sail]

Originally Posted by btrayfors

I take slight exception to that rule, RC. In my experience, recommended charging/float voltages are very conservative and not necessarily to be followed exactly.

For example, I have floated my T-105 Trojans, and now my CR235 Crowns, at 13.8VDC. That works very well. I also charge them at 14.8 - 15.0VDC in the absorption phase, and from the float level kick them up to the absorption level periodically -- in my case, every other day for a half-hour.

I have found this to be beneficial to the batteries.

In my discussions with Concorde engineers, I was told that higher float voltages for their AGMs are a good thing.

What I can tell you is that the higher float voltages work well, and do not result in extra bubbling or water loss. And, hopefully, they result in less sulfating because batteries will continue to sulfate if kept at the lower float voltages.

Bill

You and I are not really in disagreement. The problem however is quite complex. Three days ago I had lunch with a well know marine author, a battery engineer/inventor, Bruce Schwab & a marine electronics blogger. The subject of problems with charging & the failings of the charger manufacturers to work in concert with the battery makers, certainly came up.

One issue is that almost none of the charge sources out there will provide a long enough absorption cycle to bring the batteries to 100% SOC before dropping to float. We are trying to finish charging in the marine industry by using float and this is simply a poor practice.

Even programmable models often lack the ability to extend or customize the absorption time or when or how to transition to float. Good ones do allow this, medium ones allow custom voltages only and $hitty ones have dip switches & egg timers offering a; you get what you get product..

If the batteries are charged correctly to begin with, then a higher float, than what the manufacturer recommends, is not entirely necessary and can actually be detrimental to some batteries. The engineer I had lunch with on Tuesday is adamant that 13.2V be the absolute max float and he would ideally prefer no float at all. He does however recognize that in off-grid, marine & RV applications parasitic loads result in the need to float.

As it sits I just got off the phone (& text) with a consult customer who has destroyed a set of Trojan batteries way sooner than he should have. Last week he had a thermal runaway event on one battery and another battery, Batt #1, with an SG imbalance he thought he could save by EQ, just went that way too about two hours ago.

And this was after it had been isolated and cooled down a bit. Murdered far too soon due to poor charging....



When I finally dug into his system with him I found out that he has 500W of solar with an absorption of 14.1V with a maximum absorption time of two hours regardless of SOC....... Yep, murder for Trojan batteries and his particular MPPT controllers are not programmable.

We certainly have some leeway with flooded batteries & foat but less so with sealed GEL or AGM batteries and this varies by battery manufacturer.

But it all comes down to the question of; why bother paying good money for a dip-switch/egg timer product when you can buy better products that allow customization so your batteries can actually charge they way they should??

[btrayfors]

RC,

Thanks for that.

I certainly agree about the problem of absorption times, and the chronic failure to reach 100% charge before dropping into float.

I do not agree with the engineer's opinion of 13.2VDC as the absolute maximum float voltage or, preferably, no float at all. After all, seven of the ten battery manufacturers in your listing recommend higher float voltages. In my own research -- actual, not theoretical -- the higher float voltage can be beneficial as previously mentioned.

Furthermore, if batteries are left sitting with no float voltage applied, they will soon drop to 12.6 (flooded) or 12.7/12.8 (gelled, AGM) and at that voltage will be subject to more rapid sulfation.....the killer of capacity in all types of lead-acid batteries.

And, as you noted, this situation rarely obtains anyway in a real world situation on boats, where parasitic and other loads on the batteries are usually 24/7, virtually mandating some charging amperage to keep them at float level. The self-discharge phenomenon, particularly in flooded batteries, makes this even more true.

Periodic/frequent reversion to absorption voltage levels can be beneficial in at least two ways:

1. it helps to retard sulfation by kicking loose PbSO4 crystals off the plates; and

2. it helps to retard stratification of the electrolyte through substantial bubbling.

Sounds like your clients problems with T-105s are more related to the physical construction of those particular batteries than a problem with charging/equalization voltages, n'est pas?

Bill

[dave777]

Originally Posted by Maine Sail

When I finally dug into his system with him I found out that he has 500W of solar with an absorption of 14.1V with a maximum absorption time of two hours regardless of SOC....... Yep, murder for Trojan batteries and his particular MPPT controllers are not programmable.

We certainly have some leeway with flooded batteries & foat but less so with sealed GEL or AGM batteries and this varies by battery manufacturer.

Testing & seeing is believing. But is this a step we should always take?
Should we be measuring the observed voltage during float and compare to manufacturer's recommendations? I could see how we'd observe an overvoltage condition in some circumstances, but when the battery is accepting charge the observed voltage would be much lower than the programmed voltage.

Should we program to try to adjust for voltage sag caused by the wiring run between the controller and banks?

Should we observe SOC with no charging sources, and raise or lower one of the stages as appropriate?