2 or 3 stage Battery Charger, which is best?

I tried to select a title that would elicit responses from all the "converter/charger experts" here on the forum. 

Following this link will give you a better idea of what I am pondering than any explanation I might offer in my own words.

I do have my own ideas (and limited experience) on the subject, that I would be glad to share, but am most interested in your comments.

Good post, Lou (Porky), Are you a politician?  A great way to say something without getting you into the immediate cross hairs. Politicians do that, don't they?

Happy Valentine, Lou.  ;D
carson FL

I disagree that a 3 stage charger will over charge a battery under load.  Any excess current will go to the load and not the battery, so how would this cause an over charge?  We have a 3 stage charger and have never over charged the batteries under normal operation and they always have some load on them.  A more usual cause of over charging is setting the float voltage to high for the battery type, and that is true of both 2 and 3 stage chargers.

Well, the Xantrex three-stage chargers all have a fairly tight time limit for the absorption stage, so it's hard to imagine that the batteries would be overcharged to any meaningful degree.

Maybe Samlex chargers don't have such a time limit, but that isn't a reason to stay away from other three-stage chargers that do.

The other side of the coin is that the absorption stage is not merely a means of getting that last 10% of capacity.  For flooded cells, at least, the absorption charge is necessary to get the maximum life out of a deep cycle battery, because it produces enough gassing to move the electrolyte around and stir things up, and because it improves the surface characteristics of the plates.

I hear you ned.  Maybe the words "overcharge" are wrong in this case.  The theory espoused in the article is that extraneous loads will not allow the charger to drop out of the absorption stage and could therefore hold the battery at 14v+ causing excessive gassing.  (maybe not the same as over charging)  Larger loads would certainly over ride this effect.

My owners manual, for the S2012A Inverter/Charger, expresses it a little differently by stating the actual charger operational characteristics.

The charger senses the battery voltage at start up and will enter BULK charge mode if the battery is 12 volts or less.  The charger will maintain a constant current (selectable 10 to 80 amps) until the battery voltage reaches 14.5 volts.  The charger will continue at this level for an additional hour before entering the "float" stage.  The float stage is selectable at 13.2 to 13.8 volts.

I guess one could call that additional hour the missing "absorption stage".

If the battery voltage, at start up, is above 12 volts, the charger immediately enters the float stage at the selected voltage level.

The big difference between this charger and the average multistage charger is that it will continue charging at the float level, in spite of any loads, until the battery drain results in a battery draw down to 12 volts.  It will then start the bulk charge cycle over again.

I haven't had this unit installed very long, and since I don't yet have my Link-20 installed, am limited to voltage only measurements and observations, but it seems to be operating exactly as advertised.  I'm just not sure it's any better than the alternatives, over the long haul.

Lou,

lou  ---  '97 Bounder 34V (F53 w/tag), '97 Honda Accord EX
"You don't write because you want to say something, you write because you have something to say"  F. Scott Fitzgerald

Did you select your tag line to be directed at a particular member of the forum?  I have wondered about that ever since the first time I read it. Just curious.

Richard

I don't think this is answerable without talking about a specific load on a new battery and a specific 3 stage charger and (if applicable) the particular charge profile in use. There is probably some load that will just barely push any charger into its more aggressive charge phase without quite taxing the charger output enough to use the additional current thus applied.  I'm sure this crossover point could be found in a lab for some specific charger, and I'm equally sure it would have little practical effect on 99% of RVers.

As a practical matter, most of us with 3 stage chargers rarely ever get it into bulk mode anyway, since this requires a fairly deep discharge to reach. Most all the time we are operating in absorption or float, which means it is mosty functioning as a 2 stage anyway.

3 stage is something of a misnomer.  What happens is that the charger output is limited to the lower of voltage or current.  The absorption charge "stage" starts when the current starts to be reduced because the voltage limit has been reached.  As a result, the full output current of the charger is still available.  Technically, a large load could keep the charger in the "bulk" stage above float and below the voltage limit indefinitely but that would require an extremely large load so that only a handful of amps are left for charging.  That's not a real world scenario.

Anyway, the transition between so-called "stages" is a gradual one and the full output current of the charger is still available in the absorption "stage."  Xantrex and I think at least some of the other manufacturers terminate this "stage" either when the current drops below a particular threshold or when a timer expires, usually for several hours.

The "float" stage is accomplished by the charger circuitry simply reducing the voltage limit setpoint by a few tenths of a volt.

I suspect that the reason Samlex doesn't do it this way is that it requires a memory of charger STATE which generally means a microprocessor or some logic.  A "stateless" 3 stage charger that responds only to the present conditions can be built that sets the voltage limit to 14.4 if the charge current is above, say, 10 amps and to 12.8 if the charge current is below that.  No need for timers or recollection of the past.

In contrast products from Xantrex and the like have to keep track of how long it's been since the voltage limit was first reached, something best accomplished with a microprocessor.  With today's inexpensive devices that cost all of $1 or $2 each in quantity there's no reason not to do it that way other than the up front engineering costs involved.

As an aside some converters that are out there will flip back into absorption after 24 hours and exit it again once the low current limit or timer expiration is reached.  The idea is that self-discharge may cause the battery to lose the last few % of its charge even if kept on float.  I don't know whether this is a good idea or not.  Xantrex doesn't do it and none of the industrial or telco systems I've seen do it either.

A 3 stage will not over charge.

RLSharp said:

Lou,

lou  ---  '97 Bounder 34V (F53 w/tag), '97 Honda Accord EX
"You don't write because you want to say something, you write because you have something to say"  F. Scott Fitzgerald

Did you select your tag line to be directed at a particular member of the forum?  I have wondered about that ever since the first time I read it. Just curious.

Richard

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Richard, you know darn well I did, and you know who as well ;D

RV Roamer said:

Most all the time we are operating in absorption or float, which means it is mosty functioning as a 2 stage anyway.

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I understand that.  That's why the Samlex approach (no absorption level) applied unless, and until, the battery needs a deeper charge may have some merit.. 

It seems to me that a condition could easily exist that would require a higher level charger output than the float level, but never require a more significant current draw.  This situation would/could cause the batteries to continually gas. JMHO

Jammer said:

I suspect that the reason Samlex doesn't do it this way is that it requires a memory of charger STATE which generally means a microprocessor or some logic.  A "stateless" 3 stage charger that responds only to the present conditions can be built that sets the voltage limit to 14.4 if the charge current is above, say, 10 amps and to 12.8 if the charge current is below that.  No need for timers or recollection of the past.

In contrast products from Xantrex and the like have to keep track of how long it's been since the voltage limit was first reached, something best accomplished with a microprocessor.  With today's inexpensive devices that cost all of $1 or $2 each in quantity there's no reason not to do it that way other than the up front engineering costs involved.

Click to expand...

Jammer, I assure you, the Samlex is no $1 or $2 charger, and has as much applied  microprocessor power as any Xantrex.

tvman44 said:

A 3 stage will not over charge.

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Sorry, but conditions can and do exist that can cause any three stage charger to cause battery gassing.  Maybe you don't call that over charging.  I do.

Holding a battery at the normal absorption voltage level (somewhere above 13.8v) with a low current draw will cause gassing.