THIS is WAY TOO LONG, but I didn't have time to write a short note, so I wrote a long one (as Mark Twain sort-of noted, long ago).

First, I know that that a smart charger will vary voltage, the other elements of the charger and the battery banks themselves will affect the actual amperage. A "smart" charger can be forced into bulk, absorptions and finally lead to absorption voltages without necessarily having the charger put additional amperage into the batteries. As long as the smart charger is working properly, the batteries aren't too hot (or cold) so that the voltage is appropriate, and the batteries aren't "shot" through whatever. But I'm speaking about what happens when you have the "need" to charge the batteries, and when you think you have the appropriate rating for your "smart" charger to really put the amp hours back into the batteries and it doesn't happen....

Secondly, On our current J/160 which we have sailed for 16 years, I know about this first hand because of my heavy (some might say wasteful or extravagant) electrical consumption when moving along. We have relatively large battery banks. I have two engine starting batteries for each of our main and generator (100+ amp hour Optima AGM's); Our house banks are 4 Lifeline AGM's -- each with 208 ah or so. I had the generator battery being charged by a combiner with the house bank. The engine start battery was on its own with a 50 amp alternator off the engine. The main bank and generator would get their recharging from a 225 amp Balmar alternator (which never would, in real life put that amperage out). We originally had a 130 amp Xantrex (Heart) inverter charger with their most sophisticated controller. The Xantrex unit would effectively not put-out more than about 60 amps -- voltage okay, but not real juice. Offshore, if we were motoring, no big deal -- however, we were sailing and wanted to recharge the batteries after 12 hours, you would have to run the generator for 3 hours or so to get the batteries up to 90% or so capacity. I didn't notice this as an issue when cruising and we were running the generator after a 4-5 hour sail while at anchor or when we just came home after a couple of days and plugged-in to shore power. But it pissed me off that I bought a "130 amp charger" that wouldn't put-out.

After a bit of research, finally Xantrex, a couple of VERY good electrical experts, and two marine electronic types admitted that was all you got out of Xantrex (or Heart) units.

I wanted to upgrade to a larger charger that would allow me to only run the generator for a reasonable amount of time when sailing with my 20-35 amp draw average draw over a day. (I said extravagant ;^)))) ).

About 4 years ago, the local Mastervolt dealer said I could have the "best" smart charger and sine-wave inverter unit which will honestly put-out what it is listed to put-out. I ordered a 200 amp hour Combi unit. For a variety of reasons, without telling me, they installed a 100 amp Mastervolt Combi unit. I had a not very pleasant discussion with the guys that installed this, they said the 200 ah unit "wouldn't fit" -- and, no problem, "you'll get and 'honest' 100 amp output vs. [your] Xantrex's meager 60 real amp output". Off to Maine I went.

We sailed under good winds for 12 hours off of Cape May -- then I turned on the generator to quickly let the charger "fill my batteries" back up to a reasonable level. I was SHOCKED, the unit started out at 100 amps of output, but quickly (a few minutes) started dropping to settle down to about 65 amps output.

In Maine, after hours of phone calls and inspection of the installation involving Mastervolt's head office, they said there was a problem with the flow through the unit where we had installed it. (Mastervolt has a fan that "pulls" air vertically downward through the unit to discharge below -- convection would make you believe that it should pull the air up, but I think they're concerned with the disaster that water intrusion might cause.) Finally, the Mastervolt US manager admitted that his unit that he had working in his shop didn't put-out more than that. I believe they didn't realize it either, before I raised this issue. They just had never focused on it.

They said "the factory in Europe is working on a fix to provide adequate ventilation by changing the fan". Is said: "I'm on a cruise, when will you ship me the parts?" He laughed, "Oh we have to engineer it, then put in production, then test it, then...." I will omit the interchange at that point.

However, late in the evening in a state of frustration, I thought to myself: Would it just be the "air flow" of a better fan that would solve the problem? The next morning, after drawing down the batteries by 200 amp hours, I got-out our vacuum cleaner, but the hose at the bottom of the unit. (This is a portable Oreck unit that will "suck a bowling ball up into the air". I turned on the 1/2 hp (or whatever) Oreck sucked a gale of air through the unit. 100 amps of output flowed through the unit. I said, amazing, then, slowly three minutes later, the output started to drop and settled about 70 some amps. This is 65 degree Southwest Harbor, Maine -- not the 90 degree Annapolis. I called the Mastervolt senior manager and told him that his engineers wouldn't solve that problem by tweaking a fan.

After a bit, they admitted that they couldn't solve the problem -- it was endemic to their 100 amp unit. However, they said that their 200 amp unit wouldn't do that. After a whole bunch of figuring, I discovered that I can get a 200 amp Combi unit into my boat in the place that the Annapolis dealer couldn't get it in -- Mastervolt, the dealer, and I agreed to a make me "whole" so as to put me in pretty much the same position that I would have been in if I'd installed the unit originally. Two days later, in goes the 200 amp hour listed unit. Fire it up...200 amp hours of power surges forth. Then, a few minutes later it starts to drop until only 135 amps are actually being shipped out of the unit. I pull-out the vacuum cleaner, the output goes up when I suck air at gale forces through the unit to about 145 amps in the cool of Maine.

Finally the exacerbated Mastervolt manager admits that's the best "I guess you can expect in the real world"....

The reality, like Hi Fi amp ratings of output, they really aren't an accurate measure of what comes out. The heat "internally" in the charger circuit forces the output to drop. They just aren't what is advertised in the real world.

Xantrex, Mastervolt, (I'm told) Magnum, etc. all seem to do this -- "Cosi Fan Tutte" or "As Do They All"

Maybe small units are more accurately rated for their output. I like the Mastervolt, I'd buy it again -- but I'm still pissed that the ratings are overstated. There's no need for that!

ANOTHER EPIPHAMY: We have a lot of control of our Mastervolt's output. I finally realized that the measurement of voltage output needs to be tweaked to reflect the shunts at the battery banks voltage -- not what goes to the electrical panel. If you have the output measured at the combination of the current load the way that the manual says, and you have a current load of 20-25 amps on the batteries, you don't see the amperage coming out of the unit. The "Smart" charger is fooled! If the battery is supposed to put-out 14.45 volts (after temperature correction) at bulk, then you really need to set the voltage to be 14.6+ volts. That way the charger keeps the unit at full output (say 135 amps). After you use the 20-25 amps, the rest is going into the batteries. If you don't do that, the batteries take forever to get to 14.4 volts to go into "absorption". I had to adjust both the absorption and float voltages upward unit it produces the intended voltages AT THE BATTERIES (not a that panel).

The result is the charger is really performing like it is supposed-to at the batter levyel. The instruments, lights, etc. don't mind the 14.6 volts (for bulk), the 14.45 (for absorption), and the 13.8 (at float). The batteries are getting -- as Brian noted, only about 2-3 amps of power into a 840 (or so) battery bank...just a trickle at slightly higher voltage than they might otherwise. Instead of 13.6, they float at 13.8 amps.

Mastervolt doesn't realize this yet, I'll be talking with them when I get a chance.