Inverter vs Converter

[Sean]

... Can't figure out why a charger needs to be 75 amps unless it has a starting boost feature?

For starting batteries, you are probably right.  But for house batteries, 75 amps is incredibly small.

My charger (built in to my inverter) is 150 amps (at 24 volts) and I wish it was 400 amps.  At 150 amps, that charger takes about an hour per day of generator run time to sustain us while we are boondocking for long periods.  A 400-amp charger would cut that at least in half.  At over $4 per hour to run the generator, a larger charger would be a real boon.

A 75 amp charger is sufficient for a flooded bank of 375 amp-hours, or an AGM bank of just 150 amp-hours.  Above that and you can benefit from more charging capacity.

-Sean
http://OurOdyssey.BlogSpot.com

[gus]

Sean,

Thanks, never knew such high amp chargers existed!

My house battery banks are so small I never think in those figures.

[Lin]

It is important to remember, particularly with AGM batteries, that they do have a max charge rate.  Therefore, the charger would have to be set to that rate times the number of batteries.

[bobofthenorth]

For external 3-stage regulation I just installed a Balmar 614.  Ridiculously expensive IMHO but it was either that or an Ample Power and the Ample Power was more expensive.  FWIW, the Balmar clearly does what it is intended to - my alternators are pumping out power like they never did before and they do it at much lower RPMs than they did with the previous (Xantrex) system.  They also ramp up slowly - there's close to a minute lag after the engines start before the regulator starts powering up the alternators.  The other difference I notice from the Xantrex is that I never completely lose the field with the Balmar regulator.  I used to lose my tachs when the Xantrex shut the field off completely but that never happens with the Balmar.  Count me as a happy customer, even if they are overpriced.

[Sean]

It is important to remember, particularly with AGM batteries, that they do have a max charge rate.  Therefore, the charger would have to be set to that rate times the number of batteries.

Not sure why you say "particularly with AGM" since AGM will tolerate a much higher charge rate than flooded batteries.

Always go with the battery manufacturer's recommendation, but the rule of thumb is that flooded batteries can be charged at C₂₀/5 and AGM batteries at C₂₀/2.  So a 200 amp-hour flooded battery should be charged at no more than 40 amps, but a 200 amp-hour AGM can be charged at 100 amps.

Therefore, the charger would have to be set to that rate times the number of batteries.

To clarify, that's the number of batteries in parallel.  Batteries in series do not increase the charge rate.

IOTW, if you have two 200 amp-hour batteries in parallel in a 12-volt system, you could charge them at 80 amps for flooded or 200 amps for AGM.  However, if you have the same two 200 amp-hour batteries in series in a 24-volt system, that's still just 200 amp-hours and you could charge them at most at 40 amps for flooded or 100 amps for AGM.

-Sean
http://OurOdyssey.BlogSpot.com

[Lin]

Although I have been told that AGM's can take a higher charge rate. I have also been told that they do not deal with overcharging well.  I could be wrong on both accounts.  Anyway, my AGM's are rated at 134ah @ 20 hours.  The manufacturer gives C/5 as the max charge rate which puts it at 27 amps each.  Three in parallel means I have an 81 amp max.

To be clear, my batteries were designed for a UPS and, therefore, have some differences from those specifically designed for RV use.

[bevans6]

One comment on a difference between "converters" and "chargers".  Converters designed for RV use often are designed for a constant voltage output, usually around 13.6 volts, since their primary use is considered to be not killing the batteries while providing a constant voltage to the many 12 volt appliances and gadgets they need to supply stable power to while connected to a pedestal for months on end.  In other words, battery charging is the secondary application.  Some, like the Iota, have a "Charge Wizard" option or add-on to allow them to do what we consider real multi-stage charging.   Just something to consider when reading the sheets on these things.  There is a difference between a converter and a charger, and this is just a part of it.

Brian

[Sean]

Although I have been told that AGM's can take a higher charge rate. I have also been told that they do not deal with overcharging well.

Those are two separate issues.  You can overcharge a battery just as easily with C/5 or even C/10 as you can with C/2 if you don't lower the voltage after the bulk phase completes.  This is not so much a function of the charger's rate or maximum delivered current as it is of the charger's ability to detect the drop in current that signifies completion of bulk charging and then lower the voltage (usually after an adjustable absorption period) to the float setting.

...  my batteries were designed for a UPS and, therefore, have some differences from those specifically designed for RV use.

UPS/telecom batteries are a very different animal from deep cycle house batteries, designed for a very different kind of service.  It is not surprising to see a UPS AGM battery specified at a max charge rate of C/5.  I detailed some of the differences among battery types in this post from several years ago:
http://www.busconversions.com/bbs/index.php?topic=3291.msg29687#msg29687

Note that the excellent battery information web site I reference in that post has since moved and can now be found here:
http://batteryfaq.org/

FWIW, a good charger will let you set the maximum current delivered during the bulk phase.  With these chargers it is always preferable to have one which is capable of too much current rather than one capable of too little.

... Converters designed for RV use often are designed for a constant voltage output, usually around 13.6 volts, since their primary use is considered to be not killing the batteries while providing a constant voltage to the many 12 volt appliances and gadgets they need to supply stable power to while connected to a pedestal for months on end. ...

Just to be clear about this, the voltage a converter provides to the appliances is different from the voltage it provides to the batteries, as I explained in the first post I linked above.  Here is the link again:
http://www.busconversions.com/bbs/index.php?topic=16283.msg175122#msg175122

In this post I went over some reasons why one might choose a converter over a charger (whether part of an inverter or standalone):
http://www.busconversions.com/bbs/index.php?topic=8911.msg88297#msg88297

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

Sean, for what it's worth the several converters I own, including the Iota that I referenced, have only one output that is dedicated to both the battery charging and the load supply function, and they function exactly as I described.  Constant relatively low voltage.   Mind you, they are just now out of production and have been replaced by three stage versions. 

Brian

[Sean]

Sean, for what it's worth the several converters I own, including the Iota that I referenced, have only one output that is dedicated to both the battery charging and the load supply function, ...

Well, if it has only a single DC output, then I would call it either a charger or a power supply, not a converter, no matter what the manufacturer calls it.  True RV "converters" have separate connections for the DC loads and the batteries.  Otherwise it's just a crappy charger with a different name.

As I have written many times, I am not a fan of converters in any case.  To hear that someone is selling a single-connection power supply and calling it a converter only makes me more emphatic in recommending against them.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

OK, so Brian's last post prompted me to survey the RV "converter" market, something I have not done for many years (since I don't ever recommend them anyway).  And it turns out that there are now a wide range of inexpensive battery chargers being marketed as "RV converters," so I will need to modify my earlier treatise on them.  Mostly to add one more step after my current "least expensive" to include battery chargers will no additional hardware.  From my standpoint these would be the least preferable of all the alternatives, being essentially the worst of both worlds.

To relate this back to the OP, I will reiterate that with few exceptions, the chargers built into even the least expensive inverter/chargers are more sophisticated than just a single stage and will lower the voltage to float once current drops.  If you have no need of an inverter, there are certainly stand-alone battery chargers that also do this, and most quality chargers will be marketed as just that: battery chargers.

If you are concerned that bulk voltage will be too high for some of your DC loads, OR if you are concerned that one or more DC loads are heavy enough to trick the charger into continuing bulk charge past the point where it should stop, then I would recommend either the style of converter that isolates the loads from the charger, or adding a power supply and a make-before-break relay to isolate them yourself.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

And that was basically the point of my post - to let people know that there are converters on the market, or were a couple of years ago, that were basically current limiting, constant voltage "power supplies" (I like that term better too) that really didn't do much more than float charge the battery.  It's beyond me how they can be a 45 amp supply/charger and be voltage limited to 13.6 or 13.8 volts, since I have no idea now current controlled supplies work, they came along after I got out of electron pushing and into electron selling so to speak.  But Google suggest that there are commonly available constant voltage, current controlled supplies for charging some types of batteries that have a constant voltage and do indeed limit the current after the appropriate charging has taken place, I just don't know how they do that...

While I have the converters, I don't actually use them for anything except float maintenance on one battery.  I also have the three stage chargers that I use to actually do charging when that is required.  The "converters" are handy in a little truck camper because they also act as the AC breaker/distribution panel and the DC fuse box.  I agree they don't suit a large RV very well at all.

Brian

[gus]

Mine isn't limited to 13.+ volts. The first stage is something like 14.1-4 as I remember. It is too cold to go outside and look. It is three stage and works  very well.

It is a very old one that I updated with a kit last year. The old one was a battery boiler!!

[thomasinnv]

Gus, sounds like you might have the iota 9100 series with the added charge wizard module? the 9200 series is the same thing as the 9100 series but with the charge wizard built in. They look exactly the same. the voltage points are 14.4v bulk, 13.6v absorption, and 13.2v float. the iota with charge wizard also has an extended storage feature which enters a desulfate mode after 21 days of float, and cycles every 21 days thereafter until it senses a load, at which point it starts all over.

[gus]

Yeah, mine had the built in charger so I replaced it with a PD4645 45 Amp Converter Upgrade Section which I think is an Iota but not sure. It is too cold to go out and check!!

Those charge rates sound just right too.