battery charger system

[captjohn360]

okay this may be redundant but i am planing to buy a xantrex truecharge 40+ multi stage charger for my boat to use at dockside. the main reason being i want to use my reefer and hot water heater witch are european-50hz 240v.i have an inverter allready that came with the boat and seeing i cant change my shore power to 50hz i will use my inverter 110 60hz-in to batterys-to inverter-to 240v 50hz to run installed equip. anny feedback or advice or faultswith this idea would be greatley appriceated jt

[Rick]

What you propose to do is referred to as a "supported system" in that the inverter battery is supported by a separate charger designed to carry the dc load. This is one type of an uninterruptable power supply topology and is not all that unusual.

First make sure that the charger will "lock" into a regulated float voltage while delivering the current required by the inverter without causing the battery voltage to sag. This is sometimes referred to as a "converter mode" in that the charger is delivering no real current to the battery and the battery is merely along for the ride, so to speak.

Next I recommend using a MINIMUM of 20,000 micro-Farads of low ESR (equivalent series resistance) rated capacitance wired using 10 inches of 10Ga wire directly to the inverter cable attachment points. The capacitor should be rated for 25VDC on a 12V battery. One great part is a 170,000 micro-Farad cap made by Cornell Dublier (part number 500C174U025BF2B). Happens that Spectrum Engineering has these in stock..they have a web site.

The cap reduces battey current ripple and reduces inverter switch heating during operation and I recommend all battery operated inverters use caps external to the inverter (which has some internally yet there is never enough to be too much).

Regards,
Rick

[captjohn360]

thanks for the info do you know if the xantrex will lock as you say or can you rccomend a good charger? jt

[Rick]

The best that I recall regarding that specification is that if they claim to be a 40A charger then they will deliver 40A at least at float voltage. One question that you might invesitgate is just how much current your loads require in starting their loads. The reason that I mention this is that if there is a significant start-up current over several seconds then if the charger does not specifically provide that current then the battery does. Over the course of several months you might discover that the battery has actually been loosing capacity unless you peridically (once per day, week or month) re-enter the acceptance mode in order to make up for the loss.

It is not apparent, from a system design point of view, that lead-acid batteries do not recover lost capacity when only left on a float voltage under the presence of periodic load surges. They MAY not lose capacity under the presence of float voltage ONLY if there is no periodic discharge current occuring. Such a phenomenon is not necessarily the fault of the charger, only the application and/or failure to "almost" overspecify the charger current rating.

As a side note you might be aware that the nice newer technology of switch-mode chargers providing light-weight and small volume versus the "older" line frequency swithching chargers has a slight disadvantage in not being able to deliver surge currents much beyond the rated values, whereas the older "heavy iron and copper" transformer line switch ing units DO deliver 3 to 7 times their continouous current rating for some time in surge power. One HUGE disadvantage of the "older" line-frequency switching technology is that there is usually NO compencation for lagging power factor. To you that means that if you are needing to "push" your 30A shore power breaker rating then a power factor compensating converter/charger will allow you to draw more current for ship's use than is otherwise required for charging/converting. The future is with the high-frequency lightweight switcher charger/converters as long as one is able to properly specify the ratings required in order to perform as necessary for your loads.

[GordMay]

Rick:
"... One HUGE disadvantage of the "older" line-frequency switching technology is that there is usually NO compencation for lagging power factor ..."
I

Is there any point in adding a power factor correction capacitor to either the Charger output, or the offending Motor (if only 1)?

Thanx,
Gord

[Rick]

One oversimplified view of lagging power pactor is that one could merely add capacitance to compensate for a simple "lumped" value of equivalent inductance causing the lag. In truth, this may work with some types of motors in the first approximation. Yet, even then, with salient pole motors the effects are not truly sinusiod and, therefore, a simple capacitance will not make an ideal correction.

With other types of loads, such as with line-frequency chargers using triac controlled regulation (the most common) and with microwave ovens (which remove power from the input waveform only above set thresholds thereby causing "flat-topping") an overall lagging of the input current related to input voltage occurs with multiple harmonics and the resultant current and voltage waveforms are non-sinusiod.

It takes a compensating network of inductances and capacitors and resistors to properly make a correction and the size and expense is large for these components which is why you don't see such solutions outside of the military and industry. Consumers just woun't pay the price...it would probably add 50% to the cost of an inverter/charger.

So, the reason that you will begin to see specifications indicating "harmonic line correction" or similar wording is because there is no simple specification for the number of degrees allowed between the input voltage and current sinusiods. There ARE, however, US and Eueropean standards for harmonic content allowed by a product connected to the power line.

Because the size of reactive components physically decrease as the square of frequency it is much more economically feasable to make corrections with high-frequency switch-mode power supplies. In addition, switch-mode supplies can be designed so as to extract energy from the entire conduction angles of the input voltage waveform and if done in a proper manner can cause the input current waveform to be in phase with the input voltage and be sinusiodal in nature. Electric heaters and light bulbs do this, of course, by their very nature in that they are purely resistive loads (except for the fans). Motors do this as well yet change the angle between the current and voltage waveforms.

High tech motors today (up to hundreds of HP) use dc and, therefore, a properly designed off-the-line power supply can be a high-frequency switch-mode design to drive the motor. The net result is a system having around 98% efficiency and automatic harmonic correction. They are being used in referigeration systems and diesel-electric drives.

Honda,Panda and Onan (Cummins) make some high-tech generators using these principles. We will see this type of technology dominate the market if for no other reason than we cannot export products into the Eueropean markets without meeting harmonic standards and the old technology just will not compete.

Neat, huh?
Rick

[GordMay]

Yep, neat.
By which he means to say - NO, you cannot merely add P.F. capacitors (in most cases)
Thanx, Rick - as always your very erudite reply continues our (my) education.
Gord