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Conversion project: converter to smart inverter/charger

A few weeks ago I was talking to a marine electronics installer about aspects of EMI--electromagnetic interference—that some electrical equipment can spin off, generating "noise” that can impact communications and other electronic signals. The conversation turned to battery chargers and inverters and almost immediately I was over my head technically. 

So I Googled the topic and sure enough landed on a site I should have gone to first thing--the Tech Doctor, which is part of the Xantrex website. Don Wilson, who writes the blog, has worked in technical capacities in the automotive, RV, truck and marine fields and for the military since 1989. He has extensive experience in designing and troubleshooting onboard electrical systems. A former customer service manager dealing with electronic issues, Wilson now is a sales application specialist for power systems product manufacturer Xantrex. Don has also written a couple articles for Marine Electronics Journal.


By Don Wilson, Tech Doctor
A few of the Tech Doctor articles generally cover adding an inverter. However, I am often asked about installing an inverter/charger to replace an existing converter, or battery charger. I enjoy these conversations because it tells me that folks are smart in their thinking. 

We know that a quality three-stage charger can charge batteries more effectively than a static converter, making the batteries last longer, naturally. Some brands of converters have historically suffered in reliability and quality. In fact, when you find that the converter is faulty and being replaced, this is an ideal time to upgrade to an inverter/charger.

Some modern inverter/chargers have "power factor correction” (PFC), which can use your 120 volt current more efficiently than older chargers or converters. Plus, a combination inverter/charger gives you "power share,” which you can’t get when you have a separate charger and inverter. Lastly, an inverter/charger gives the advantage of having AC power available when the generator is not available/advisable, or there is no shore power . . . and the converter is built right in! 

What are the benefits? 
As I mentioned before, the three-stage charger ensures the batteries are fully charged before allowing the charger to "float” them at a full charge. This uses higher voltages to bring the batteries to "full.” After that, it lowers the voltage--low enough to prevent over-charge of the battery, but high enough to be the source of any needed DC current. This will protect your batteries from the most common causes for premature failure, or sulfating.

PFC is a way for electronics to use the power factor of alternating current more effectively, decreasing the input amperage to provide more output amperage. In short, your PFC charger will require less energy than a non-PFC charger to charge at the same rate. 

"Power share” is the ability for the charger to de-rate itself based on the measured load downstream from a combination inverter/charger (combi), which cannot happen with a simple inverter and transfer switch in conjunction with the standard converter. For instance, if your combi is set for 15A power share, and you turn on your microwave that uses 10A of power, the charger "knows” that it only has 5A available, so it limits the charger’s amperage output to compensate. The major benefit is that the shore or generator breaker won’t trip at odd times when using heavier loads. Because chargers that are separate from the inverter cannot know what the system amperage draw is, they cannot power share. 

How do I wire the new combination inverter/charger? 

The existing converter or charger is fed by a breaker. Let’s say that breaker is a 15A breaker and the load you want to be powered by the inverter, like the entertainment center, is also fed by a 15A breaker (sometimes this is a 20A, but the concept is still the same).

Step 1: Replace the converter/charger breaker with a 30A 

Step 2: Feed 10AWG wire to the input of the combi inverter charger 

Step 3: Take the entertainment center 120V wires (all three, hot neutral AND ground) out of the breaker panel
Step 4: Feed that through a small sub-panel breaker that is fed by the inverter, an inverter-mounted breaker, or put a male plug on it and plug it into the receptacle mounted on many inverter/charger models. 

Finally, make sure the DC cables/fuses are sized appropriately for the combi inverter/charger (even though the new charger may be the same amperage as the old, consider the amperage demand of the inverter which could be more than the charger). As far as the old converter/charger, either remove it altogether, or tuck the wires away in the load center, or converter compartment (for those models that have the converter built into the load center), and leave it there. That’s it. Clean, simple installation. Just sit back and reap the benefits of inverted loads and a quality three-stage charger. 

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