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Ran D. St. Clair · 08-21-2009, 07:01 PM

RTFM – Read the F--ing Manual

The second of my assumptions to fall was the idea that I would use a more or less standard RV style inverter – charger – transfer switch.

I had been considering the Tripp-Lite RV750LHW, (Inverter - 750W, 1125W<1hr, 1500W<10sec, Multistage Charger - 45A, and Transfer Switch) or something like it. There are similar models made by Xantrex, Magnum Energy, and others. As far as I can tell, they all have the same killer problem for my application. When they see a valid AC input they want to turn on their transfer switch and pass that AC to their output, or in other words, to all the AC appliances.

The problem is, my wimpy little EU2000i is no where near strong enough to handle the air conditioner, battery charger, and all the AC appliances that could potentially be turned on all at once. If I was connected to shore power then turning on the transfer switch would be a fine plan, but they gave me no way to disable the transfer switch. It’s a shame, because technically speaking it would probably be an easy thing for them to do, but they didn’t.

On the other hand, my mighty little EU2000i is plenty strong enough to provide all of my daily energy needs, even in a worst case scenario. It just can’t meet the potential peak demand. I could get a 2nd EU2000i and put it in parallel for twice the current, but I don’t want to have 2 generators running if I can avoid it.

The solution is simple enough. I just need to buy a separate inverter and battery charger. A transfer switch might have been nice in some situations, like when on shore power, but I don’t really need one. The only down side is that the inverter is running all the time, but it is made for that, and the energy wasted is pretty trivial.

Another small problem is that inverters put a small load on the batteries even when no AC power is being drawn. I say a small load, but depending on the size of the inverter it could be a few amps. Some inverters sense the load, or lack thereof, and go into a sleep mode to minimize this current draw. I might want to use a relay with a switch inside the living quarters to turn off the DC power to the inverter at night when I don’t need AC for anything. On the other hand, I will be running the generator every day so the overnight loss of energy isn’t much compared to my daily needs.

To help with any back of the envelope calculations you might do, I will offer these conversion factors for AC to DC (Battery Chargers), and DC to AC (inverters). These are just approximate as battery voltage varies with load, temperature, and state of charge, and conversion efficiency varies with technology and by individual brand or model.

AC to DC (Battery Chargers) 120VAC/14VDC x.95 = 8.14, or 1A AC translates to roughly 8.14A DC. If you know the DC current you need then divide by 8.14 to get the AC current into the battery charger.

DC to AC (Inverters) 12VDC/120VAC x.9 = .09, or 1A DC translates to roughly .09A AC. If you know the AC current you need, then divide by .09 to get the DC current into the inverter.

To be continued…
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08-21-2009, 07:01 PM	#41
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	RTFM – Read the F--ing Manual The second of my assumptions to fall was the idea that I would use a more or less standard RV style inverter – charger – transfer switch. I had been considering the Tripp-Lite RV750LHW, (Inverter - 750W, 1125W<1hr, 1500W<10sec, Multistage Charger - 45A, and Transfer Switch) or something like it. There are similar models made by Xantrex, Magnum Energy, and others. As far as I can tell, they all have the same killer problem for my application. When they see a valid AC input they want to turn on their transfer switch and pass that AC to their output, or in other words, to all the AC appliances. The problem is, my wimpy little EU2000i is no where near strong enough to handle the air conditioner, battery charger, and all the AC appliances that could potentially be turned on all at once. If I was connected to shore power then turning on the transfer switch would be a fine plan, but they gave me no way to disable the transfer switch. It’s a shame, because technically speaking it would probably be an easy thing for them to do, but they didn’t. On the other hand, my mighty little EU2000i is plenty strong enough to provide all of my daily energy needs, even in a worst case scenario. It just can’t meet the potential peak demand. I could get a 2nd EU2000i and put it in parallel for twice the current, but I don’t want to have 2 generators running if I can avoid it. The solution is simple enough. I just need to buy a separate inverter and battery charger. A transfer switch might have been nice in some situations, like when on shore power, but I don’t really need one. The only down side is that the inverter is running all the time, but it is made for that, and the energy wasted is pretty trivial. Another small problem is that inverters put a small load on the batteries even when no AC power is being drawn. I say a small load, but depending on the size of the inverter it could be a few amps. Some inverters sense the load, or lack thereof, and go into a sleep mode to minimize this current draw. I might want to use a relay with a switch inside the living quarters to turn off the DC power to the inverter at night when I don’t need AC for anything. On the other hand, I will be running the generator every day so the overnight loss of energy isn’t much compared to my daily needs. To help with any back of the envelope calculations you might do, I will offer these conversion factors for AC to DC (Battery Chargers), and DC to AC (inverters). These are just approximate as battery voltage varies with load, temperature, and state of charge, and conversion efficiency varies with technology and by individual brand or model. AC to DC (Battery Chargers) 120VAC/14VDC x.95 = 8.14, or 1A AC translates to roughly 8.14A DC. If you know the DC current you need then divide by 8.14 to get the AC current into the battery charger. DC to AC (Inverters) 12VDC/120VAC x.9 = .09, or 1A DC translates to roughly .09A AC. If you know the AC current you need, then divide by .09 to get the DC current into the inverter. To be continued… ?