Looking for advice on a pure sine wave power inverter

[ERD50]

... The battery might be needed if for example you buy lunch at a store and stop at a roadside table to eat it (near the car).

There is another reason I think you may need the battery - the start up current on the unit. Since it plugs in a wall, and only draws 350 watts, the designers would not be concerned at all about a big multiple on the surge current.

But the surge will create an increased voltage drop on the wires from the vehicle battery to the inverter near the back of the vehicle. Having a battery very close to the inverter will help with the surge.

I've got a 2500 W inverter, and it wasn't easy to start a freezer that only draws ~ 450 watts. You really need heavy, short wires and very, very solid connections. Even though I know that, I was still a bit surprised at just how fussy it was.

And I still question the constant 30 A added draw on the alternator. I know they are rated at 90 A or more, but that was a design spec based on normal use. Is there really enough margin for another 30 A continuous?

But that switch that isolates when V < 13.x volts is really clever! That's really a simple and optimal way to do it, since you wont have much above 13V w/o the alternator active. Smart.


-ERD50

 

[RobbieB]

You may want to consider a 24 volt inverter and 2 AGM batteries in series. I think they have a "split" isolator / charger that can charge them from 12 volts. You won't get such a heavy draw from the series connection.

 

[NW-Bound]

... look at the tow vehicle's OEM wire gauges for the charge wire to the 7 way connector. Many are 12 gauge. Add in similar wire gauge in the trailer with the battery in the back, plus a 12 volt refrigerator running while towing pulling 10 to 20 amps and you've got a dead trailer battery at arrival...

Come to think of it, what RV 12V refrigerator draws 10-20 amps?

Ammonia absorption fridges that are common in RVs and the one I have draw more than 350W. That would be 30 amps on a 12V circuit. There are new fridges that have a 12V compressor and these draw quite a bit less current, but are not yet common.

My class C came with a big AWG2 wire linking the engine battery to the house batteries. It snakes under the chassis, and is routed to the genset (to start it), hence is definitely longer than 15 ft.

My fridge is a 2-way, meaning its heater is either on propane, or 115V. A 3-way would have a 12-volt heater, which mine does not.

When driving, I run the fridge on the inverter which is attached to the house batteries. The big wire has no problem transferring the juice. The idea is to save on propane. I'd rather pump more gasoline than driving around strange places looking to buy propane. Hence, the idea of running the fridge on electricity whenever possible, although it is cheaper to run it on propane.

I installed a DVM panel with a large display that my wife can look back and see from her passenger front seat. This is to make sure that the house batteries do not get drained. That DVM panel saved me the dead batteries once, when the isolation relay failed due to a poor contact on its coil.

Still, there's a hassle with manually switching the fridge back to propane when we stop. So, I installed an automatic relay that cuts out the electric feed when the house battery voltage drops below 12.8V. When driving, the voltage back there is usually around 13.2-13.5V. No more forgetting.

 

[NW-Bound]

By the way, for work like this, one needs a clamp-on DC ammeter. This is not the same as the common AC ammeter which has an inductive coil for the pickup.

To measure DC current magnetically, a Hall-effect sensor is needed. This type is getting cheaper. I bought a Craftsman DVM from Sears a few years ago, and paid $60. Don't work on batteries, auto and RV wiring without one like this.

 

[travelover]

Come to think of it, what RV 12V refrigerator draws 10-20 amps?.........

Mine does. I have a little Norcold 323 and it draws 10 amps. A fridge that draws 30 amps is a big sucker.

 

[NW-Bound]

OK. Here's yours, and here's mine. 3x the volume, 3x the power drawn. It's fair.

By the way, the photo is deceiving. Mine, which is the de-facto standard for class C's, is only 6.1 cu.ft. It's very shallow and does not hold as much as the photo suggests.

The trend now in class A's and 5th-wheels is to put in full-sized residential refrigerators. Side-by-side or french-door. Your pick.

 

[travelover]

That's fair. I'll show you mine if you'll show me yours.

Generally, the DC heater is not as powerful as the AC heater in the fridge. Given the inefficiency of absorption refrigerators, I think they try to keep it to a minimum to just maintain temperature in moderate conditions.

Back to the matter at hand, I think the right solution is very heavy wires from SUV battery to AGM battery to allow both batteries to provide grunt to the inverter. Again, I'd bench test it with the AGM battery at 50% to be sure the concentrator will always start.

Another possible avenue to explore is a soft start on the concentrator, but given how critical it is, you might not want to mess around with add ons. These are used on RV air conditioners to allow small inverter generators to run AC units with high surge loads.

 

[aja8888]

That's fair. I'll show you mine if you'll show me yours.

Generally, the DC heater is not as powerful as the AC heater in the fridge. Given the inefficiency of absorption refrigerators, I think they try to keep it to a minimum to just maintain temperature in moderate conditions.

Back to the matter at hand, I think the right solution is very heavy wires from SUV battery to AGM battery to allow both batteries to provide grunt to the inverter. Again, I'd bench test it with the AGM battery at 50% to be sure the concentrator will always start.

Another possible avenue to explore is a soft start on the concentrator, but given how critical it is, you might not want to mess around with add ons. These are used on RV air conditioners to allow small inverter generators to run AC units with high surge loads.

I will bench test this once I have all the components in hand. I'm not going to install this and take a chance.

 

[NW-Bound]

Sure. It does not take long to lay out all the components and wiring alongside the vehicle to check things out. Again, I would be very surprised if you have any problem. Are you going to use the vehicle chassis for the return ground? That's what is done in my class C, and the vehicle frame offers a much lower resistance than an AWG2 wire for the negative lead. You save some money as well.

On eBay, there are many vendors offering AWG2 stranded welding cable. They are much better than the cheaper battery cables with fewer strands, in that the flexibility makes it easier to route through corners. For a bit more, I bought marine cable, which is also multi-stranded and tinned. It has better corrosion and oxidation resistance.

 

[aja8888]

Sure. It does not take long to lay out all the components and wiring alongside the vehicle to check things out. Again, I would be very surprised if you have any problem. Are you going to use the vehicle chassis for the return ground? That's what is done in my class C, and the vehicle frame offers a much lower resistance than an AWG2 wire for the negative lead. You save some money as well.

On eBay, there are many vendors offering AWG2 stranded welding cable. They are much better than the cheaper battery cables with fewer strands, in that the flexibility makes it easier to route through corners. For a bit more, I bought marine cable, which is also multi-stranded and tinned. It has better corrosion and oxidation resistance.

My plan is to use the chassis for the ground since it will make for only one long cable. I just have to drill a hole under the rear somewhere to feed the positive cable into the rear hatch area. I can us that same hole to drop a ground into and find a spot on the frame nearby to drill a hole for the ground lug. I have this all planned out in my mind right now.

 

[travelover]

..........On eBay, there are many vendors offering AWG2 stranded welding cable. They are much better than the cheaper battery cables with fewer strands, in that the flexibility makes it easier to route through corners. For a bit more, I bought marine cable, which is also multi-stranded and tinned. It has better corrosion and oxidation resistance.

Just watch out for copper coated aluminum wire, which seems to come from China. Sometimes it is not accurately identified. Chinese jumper cables, for example tend to be made with this wire.