Coach Electrical Systems

[sledhead]

at 7 c feet that is small for a fridge

most people see the low price for a home style fridge compared to a r/v fridge price and with all the fires from some r/v style fridges I can see the change to a home style fridge

but yes you will pay the power price from a home style fridge

Dave

[PNWorBUST72]

The bus we are looking at has an older residential fridge but we could be willing to go propane if the electrical needs where too much.

I would like to be able to run the fridge, AC and maybe a  TV and some lights while we drive.

I am down for doing the JD route and get all the bells and whistles but I would prefer to do it in logical stages for a cost perspective.

[gumpy]

It's a toss up. You pay now, or you pay later.

I chose to go residential 120v fridge. 11 cf. $300. However, I have $1100 of Trojan batteries and a 4000 watt sine wave inverter that I got a deal on at $1200. I also have a 7.5WK generator. My house bank is 24v and I charge it off the big 50DN alternator while driving. I can run the house a/c off the inverter if necessary while driving, though it doesn't compete to the bus a/c when it's working properly. That's what I wanted, and I'm very happy with how that has all worked.

[richard5933]

I like my beer COLD.  RV refers suck-- not cold enough, and ice cream isn't frozen. Plus you have to vent them out the roof, cost too much new, and are too small.  I never heard of 12v RV reefers without also having propane (I know, Summit, but are they for boats?).

Both of our coaches had/have 12v/120v Norcold refrigerators (without propane). They use 12v DC compressors. Both kept ice cream frozen and beer cold, and they both were able to run for quite a while without using much from the batteries. A few different manufacturers make them, and they're more common on boats than RVs, but they can be found in RVs. Apparently Custom Coach was fond of them back when they were in business. Personally, I would not have a propane fridge in my bus for any reason. The danger people see from RV fridges are the propane units - the 12v compressor fridges don't seem to have a problem with fires like the propane ones.

For just two of us we don't really have need for a huge refrigerator. It's not like we're trying to stock up for a week or more at a time. We had 7 cu ft in our first coach, and it easily held enough for the two of us. I think our current one has only about 5.5 cu ft. It's just about as small as I would consider useful. If we were going to be on an extended trip away from stores I'd probably look into adding a small 12v chest style cooler in the basement for added storage.

If price is a factor, then it's absolutely going to be the residential units that win the contest. But then you've got to add enough batteries and charging capacity to run them.

Guess it's all in what one considers better suited to his/her needs. Luckily there are enough options for us to be able to have this conversation.

[Oonrahnjay]

Not a chance this is accurate if its actually running on DC.  Maybe it draws 2 amps @ 12 volts while running on propane but there's zero chance it draws that running on 12 volts. 

Absorption fridges are notoriously inefficient so that may make a case for inverting up to 110 but the other option would be a 12 volt compressor fridge.  Either way, its going to draw significantly more than 2 amps @ 12 volts while its running. 

        An issue with refrigerators is that they aren't "steady draw" like a light bulb.  They use more power in hotter conditions, if they have unchilled food put in, when they're drawing down at startup, if the door is opened more often, etc.  At a steady average run, my NovaKool is rated at 48 - 50 watts; that's about 4 Amps at 12V (nominal) or about .4 Amps at 120V.  It's an energy sipper, works great in the fridge and has a powerful freezer, and was breathtakingly expensive. 

[Geoff]

I can tell you how my 10.3 cu ft works out.  I have four golf cart batteries.  The Trace inverter jumps between 1&2 amps draw @120v.  The batteries will keep the reefer going for about 2.5 days before the Trace shuts down to save the batteries.  I could set up the Trace to automatically start, but haven't felt the need.

[richard5933]

I can tell you how my 10.3 cu ft works out.  I have four golf cart batteries.  The Trace inverter jumps between 1&2 amps draw @120v.  The batteries will keep the reefer going for about 2.5 days before the Trace shuts down to save the batteries.  I could set up the Trace to automatically start, but haven't felt the need.

Is that the draw on the 120v side? If so, how much is it actually pulling from the batteries in 12v?

If it's only pulling that low of a reading on 12v, then you're doing really well.

[Oonrahnjay]

Is that the draw on the 120v side? If so, how much is it actually pulling from the batteries in 12v?

If it's only pulling that low of a reading on 12v, then you're doing really well. 

    Watts - Watts - Watts!  If it's pulling 1-2 Amps at 120V, that's 120 - 240 Watts.  At 12V, that would be 10 - 20 Amps.  But instantaneous readings don't tell you much about refrigerators.  If a fridge is cooling down, it will use more power (it will run almost continuously until it reaches thermostat setting).  Then, it will come on as the thermostat calls for it and goes off when the thermostat thinks it's cool enough.  To get a good idea of how many days a fridge will run before the batteries go to "low voltage", you would need to get an average of wattage use over time.  A single point in time tells you either nothing or very little.

[richard5933]

    Watts - Watts - Watts!  If it's pulling 1-2 Amps at 120V, that's 120 - 240 Watts.  At 12V, that would be 10 - 20 Amps.  But instantaneous readings don't tell you much about refrigerators.  If a fridge is cooling down, it will use more power (it will run almost continuously until it reaches thermostat setting).  Then, it will come on as the thermostat calls for it and goes off when the thermostat thinks it's cool enough.  To get a good idea of how many days a fridge will run before the batteries go to "low voltage", you would need to get an average of wattage use over time.  A single point in time tells you either nothing or very little.

I'm not sure why you're talking about watts. Watts is just a calculation of volts x amps. Not sure what it has to do with this conversation. 1-2 amps at 120v is 10-20 amps at 12v, in round numbers. That's my point. When you pull amps from a battery bank to power the inverter at a higher rate than you do to power the DC fridge directly you'll run out of juice faster.

For an inverter to make the AC power it's pulling DC power. It sounds like such a little bit when said as 1-2 amps, but if the inverter is pulling 10-20 from the battery bank then it's using more than if he here doing it with a 12v compressor only pulling 2-5 amps.

I understand about the power cycle thing on a fridge. That is the exact same whether a DC or AC powered fridge. The concept of thermostats and such works no differently on my DC fridge than it would on a fridge powered by AC.

I think that bottom line is this... There is a loss to heat when power is converted from DC to AC. This is why inverters have cooling fans. There is simply no way around this. There is also a loss just having the inverter turned on. Also no way around this. I'd love for you to show me the math of how to make a 120v fridge last as long on a battery of a set size as a 12v DC fridge pulling fewer amps.

All that said, I understand that for some people the residential fridge works for them. Great. For us, I'll stick with the energy-sipping 12v compressor fridge. It works for us. There is no right answer as to which one someone should choose.

[buswarrior]

This thread is a mess.

The reason to use watts in a discussion like this, is so we are comparing properly.

So, a busnut might say "my DC refrigerator uses 450 watts when it is running flat out"

Another might say "my dollar store bar fridge uses 500 watts when running and using a kill-o-watt device, it uses 4 kwh a day averaged over a 3 day test, door closed."

Another busnut with engineering knowledge and some fancy test tools will chime in with "that new pure sine wave inverter I got off ebay that everyone thought was crap, turns out it runs at 94% efficieny!"

There is supposed to be a difference between a moderated board and Facebook...?

Happy coaching!
Buswarrior

[richard5933]

I'm not sure why this is a mess. Some manufacturers list their specs as amps, some as watts. Use the formula below however you need to make the comparison.

The formula is pretty straightforward.

     watts/volts=amps

Since most people rate their battery banks in available amp hours, for me doing the calculations in amps makes more sense.

I just looked at the Norcold site. For a brand new 7 cu ft. fridge it uses 0.4 amps on 120v and 3.2 amps on 12v. If you run an inverter to make the 120v you'd be using approximately 4 amps to run this fridge. The difference between the 3.2 and the 4 amps would mostly be the energy loss to the inverter itself. This is a very efficient fridge whether on AC or DC.

Compare that with a residential fridge and you will see why I lean towards this type of fridge. Let's say that you have a residential unit that uses 'only' 1-2 amps. That's still 2 to 4 times the power consumption of the RV model.

If someone wants to make this comparison using watts instead of amps, the formula above can make that easy. Regardless, the end result will be the same.

All that said, if someone wants or needs the larger residential fridge and has the ability to make it work for them, then that's what they should do. I'm only stating what works for me, and that's the smaller more efficient unit.

[PNWorBUST72]

Ok, not trying to get into a amp  vs watts argument because I doubt I understand a 1/3 of it...

We want to run AC(thinking roof mounts but the noise is a question), maybe 2 maxxfans (at least one for the shower/bathroom area), 2 27in-ish TV's, propane tankless hot water heater, propane RV oven, smaller fridge(7-10cu ft - most likely residential), assorted lighting of course, USB charge ports, maybe a Playstation 4.

Can I run the 24v starter batteries like normal and just build out the rest of the coach side as a standard RV, 12v and 110v?  As long as I dont try to use the 24v alternator to charge my 12v batteries OR dont draw anything from the 24v side, I can built them as completely separate systems no? 

I only need an isolator if I plan on drawing from both sides of the system correct?

[richard5933]

Ok, not trying to get into a amp  vs watts argument because I doubt I understand a 1/3 of it...

We want to run AC(thinking roof mounts but the noise is a question), maybe 2 maxxfans (at least one for the shower/bathroom area), 2 27in-ish TV's, propane tankless hot water heater, propane RV oven, smaller fridge(7-10cu ft - most likely residential), assorted lighting of course, USB charge ports, maybe a Playstation 4.

Can I run the 24v starter batteries like normal and just build out the rest of the coach side as a standard RV, 12v and 110v?  As long as I dont try to use the 24v alternator to charge my 12v batteries OR dont draw anything from the 24v side, I can built them as completely separate systems no? 

I only need an isolator if I plan on drawing from both sides of the system correct?

You can certainly have them as two totally separate systems. The problem would be trying to figure out how to charge the batteries while on the road, since you'd only be able to charge while plugged in or while running the generator. If you pulled up anchor in the morning after drawing down your batteries overnight, you'd arrive at your next campsite at the end of the day with uncharged house batteries unless you ran your gennie while driving.

Couple of quite definitions:

An equalizer is used to pull 12v from a 24v battery bank. It pulls from both batteries evenly so you don't end up with one being overcharged and one being undercharged. It is possible to just tap into the point between the two 12v batteries in a 24v battery bank to get 12v without an equalizer, but this will quickly destroy your batteries.

An isolator is different. This is used to charge two separate battery banks from one charging source while keeping the two sources isolated from one another. They do this with diodes usually. For example, if you had a 12v start system and a 12v house system you could use the engine's alternator to charge both battery banks while driving. The isolator is there to keep the house system from drawing down the start batteries while camping. It basically make for a one-way pathway from the charging source to the batteries. On our 4106 we used a solenoid instead of an isolator with a manual switch to bridge both the house batteries and start batteries together while driving down the road. This allowed us to use the bus engine's alternator to charge the house batteries. When we shut down the engine we flipped the switch and disconnected the two battery banks from each other. The isolator allows them to stay physically connected but provides an electronic barrier instead.

If you went with a 24v house system (using a bank of deep cycle batteries separate from your start batteries) then you can easily charge them while on the road using the bus alternator by using an isolator or a solenoid. If you did this and had a 24v house system, then you would need to use an equalizer or 24v-to-12v converter to pull 12v for those appliances which required 12v. Probably a converter would provide enough 12v current in your case.

We went with a system like you're describing (totally separate 24v start and 12v house systems), and to charge the house batteries while on the road I'm installing a 24v-to-12v battery-to-battery charger. It will charge the house batteries at 70 amps using a 3-stage charge just like any other plug-in charger. It only charges while the alternator is operating on the bus engine so it shouldn't draw down our start batteries, plus we're adding the remote on/off switch to be certain. This battery-to-battery charger is made by Sterling. We also have a Progressive Dynamics 70-amp 120v plug-in charger for when we have power from pole or gennie. I know that our system has some duplication, but it works for us.

It looks like you'll have mainly a 120v load, so running a 24v inverter may make more sense as they can be more efficient and require lighter gauge wiring. That would be a decision you'll have to make. The remaining 12v appliances you plan to run are not high-draw items, so they could probably run from a 24v-to-12v converter.

I'm sure that others will offer other options which are equally valid. The problem is always trying to decide which will work best for you.

[PNWorBUST72]

Great reply Richard, thanks.

So how does the equalizer get wired in?

AC Side - Shore Power/Generator > Transfer Switch > 50amp Breaker Box > Inverter > Battery Charger > Coach Side Battery Bank
DC Side Chassis - 24v Alternator > 24v Battery Bank >24v Loads
DC Side Coach - Inverter from AC Side > 12v Fuse Boxes > 12v Loads


Does multiple alternators of the same voltage charge the batteries faster or more fully?  I could add a 12v alternator to change just the 12v Coach Side?

[eagle19952]

why ?
they make 24v inverters
and 24v LED's
12v makes no sense to me.