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Author Topic: 24v or 12v inverter?  (Read 5712 times)
Stolaas
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« on: September 06, 2010, 08:42:03 PM »

I've got a question here..

I currently have a 6 batt 6v bank tied together for 12v and have a converter/charger.

I am installing an inverter for the 120v needs like the refrigerator, TV, DVD etc.

Since I have not got a inverter yet (well I do have one laying around but it's not a Pure Sine Wave) now is a good time to get what I need and not have to Upgrade later...

The main question is, do I opt for the 12V inverter which will be easy to install and the bank will be recharged by shore power or Gen,  and use all 6 batteries.

or go for the 24V and pull 4 of the 6V batteries out of the bank for a"second" bank.  This bank would be used strictly for the 120VAC while the other two 6V (series for 12V) will power the lighting and water pump etc.   The advantage I see is I could have the bus alternator charge the batteries over the road which would be nice for the fridge, and I can use smaller gauge wire to the inverter (which isn't that far a distance however)

What are your thoughts?
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« Reply #1 on: September 07, 2010, 04:34:55 AM »

your limited on out put on 12v--around 3000 watts where 24 volt are much higher...use recommended wire size..other more elect savvy will chime in . Has been reviewed  in past here.try search function...again depends on use.... Bob
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« Reply #2 on: September 07, 2010, 05:01:43 AM »

I have just finally finished up the installation of my inverter.  My need was to be able to run my 15K btu rooftop AC on the road from the bus alternator, and to be able to use typical 120VAC items when I wanted.  I went with a simple straight PSW inverter, not  an inverter/charger, but I think in hindsight an inverter charger may be a good choice as well - downside is they are a lot more expensive.

I chose a Samlex America 24 volt 3000 watt pure sine wave inverter, connected to the bus batteries and alternator via a disconnect switch.  I chose a manual battery combining switch for now, I will probably install an automatic switch later.  I installed a house battery bank of four 235AH 6 volt batteries, and used my Vanner to equalize them so that I can draw all 12 volt needs from them, up to 60 amp limit of the Vanner although my 12 volt needs are actually quite small.  I use a stand-alone charger, currently a quite small 3 stage 24 volt charger, and I will upgrade to a larger charger when this one shows it can't keep up.  My 24 hour typical electricity needs are very small (as long as I'm not using the air conditioner, which I wouldn't run from batteries anyway), so I may not have to upgrade.

Advantages:  24 volts allow use of smaller DC wiring.  Bus alternator charges house bank and supplies the power for the air conditioner.  House batteries can be combined with bus start batteries for starting boost in cold weather.  Charger can maintain the start batteries and the house batteries when the bus is not being  used.  The Samlex inverter has a direct connect AC option, and has a simple remote control and monitor panel.  It starts my rooftop AC unit effortlessly.

I used cascaded auto-transfer switches so that the various AC feeds are prioritized and switched automatically.  I went with pedestal first, generator second, inverter third in the priority list.

Disadvantage - not having an inverter/charger means I can't use the  inverter to boost current level when starting multiple AC units.  Sean recently pointed out how he uses his inverters to allow him to run two rooftop AC units reliably on a single 30 amp pedestal.  His inverter synchronizes to the incoming AC feed, is normally charging the batteries, but when the starting surge for the second AC unit hits it seamlessly changes to supplying the excess current required so both AC units can run.  I have no idea if all inverter/chargers can do this, or if you need the really  sophisticated kind that he has, with additional programmability, but it struck me as a really neat trick.

I am very happy with my setup, it works perfectly for my needs.

Brian
« Last Edit: September 07, 2010, 05:11:09 AM by bevans6 » Logged

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« Reply #3 on: September 07, 2010, 06:12:21 AM »

As bevans6 mentioned, Sean has the ideal setup.  Mine is quite similar.  Xantrex sw4024 inverter, with a vanner equalizer for your 12v lights and such.  I can run 2 roof airs while going down the road while pulling 24v from the buses alternator.  This model also has the automatic generator start/stop function that i love!  Batteries getting too low while you're not at the coach?  No problem!  Generator starts automatically.  Don't want it to run at night?  No problem.  It will wait until absolutely necessary.  This way you can use your whole bank and get the most bang for your buck.  Good luck!

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« Reply #4 on: September 07, 2010, 07:21:25 AM »

For me 24v is the only way to go. Now with that said you need to know what you want to happen with the inverter/electrical system. I have a setup similar to Sean's Gumpy's and others here. You will find the more watts, auto start and load sharing the inverters do, will cost you some good money. I had to build a transfer switch to allow the neutral to go back to the shore source. I had to buy a good equalizer to convert 24v back to 12v for lighting and such. These things all cost money but in my mind well worth the expense. I have to admit when I was  planning all  of this, I wasn't  aware or didn't make myself aware of the final cost of doing it right.

Good luck
John
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« Reply #5 on: September 07, 2010, 08:10:57 AM »

I didn't think of using a Vanner to pull the 12V from the lager 6 bank instead of keeping 2 separate for a 12Vbank.  I have a Vanner now but I have it hooked to the coach batteries to run the toad lighting and ecc. items in the dash to keep the all the coach stuff separate from the house.

I do only have a 12V water pump and a fair amount of 12V lights so that would work.  Of course I already have a nice converter/charger from the PO that I hate to discard.
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« Reply #6 on: September 07, 2010, 08:32:06 AM »

I was keeping coach 12v separate from house 12v too, until I set things up so that the alternator could charge the house batteries.  Then, while the combiner switch is thrown, there really is no difference between coach and house systems since they are connected.  I only have a little dash radio, my brake controller, and my electric seat motors that run off the bus 12v system anyway.  So I ran two separate 30 amp fused circuits from the 12v tap on the house bank, one to the house and one to the bus (terminal 6 in the AC breaker panel in the bay).  I also have a nice, new 12 volt smart charger with a built in AC and DC distribution panel that I installed last year, I just pulled the 12 volt charger out and shed a small tear (about a $150 tear, to put a point on it), and carried on with life.  I'll  use it somewhere, for something.  It's a pretty nice smart 45 amp charger, has to be a use for it around here somewhere!

Brian

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« Reply #7 on: September 07, 2010, 08:45:46 AM »

I didn't think of using a Vanner to pull the 12V from the lager 6 bank instead of keeping 2 separate for a 12Vbank.  I have a Vanner now but I have it hooked to the coach batteries to run the toad lighting and ecc. items in the dash to keep the all the coach stuff separate from the house.

I do only have a 12V water pump and a fair amount of 12V lights so that would work.  Of course I already have a nice converter/charger from the PO that I hate to discard.

I had to put one 12v battery after my Vanner. The Vanner kept shutting down. With this buffer battery between the Vanner and the 12v loads it seems to have solved all my problems.

John
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« Reply #8 on: September 07, 2010, 02:36:10 PM »

What size of battery isolator have you all used to hook up your 24v bank to your alternator so it will charge both coach and house banks?
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« Reply #9 on: September 07, 2010, 02:51:56 PM »

I have not hooked it up yet. I have a White-Rodgers type  586-114111-3 coil 24v DC cont. I have gotten some advice from Sean (Quote) Because of the nature of the Relay signal from the alternator, I recommend you use this only to drive a 12-v cube relay.  Then use that to switch a 24-volt circuit to operate the solenoid.  If you still have the blower circuit, you can just use that
I have been without work and have been working on getting a job so my time has been limited lately. But plan to hook this up soon.

John
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« Reply #10 on: September 07, 2010, 08:03:13 PM »

Hey John, it looks like we have very similar buses Smiley Not sure what you mean by the solenoid.  I was planning on putting an isolator at the connection under the coach battery cut off, or near there and just run a wire from the +pole where the alternator wire and coach wire meet at the switch.  I may just have one wire coming from that pole connecting to the isolator and then continue to the 24V bank and leave the "coach side" of the isolator empty since the coach batteries will still be getting juice at the switch junction.   

Can an isolator be hooked to only one side?
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« Reply #11 on: September 08, 2010, 05:12:40 AM »

There are a couple of ways to go about the joining of the bus battery/charging system to the house battery system, presuming that they are both 24 volts.  There are some  automatic devices that you can get (I have not researched this at all) that can sense when the alternator is charging and connect without any manual control.  You can build a relatively simple system with a relay, or you can get a marine type battery combining switch.

If you go with a relay based system, you get one like this:  http://www.texasindustrialelectric.com/Relays_24824_01.asp which is a 225 amp continuous, 600 amp surge relay designed for this application.  You connect it to bridge the two battery banks, and control it with a switch or low current relay of some type.  Your alternator is set up, stock, so that it doesn't start to charge until air pressure builds up and there are a number of systems (like the stock AC heating and cooling fans) that only can be powered after the alternator has switched on.  You control the relay  from the same source so that it connects the two battery banks only when the alternator is charging.  You can also add the ability to connect them at any time with an over-ride switch, useful if you want to boost the bus start batteries with the house bank, for example.  Now you have an auto connect system that only bridges the two battery banks when the alternator is charging, with an over-ride capability.

I went with a simple battery combining switch from a marine store.  It has the ability to turn two battery banks on or off, and to combine them.  For some reason I wanted to be able to completely disconnect the inverter from the battery so I wanted the on/off switch, and it simplifies combining the start and house batteries, but obviously is manually switched and I have to be in the electrical bay to do that.  So far it is working perfectly.  I start the bus for the first time in the morning with the switch to not-combine, after the bus warms up and the alternator is running I switch to combine, and leave it there all day.  While it works fine, I think that the relay based automatic system is better and I will probably upgrade later.  I like to do things in stages and prove in the steps before adding more complexity.

Hope these ideas help a bit.

Brian
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« Reply #12 on: September 08, 2010, 06:21:01 AM »

Brian said what I was trying to. Thanks Brian. The only time my house and Bus banks will be close to being connected is when the alternator is charging. If I need power from the house bank for starting I will use jumper cables. I do not want to ever forget that the manual switch was engaged and now have dead start batteries also. I also made my pickup tubes for my webasto and generator from my diesel tank shorter than the bus engine pick up. That way I should have fuel to get back to town. I have tried to make the house and bus systems separate with the bus engine getting priority

John
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« Reply #13 on: September 10, 2010, 08:51:24 AM »

Thanks for all your help guys!

Here are two crude drawings I made to use my 6 6V batteries.  The 12V is what is currently being used.  But in order to have 24V out of 6 batteries I have to make one Series of two  and then two parallel series.  Would this work right and charge correctly or is it not a good idea to mix it up like that and just either parallel or series and not both. 

Trying to find a way to use all 6 instead of dropping two or adding two.



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« Reply #14 on: September 10, 2010, 10:03:22 AM »

... in order to have 24V out of 6 batteries I have to make one Series of two  and then two parallel series.  Would this work right and charge correctly or is it not a good idea to mix it up like that and just either parallel or series and not both. 

Trying to find a way to use all 6 instead of dropping two or adding two.


Not advisable.  Use four or eight, not six.

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« Reply #15 on: September 10, 2010, 10:28:15 AM »

Are you going to use a Vanner type battery equalizer?  If you are, you should look at the Vanner manual page 8 for how to connect up un-equal batteries for heavy 12v loads.  You could put 4 batteries in series to create your 24 battery, and two batteries in series to create the 12 volt battery that would be charged by the Vanner 12v output.  The idea is to prevent over/under charging of any of the batteries.

http://www.vanner.com/client/images/manual_Battery_Equalizer.pdf

Edited out something that was close, but no cigar...

Brian
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« Reply #16 on: September 10, 2010, 11:53:44 AM »

Brian,

Vanner explicitly recommends against having different sizes between upper and lower sections.  I agree with this recommendation.  One or the other will be over or under charged.

The acceptable (per Vanner) configuration would have a 24-volt only bank connected between the ground and 24 terminals, and a 12-volt only bank between the ground and 12 terminals (no connection between the 12 terminal and the center tap of the 24 volt bank), as shown in the left side of the diagram on the top of page 8 of the Voltmaster installation manual.

That would be one way for the OP to use the other two batteries, but they would not be able to contribute to powering the 24-volt inverter.

FWIW.

-Sean
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« Reply #17 on: September 10, 2010, 12:08:05 PM »

Sean, I agree, that's why I deleted my post and added the link to the Vanner manual page 8.  this whole business of keeping batteries healthy is an ongoing struggle.  I  made sure my own batteries are identical and balanced, and I still have problems sometimes. 

Brian
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« Reply #18 on: September 11, 2010, 02:15:57 PM »

I know this is probably a stupid question, but has anyone tried plugging in a converter to their 24v inverter's output for 12V?
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« Reply #19 on: September 11, 2010, 02:55:05 PM »

I know this is probably a stupid question, but has anyone tried plugging in a converter to their 24v inverter's output for 12V?


That would work.  However, you lose about 10% of the energy converting from DC to AC, and about another 6-8% converting back to DC again.  Call it 15-17% or so energy loss.  By contrast, using an equalizer and pulling the 12v off the center tap costs about 2%.  Much more efficient.

The numbers get even more lopsided if you set things up so the inverter only runs with a load (many inverters can be set this way).  Now you have to run the inverter 100% of the time you have anything on, AC or DC.

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« Reply #20 on: September 12, 2010, 06:54:22 AM »

I figured I would loose some efficiency in the process but trying to work with what I currently have.  A Vanner would be nicer but the one i have is an old one and only rated for 15 amps. 

I was thinking of just having the converter hooked into an A/C breaker that is powered by the inverter.  When hooked to shore power the inverter supplied panel would then be shore supplied and the converter would then run from the shore.

This case the only real noticeable efficiency loss would be when I run the DC lights or Water pump when neither power plant is running or shore power connected

If I only connect the 4 btteries to the 24 and use the other two for the 12V the converter will work only as a charger when no show power is connect.

Of course the main problem is can I run the A/C fridge over night on just the four batteries...
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« Reply #21 on: September 12, 2010, 07:13:05 AM »

I figured I would loose some efficiency in the process but trying to work with what I currently have.  A Vanner would be nicer but the one i have is an old one and only rated for 15 amps.

15 amps is probably more than enough.  Remember that an equalizer is not sized for the maximum demand, but the total demand over time.  IOTW, a 15-amp equalizer is capable of providing 180 amp-hours of 12-volt power every day, which is more than you will likely ever use if your inverter is on the 24-volt side.

  
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I was thinking of just having the converter hooked into an A/C breaker that is powered by the inverter.  When hooked to shore power the inverter supplied panel would then be shore supplied and the converter would then run from the shore.

This case the only real noticeable efficiency loss would be when I run the DC lights or Water pump when neither power plant is running or shore power connected

No.  The converter will be using power 100% of the time it is connected, whether or not it has a load.  It will also force your inverter to be inverting 100% of the time, defeating any idle or standby capability.


Quote
If I only connect the 4 btteries to the 24 and use the other two for the 12V the converter will work only as a charger when no show power is connect.

Again, using a battery bank to charge another battery bank through an inverter and charger is highly inefficient and will drain the donor bank all the way to zero, even if there is no load at all.  By contrast, with an equalizer the drain on the 24-volt bank will stop when the halves are equalized if there are no loads.

If it were my coach, I'd sell the converter on eBay and hook up the 15-amp equalizer, then take the 12 volts from the center tap.  It is the simplest and most efficient method and will cost you nearly nothing since you already have the equalizer.

-Sean
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« Reply #22 on: September 12, 2010, 04:13:12 PM »

  Not to sidetrack the discussion, but if you guys were starting from scratch, wouldnt it be more efficient to keep everything DC?

  I was googling around, and it appears modern DC motors can match, or possibly surpass AC motors in efficiency. They make DC Compressors, it would seem that you could keep the entire electrical system DC, have a DC Generator, and not really need any large inverters.
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« Reply #23 on: September 12, 2010, 04:37:19 PM »

  Not to sidetrack the discussion, but if you guys were starting from scratch, wouldnt it be more efficient to keep everything DC?


Yes, it is more efficient.  However I challenge you to find air conditioners, decent sized microwave ovens, Blu-Ray players, nice blenders, a decent coffeemaker, or myriad other appliances built for DC.  Even the chargers on battery powered tools and similar items are AC.

Appliances that are available that way (for efficiency on DC systems like RVs and boats) are very expensive compared to commodity AC items -- our minuscule DC fridge cost over $1,000, and a comparable AC model would be about $150 at Wal-Mart.

When all is said and done, using an inverter to power more conventional appliances is cheaper in the long run, even if that means you need more batteries and/or a larger charger due to the roughly 10% loss in the conversion.

Remember, too, that 12- or even 24-VDC cables to supply high-power appliances would be enormous compared to 120 VAC.

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« Reply #24 on: September 12, 2010, 06:16:22 PM »

Thanks for all the help.  Here is the next question for those who have experience. Which is better, a relay or diode isolator?  I was going to opt for the really so I could tie the batteries together if I ever had to.  But I have been doing some research and have read that a diode isolator is better if you have different batteries.  My coach batteries are the 8d liquid type but the house batteries are 6v gell cell. 

With this different type of banks, is the solid state diode better?
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« Reply #25 on: September 12, 2010, 06:18:06 PM »

Yes, it is more efficient.  However I challenge you to find air conditioners, decent sized microwave ovens, Blu-Ray players, nice blenders, a decent coffeemaker, or myriad other appliances built for DC.  Even the chargers on battery powered tools and similar items are AC.
-Sean

  They are making a/c compressors that run on DC. And that was my interest in the question. AC would be the largest drain in power. If the overall costs were the same, the DC compressor would win hands down. But it could still win if the cost of the AC unit were reasonable, negating the use of a giant inverter. As for the other items, you could either get by with a much smaller inverter, or have inverters dedicated to the particular appliance.

   Just a thought, thanks for the input.
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« Reply #26 on: September 12, 2010, 09:46:17 PM »

Thanks for all the help.  Here is the next question for those who have experience. Which is better, a relay or diode isolator? 


That's a religious discussion, and you will find plenty of arguments on both sides.  Go back through the archives, we've discussed this topic here frequently, including just last week.

The are pros and cons on both sides.  As it happens I just wrote something up for a thread on the other board:
Quote
1. With an isolator, if you need to bridge the two banks together, say for emergency starting purposes, you'll need to use a switch, solenoid, or jumper cables downstream of the isolator. If you use a solenoid for this, your paying for (and installing) a device that could have done the whole job had it just been wired the right way.

2. The alternator's regulator adjusts the alternator output based on sensing battery voltage right at the batteries. With an isolator, you need to pick which set of batteries will have the sense lead connected. The other bank gets "pot luck" on charge voltage. This means whichever bank is not sensed might get undercharged or even overcharged.

3. When the batteries are depleted, the alternator will be operating at maximum output. The inherent loss in the isolator means that and extra 5% of your alternator output is being wasted as heat in the isolator -- what those big fins are for. With a solenoid or switch, that energy goes into the batteries instead.

4. With an isolator instead of a solenoid or switch, there is no way to disconnect the house load from the alternator, say in order to get back that ten horsepower for hill climbing or whatever. Unless, of course, you also install a switch for that purpose, but then see my comment at #1.

On the plus side, there are no moving parts, the wiring is arguably simpler, and you can't accidentally leave the switch in an incorrect position (in the case of solenoid installations with over-ride switches). There will be no flow of energy from one bank to the other for any reason.


I recommend the solenoid be connected only when the alternator is charging, and not when the house battery charger is operating.

They are making a/c compressors that run on DC. ...

Well, yes, but not in RV air conditioners, and also not generally on 24 or 12 volts.  The only residential size commercially available unit that I'm aware of runs on 48 volts, and making 48 volts on a bus is even less practical than just making 120VAC.

It's not a matter of technical feasibility, it's a matter of market dynamics.  I set my coach up with as many 24-volt items as I possibly could, right down to 24-volt fridge, water pumps, and lighting, but still I need 120vac for air conditioners, microwave, coffee maker, vacuum cleaner, air compressor, and even  electronics (just try to find a 32" TV that runs on 12 or 24 volts).

Once you have the inverter because at least one thing on the coach needs it, it's far cheaper and simpler to just use it for anything else you need.  For loads that run only intermittently, like a microwave, coffee pot, or vacuum, the energy delta is negligible.  Full-time loads like refrigerators start to tip the scales towards dedicated DC systems.

FWIW.

-Sean
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« Reply #27 on: September 13, 2010, 06:29:59 PM »

Well I thought my Vanner was an equalizer, but no, it's a dc to dc 24-12 converter Sad but it's what I am using for my coach side 12v including trailer lights.  Got an awesome 24v inverter/charger today though. Should be in in a couple of days  Grin

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Sean
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'85 Neoplan Spaceliner "Odyssey"


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« Reply #28 on: September 13, 2010, 07:10:44 PM »

Well I thought my Vanner was an equalizer, but no, it's a dc to dc 24-12 converter ...

What's the model number?  Most Vanner equalizers could be used either way.

-Sean
http://OurOdyssey.BlogSpot.com
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Full-timing in a 1985 Neoplan Spaceliner since 2004.
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