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Author Topic: 24v or 12v inverter?  (Read 5630 times)
bevans6
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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
« Last Edit: September 10, 2010, 11:15:42 AM by bevans6 » Logged

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Sean
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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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bevans6
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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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Sean
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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.

-Sean
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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.


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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.

-Sean
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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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Sean
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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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« 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
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