24V-12V changeover

[bemcadam]

Can onyone out there tell me if there is an easy way to convert to 12V for charging house batteries off the 24V bus alternator.  I will be running 4  4D deep cycle batteries for house power and would like to be able to charge while running as well as with the converter.  Also plan on solar panels on the roof, but thats down the road a ways.  Darn electrical, anyhoo.!.!.

[Bill B /bus]

You have two options: 1 - stay 24 and use an equalizer for the house 12V or 2 - convert the entire bus to 12V. That means wire size increased to handle the doubled current flow.

Most of us facing this leave the bus 24V, batteries/starter/alternator etc and use 12V battery equalizers to produce the 12V for the motorhome stuff.  You can find the equalizers in the $100 or less range. You should already have one for the 12V accessories.

If you tow a car then you convert to 12V for the lights with a relay. 24V coil contacts passing the 12V. With this you can convert your rear lights to 12V LED. The front markers use either the relay or add a resistor in series with the new LED.

Bill

[Lin]

A Vanner equalizer can be used in two ways (either or, not both).  One is to wire it to draw 12v for house functions while keeping the batteries balanced.  The other is to wire it to merely supply 12v as if it were a 24v to 12v battery charger.  The latter is what we do, and it works fine.  There is no reason to go through the headache of converting the entire bus to 12v.

[hargreaves]

I use a 100 amp vanner with a 1 ought cable going to the 8 golf cart batteries. The 1 ought cable has a constant load relay that switches on when I start the motor. I can run one ac unit off the batteries while traveling with the 270 amp alt. putting in 100 amps to the batteries through the vanner.  System seems to work good.  Checked the batteries after a 5 hr run and the were right up.

[bemcadam]

I knew of several ways to convert 24 to 12, the busses always seem to have a little quirk or 3 that the trucks & heavy equipment I work on have never seen or heard of.   That said, I come here to the experts who have found, by trial and error, the best ways to handle a problem on the bus.  Kudos & thanks for the replies.
Mac

[jjrbus]

 When I looked into this, the cheapest, fastest, easiest  way was to use a 24V inverter. The people that seemed knowledgeable designed a system. Cross tying  the bus and house system with a switch or solenoid. Then tapping what little 12V was needed off the house bank. With little effort I used almost no 12V.

A big factor at the time was the wire,  with a 24V system wire can be up to 6 times smaller than 12V! Of course my opinions are worth what you pay or them.

Made sense to me                       JIm

[bevans6]

I have my house bank configured as a 24v bank, since my "killer application" is the 3000 watt inverter.  It runs from the 24 volt supply and runs off the house alternator when the engine is running.  All of my 12V needs are taken from the center tap of the 24 volt configuration, with a 60 amp Vanner to equalize.  It works really well.  I am adding a 24V 45 amp smart charger this year, that will be able to be switched to act as a converter, or to charge either or both of the house and start batteries.

I really like the way this works for me.  24 volt reduces the cable sizes to the alternator and the inverter, and the equalizer keeps everything equal when I use the 12V supply for the small stuff like the radio, the lights, and the furnace fan.  I highly recommend this approach if your electrical needs are like mine - fairly small 12v loads, and a honking big inverter load.

Brian

[Geoff]

Do you have the V730 transmission with the AC pulley?  If you do the simple solution is to mount a 12v truck alternator to the PTO.  That's what we do on our RTS buses with the V drive.  It has worked for me for many years.

--Geoff
'82 RTS AZ

[Sean]

FWIW, I wrote a feature-length article on the subject of 12-volt vs. 24-volt house battery systems in the April, 2010 issue of Bus Conversions Magazine.  One of these days I will get around to formatting it for my web site.

Frankly, I do not understand the obsession with 12-volt house systems.  24 volts is a technically superior system in every way, and if your coach already has a 24-volt alternator that should really seal the deal for you.  Only a very small handful of items come only in 12-volt configurations, including FanTastic fans, LP alarms, and car stereos, and all those items can easily be accommodated from a 24-volt system using a small equalizer in the 10-20 amp range.

The cost and complexity of running a 12-volt house system on a 24-volt coach far exceeds the cost of a small equalizer and the slightly higher up-front cost of 24-volt items like lamps and pumps.  Often the differential cost of 24-volt lamps and other items is more than made up for by the savings in copper wire, switches, relays, and other items that can now be downsized to half the load.

JMO and YMMV, as they say.

-Sean
http://OurOdyssey.BlogSpot.com

[jjrbus]

I used some 24V DC low voltage lighting in my bus. It was standard 120V house lighting that was transformed to 24V DC. it was as simple as eliminating the transformer.   The lighting was of course in the bargain bin at the store.

RV type furnaces also require 12V.                                       

[bemcadam]

Geoff,
Our bus has the 8844 Spicer 4-speed with an 8V71 DD.

Sean,
I will try to find that issue of BCM and read that.  Seems like there are as many ways of doing this as there are busses out there.  From what you have told me and FWIW, I will probably leave it 24v and just use a small inverter.  Thanks again for all the replies.
Mac

[Oonrahnjay]

(snip)  I do not understand the obsession with 12-volt house systems.  24 volts is a technically superior system in every way, and if your coach already has a 24-volt alternator that should really seal the deal for you.  (snip)   

    Yes.  Make your "core system" 24 volts - get all the benefits of it (ESPECIALLY if you're going to be using an inverter) and pull what little 12v loads off of it that you need.  (I think that this is the gist of Brian Evans's post too.)

[Geoff]

This debate has been going on for years-- 24v vs.12v house systems.  I could take almost everyone of Sean's comments and substitute 12v everytime he says 24v and it makes more sense to me.  I purposely got rid of my 50DN 24v alternator with it's small 12v converter and mounted a 12v 160amp truck alternator and small 42 amp 24v alternator off the transmission PTO in my RTS.  I use a SW2512MC inverter with 4 golf cart batteries configured for 12v.  The only thing on my bus that is 24v is the starter and the driver's defrost motor, so I have two Group 31 batteries/alternator @24v just for this purpose.  Converting a bus using 12v is much easier because everything you buy that is DC is readily available in 12v.  The RV industry doesn't use 24v anything, so you pay dearly for 24v lights and pumps.  From a technical standpoint, 24v is more effecient because it can use smaller wiring but that's about it.  And wiring for 12v is no problem at all in a 40' bus conversion.

--Geoff
'82 RTS AZ

FWIW, I wrote a feature-length article on the subject of 12-volt vs. 24-volt house battery systems in the April, 2010 issue of Bus Conversions Magazine.  One of these days I will get around to formatting it for my web site.

Frankly, I do not understand the obsession with 12-volt house systems.  24 volts is a technically superior system in every way, and if your coach already has a 24-volt alternator that should really seal the deal for you.  Only a very small handful of items come only in 12-volt configurations, including FanTastic fans, LP alarms, and car stereos, and all those items can easily be accommodated from a 24-volt system using a small equalizer in the 10-20 amp range.

The cost and complexity of running a 12-volt house system on a 24-volt coach far exceeds the cost of a small equalizer and the slightly higher up-front cost of 24-volt items like lamps and pumps.  Often the differential cost of 24-volt lamps and other items is more than made up for by the savings in copper wire, switches, relays, and other items that can now be downsized to half the load.

JMO and YMMV, as they say.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

... From a technical standpoint, 24v is more effecient because it can use smaller wiring but that's about it.

Actually, that's not "it" at all.  For one thing, smaller wire size is only one of several benefits, and not even the greatest of them.  Doubling the voltage means halving the current, so not only are wires smaller, but so are the internals of many components, thus:

• A 12v 50DN produces 4,200 watts, while a 24v 50DN produces 7,560 watts.
• 12v inverters top out around 3kW, while 24v inverters are available up to 5kW
• 12v battery chargers top out around 2kW while 24v chargers top out around 4kW
• 24v motors, charge controllers, and power supplies are all half the size and weight of their 12v brethren

I'm also not sure what you mean by "more efficient", but there are three principle benefits of smaller wire size alone:

• Cost of the wire itself: Copper is running about $3.75/lb right now; using 100lbs of wire in your bus instead of 200lbs saves you $375 right off the bat.  Many coaches could have twice that amount, depending on number of batteries, devices, etc. and the savings would be that much greater.
• Cost and size of switches, relays, and fuses:  A 200-amp contactor is much smaller and less expensive than a 400-amp one, likewise for disconnect switches.  Fuses change size and price when you have to go to the next larger physical size.
• Ease of installation.  Anyone who has had to wrestle a pair of 4/0 cables into a tight space could appreciate how much easier that would be with a pair of 0-gauge instead.  Also the minimum allowable bend radius for the smaller sizes is much tighter.

... I purposely got rid of my 50DN 24v alternator with it's small 12v converter and mounted a 12v 160amp truck alternator ...

Thus going from an available 7,500 watts to just 2,200 watts.  Depending on how you use your coach, that might not be a problem for many folks, but I can tell you from our experience that our house batteries use a good 4,000 watts or so while we are underway to recharge.  We seldom stay in RV parks or anyplace else with a power outlet, so a large battery bank and a good alternator are essential for us; for coaches that mostly stay plugged in, it's not so much of an issue.

...  Converting a bus using 12v is much easier because everything you buy that is DC is readily available in 12v.

That part is true -- finding 24v items takes a little more time and patience.  And even just ten years ago, it would have been a much greater challenge, but in today's Internet-enabled world, 24v items are readily available on-line.  FYI, 24v is a very common voltage in Europe as well as in the marine market.

...  The RV industry doesn't use 24v anything, so you pay dearly for 24v lights and pumps. ...

Actually, the lights don't cost you anything extra if you've planned for it ahead of time.  Incandescent lights don't distinguish -- there's no such thing as a "12v" or a "24v" fixture, they are all rated for any voltage up to 30.  You just need to replace the 12v bulbs with 24v items; I think that cost me an extra $50 for all the lights in my coach.  LED items just get wired in pairs.  Fluorescents, if that is your preference, require a different ballast, which might add a bit to the cost, but probably all the lighting loads in your coach added together are well below the capacity of even an inexpensive equalizer.

Pumps and other motor-driven items will cost you from 20%-50% more for 24v models in some cases.  In many cases, there is no upcharge because the motor itself is smaller; this is true for coolant pumps, blowers, and the like.

In short, if you start out with a 24v coach, your complete BOM to install a 24v house system will be less expensive than the complete BOM to install a 12v house system.  The only way a 12v house system will cost you less money up front is if the coach is a 12v coach to begin with.  This is not a matter of speculation, it is demonstrable.

I will even go so far as to say (as I did in the article), that for some folks, it even makes sense to install a 24v house system on a 12v coach.  For example, if your plans require more than 3kW of inverter capacity, or if you plan for more than about 10kWh of battery capacity, it will almost certainly be cheaper and easier to go with a 24v house system even though it means adding a 24v alternator.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Geoff]

I knew Sean would have all the answers, but personally for my conversion I don't need all the extras 24v would give me.  I think my setup is cheaper, and meets all my needs.  My inverter works fine with my battery bank, and that 160amp 12v alternator will run the front air conditioner with the 2500k inverter.  My diesel generator takes over if I need more air, or want to use the microwave/convection oven for extended times.  I know there are some Prev'os out there that use two Trace SW4024 for 240 volts.  I don't need that either.  And those 50DN's are great when they work, but are a hand-grenade waiting to go off and take your engine gear train with them..  I'm glad to be rid of mine.

Hope this helps people that have to make that 24v-12v choice, and do a price comparison first.

--Geoff
'82 RTS AZ