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Running a 24v acessory from a 12v car.

 
 
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Nathaniel
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Joined: 13 May 2003
Odometer: 17901
Location: North, North Yorkshire


1979 Suzuki LJ

PostPosted: Tue Feb 05, 2008 10:34 pm    Post subject: Running a 24v acessory from a 12v car. Reply with quote

Ok guys, gather ones brainpower!

I've been thinking about this, and I MIGHT have figured it out, but need other's ideas...

I've seen the convertors that make 12v into 24v for the truck drivers but I can't see any rated to 25A +?

I need an elegant soloution - I have a 24v heater I want to fit to the Fourtrak, it will pull about 25A for a max of 3 mins on startup then about 3A thereafter.

I will be fitting another battery to the back and want to fit a switch so that I can have it in 1 position so it charges the extra battery off the cars battery and to be able to flick the switch which takes it off charge and makes the battery output 24v to power the heater.,
I'm not bothered about outputting and charging simultaniously so that makes it easier!?

I have a couple of 30A 5 pole relays here...

Can it be done? Can you draw me a picture? Razz

Nat

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Anthony
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Joined: 05 Mar 2007
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PostPosted: Tue Feb 05, 2008 11:03 pm    Post subject: Reply with quote

The Toyota 80 Series Landcruiser 4.2 TD is nominal 12V but on start up uses 24V. Their are 2, 12V batteries linked in parallel for normal running. When starting, change over style relays put the 2, 12V Batteries into a series connection making a nominal 24V for driving the starter motor. One started the relays revert the 2 batteries to parallel and 12V.

I hope that makes sense. Perhaps this system might help you achieve what your after. Smile

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:)
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Joined: 06 Feb 2006
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Location: Norfolk


1995 Land Rover Defender

PostPosted: Wed Feb 06, 2008 7:56 am    Post subject: Re: Running a 24v acessory from a 12v car. Reply with quote

Nathaniel wrote:
I've seen the convertors that make 12v into 24v for the truck drivers but I can't see any rated to 25A +?

That is at least 600W (720W if you want to charging voltage), which would be a big and expensive DC-DC converter.

The two battery idea seems to be the one to go for. I suggest two possible solutions:
1. Use a DPDT (Double-Pole-Double-Throw) relay to do a parallel/series switch as described by Anthony. You must use a break-before-make relay - most are - but it would a dead short if not! You could use big toggle switch with a similar configuration, but I'm not sure they are available rated at 25A. This is the wiring:

As shown you only get the 24V when the relay is in that position, otherwise you get nothing. Alternatively, you could tap the + terminal of the AUX battery which would then give 24V or 12V depending on the relay position.

2. Permanently wire the batteries in series to give 24V. Use a small DC-DC converter to take power from the main battery to charge the second. This has the advantage of allowing continuous charging, but would be more expensive due to the DC-DC converter cost. You would need a 12V in and an isolated 13.8V out. If the output could be tweaked to 14 or 14.2V that would be better. It doesn't need to be rated at a high current, 3 to 5A output would probably be OK but must be current limited, industrial ones always are.

You will need to switch the DC-DC off when the IGN is off otherwise the main battery will be discharged, while it continuously chargers the AUX.

Either of these ideas any use?
--
Tim.

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Nathaniel
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Joined: 13 May 2003
Odometer: 17901
Location: North, North Yorkshire


1979 Suzuki LJ

PostPosted: Wed Feb 06, 2008 12:48 pm    Post subject: Reply with quote

Thanks Tim, that top one is exactly what I was thinking of, I just needed to check that I wasn't gonna have (another) Elecrtical fire Laughing

Anthony, I've wondered before how Toyota got the starter to run on 24v and the lights on 12v...

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Anthony
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PostPosted: Wed Feb 06, 2008 1:08 pm    Post subject: Reply with quote

It works very well, a bit more complicated than just starting on 12V as well but is reliable enough. The only problem I have had is when the batteries lost some charge and they were a bit old and I recall I may have had a cell down, no fault of the system. I just got clacking or nothing at all, go away and curse, come back and it would then work. Two brand new batteries and absolutely no problem at all, one would not even know it was a duel 12/24V system. Very reliable. Very Happy
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Nathaniel
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Joined: 13 May 2003
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Location: North, North Yorkshire


1979 Suzuki LJ

PostPosted: Wed Feb 06, 2008 7:37 pm    Post subject: Reply with quote

Now for the next chalenge, where the hell do I get a 12v 25A DPDT Realy from????

I have 2 SPDT 30A Realys at work, I expect these will work ok, but I'd like less wires!

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Joined: 06 Feb 2006
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1995 Land Rover Defender

PostPosted: Wed Feb 06, 2008 8:35 pm    Post subject: Reply with quote

Nathaniel wrote:
Now for the next chalenge, where the hell do I get a 12v 25A DPDT Realy from????

I have 2 SPDT 30A Realys at work, I expect these will work ok, but I'd like less wires!

NO you must not use two separate relays.

It has to be one becasue if one was to change even slightly before the other you have a short circuit across the battery. By using a single DPDT relay the two contacts have to move together becasue they are operated from the same armature. This point can't be over emphasised, it is too risk to use two separate relays, they also have to be break-before-make.

THIS (RS:376-880) is the sort of thing you need.
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Tim.

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Nathaniel
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Joined: 13 May 2003
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Location: North, North Yorkshire


1979 Suzuki LJ

PostPosted: Wed Feb 06, 2008 8:57 pm    Post subject: Reply with quote

Thanks for that link, I couldn't find one!
Have ordered one.

I suspected there would be an issue aout using 2 relays, especially if one failed, you know me and FIRE! Evil or Very Mad

Thanks once again

Nat

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