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Old 10-26-2020, 01:32 PM   #1
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How to get 24 volts from the coach batteries

I have four six volts batteries connected in series and parallel to provide 12 volts output to the Inverter. Can I retain the existing batteries configuration for powering all 12 volts accessories in the motorhome and add another configuration which would have the four coach batteries connected in series to get 24 volts to a dedicated device requiring that voltage.

If it is possible to have two batteries configurations at the same time, would you simply connect the four batteries in series and run a line (24 volts) to the dedicated device?

If this is not possible, do you know another way of getting 24 volts continuously without interruption to the device.

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Old 10-26-2020, 01:41 PM   #2
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Quote:
Originally Posted by Gaetan Lavoie View Post

If this is not possible, do you know another way of getting 24 volts continuously without interruption to the device.

"Device" isn't very descriptive. You might be able to use a 12V-24VDC converter, but it depends on the power requirements of the device you're intending to power.

https://www.amazon.com/Cllena-Conver...3737653&sr=8-5
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Old 10-26-2020, 02:06 PM   #3
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I too suggest a converter. Used lots of them to down convert 24/12 in crop dusters. The other way is just as simple.

Some old trucks used to do a 12/24 setup but I do not recall seeing a schematic for it. The 24 volts was used to turn the engine over and it reverted to 12 volts.
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Old 10-26-2020, 02:14 PM   #4
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To answer your question, No. You can't get 12 V and 24 V off the same four 6 V batteries at the same time.
If you had two 12 V batteries, it would be possible to connect to one battery for 12 V and the two in series for 24 V, but you would have to do disconnects for charging purposes.
When you have batteries connected in series, connecting them at the same time in parallel creates a dead short circut across the batteries. The same is true if you start out with parallel connections and try to add a series connection at the same time.
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Old 10-26-2020, 02:38 PM   #5
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Doesn’t this work ?

Click image for larger version

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Size:	88.0 KB
ID:	306070
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Old 10-26-2020, 03:20 PM   #6
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That works, but probably not long term. The OP wanted 12V parallel for the power to the invertor and 24V for something else. Doing it your way will work but only provide 1/2 the power to the invertor that a parallel connection would provide using all the batteries.
Charging will be another problem since two batteries will have a much smaller load than the other two. All the current needed to charge the depleted batteries will have to pass through the charged batteries which will over charge them, basically boiling them or cooking them out. Those two will have a much shorter life span. Dual 12V chargers may be an option.
Usually, the purpose of having multiple batteries is to use them all for the power they provide when used combination.
I think you could find your type of circuit used when only a very small amount of power is needed at 12V, but nothing like what would be needed to power an invertor.
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Old 10-26-2020, 03:40 PM   #7
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Actually, charging at 24 volts while drawing off 12 volts from them will not overcharge any batteries.
It will undercharged the two used as 12 volts.

Had it happen when my operators tapped in 12 volt radios to the 24 volt system.

In a few weeks, the 12 volt end was dead.
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Old 10-26-2020, 06:47 PM   #8
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This has been done in vehicles of all types for years (boats, commercial busses, ambulances, etc), you have to use an equalizer on the batteries. Look into Vanner Equalizers, for example. They do take some power all the time, about half a watt for one I looked at.
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Old 10-27-2020, 08:06 AM   #9
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Actually, charging at 24 volts while drawing off 12 volts from them will not overcharge any batteries.
It will undercharged the two used as 12 volts.

Had it happen when my operators tapped in 12 volt radios to the 24 volt system.

In a few weeks, the 12 volt end was dead.
When doing that I am sure you found the resolution was a good cutoff switch. All too often the (operators) neglected to turn off the radios and ended up with sick battery banks. Timers are often a good solution. I am positive you know this but an old radio tech just can't resist adding his two cents.
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Old 10-27-2020, 08:07 AM   #10
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What device needs 24 volts? There might be other solutions based on the knowledge of the current and voltage requirements of that load.
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Old 10-27-2020, 08:57 AM   #11
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When doing that I am sure you found the resolution was a good cutoff switch. All too often the (operators) neglected to turn off the radios and ended up with sick battery banks. Timers are often a good solution. I am positive you know this but an old radio tech just can't resist adding his two cents.
Actually the solution was a plug and play 24 to 12 volt converter from Cat. Sometimes you need to look in the book.

We used them in other machines by cutting off the plugs.

They never used the battery disconnect.
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Old 10-27-2020, 09:24 AM   #12
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The device is a Diesel Parking Heater that can draw as much as 10 to 12 amp when the glow plug is on. Since i need to run the inverter for the residential fridge on the road, I am inclined at putting a 110 AC Power Supply 24 volts 20 amp over a Step up Converter 12 to 24 volts which are not that reliable based on what I have read.
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Old 10-27-2020, 10:47 AM   #13
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Actually the solution was a plug and play 24 to 12 volt converter from Cat. Sometimes you need to look in the book.

We used them in other machines by cutting off the plugs.

They never used the battery disconnect.
We too used lots of converters. Airplanes etc. However those electric cart/cars often had 72 volts so tapping was about the only solution in those days. They tapped them themselves as well. Sometimes life is compromise.
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Old 10-27-2020, 10:52 AM   #14
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Doesn’t this work ?

Attachment 306070

That diagram, as drawn, absolutely WILL NOT WORK!


Why? Because whatever 12v device you are powering will be floating approx 12volts above the frame ground. Either the device would short out the other 2 batteries if it contacted frame, or it would have full 24v across it if frame contact was made.


If you want to use this configuration, it is important that the 12v device ground/return be the same as the rest of the battery bank. Use the bottom 2 batteries, not the top.
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