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Old 04-19-2018, 07:30 PM   #1
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Why 12vdc over 24vdc?

I see everyone running 12volt systems in their skoolies. Wondering why?

24-->12 step down converters seem pretty cheap
You can run a smaller MPPT Charge Controller saving $$
24Vdc is supposed to be more efficient right?
I guess number of batteries in your bank may be a hinderance. Can't run 6 6v batteries and make 24v.

Is there any other reason not to run 24v?
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Old 04-19-2018, 07:35 PM   #2
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Originally Posted by dspizzle View Post
I see everyone running 12volt systems in their skoolies. Wondering why?

24-->12 step down converters seem pretty cheap
You can run a smaller MPPT Charge Controller saving $$
24Vdc is supposed to be more efficient right?
I guess number of batteries in your bank may be a hinderance. Can't run 6 6v batteries and make 24v.

Is there any other reason not to run 24v?
I would hazard to guess it's because the vast majority of them are factory built for 12v. It's sort of the standard of the automotive industry, and manufacturers and school districts like being able to buy widely available, reasonably priced parts as opposed to special-order 24v parts.
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Old 04-19-2018, 07:38 PM   #3
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You can run 6V batteries in a 24V system.

You string 4 in series, then run as many of the strings in parallel that you like.
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Old 04-19-2018, 07:42 PM   #4
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The question was more for the house batteries. Why dont people run 24v systems more often?
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Old 04-19-2018, 07:52 PM   #5
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The question was more for the house batteries. Why dont people run 24v systems more often?
Because many more appliances and fittings are available for 12V, and at more reasonable prices, than can be found for 24V service.
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Old 04-19-2018, 08:20 PM   #6
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Makes sense. I kind of talked myself out of it simply by the number of Batteries I can have. Was planning on 6 6v. So 24v seems like a no go from that alone...
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Old 04-19-2018, 08:41 PM   #7
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Makes sense. I kind of talked myself out of it simply by the number of Batteries I can have. Was planning on 6 6v. So 24v seems like a no go from that alone...
Yes, you would need either 4 or 8.

Remember that batteries have the same amount of actual power in them regardless of the voltage it is delivered at.

So 4 x 6V 225 amp hour batteries have the same power whether they are configured as 12V @ 450 Amp hour, or 24V @ 225 Amp hour.

The measure that ignores voltage is kilowatt hours. If you run the numbers you will find they are the same.
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Old 04-19-2018, 09:10 PM   #8
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Right. I was more interested in regards to voltage loss, and wire sizing.

As I understand it, its much easier to have higher efficiency with a higher voltage. But maybe it doesn't make a big enough difference for the price.
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Old 04-19-2018, 09:35 PM   #9
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Right. I was more interested in regards to voltage loss, and wire sizing.

As I understand it, its much easier to have higher efficiency with a higher voltage. But maybe it doesn't make a big enough difference for the price.
Your understanding is correct. That's why long-distance transmission lines run anything up to 400 000 volts.

When we convert a bus it's a bit different. Do your calculations based upon a 3% maximum voltage drop then go up at least one size on the wire. We aren't buying enough for it to break the bank.

Vehicle manufacturers buy the stuff by the mile, so for them it makes sense to use the smallest wire that can be made to do the job. We have a little more luxury than that.
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Old 04-20-2018, 07:05 AM   #10
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In general the higher voltage systems are more efficient, not only in copper losses. Higher voltage means lower amperage for the same load.
The brushes in your starter / wiper / fan motor can live longer because of reduced current.

the switching transistors , mosfets, igbt's have less voltage drop with a lower current as well. so if you heavily rely on an inverter / solar charger / solar then higher voltage would make a lot of sense.


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Old 04-20-2018, 10:07 AM   #11
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A philosophical question

I had been thinking of this recently. For those of you who went with a higher-than-12 volt system, do you miss that you cannot interconnect with your chassis systems. I had been thinking that one compelling reason to go with a 12v storage system for the house was that you were in effect carrying a great honking set of jumper cables around with you.

Is this wrong? If you had say 450-600 AH @12v to work with, would it damage your house batteries to pull 800A from them to jump start yourself?
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Old 04-20-2018, 11:11 AM   #12
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Is this wrong? If you had say 450-600 AH @12v to work with, would it damage your house batteries to pull 800A from them to jump start yourself?
For occasional use in an emergency, your house batteries can start your engine.

However, they are not designed for a heavy rate of discharge, and doing it regularly would hurt them.

Vehicle start batteries are designed to give up a small amount of their capacity, very quickly. Deep-cycle batteries are designed you yield their stored capacity more slowly, but more deeply.
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Old 04-20-2018, 03:38 PM   #13
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Vehicle start batteries are designed to give up a small amount of their capacity, very quickly. Deep-cycle batteries are designed you yield their stored capacity more slowly, but more deeply.
Right. My (sort of implied) question is/was, can you damage house batteries by subjecting them to a few short bursts of a very high rate of discharge on an emergency basis? I certainly wasn't suggesting that you assign engine-starting responsibilities to them on an ongoing basis.

To restate, would a house battery bank (assume FLA; t-105s for instance) be damaged by subjecting them to ONE event of discharge at like a C0.25 or C0.1 rate (so to speak) for 10-20 seconds? It's not a piece of info I've been able to tease out of a mfr's website or spec sheet.
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Old 04-20-2018, 04:52 PM   #14
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Right. My (sort of implied) question is/was, can you damage house batteries by subjecting them to a few short bursts of a very high rate of discharge on an emergency basis? I certainly wasn't suggesting that you assign engine-starting responsibilities to them on an ongoing basis.

To restate, would a house battery bank (assume FLA; t-105s for instance) be damaged by subjecting them to ONE event of discharge at like a C0.25 or C0.1 rate (so to speak) for 10-20 seconds? It's not a piece of info I've been able to tease out of a mfr's website or spec sheet.
So ... while not an ideal way to use the house batteries, as I said above, I doubt it will do a great deal of harm occasionally and if it gets you out of a hole it's probably the smart move.
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Old 04-20-2018, 07:21 PM   #15
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Batteries should have a data sheet available in which the manufacturer indicates the maximum recommended discharge rate (see stackexchange: What is a safe max. discharge rate for a 12V lead acid battery?).

Depending on the Ah capacity of the house battery bank, starting off the house battery could range from "do that only when in dire need" to "it's less of a strain for the house bank than it is for the starter battery bank!"
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Old 04-20-2018, 09:22 PM   #16
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Even just DCDC conversion is inefficient of precious stored energy.

And good converters aren't cheap.

Some high-current devices may require higher voltages.

If you don't need it for that, all it gets you is lighter copper gauges, in the end much cheaper and more efficient to just stick with the standard 12V.

Can always boost to higher V if say a winch needs it.
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Old 04-21-2018, 05:10 AM   #17
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For those of you who went with a higher-than-12 volt system, do you miss that you cannot interconnect with your chassis systems.
In my case, both the chassis and house are 24 volt systems. Regardless, I have never needed to connect the two (but never know when an emergency may strike).

I am very happy that I went 24VDC on the house side. I suspect the reason that many folks go with 12VDC is simplicity and stick-to-what-they-know.
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Old 04-21-2018, 06:45 AM   #18
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I am very happy that I went 24VDC on the house side. I suspect the reason that many folks go with 12VDC is simplicity and stick-to-what-they-know.
Can you explain why you are happy with it, what runs on 24vdc, and what you use a step down converter on?
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Old 04-21-2018, 07:31 AM   #19
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Can you explain why you are happy with it, what runs on 24vdc, and what you use a step down converter on?
Sure! It might be easier to see my electrical diagram.


Basically the 3000 watt inverter, water pump, and some of the LED lighting run on 24VDC. I wish it was more but not every component is available in 24VDC (such as the hose/shore power reels, dump valve, some LED lights, CPAP machine, etc.).

I'm happy with it as I was able to run smaller diameter cables/wires (saves money and easier to work with) and the capability of the Morningstart MPPT charge controller is effectively doubled (vs 12VDC).

Looking at the diagram reminds me... (sorry, I forgot), I do have what amounts to a trickle charger from my house to chassis batteries (one way only).
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Old 04-21-2018, 11:46 AM   #20
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I use 24 vdc In Dory, House and starter battery are the same. At the moment two 12 Volt 8D batteries in series.
The advantage is mainly that the efficiency is higher with high current draws like a induction stove or microwave.
Another issue is that putting batteries in parallel is in general a bad idea. There is no way to ensure that all battery strings are helping equally with the total current. At high currents the resistance from cables and battery posts are not negligible resulting in different discharge levels per parallel battery string.
Often when left unused the better battery gets pulled down by poorer battery destroying them both in the process.
But if you are not cooking electric or not using a microwave then it is easier to stay with 12 vdc.

good luck
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