12 and 24Vdc from one bank?

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Happyjax

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Okay, maybe it's my goofy brain but I have a hypothetical question for you brainiacs:)

As most of us know, when you use an inverter on a 12v system you are drawing 10 times the current you would draw from your home. So a simple Keurig which draws about 10Amps for a cuppa joe will draw 100 Amps from your battery bank. OUCH!

Soooooo my thought (okay some smoke involved.....) is how practical or possible would it be to have 2 large 12v batts in series (not parallel duh) hooked up to 2 solar panels in series with an MPPT controller giving you a 24V bank for the inverter and also have one of the batteries tapped off for 12V operation?

I know that the battery supplying the 12V would be used at a different rate then both together but how much would it be affected and could the same battery supply 12V to an output while remaining part of a 24v system?

Is it just me or has anyone else every had this hare brained idea?????

Have fun with it, I can take the punishment :)
 
Sorry I meant that.... 2 is series but one tapped off to be a single 12V supply while still being in the 24V setup. I'm crazy aren't I :)
 
From a theory perspective, that will work. In practice, I am not sure how effective it would be. The batteries themselves might be an issue. There may be information on the web that has more to offer than an old one wire electrician like me.
 
It can work fine. The Trolling motor in my boat is an old 12/24 volt model. It has 3 speeds and is is hooked up to both 12 + 24 volts. So it's like having 6 speeds. The reality is that I rarely use the 12 volt setting. It draws hard on a single battery. Then that one want's to draw down the other. It works but the 12 volt draw will cause battery inbalances in the system.

Hook up is simple. Neg - cable to 1st battery neg terminal.
Pos + terminal of 1st battery to Neg - of 2nd battery.
Pos + terminal of 2nd battery to Pos cable to 24 volt load.

12 volt tap comes off the Pos + terminal of 1st battery.

I've thought about the coffee maker thing myself. I carry a mister coffee for locations and campgrounds that have shore power. But I've found that a decent percolator on the stove works fine. Just a little more clean up. Some folks sew the coffee inside a coffee filter to cut down on cleaning up grounds.

I personally intend to stay as far away from invertors as I can.
 
I don't know for sure...and maybe I'm misunderstanding your plan...but:
Doesn't an ordinary inverter "expect" to be fed 12 volts DC? So it might not work -- maybe :"die" -- if fed 24 volts?
 
That was kinda what I was thinking Squatch but never heard of anyone doing it. Just a thought that made me curious and since I didn't want to die from curiosity I thought I would first....lol

I do so love my Keurig but will probably have to forego or get a generator. Still need an inverter for my laptops and camera gear but nothing there will draw 100 amps.......
 
Personally, I prefer a filter cone or the Aerobie Aeroflow for camping. However, I know many like the convenience of the Keurig coffee makers.
Have you looked at this substitute for a large power hungry machine for camping?
Even cheaper at Amazon. Less than the cost for the connecting cables for creating your 12/24 volt battery system.

Paul
ps. If you boil water the night before & store in a thermos, you wouldn't even have to wait for water to heat in the morning for that first dose of ambition.
 
In addition to the (possible) problem of feeding 24 volts to a 12-volt inverter... is there a big advantage to drawing fewer amps -- at a higher voltage? Either way, you're drawing the same amount of power from the battery. I guess you'd need smaller-gauge wires at the lower current...that would be something.
 
Interesting idea. Technically, I think it can be done, if the inverter is designed to accept 24v. There are efficiencies to be had with a 24v system, and I understand that big RV's and home PV setups use them.

That said, is there a problem with drawing 100A for a short time? Your starter motor draws many times that when firing up the truck engine, so it isn't an unusual scenario.

Your batteries will become unbalanced tho, which can dramatically reduce their life. I know my Victron solar controller/monitors have the option of detecting very small imbalances between my 6v batteries, so they can't be good.
 
I was just thinking that keeping the high amp draw down would be good for the batteries (less heat) but then there are new issues added so it probably would not work nearly as well in real life as it wanted to in my head :)
 
The amp flow is the same in both cases, 5 amps per battery. Two batteries in parallel, 12 volts out, each supply 5 amps to the 12v-120v inverter, 10 amps total/120 watts. The amp flow is 5 amps through the first battery and then 5 amps into/out of the second battery in series, 24 volts out/120 watts. Any efficiency gains depends on the inverter design, maybe yes/maybe no. 24 volt motors are usually more efficient than 12 volt. Large RV's routinely charge batteries in parallel, 12v, and consume current in series, 24 v. My grandfathers Class A motor home had a 12 volt system, a 24 volt system, a 50 volt (48v?), and a 120 volt system. Just think of the wiring.

edit: I am focusing on battery heat generation which is significant not wire heat production. Proper wire sizing should eliminate wire heating.
 
We're in the French press camp too. But who wants to be boiling water at 5:)0 AM (when I get up) and Sioux sleeps till 7:00?

So the night before we pre-heat a very high quality thermos we got from REI with hot water, make the coffee on the french press before we go to bed and voila- steaming hot coffee at 5:00 AM! (And without disturbing the sleeping lion... I mean the missus!)

Then after breakfast we do it again to have hot coffee for lunch.

Works for us and with zero amp draw! :)
 
I have an old glass lined thermos I have used for years. When traveling or going for an overnighter. I preheat it with hot water. Then fill with coffee. Then I wrap it in a couple of old rag bath towels. I can leave this in the truck or the boat on a cool night and still have hot coffee in the am.
 
24VDC vehicle people say to never, ever tap between batteries as the dissimilar discharge causes all sorts of problems and usually results in one battery dying before the other. There are 24VDC to 12VDC converters out there specifically for this reason.
 
If I were to attempt to wire up what you suggest, I'd have a smoking hole in the ground! I have to subscribe to the KISS principle when it comes to electricity!

If you are able to work out the specifics and perform the installation, please post how you achieved it.
 
It could be possible, but as others have stated not an ideal situation, and not worthwhile unless you have a lot of money, free time, and like to experiment.

2 x 12v batteries interconnected with a large diode. You would need 2 separate 12v chargers, 1 for each battery. The diode keeps them separate on the charging circuits. 12v loads should be split between the 2 batteries to keep them as balanced as possible. Not sure how the chargers would handle it if any 24v loads are connected during charging.

We use the large thermos idea, heat water at night, have ready and hot in the AM. We use the Stanley growler, that may or may not have been previously filled with a fermented beverage...
 
I was thinking that the batts would be charged as a 24V bank running a 24V inverter and if I had a need for a 12 V item I would just take it off one battery. The 24v system would cut the amp draw in half creating less heat??? and maybe giving the batteries a bit longer life. I know many house systems run off 48V systems with 48V inverters. Really makes cable runs easier :)
 
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