24V bank, draw 12V using switching, diodes, etc

[skyking]

Greetings! Glad to find such a place.
I am designing a future system to go with a ground up travel trailer build. What I have learned over the years:
A pair of 6V golf cart batteries in series are the best, if you can design for it and handle the extra weight. If you want more storage, then two sets of series 6V.
Can I design a system that lets me use the far more economical ( $ per watt )38~40 VOC house panels and charge a 24V bank, and then tap 12V at the battery bank level of things?
It seems quite do-able, but I expect if it isn't you can explain why.

[bcroe]

All you need is the right MPPT charge controller for your batteries, 12V or 24V. Don't think about using a 12V battery tap
on 24V batteries, that will ruin them in short order. Bruce Roe

[Logan5]

You use a common 24v to 12v step down converter. 20 to 50 dollars. Never bond a load to the bonded post of any series battery bank.

[inetdog]

First, it is not any problem using higher voltage panels with a 12V battery bank as long as you use an MPPT type charge controller (CC). It will reduce the voltage and increase the amperage of the DC as needed. If you get a CC with a high enough input voltage range you can even put two or more panels in series to simplify the wiring.

Second, a higher voltage battery bank (6 or 12V batteries in series) is easy to use if your primary loads will be driven from an inverter.
But if you have a significant number of DC loads, like water pumps, lighting, etc,, and they are currently using 12V DC there is no simple and cheap way to get 12V from a higher voltage battery bank.
Diodes, or even switching, will not let you connect both 12 and 24V loads at the same time or charge at 24 and draw at 12 at the same time. The problem is that when you tap just 1/2 of the battery bank you are draining only half of your batteries while charging all of them at the same rate. That will quickly cause unbalanced State Of Charge (SOC) among your batteries.
There are specialized 24 to 12V converters that draw exactly as much power from the upper half of the bank as from the lower half of the bank, but they are expensive. There are also 24V to 12 converters which are basically just DC to DC power supplies. They sacrifice efficiency doing the conversion but will give you nicely regulated 12V DC output. You would use a 24V input inverter for large AC loads.

[Sunking]

That is what MPPT Controllers are designed to to do. It allows you to run much higher voltage via using less expensive GT panels and wiring them in series. Last thing you want is anything in parallel batteries or panels. Just make sure you are buying a real MPPT controller and note the Voc input limit. You can charge a 12 volt battery with a 600 volt panels.

[skyking]

I too prefer no parallel batteries, but I may want that much capacity. In the portable scheme of things 4 golf cart batteries are attractive.
I had not thought of a 24~12 converter. There are only a few loads that cannot be done in 24V. All things equal I'd rather run a high voltage bank if I can. Wiring efficiency, etc.
I was planning a marine compressor style refrigerator, and those run on12v or 24V. That's the big amp user.
The water pump can be 24V
I plan on making my own lighting, so that is easy to do 24V
The only loads that are not easy are about 3 amps for the furnace fan, and milliamps for the gas hot water heater ignition. A few amps for cigar adapters for charging phones and such.
The problems are, 4 batteries. I would be committed at that point to 24V. There would be no going back really.
EDIT: The only planned large inverter load is the microwave. inverting from 24V surely has to be better than 12V

[skyking]

First, it is not any problem using higher voltage panels with a 12V battery bank as long as you use an MPPT type charge controller (CC). It will reduce the voltage and increase the amperage of the DC as needed. If you get a CC with a high enough input voltage range you can even put two or more panels in series to simplify the wiring.

Second, a higher voltage battery bank (6 or 12V batteries in series) is easy to use if your primary loads will be driven from an inverter.
But if you have a significant number of DC loads, like water pumps, lighting, etc,, and they are currently using 12V DC there is no simple and cheap way to get 12V from a higher voltage battery bank.
Diodes, or even switching, will not let you connect both 12 and 24V loads at the same time or charge at 24 and draw at 12 at the same time. The problem is that when you tap just 1/2 of the battery bank you are draining only half of your batteries while charging all of them at the same rate. That will quickly cause unbalanced State Of Charge (SOC) among your batteries.
There are specialized 24 to 12V converters that draw exactly as much power from the upper half of the bank as from the lower half of the bank, but they are expensive. There are also 24V to 12 converters which are basically just DC to DC power supplies. They sacrifice efficiency doing the conversion but will give you nicely regulated 12V DC output. You would use a 24V input inverter for large AC loads.

Thanks again for the converter idea. That may be the ticket. Since it is a ground up build going 24V on the water pump is the only oddity that might bite if it goes out on the road. Other than that, the fridge is either/or 12v/24v and 120VAC as well. With 4x 225AH 6 volts It is likely I am overkilling it, but not having to run a genset is priceless, IMO.

[skyking]

This has been informative. All the RV sites try and sell these lower voltage panels at greater than $2/watt, and all it takes is a decent MPPT CC to utilize the sub $1/watt big panels. The only consideration is panel size.
This panel fits the size requirements for me. I know nothing about it though, it may be so much junk.

http://www.wholesalesolar.com/1530190/topoint/solar-panels/topoint-jtm190-72m-silver-mono-solar-panel

[Sunking]

I too prefer no parallel batteries, but I may want that much capacity.

Real simple if you need 450 AH batteries, buy 450 AH batteries, not 225 AH batteries silly. There is no reason in the world to use battery panels other than to milk your wallet.

[skyking]

something like this?

http://sunelec.com/batteries/deep-cycle-battery-rolls-s-600-6v-450-ah.html