24v Battery Bank for 12v Load

KhellSk · March 2019

Is it healthy for batteries to do that ?

PNW_Steve · March 2019

I have found 24v water pump and USB charging jacks.

I have pairs of 12v LED lights wired in series to run from 24v.

I will run the remaining 12v loads from a 24v to 12v converter. https://www.amazon.com/uxcell-Converter-Regulator-Transformer-Waterproof/dp/B01LYK6G2Y/ref=sr_1_4?hvadid=177299294839&hvdev=t&hvlocphy=9033806&hvnetw=g&hvpos=1t1&hvqmt=b&hvrand=15785267131845784753&hvtargid=kwd-1142292102&keywords=12v+to+24v+dc+converter&qid=1553287278&refinements=p_72%3A2661618011&rnid=2661617011&s=gateway&sr=8-4&tag=googhydr-20

BB. · March 2019

If you have a 24 volt battery bank (say 2x 12 volt or 4x 6 volt in series), and "tap" 12 volts off the center of the battery bank--Discharging does not "hurt" anything--Other than the "low 12 volt" battery is discharged, and the "high" 12 volt battery is not discharged.

The "damage" comes when it is time to recharge the battery bank... If you use a single 24 volt charger and need to recharge the battery bank... The "high" 12 volt batteries will be "overcharged" and the "lower" 12 volt batteries will be undercharged.

There are a couple of solutions. First would be independent 12 volt chargers. One on the "high bank", and the other 12 volt charger on the "low bank". You have to be a little careful with the "high" 12 volt charger, especially with solar chargers... That means the "negative" lead on the high charger has +12 volts on it... And should not be connected to a computer or networked to the second charge controller because of "offset" ground voltage (isolated comm circuits do wor).

The other is to get a 12/24 volt battery balancer. These are commonly used on RV bus conversions and others that have 24 volt vehicle battery bus and 12 volt "house" battery bus. A balancer keeps both the High and Low 12 volt batteries at matched voltage. And the balancer will "borrow" current from the high bank and move to the low bank during discharging--And if you charge the low bank, it will move current back to the "high bank":

http://www.eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/Transportationsolutions/PowerConversion/Equalizers/24Vto12VBatteryEqualizers/index.htm

Here is an RV site that talks about some of the options/issues:

http://bus.getdave.com/Docs/12Von24V/

The short answer--Avoid "tapping" 12 volts from a 24 volt battery bank. Try to design to do something else (24 volt loads, 24 to 12 volt DC to DC converters, use a 120 VAC inverter to power your loads with AC power, etc.).

-Bill

KhellSk · March 2019

https://forum.solar-electric.com/discussion/comment/396379#Comment_396379

ant you just put ur load on terminals of half you bank for 12v output ?

KhellSk · March 2019

ok thnx you all

BB. · March 2019

No, you cannot just put the 12 volts loads on a 1/2 of a 24 volt battery bank.

The "high" and "low" batteries will be discharged and charged unevenly.

Regarding the 12 volt LED lights in series... I would suggest getting DC LED lights that are rated for 24 volts:

https://www.amazon.com/12V-24V-Voltage-Incandescent-Lighting-Daylight/dp/B07DKJT2HX

https://www.amazon.com/LEDwholesalers-Waterproof-12-24V-ETL-Listed-1533WWx6/dp/B079G8P8FQ

There are quite a few 24 VDC (and sometimes VAC) lamps and fixtures out there.

The reason I don't suggest that you put two 12 VDC lamps in series--The electronics may not be "stable" when put in series (current mode switching power supply?). If they are simple resistor ballast, probably not an issue.

If your 2x 12 volt LED bulbs in series are working OK... Then why not use them.

-Bill

706jim · March 2019

One of the problems you are faced with is the limited availability of small 24 to 120 volt inverters. If you could find one, that would be my solution; just run it 24/7. As has been pointed out, a large inverter consumes power just waking up and this is what you are trying to avoid. Faced with a similar dilemma, I elected to keep my 24 volt inverter running constantly. This old Trace unit idles at 350ma so doesn't waste a lot of energy out of standby mode yet solves the problem of keeping my refrigerator electronics awake.

PNW_Steve · March 2019

Limited availability?

News to me, there are a few obscure manufacturers that offer a pretty good selection: Trace, Xantrex, Cotek etc....

The sponsor here has 59 different ones to choose from: https://www.solar-electric.com/residential/inverters.html?nav_inv_input_voltage=437

KhellSk · March 2019

I have to keep my loads on 12v. I refuse to use an inverter. I buy car charger for the laptop, drills, 12vto 5v usb adapter for my phone, flashlight and frs radios. Everything in the 12v is readily available, i have no waste and expenses from the inverter that might died at anypoint. Also allows to keep the investement on the solar panels rather small and physical space occupied.

Seems like the situation will resolve itself when ill get myself modern "12v"panels with higher Vmp and put the array in a field rather than in the wood.

Thanks for all the help boys.

mountainman · March 2019

As@BB suggested 24 to 12 DC converters. are a fairly inexpensive option. But still you have some loss converting voltages. One thing I've had issues with using DC is the range of voltages from a low battery soc to charging voltage 15or more in cold weather. Fried my Samsung phone. Rv Heater and gas refrigerator control board's. IMHO A converter be it 24 to 12 or 24 to 120 AC has more stable voltages. Much easier on equipment.

BB. · March 2019

There is not that much difference between a DC to DC converter and a DC to AC converter (inverter). The DC to DC units tend to be a bit more efficient than AC inverters. But you are still left with distributing low voltage DC (12 or 24 volts) which has issues with voltage drop (need heavy/short power cables) and you should use property DC current/voltage rated fuses/breakers/switches. DC tends to sustain arcs much better than AC.

In the old days where AC appliances with cheap utility power wasted lots of energy--Using the DC versions gave you a very nice efficiency increase.

Today, with Energy Star rated appliances and the whole green energy kick--The AC versions of appliances today don't use much more energy than the DC versions.

And wiring AC through a cabin/home/longer distances is so much easier and cheaper with 120 (or 240) VAC vs doing the same thing at 12 VDC--That the "downsides" of using an AC inverter vs 12 VDC battery bus (and the 10.5 to 16.5 volt working bus voltage) or 24 to 12 VDC down converters--The AC systems just seem to work.

But if you want or need 12 volts, you want/need 12 volts... Not going to argue that.

-Bill

KhellSk · March 2019

@BB. dont have appliances and never will. I dont have wire through my walls and never will.

(Grumbling, can be skipped)

Appliances are a racket making it based on programmed obselance. They couldnt sell you 2 compressor for one fridge so they made it weak thise covering a second one "covered" buy the warranty that you pay. Fridges can be built in the ground, you can wash clothes in a tub or bucket. Anyway this isnt an economical forum but I lost faith in the corporations and im gonna limit the amount of things I buy from now on. Someone has to say Enough.

(Sorry about that)

Thanks for help tho. Its okay ill just get mordern panelseventually fixing my problem.

KhellSk · April 2019

@BB. @mountainman

Let say I was to put a 24v to 12v converter. Would it act as a Regulator at the same time ?

BB. · April 2019

K,

What do you mean by "converter"... The RV world sort of has a different meaning vs what I think you are asking... A 24 volt to 12 VDC "buck type" or "down converting" DC to DC switching power supply (aka "converter") is, more or less a relatively to the AC Inverter.

The DC to DC down converter takes a higher voltage (probably something like 5-7 volts or more--Depends on the exact model/design) and converts that to 12 volts (or 13.8 volts is a "favorite voltage" for the HAM radio world-->=13.8 volts = "full power" transmitter rating).

Generally, the DC to DC converters will regulate the output voltage and/or current pretty accurately (many DC to DC down converters have an adjustable output voltage and even an adjustable current limit).

If you want 10 amps at 13.8 volts, you can set that. And the input voltage can run from ~19 volts to 45 to >70 VDC--Again, depending on brand/model. And, normally, the output voltage (and/or current) is very well regulated -- And wide variations in Input voltage will not have much effect.

They typically are something like 90-95% efficient (a little bit more efficient than an AC inverter, but not much).

Because these are switching power supplies, they have limits in their output current (not like a battery that can output 100's of amps of surge current), and they can be electrically noise (the supplies may radiate radio frequency interference and/or there can be noise on the DC output too).

If you are are working with HAM AM radio, have high quality audio gear, or like listening to AM broadcast radio, you may need to be careful about what DC to DC converter you choose. And you may have to do things like build a metal box/copper screen around the supply and filter the output a bit.

Most DC to DC converters are "one way" devices. They will take power from the >19 volt bus, and downconvert to the 12 VDC bus.

If you tried to feed power backwards (12 volt solar panel on 12 volt bus, it will not feed back power to the 24 volt bus). There are bidirectional converters, but a bit on the rare side.

And since you are drawing 24 volts from a 24 volt battery bank, there is no "out of balance" issues on the 24 volt bank (other than the normal stuff we see that is usually addressed by equalization charging every month or so, if needed).

24v Battery Bank for 12v Load

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