Running 12v appliances off a 24v pv system

Caneman

Planning a SHTF backup/emergency solar power system (pv-cc-battery bank-inverter)... call me a "prepper" if you want, imo i just think we are going to see power outages in the near future ranging from 2 weeks to 3 months at a time and this being "normal" for quite some time... i know, call me a nut job like my wife does anyway, i have some questions i am not sure about:


1) What are the problems of running low amp 12v appliances like lights or fans off of a 24v system? i assume you would need a 24v/12v dc stepdown converter?


2) Any problems with using a 24v dc to 120 ac inverter to run low amp a/c appliances like a laptop, recharge cell phones, a tv, or a fan?


3) If the 24v battery bank dies and can not be recharged, can dc or ac appliances still be run off of the charge controller?

niel

Re: Running 12v appliances off a 24v pv system

1> none as long as the converter and battery can deliver the current properly.

2> no, but just as i stated in #1 it has to be sized properly.

3> no.

techntrek

Re: Running 12v appliances off a 24v pv system

Just make sure you don't tap one 12 volt battery (or 2 6 volt batteries) in a string to get 12 volts - that will cause an imbalance... i.e. problems.

There is one exception to #3. It is possible to run a water pump with a properly sized PV array and specialized pump controller, with no battery. Not the system you have, but it is possible to do something w/o batteries.

Caneman

Re: Running 12v appliances off a 24v pv system

what about using some type of 120v grid tie inverter, like the Sunny Boy 700, could you use this inverter from the PVs to use the 120v ac output?

Cariboocoot

Re: Running 12v appliances off a 24v pv system

Caneman wrote: »

what about using some type of 120v grid tie inverter, like the Sunny Boy 700, could you use this inverter from the PVs to use the 120v ac output?

Nope.
GT inverters shut down without an AC sync signal.
With no batteries to provide consistently available power you can't run any inverter alone. Panels put out a wide variety of current and Voltage and need the batteries to stabilize this output to provide a consistently available power source. The pumps techntrek mentioned are designed to work off this varying DC source; their speed changes with the power available. This won't work with an inverter.

65DegN

Re: Running 12v appliances off a 24v pv system

Technically, I suspect you could get away with running small loads off of an inverter or directly off of DC with no batteries if you replaced the batteries with a large supercap or bank of supercaps. You would need a voltage regulator to insure that the input voltage to the inverter or DC devices never exceeded it's max input voltage.
The advantage to the supercap is that it will last far longer (probably the rest of your life) than a battery but the downside is that it can not store much energy long term. So most of the power would need to be used as it is being made available in real time.
I suspect it will act sufficiently as a resevoir for power to be useful.
Also for long term plans you might look into NiFe ( Nickel Iron) batteries. They have been known to last for 50+ years with just a replacement of the electrolyte (typically a potassium hydroxide solution available as a powder) which can be purchased and stored.

SolaRevolution

Re: Running 12v appliances off a 24v pv system

I call you an "end of the worlder".;)

1. Many 12 volt appliances will "let their smoke out" if you connect them to voltages much higher than about 15v. 24VDC to 12VDC converters are available. Be sure to check for high effciency and low "phantom" loss.

2. No problem. You might want to check out and test your inverter's "Idle current" draw. Some inverters consume as much as 90 Watts just to be on, even with no loads. Most decent quality inverters will draw 7-22 watts constantly just to be "on".

3. If you have a battery in you system and it "dies", it's voltage will still rise as long as the sun is on your PV panels and your charge controller will allow the solar to pass thru. Some charge controllers need a minimum battery DC voltage to allow PV to pass to the battery. The inverter will turn on if it has the proper voltage present. As long as your inverter and loads do not draw more than your PV is producing, you will still be able to power loads during the day, even with "dead" batteries.

Alex Aragon

2manytoyz

Re: Running 12v appliances off a 24v pv system

At work we have M-113 Armored Personnel Carriers that have a 24V electrical system. Our communication equipment is 12V. Our solution was to use a Samlex SDC-20 DC-DC converter. Does a nice job of converting 24VDC down to 12VDC (13.8 VDC +/- 0.1 V).

These are well built, and hold up to the hot, humid, and sometimes wet conditions inside this APC. The guts of the converter are all potted.





This one is rated at 15A output. They have a 20A and 60A version as well.





I avoided the issues with my setup at home. I stuck with a 12V setup. I do have to use fatter cables to the batteries and inverters, but all my 12V items are compatible. My "opinion" is the cost savings of using a 24V system due to thinner wires is negated by the extra cost of 24V inverters & appliances or converters. JMHO.

Caneman

Re: Running 12v appliances off a 24v pv system

^^^ cool pics, that looks fun

thanks for you insights... so with dead batteries the inverter will still work during daylight hours so long as the load is not greater than what the panels are drawing...

24v vs. 12v, that is my big decision right now... I can double the power pumped by the cc if I use 24v and it runs at lower amperages (not just smaller wire, but for me it is safer as well)... but the 12v system is compatible with all the 12v appliances... i am leaning at this point toward the $75 cost of the 24/12 converter being much cheaper in the long run by being able to have much more power available to the battery bank when running at 24v and being able to expand the system, the 12v seems too limiting and not as resource efficient as the 24v system.... still not sure though, and thinking about it...

westbranch

Re: Running 12v appliances off a 24v pv system

take an extreme example, same 4 batteries in 12 & 24 v config. in 24v there are 2 "batteries" in parallel , in 12 v there are 4 batteries in parallel, now think of how you will run the wires to connect them to the CC and inverter etc??? anmd keep all the cables the same length...

remember KISS!

hth

mike95490

Re: Running 12v appliances off a 24v pv system

Caneman wrote: »

^^^ cool pics, that looks fun

thanks for you insights... so with dead batteries the inverter will still work during daylight hours so long as the load is not greater than what the panels are drawing.........

Most charge controllers require a minimum voltage before they will "boot up". Even oldies like the C-12, PWM, need 9v of battery power before they will boot and allow charging. You may need a "jump start" button, to feed panel output to batteries, till they are able to wake up the controller. Or maybe a large capacitor would do it.

The other issue with weak batteries, is DC ripple. As the inverter sucks power from the batteries in 60Hz sips, it's un-even load as it creates a sine wave out of DC, causes the DC source to "ripple" or fluctuate. This can crash the controller, inverter or both. Larger house size inverters, and Grid Tie inverters have special capacitors to manage this "ripple" voltage.

Caneman

Re: Running 12v appliances off a 24v pv system

mike90045 wrote: »

Most charge controllers require a minimum voltage before they will "boot up". Even oldies like the C-12, PWM, need 9v of battery power before they will boot and allow charging. You may need a "jump start" button, to feed panel output to batteries, till they are able to wake up the controller. Or maybe a large capacitor would do it.

The other issue with weak batteries, is DC ripple. As the inverter sucks power from the batteries in 60Hz sips, it's un-even load as it creates a sine wave out of DC, causes the DC source to "ripple" or fluctuate. This can crash the controller, inverter or both. Larger house size inverters, and Grid Tie inverters have special capacitors to manage this "ripple" voltage.

I am considering the Morningstart Tristar MPPT 60... the manual says it has a low voltage shutoff of 7 volts, and won't restart until the battery gets to 8 volts (manual doesn't say if this is for a 12v system, or a system of any voltage- not sure)... anyway, if my battery is below 8 volts, what is a "jump start" button?

SolaRevolution

Re: Running 12v appliances off a 24v pv system

Caneman wrote: »

anyway, if my battery is below 8 volts, what is a "jump start" button?

Brief, direct contact of the PV to the battery bus to bring the voltage up enough for the controller to turn on.

There are different degrees of "dead" for batteries. Manufacturers ratings may consider a battery "dead" when it has lost 10% of it's rated capacity. A battery that cannot hold a C/20 load for an hour is "dead" but in an end-of-the-world scenerio it will still be quite useful. You may still be able to get several hours powering a single LED or CFL light.

The capacitance of a dead battery will get worse over time and may eventually be no better than no battery at all. How long will it still last? Who knows? Eventually it could be dangerous. Make sure it does not go dry and you may eventually want to lower the charging voltage if it is unable to hold a charge anyway.

Caneman

Re: Running 12v appliances off a 24v pv system

SolaRevolution wrote: »

Brief, direct contact of the PV to the battery bus to bring the voltage up enough for the controller to turn on.

that is pretty interesting, how would that work? would I disconnect the cc from the batteries, then direct connect the pv to the battery terminals just long enough to "wake them up"? would I know the batteries awake by watching the battery monitor?

SolaRevolution

Re: Running 12v appliances off a 24v pv system

Caneman wrote: »

that is pretty interesting, how would that work? would I disconnect the cc from the batteries, then direct connect the pv to the battery terminals just long enough to "wake them up"? would I know the batteries awake by watching the battery monitor?

Keep the CC connected to the batteries. Disconnect the PV from the CC and touch the PV directly to the batteries. When the battery voltage rises, the CC should come on. Be careful! It may be quick and easy or the battery voltage may be slow to rise and then fall before you can reconnect the PV to the CC.

Please be sure you still have over current protection (fuse or breaker) between the PV and the battery. Even a "dead" battery may be able to deliver a lot of current into a fault.

waynefromnscanada

Re: Running 12v appliances off a 24v pv system

Easiest to use a jumper wire from CC + input from the PV, to the CC + out to the batteries, without disconnecting PV, CC or batteries. Just touch the jumper across those terminals, holding it there only as long as required to get things going. Depending on the condition of batteries etc, it may take only a second, or it could take minutes.
There is no problem as long as you don't allow the battery voltage to go to such a high level that might be dangerous to the CC, which might happen depending on the PV voltage, if the batteries are acting as an open circuit and not accepting any current.

Cariboocoot

Re: Running 12v appliances off a 24v pv system

The safe way which should work with any system Voltage, array Voltage, and charge controller:

Disconnect PV from charge controller.
Disconnect batteries from charge controller.
Connect PV to batteries.
Monitor battery Voltage with DMM.
Allow batteries to charge until Voltage is at least above nominal.
Disconnect PV from batteries.
Connect batteries to charge controller (it should now "fire up").
Connect PV to charge controller and allow charge to "finish".

Even if the array Voltage is above the system Voltage this method should be able to drive enough current to the batteries and raise them to a functional level. If not, replace batteries because you're going nowhere.

Some charge controllers can be sensitive to having the PV attached to their output, even momentarily.

SolaRevolution

Re: Running 12v appliances off a 24v pv system

waynefromnscanada wrote: »

Easiest to use a jumper wire from CC + input from the PV, to the CC + out to the batteries, without disconnecting PV, CC or batteries. Just touch the jumper across those terminals, holding it there only as long as required to get things going. Depending on the condition of batteries etc, it may take only a second, or it could take minutes.

That would definately be the easiest for a PWM controller. I would hesitate to jumper the battery to the PV side of an MPPT CC. All of the advice I've heard from the Midnite and Outback folks says this will be bad.

"Whatever you do, don't cross the streams!" - Ghostbusters


Alex

BB.

Re: Running 12v appliances off a 24v pv system

I hope that nobody is using a $600 MPPT solar charge controller with $5 battery cores to make up their off grid solar PV system...

Jumpering around a MPPT solar charge controller will probably bypass and confuse a bunch of its internal electronics--But it probably will not kill the controller to "jump start" a dead battery bank... Once the jumper wire/switch has been opened, the controller should operate normally again.

However, charging a battery with no real active material left is probably going to generate a lot of hydrogen gas and electrolyte fumes. Add that you may overheat/boil the battery dry and start a fire that way too--I don't see the need to do this for an ongoing basis.

And, if you have a battery that was taken dead (below the boot voltage for a charge controller)--More than likely the battery will have either no or up to a couple of days of useful life left before it just goes dead again over night and/or when you try to put any loads on it (especially if the battery bank is more than a year or two old).

Interestingly, in times past (present too?) the phone company had electrolytic cells that were used to moderate voltage swings in their equipment--These were not batteries but just cells with metal electrodes and some sort of electrolyte. I ran across a small article that talked a bit about them--But don't remember where or much about it. Took a quick look and did not find anything... Wonder if they generated hydrogen gas as a way to dump excess current/power? I don't think they were wet capacitors in the traditional sense...

-Bill

Caneman

Re: Running 12v appliances off a 24v pv system

^^^ BB, this was just a "what if" SHTF situation in case the battery bank went dead, and you couldn't get any replacement batteries for a while, and you wanted to run some power during the day right off the PV...

marsofold

Re: Running 12v appliances off a 24v pv system

In a SHTF situation, could one just use a small 12 volt solar panel across the output of the solar charge controller to "deceive" it into thinking that there is a battery present, then once it turns on just use it to power loads afterward connected to the charge controller's output?

vtmaps

Re: Running 12v appliances off a 24v pv system

marsofold wrote: »

In a SHTF situation, could one just use a small 12 volt solar panel across the output of the solar charge controller to "deceive" it into thinking that there is a battery present, then once it turns on just use it to power loads afterward connected to the charge controller's output?

No. virtually all charge controllers need a battery to work properly. In particularly they need the capacitance of a battery. The battery can be nearly dead as long as it has some capacitance.

By the way, the answer I have given has already been given and explained in this thread.
--vtMaps

westbranch

Re: Running 12v appliances off a 24v pv system

Jim, have a read of section 10 in this manual it gives a good explanation of charging and sulphation, from a different battery maker.
Ps they have a different view of charging AGMs

http://www.cdtechno.com/pdf/ref/rs_02109_0911.pdf

GWILESKI

Re: Running 12v appliances off a 24v pv system

I am new to this and don't see how these responses relate to running 12v appliances off of a 24v system. I want to run 12v lighting and water pump and radio from 24v system that runs inverter.

vtmaps

Re: Running 12v appliances off a 24v pv system

GWILESKI wrote: »

I am new to this and don't see how these responses relate to running 12v appliances off of a 24v system. I want to run 12v lighting and water pump and radio from 24v system that runs inverter.

The OP wrote: " I assume you would need a 24v/12v dc stepdown converter? "

Several others confirmed that, but suggested alternatives (super caps, battery balancer with center tap).

Same thing in your other thread... you need a DC converter.

The OP also asked about batteryless operation of a solar charge controller, and much of this thread is about that.

Lesson to learn: Don't ask too many questions in one OP... it encourages a natural tendency to wander...

--vtMaps

Mark40930

Re: Running 12v appliances off a 24v pv system

Robert, thanks for the info on the Samlex. This now gives me more options for designing my off-grid setup and gives me more flexibility in adding an inverter for AC loads.
Just an FYI, DX Engineering is selling these and it was about $340 or so for the 60A version.

Blackcherry04

Re: Running 12v appliances off a 24v pv system

I have been using these from Victron in the Marine environment never a issue yet.

http://www.shop.pkys.com/DC-DC-Converters_c_14.html