12v vs 24v

Absolute beginner at solar power. Just looking for something small and simple.

Setup A: 1x 12v 100Ah battery, charged at 10A.
Setup B: 2x 12v 50Ah batteries in series, charged at 5A.
Setup C: 2x 12v 50Ah batteries in parallel, charged at 2x5A.

Assuming the price of all setups is the same, what are the pros and cons of the setups?

I have a larger system.  I selected 12v as a safety concern.   I am sure you will encounter other views.  I took the simplistic approach of asking myself if I want to get shocked by 12v, 24v, or 48v.

I like the A option.  Like John, I like low voltage.  I also like only 1 panel and 1 battery.  I always try to design around the least amount of parts and maintenance.  

12v is really versatile; lots of stuff available, charge a car, charge it from a car..

24V scales up farther, and cuts wiring costs. I really like having it as an option, because I have a battery powered welder and a couple pieces of heavy equipment that are 24V... but for me this just means making sure that panels and controllers are selected that can handle 24V.

If you do this, choice of battery and system voltage is not a big deal on a small system. The battery usually is the first thing to be replaced, and the expense is minimal on this size of system, so you can readily switch voltages when you change batteries.


Perhaps you have already done this, but v. important in my opinion to start with the loads and work back, when speccing any solar system!

What are you intending to do? It's best to start from what you want to accomplish, and work backwards from that.

If you have very long cable runs, a 24VDC system will reduce your line losses somewhat. A long wire at 12VDC is sort of a toaster, unless it's massively thick, and you lose a lot of energy along the way.

How are you planning to charge these? The advantage to a 2-battery system is that you can disconnect one in a pinch and charge it more quickly, to get yourself up and running. The bigger the pail, the longer it takes to fill.

EDIT: Gah! D Nikolls beat me by 20 seconds!

You need to plan your future battery bank capacity before making this decision.

In order to increase available amp hours, you have to add parallel strings of batteries.  If you get more then 3 parallel strings, you will find that the batteries will not charge and discharge equally and will become out of balance.

For small systems while you learn how solar operates I think you'll be fine with 1 big 12volt battery, but as you build a larger system I think you'll find 24v is ideal.

The reason I mentioned the 2x12v option is mentioned above, less losses and little more flexible.
I'm a bit worried about the following.

Two batteries in series are charging.
Battery A is 99% full.
Battery B is 40% full because it has a load attached to it.

It's my understanding (nearly) empty batteries are charged at high amps and as they fill the amps go down.
Does that mean Battery-A sets the max amps at say 1A, while Battery-B can safely handle 10A.

I that's true I'll just drop the series idea. Perhaps I'll still use 2 batteries in parallel and in rare situations (perhaps when dark) unhook and place in series.
---------

Additional question.
If I have a 100 Ah battery how much will be available for actual use? Not just talking about conversion losses of all sort, but also how much the battery can be discharged without dramatically shortening its lifespan?
Is there a rule of thumb that states "Don't discharge a battery below x%"?

I'd be helpful to state your purpose (anticipated LOAD(s)).  Generally speaking, the greater the load, the higher Amp-Hour capacity batterie(s) are required.

Tony Masterson wrote:Two batteries in series are charging.
Battery A is 99% full.
Battery B is 40% full because it has a load attached to it.

It's my understanding (nearly) empty batteries are charged at high amps and as they fill the amps go down.
Does that mean Battery-A sets the max amps at say 1A, while Battery-B can safely handle 10A.

Not recommended to load a single battery of a bank.  SERIES connected batteries act independently and both batteries in a bank should be maintained at close to same DOD Voltages... and likewise, charged as if a single 24v battery (i.e. two 12V batteries in series = 24V (charged @ approx. 27-28V). If you wish to load a single 12V battery, then you'll need a 12V charger.

Tony Masterson wrote:Additional question.
If I have a 100 Ah battery how much will be available for actual use? Not just talking about conversion losses of all sort, but also how much the battery can be discharged without dramatically shortening its lifespan?
Is there a rule of thumb that states "Don't discharge a battery below x%"?

I have four 12v FLA's in series on solar, and I never allow the bank to drop below 50% DOD.

Eugene Kenny wrote:

Tony Masterson wrote:Two batteries in series are charging.
Battery A is 99% full.
Battery B is 40% full because it has a load attached to it.

It's my understanding (nearly) empty batteries are charged at high amps and as they fill the amps go down.
Does that mean Battery-A sets the max amps at say 1A, while Battery-B can safely handle 10A.

Not recommended to load a single battery of a bank.  SERIES connected batteries act independently and both batteries in a bank should be maintained at close to same DOD Voltages... and likewise, charged as if a single 24v battery (i.e. two 12V batteries in series = 24V (charged @ approx. 27-28V). If you wish to load a single 12V battery, then you'll need a 12V charger.

The usual way to power 12V items from a 24V bank is a DC-DC converter. My Samlex has been perfectly reliable for 3 years so far, but there are certainly other options in both directions on the price continuum.

Powering loads directly from one battery in any series array is not good practice, and should be avoided..

I'm learning a lot here... !
I got some solid advice on my first post. Building on that advise my next question about connecting loads and efficiency.

If I have a single 12v battery what would the best choice for stuff I power with it?
Let's assume I want to buy a 70watt rice cooker that comes in a 12v, 24v and 110v versions.
Which one would be the best option efficiency wise?

Personally I would say 12v because I can directly connect that to the battery. That would be 100% efficiency opposed to using a convertor which usually has only 80% efficiency.
Correct?
Or is directly connecting to the battery not advised?

That said the benefit of powering at 110v is clear. There is far more choice of 110v equipment than 12v.
If I would go for 110v output would a higher input voltage (24, 36) would that increase efficiency?

I think you are correct in thinking every time you change voltage you loose energy.  To me watts is watts so 12v @ 10 amps is the same as 24v @ 5 amps.  The main problem with low voltage is voltage drop.  In order for current to flow there needs to be a voltage drop.  This is resolved by shorter distances to loads and larger wire size.  The main need to go higher voltage is distance.  For a give distance if you drop 1 volt over it, it will leave you with 11v verses 23v for a 12v or 24v system.  One thing that seems to confuse people is that 12v systems are really 12.6v.  A fully charged battery is 12.6 volts. Charging is done above battery voltage or it will not push the current into the battery.  Most batteries are charged from 13.5-14.5v.  I like to set up my systems so my batteries never get below 12v.  Most solar chargers have a load hookup that will shut the load off at a voltage so the batteries will not be drawn too low.  Good chargers the low voltage cutoff is adjustable to be tailored per system.

Running an inverter typically do much better with the higher voltage inputs (24v).  This is evident when trying to start a heavy load. The voltage will drop too much and the inverter will shut down.  Batteries are chemically driven, so it takes a certain amount of time to get the electrons flowing.  Higher voltage simply means they have more push.  I resolved this with our inverters at 12v by using super capacitors.  Capacitors will unload their electrons as fast as you can take them. I have a 2000 watt inverter hooked to a 200 amp hour battery with a 58 farad super capacitor in parallel with the system.  It will start a rip saw and cut 2 x 4's all day long.  I used this as a demo for the makers group and our local farm days.  

I have made many 12v tools for our farm.  I have a corn sheller, corn seperator, grain grinder, grain threasher, refrigerators, large cooling fans, small greenhouse circulation fans, water heaters, water coolers, water circulation pumps and of corse lights.  My favorite places to get 12v ideas is truck stops.  They have everything from coolers to coffee makers that are 12v.

Tony,

There is a lot of good information on this thread, but the ultimate choice of voltage really comes down to your final application.  I will try to explain.

Are you looking for something that can be used as a rolling battery backup for main power up to maybe about 2kw?  In this case I would consider going with 12 volts just for simplicity sake as there is a lot of 12 volt equipment out there that is relatively affordable.

Are you wanting to run a fixed system that might run up to perhaps 4-5kw?  I would seriously consider 24 volt power here as it lowers the amperage needed for a given wattage.

If you are thinking about going over that 5kw Mark, there are 48 volt and higher systems out there.

Generally running at higher voltage is going to give better performance for much of the electrical equipment and this becomes especially true at higher wattage.  My figures are all approximate and you will want to check anything I have said just to be clear (and to be suitable for your particular application).  I am planning on building a battery power ac backup supply powered by a 12v battery and running up to a 2kw inverter (still debating this though).

Good Luck and if you can fill in any of the potential applications, the better.

Eric

Tony Masterson wrote:I'm learning a lot here... !
I got some solid advice on my first post. Building on that advise my next question about connecting loads and efficiency.

If I have a single 12v battery what would the best choice for stuff I power with it?
Let's assume I want to buy a 70watt rice cooker that comes in a 12v, 24v and 110v versions.
Which one would be the best option efficiency wise?

Personally I would say 12v because I can directly connect that to the battery. That would be 100% efficiency opposed to using a convertor which usually has only 80% efficiency.
Correct?
Or is directly connecting to the battery not advised?

That said the benefit of powering at 110v is clear. There is far more choice of 110v equipment than 12v.
If I would go for 110v output would a higher input voltage (24, 36) would that increase efficiency?

You should be going through a fuse or circuit breaker before any loads, but in terms of efficiency you're correct.

Better than 80% efficiency is common these days, but no conversion is free of loss.

The advantage of 110v isn't just selection, but often price. In turn, there is often more variety and lower prices on 12V stuff vs 24V.

Several times it was asked, what the loads are, what I'm using it for etc.
I'm not intending powering a whole house with it. So my solar setup won't be connected to the wiring in my house.
It's purely an emergency thing. That means it may take years before I first use the setup (aside from a test run)
Solar panel wise I'm thinking about 100-300 Watt.

Charge a flashlight, phone, charging a laptop, some basic electric cooking* (for haybox)

So a roof full of panels is not going to happen. Just 1 or 2 'portable' ones.
Then depending on how much such panels generate I would size my 12v battery. (no use having 500Ah if I only can charge 50Ah/day)
Is there a online resource where I can lookup a realist amount of power I can generate at my location with just 1 or 2 panels?


*=Yes I have a rocket stove but now want to focus on a woodfree solution. Just see if it's realistic

Tony,

If you want to start out cheap and simple, I built a system that will do most of that all running just on 12v dc.  I deliberately built this system as a twofold project.  Goal #1 was to have some emergency electrical power.  Goal #2 was to develop a little bit of experience and gain some basic knowledge before building a larger unit.

I found a basic build I liked on YouTube that came with an Amazon parts list.  It is powered by a 12v, 15 ah sla battery placed in a .50 cal ammo can.  The power outputs include 12v car style power, 5v usb and a 12v generic banana jack output for whatever I dream up (I might add a buck converter that will boost the 12v up to about 20v to charge a laptop).  I bought a 20 watt solar panel for off grid charging and I found an old printer power supply I modified (clipped the ends and wired to a banana jack) to charge via ac.  The build minus the solar panel was about $100.  The solar panel was about another $20.

I recommend starting with the small build as you get some experience before going big.  Also, it’s not that expensive (the big units add up but are way cool!) and you get a functional unit fairly early.  This can really help with conceptualizing the next build which is a pretty intimidating task to start with little in the way of electrical experience.

I hope this helps.

Eric

There's a few search terms that will help you find out how much solar energy you can expect to harvest, "solar potential" is one. Google brought this map up that seems to be useful:

https://globalsolaratlas.info/map

Your location and the dataset available for it may vary.

12V sounds reasonable to me if you aren't planning on running much with it, and only using occasionally. My house runs on a 48V bank - I once very briefly and accidentally shorted out a battery cable - the amount of energy released (and thickness of wire melted) was astounding. So, yeah, 12V is easier and safer to work with for sure...

Being able to swap different batteries in and out is a plus of a 12V system (charge car battery, truck battery, tractor battery etc etc.

Is stuff like this ok?
https://www.aliexpress.com/item/4000473412279.html
https://www.aliexpress.com/item/32626848534.html

Tony,

I would say those look very good.  The charge controller looks almost identical to what I have in my unit.  My solar panel connections are a little bit different but that’s because I used a banana jack input.  For my larger unit I will use a more standardized connector.

Eric

Panel specs:
150W at an irradiance of 1000w/m2 (doubles at double irradiance?)
The cell efficiency is 19.5%.
Area: 0.63 m2

https://globalsolaratlas.info/map
Today at my location: Direct normal irradiation: 2.469 kWh/m2 per day
I need help how to calculate the daily harvest in kWh.

2.469 x 0.195 x 0.63 = 0.3 kWh/day ???

Tony Masterson wrote:Is stuff like this ok?
https://www.aliexpress.com/item/4000473412279.html
https://www.aliexpress.com/item/32626848534.html

I have not had great luck with direct from china solar stuff.

The panels were basically alright; the controller, wiring, connectors and hardware(portable panels with hinges, latches, folding legs) all failed in a year of gentle use... non-uv rated wire and connectors simply fell apart. Terrible soldering, ditto. Everything 'stainless'.. was not.

Flexible panels tend to not last as long as rigid...


YMMV...

Tony Masterson wrote:Panel specs:
150W at an irradiance of 1000w/m2 (doubles at double irradiance?)
The cell efficiency is 19.5%.
Area: 0.63 m2

https://globalsolaratlas.info/map
Today at my location: Direct normal irradiation: 2.469 kWh/m2 per day
I need help how to calculate the daily harvest in kWh.

2.469 x 0.195 x 0.63 = 0.3 kWh/day ???

Yes, kind of... but then you will lose some to charge controller(a LOT if it's a pwm controller on a cloudy day), and some to battery charging...

Tony Masterson wrote:Several times it was asked, what the loads are, what I'm using it for etc.
I'm not intending powering a whole house with it. So my solar setup won't be connected to the wiring in my house.
It's purely an emergency thing. That means it may take years before I first use the setup (aside from a test run)

There are many small (12V single battery) solar systems in use, but i'm not inclined to recommend one here.  Why?  To be genuinely useful,
a 'back-up' system should be able to power minimal essentials (fridge/freezer, a small cooktop, and a few lights) .
A single, 12V 100Ah battery won't last long (1 maybe 2 hours at most powering just the fridge).  Even 2 batteries would be marginal.
And when the batterie(s) expire,during 'emergency' use, on a cloudy day, or in the middle of the night, how do you plan on recharging?

I'm far more inclined to recommend a small 2kW inverter generator. Spike the fuel w/stabilizer and test run it for 30 min, once a month.  

I currently on 1.2kW solar system. Its primary use is for a 1100W induction cooktop, a 700W microwave, a 1.2cu/ft fridge and a couple of LED's.
But I also have a 3500W Honda and a 1400W Yamaha for backup.  

My very first system was 12VDC to 120VAC.  It worked, sort of.  I could see lights dimming occasionally, here and there.  I didn't try to run more than a few lights and a TV for an hour or so.  I then graduated to a 48V system.  OMG, what a difference!  My new system can power my 240VAC well-pump, a cement mixer, and a big circular saw, all at the same time.  This is a totally off-grid system, not a grid-tie.  I will never again even touch anything 12V again.

Once you leave 12V and enter the realm of 24-48VDC you get into the serious whole-house electronics who's capabilities far out-match anything 12V is designed to do.  I'm talking about stuff an electrical code inspector would sign off on.  These are in the arena where inverters don't even have NEMA plugs;  they are designed to be hard-wired directly into your cabin's main electrical panel.

I now have a 1hp 240VAC well-pump, a standard refrigerator/freezer, air-conditioning, and all the extras you'd expect in a normal home, all running totally off-grid on solar only.  For years now!

My advice is not to invest 1 penny into anything 12V, not even 12V solar panels.  The key to getting inexpensive solar is to buy locally, and bypass 12V completely.  Locally, I can get 1000W of high-voltage grid-tie panels for 220$/kW.  This is at a cash and carry place in a nearby town.  It's shipping that's the killer with solar panels.  Even when they say "free shipping" the expense of shipping is baked into the price.  Buy locally, and you will save big.  What you really want to pay good money for is the electronics.  Newer MPPT controllers and Sine-Wave inverters beat the pants off of stuff made just a few years ago.

I mount my panels on single-pole ground-mount arrays, that can be rotated to track the sun.  They are non-motorized, so I have to go out and turn them by hand.  I call it hillbilly solar tracking.  It works!  I can increase the daily output of a 1kW array by 50% by just rotating the arrary about 3 times per day.  Here are pics of some of my arrays.

Michael,

That looks like a pretty nice 1Kw solar setup.  I am just curious, what does your battery setup look like?  Are they 12 volt batteries wired in series?  What kind of amp hours are you getting.  I am playing around with the idea of making a backup system, but nothing like what you are describing.  Basically I am just looking for ideas.

Thanks in advance,

Eric

Eric Hanson wrote:Michael,

That looks like a pretty nice 1Kw solar setup.  I am just curious, what does your battery setup look like?  Are they 12 volt batteries wired in series?  What kind of amp hours are you getting.  I am playing around with the idea of making a backup system, but nothing like what you are describing.  Basically I am just looking for ideas.

Hello Eric, I actually have two systems, my 48V cabin system, and my 24V workshop system.  For the cabin, I have eight 6V Trojan L-16 batteries, 377Ah, for 18kWh of storage.  The workshop has three 8V Rolls batteries with 568Ah of storage, for a total of 13.6kWh.  I like the Trojans, but I really, really like the Rolls.  The problem with them though is that they weigh about 300lb each, and two grown men can barely lift them.

Rolls however, makes a 4V version of the same battery, which is what I want to upgrade my cabin to.  They only weigh ~150lbs, so one grown man (me) could lift them.  Twelve of them in series is going to give me 27kWh of storage.  That will let me run the air-conditioner 24/7 in the hottest part of the summer.

Glad you like the array!  I'm very proud of it.  Funny thing is, now that solar panels are getting so cheap, I payed more for the steel to build the array then what I paid for the panels themselves.

Michael,

These are some impressive battery figures.  This is a lot better figures than my plans call for so I am definitely impressed/intrigued.

What type of battery chemistry are you using?  It does not seem like lead-acid or lithium-ion. I am not certain what type of cell can give you 4 volts so I am definitely interested.

Thanks in advance,

Eric

Eric Hanson wrote:
These are some impressive battery figures.  This is a lot better figures than my plans call for so I am definitely impressed/intrigued.

What type of battery chemistry are you using?  It does not seem like lead-acid or lithium-ion. I am not certain what type of cell can give you 4 volts so I am definitely interested.

Sure, they are flooded lead-acid.  I have both Trojan and Rolls, but plan on upgrading the cabin to this 4V Rolls battery.  I will need twelve of them to make a 48V bank.
https://www.rollsbattery.com/battery/4-cs-17p/
Rolls makes batteries in 2,4,6,8, 12, and 32V lead-acid batteries.
Trojan makes 2, 6, and 12V lead-acid batteries.

Related question...

What's the practical capacity of a 100Ah, 12v deep cycle battery?
Hopefully I'm overly pessimistic but I think only 20% or 20Ah.

100%
minus 40% because you can't run down a battery to 0% without damaging it.
minus 10% for reduced capacity due to age of the battery.
minus 10% for loss in wiring, connectors, etc.
minus 20% conversion/heat losses. ***
----
20%


***= examples
- When powering a rice cooker some heat escapes to the environment.
- When using it to charge a phone, laptop, etc there will be conversion losses.

Tony Masterson wrote:Related question...

What's the practical capacity of a 100Ah, 12v deep cycle battery?
Hopefully I'm overly pessimistic but I think only 20% or 20Ah.

Here's a graph of a Trojan deep-cycle 12V battery.  As you can see, the longevity of the battery is directly related to the depth of discharge.  The manufacturers usually recommend no more then 50%.  So, that works out to be 50Ah X 12V = 600Wh.  So, that is four 25W lights on for 6 hours, 10 hours of TV running at 60W, or 5 hours running a computer using 120W.

As you can see, that's really not that much.  That's why serious off-grid systems are better at  24 and 48V, and why much larger Ah batteries are usually selected.  With my system, my large 24V battery in general doesn't get depleted more than 20% a night, which more than doubles the longevity of the battery.