24v battery charging with 12v system.

bob5731

I was at Walmart today looking for deep cycle battery and the smallest voltage I saw was 24v.
But I run a 12v system. My system is the harbor freight fortify what floor panel kit. http://www.harborfreight.com/solar-panel-kit-45-watt-68751.html

So how do I charge the battery up if I get it.
It's only $69.00

BB.

Re: 24v battery charging with 12v system.

Welcome to the forum Bob!

I am not sure you can buy 24 volt batteries at Walmart... Most would probably be 12 volt or 6 volt (aka "golf cart" batteries).

The Harbor Freight units are pretty small at 40 Watts unless you are running small amounts of power (some LED lighting, charging cell phones, possibly a laptop computer a few hours a day).

In general, I like to start with your power needs (5 amps * 12 volts * 3 hours = 180 Watt*Hours--an example of the math).

And for off grid solar, even before starting with the loads, usually you want to use as energy efficient loads as you can--Then measure/estimate your usage.

We can start with something else--Say you have 40 Watts of solar panels--The largest 12 volt battery I would recommend (assuming 5% minimum rate of charge):

40 watts * 0.77 panel+controller deratings * 1/14.5 volts charging * 1/0.05 rate of charge = 42.5 AH @ 12 volt battery

That is about 1/2 the size of a "typical" 12 volt car battery--And of course for solar power, you should be looking at a deep cycle battery.

So, here is an AGM (sealed) type battery at 35 AH @ 12 volts for ~$60 per shipping:

http://www.solar-electric.com/unba35amagms.html

You should not use any larger batteries than ~43 AH @ 12 volts--There is not enough solar panel to properly recharge your battery bank and run your loads (in general).

-Bill

bmet

Re: 24v battery charging with 12v system.

bob5731 wrote: »

I was at Walmart today looking for deep cycle battery and the smallest voltage I saw was 24v.
But I run a 12v system. My system is the harbor freight fortify what floor panel kit. http://www.harborfreight.com/solar-panel-kit-45-watt-68751.html

So how do I charge the battery up if I get it.
It's only $69.00

24 Volt batteries from Walmart are most likely for specific products such as power tools, electric trimmers, electric mowers, etc. They need the charger which was designed for them (which are usually a separate purchase).

bob5731

Re: 24v battery charging with 12v system.

It says 24DC on the battery

BB.

Re: 24v battery charging with 12v system.

Is this the battery you are looking at?

http://www.walmart.com/ip/EverStart-Group-Size-24DC-Marine-Battery/20531540

Attachment not found.

Probably around a ~73 AH @ 12 volt battery (at 20 Hour Rate).

24DC is a BCI battery "size code"--Not a direct description of the battery voltage.

I would recommend a minimum solar array size of (5% minimum rate of charge--5% to 10% to 13% typical recommended rates of charge):

73 AH * 14.5 volt charging * 1/0.77 panel+controller derating * 1/0.05 rate of charge = 69 watt minimum array

Marine batteries are not really recommended for deep cycling. Walmart has a "Maxx" series of deep cycle batteries which should last longer--I think (the Marine battery may last as short as 1 year in heavy cycling applications).

-Bill

bob5731

Re: 24v battery charging with 12v system.

or Im going to get Sam's Club golf cart battery. http://www.samsclub.com/sams/energizer-golf-cart-battery-group-size-gc2/prod6750008.ip

BB.

Re: 24v battery charging with 12v system.

That should be a good battery... However, you will need two in series to give you a 12 volt 208 AH battery bank.

You will need new panels and a larger/better charge controller... The recommended panel size would be:

• 208 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 196 Watt array minimum
• 208 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 392 Watt array nominal
• 208 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 509 Watt array "cost effective maximum"

With a 10% sized array, an ~4 hours of sun per day (usual 9 months average minimum daily sun):

• 392 watts * 0.52 system efficiency * 4 hours of sun = 815 Watt*Hours minimum of 120 VAC power per average day

A 208 AH battery bank's typical 120 VAC load would be (assuming 2 days of storage and not going below 50% state of charge):

• 208 AH * 12 volts * 0.85 typical inverter eff * 1/2 days of storage * 0.50 max discharge = 530 Watt*Hours of 120 VAC battery power per day

Are these numbers helping you to understand how this all works together?

If your loads are DC, you can get ~1/0.85 times more power (AC inverter losses). If you use power mostly during the day, then that is another 1/0.80 in savings (battery efficiency).

There are lots of "derating factors" in solar off grid power. And lots of places where you can gain efficiency (and use less power)--And those power savings are usually worth the costs (vs just building a larger solar power system).

-Bill

bob5731

Re: 24v battery charging with 12v system.

No is not help me.
I want a lot of 110 AC appliances.
I'm looking for a space heater, coffee maker, TV, that will run on DC power.

Cariboocoot

Re: 24v battery charging with 12v system.

bob5731 wrote: »

No is not help me.
I want a lot of 110 AC appliances.
I'm looking for a space heater, coffee maker, TV, that will run on DC power.

DC from the battery can be inverted to AC to run standard plug-in devices. That is not a problem. Some of these are available as DC units, but those are not necessarily as efficient as their AC counterparts even with the inverter factored in.

Run things like a space heater or coffee maker from batteries either way is not a practical thing to do. The amount of stored power needed to handle electric heating is very large and therefor impractical.

The basic calculations (without the details of losses) are fairly simple. You need to know two things to start with: the maximum Watts you will be using at any one time (from all loads) and the total Watt hours you will consume in a day. Without figures for those two things all you can do is guess, with a very high percentage of getting the design wrong.

BB.

Re: 24v battery charging with 12v system.

Get a Kill-a-Watt type meter (local hardware stores sometimes have them, in some places, even libraries even check them out).

Until you have measured your power usage--Everything is guess work.

To give you an idea--Very roughly a typical off grid power system will produce power for (very roughly) $1-$2+ per kWH (hardware+inverter+batteries+replacement batteries every X years) -- That is about 10x the cost of utility power. Therefore, you really need to figure out how much power you need and if electricity (vs a gas/propane stove/etc.).

A gallon of gasoline is ~33,000 WH per gallon... At ~40% efficiency:

• $4 per gallon * 1/33 kWH per gallon * 1/0.40 eff = $0.30 per kWH of "heat"

A few people, with good deals on batteries/doing their own work, have said they can get down towards $0.50 per kWH... But as you can see, getting heat from solar electric resistance (or even using induction hot plates) is still pretty expensive when compared with fossil fuel.

As an example, say you would use 1 quart of gasoline (or similar) of fuel per day for cooking... That would be:

• 33 kWH * 1/4 gallon * 0.40 efficiency = 3.3 kWH per day of power.

The size of battery to store that amount of power:

• 3,300 WH * 1/0.85 inverter efficiency * 1/12 volt battery bank * 2 days of "no sun storage" * 1/0.50 maximum discharge = 1,294 AH @ 12 volt battery bank

Assuming ~220 AH in a 6 volt golf cart battery, that would be:

• 1,294 AH * 1/(220 AH) = 5.9 = 6 "golf cart" batteries

And if you get >4 hours of sun per day for ~9 months of the year:

• 33 kWH / 4 = 8.25 kWH of electric power (equivalent to our mythical 1 quart of gasoline in a camp stove)
• 8,250 WH * 1/0.52 end to end system efficiency * 1/4 hours of sun per day = 3,966 Watt solar array

So, 6x golf cart batteries and a 3,966 Watt solar array + AC inverter + Charge Controller to make the same amount of "heat" as a quart of gasoline in a camp stove.

Again, not trying to say it cannot be done--But want to understand your needs and how to overlay them onto a solar power system.

-Bill

bob5731

Re: 24v battery charging with 12v system.

12v coffee maker, 12V Heater, 12v A/C. Where can I get the items?

BB.

Re: 24v battery charging with 12v system.

Look in RV stores:

http://www.campingworld.com/category/12v-appliances/1199
http://www.campingworld.com/shopping/item/12v-direct-hookup-ceramic-heaterfan/56984 (25 amps @ 12 volt 300 watt heater)

Also, you can get 24 volt appliances from Long Haul Trucking and some Marine stores.

-Bill

bob5731

Re: 24v battery charging with 12v system.

I was looking for something like a space heater for my house.

BB.

Re: 24v battery charging with 12v system.

Any sort of heating devices pull a lot of power (Watts and Watt*Hours).

Do you have an idea of how much power you need? If you have 120 VAC power available, you can get a kill-a-watt type meter (local hardware stores sometimes have them too) and use a standard AC heater (or appliances) to heat/cook your normal meals (grid power, or even on a genset) and see how much power/energy you would use in a typical day.

Then we can use that information to pencil out an Off Grid system that can meet your needs/expectations.

-Bill

RobW

Re: 24v battery charging with 12v system.

Bill, great basic discussion here. The light finally came on running through your formulas. For a 24v system, can 24 be substituted for 12 in those numbers?

BB.

Re: 24v battery charging with 12v system.

Rob,

Yes--But you have to look at which equation you are doing the substitution in.

If I am looking at a Load to Battery calculation:

• 1,000 Watt*Hours per day * 1/0.85 inverter eff * 1/12 volt battery * 2 days of storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank
• 1,000 Watt*Hours per day * 1/0.85 inverter eff * 1/24 volt battery * 2 days of storage * 1/0.50 max discharge = 196 AH @ 24 volt battery bank

In the above calculation, the 12 vs 24 volt calculations give you the same "size battery bank in Watt*Hours of storage:

• 392 AH * 12 volts = 4,704 Watt*Hours of storage
• 196 AH * 24 volts = 4,704 Watt*Hours of storage

Because Power = Volts * Current -- You double the voltage and 1/2 the current, you still get the same answer.

But, if you do the voltage swap in another set of math such as calculating the optimum solar array for a battery bank:

• 208 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 392 Watt array nominal
• 208 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 783 Watt array nominal

So, in this example, you have either two 6 volt @ 208 AH Golf Cart batteries for a 12 volt bank vs four Golf Cart Batteries in series for a 24 volt bank with 2x more storage (so you need 2x larger solar array for same rate of charge).

If you wish, create your own thread and we can discuss your questions regarding your system directly--Will be less confusing for Bob in this thread.

-Bill

RobW

Re: 24v battery charging with 12v system.

Don't want to hijack this thread, so I'm continuing over on a previous thread I started a few weeks ago: http://forum.solar-electric.com/showthread.php?21064-Newbie-Designing-a-System

Bob, thanks for starting this good discussion.

bob5731

Re: 24v battery charging with 12v system.

Can you use one 6V golf cart battery for 12V system?

rich

Re: 24v battery charging with 12v system.

Short answere, no.

Blackcherry04

Re: 24v battery charging with 12v system.

bob5731 wrote: »

Can you use one 6V golf cart battery for 12V system?

Long answer is you can't use just one, you need to put two in series to give you 12 V.

bob5731

Re: 24v battery charging with 12v system.

3 12v 35amp batteries. Backup charging using 110v Power?

BB.

Re: 24v battery charging with 12v system.

The range of charging is from ~5% minimum to 20-25% maximum rate of charge:

• 3*35 Amp*Hour batteries = 105 AH of storage (at 12 volts)
• 105 AH * 5% rate of charge = 6 Amps minimum
• 105 AH * 10% rate of charge = 10.5 Amps nominal
• 105 AH * 20% rate of charge = 21.0 Amps--Pretty high rate of charge

If you where just storing/floating the batteries you could get down to ~1% to 2% rate of charge (batteries unused for > 1 month of storage).

Note that you should really get multi-stage charge controllers that will charge the battery bank at the "right voltages" (dependent on Flooded Cell vs AGM vs GEL, etc.)... Many automotive chargers will over charge batteries and boil them dry if left permanently connected and powered from 120 VAC (if this is for long term permanent installation). Automotive chargers can work in a pinch (quick charge with battery clips).

-Bill

bob5731

Re: 24v battery charging with 12v system.

I was looking for something like this 12V 5AMP POWER TENDER PLUS. Will it work for keeping the batteries charge when the sun is not very powerful? But was looking for something a little bit cheaper.

BB.

Re: 24v battery charging with 12v system.

If you have 120 VAC power, that is 24 hours per day--Much more than the ~5-6 hours per day that most people see. So smaller supplies can successfully recharge a battery bank after an outage. Longer term, for true (larger) deep cycle batteries, you need a minimum of 2.5% to 5% minimum current to properly equalize a battery bank. Some mfg. (Trojan and others) will recommend 10% rate of charge for their batteries that are cycling daily.

Perfectly OK to maintain a bank or recharge it between power outages, occasional portable power, etc... It is a bit on the small side if you were going to be cycling the batteries daily--Such as many countries where they loose power in the afternoons due to insufficient local electrical grid capacity (Caribbean, etc.).

If you have other power sources (solar), the two together (or larger solar) will keep your bank happy.

-Bill

bob5731

Re: 24v battery charging with 12v system.

So what item do I need? 7 12v Harbor freight panels. And 4 12V off offbrand panels. I am currently at the moment charging the panels at 0.05 to 0.08 AMP. The batteries are currently sitting at 11.25 according to a voltmeter and according to the Harbor freight 500 what charge controller charge one controller 11.8 and according to the Harbor freight 45 W charge controller kit 11.5

BB.

Re: 24v battery charging with 12v system.

You may make the Harbor Freight panels and controller work... You might have wiring issues, a failed controller, or one or more failed panels.

Do you want to "experiment"? Or are you tired of working on the H.F. product and want to have something that "works" and will fill your needs?

What "tools" do you have to diagnose your system (Digital Multi-Meter, Hydrometer, Kill-a-Watt type meter, etc.)?

In any case, I would highly suggest a DC Current Clamp DMM (this one from Sears is frequently on sale at $51.00 and "good enough" for our needs).

And, for flooded cell batteries, you should get a decent glass hydrometer so you can measure and log specific gravity.

Next--The batteries are near dead, and if they have been at that voltage (or below) for weeks-months--They are fast sulfating and probably nearly a pile of scrap lead.

If the batteries are relatively new and still in good shape, you should get them on a 12 volt AC battery charger (car type) and get them >90% full (i.e., put them on the charger, watch the voltage and when you see >14.5 volts or so for several hours--They are at least back in the "safe storage/use" zone).1

But, in any case, if you want to try to learn about your present H.F. system--They will work for now.

With the HF system--You can start by diagnosing what is going on. A 45 watt system and 11.x volt battery bank should see roughly:

• 45 watts * 0.77 panel+controller derating * 1/14.5 volts = 2.4 amps around noon time/full sun (typical maximum--At lease >1.2 amps anyway)

If you are not seeing that (depending on how you are measuring the current)--You need to go with the basics:

1. Measure Panel open circit voltage (Voc)--Should be >~17 volts in any direct sun light for each panel
2. Measure Panel short circuit current (Isc)--Should be:
   • 15 watt * 1.25 fudge factor * 1/17 volt Vmp = 1.1 amps in full sun (~0.5-11 amps depending on weather)
3. Connect panels directly to battery bank and measure Imp-array (current maximum power array):
   • 15 watt * 1/15 volt Vmp = 0.9 amps in full sun (~0.4-0.9 amps depending on weather)
4. Check each panel, and then connect all three together and connect them to your 12 volt battery. Make sure you connect + array to + battery. If you connect them backwards, the panels will be destroyed by the battery short circuit current. The current should add. If you get expected current flow, then the panels are OK.
5. Then connect the charge controller and measure current to the battery bank. If the Batteries are less than ~13 volts, you should see "full array" current to battery bank. If not, then the charge controller is probably bad.

If nothing else, you will get some experience working on a simple solar power system and understand the basics of each piece.

At that point, you can do further experimentation (got the HF system working) with a small DC load or AC inverter+small loads and see what happens. Even if you "mess up"--You are not out much money.

Not that DMM (digital meters) that you connect in series with the wiring to measure current flow are easy to damage (wrong current scale, short circuit, etc.) can pop a meter fuse or even destroy the meter (in current mode).

The DC Current Clamp Meters are much safer and will not be damaged by typical wiring errors that can take out a standard DMM.

-Bill

bob5731

Re: 24v battery charging with 12v system.

Harbor freight five I want charge controller is now reading 12v.
Harbor freight 45 W charge controller kit is now reading 11.6 to 11.8
Digital voltmeter is now reading 11.77 to 11.85
And panels are producing 14v 1.2 AMP
i have a Kill-a-Watt.

bob5731

Re: 24v battery charging with 12v system.

What settings do I need to use for a automotive charger?

westbranch

Re: 24v battery charging with 12v system.

those chargers settings are usually fixed by the manufacturer. you will have to measure your to see what they are. Some can go to ~ 14.4 maxV

PNjunction

Re: 24v battery charging with 12v system.

bob5731 wrote: »

I was looking for something like this 12V 5AMP POWER TENDER PLUS. Will it work for keeping the batteries charge when the sun is not very powerful? But was looking for something a little bit cheaper.

That is a very good charger for agm's - I often recommend their 5A unit over the littler tenders. However, for something cheaper, look into a 6A Stanley BC-6809. Charges up to 14.5 volts absorb, and floats at 13.5v and doesn't pull any optimization tricks like some other speed-chargers do behind your back.

Even so, I'm not sure it will fit your needs, but it is cheap, automatic, and better than what you have now. Maybe this will be enough to keep those cells alive until you do all the calculations for more appropriately sized gear.