2V and 6V battery amp-hour comparison

kevinjones · January 2017

Hello,

Is the following true:

Connecting 3 2V/500Ah batteries in series will result in the equivalent of 1 6V/500Ah battery

3 x 2V/500Ah batteries == 1 x 6V/500Ah battery

Thanks,
Kevin

Vic · January 2017

YES, and the stored energy of the series-connected 2 V batteries will be three times that of the single 2V battery. Vic

Photowhit · January 2017

Think about the truth of it...
All lead acid batteries are 2 volts!

A 6 volt 'battery' is actually a battery bank of 3 batteries in a convenient case.

BB. · January 2017

Remember that Amp*Hours is only a "partial description" of the stored energy in a battery bank:

Amp * Hours = storage at "some voltage" (2 volts, 6 volts, 120 volts, etc.)
Energy = Amp * Hours * Voltage
2 volts * 500 AH = 1,000 Watt*Hours
6 volts * 500 AH = 3,000 Watt*Hours
2x 6 volt batteries * 500 AH = 12 volts * 500 = 6,000 Watt*Hours (i.e., the energy in 2x 6 volt batteries in series)
6 volts * 2x 500 AH batteries in parallel = 6 volts * 1,000AH = 6,000 AH

So--AH is only a partial way of understanding the energy in your battery bank. If every thing is equal voltage (i.e., comparing 12 volt battery banks of different AH capacity)--Then you can just look at AH.

However, when you are comparing 2 volts, 6 volts, 12 volts etc... You need the "complete" description of Watt*Hours.

We try to work with Watts and Watt*Hours when talking about the system. You may have a 12 volt battery bank and a 120 VAC inverter:

10 amps * 120 VAC = 1,200 Watt load (at 120 VAC)
100 Amps * 12 VDC = 1,200 Watt load (same load, but different working voltage)

-Bill

kevinjones · January 2017

@BB. I know you're not going to get 100% efficiency going through your inverter (1200W out !== 1200W in). Is there a rule of thumb efficiency factor for this?

Or more specifically (sorta), I have two Trace Sine Wave inverters connected for 220V running a well pump (Sorry, the model is not handy to me at the moment, they are about 15 years old). I am wondering, at least as a ballpark figure, what kind of efficiency I should expect.

Photowhit · January 2017

You can find the specs on line, Likely in the 90% area.

That is just the inverter, complete system losses may be 35-50% if you are referring to charging a lead acid battery (@20% loss), charge controller 2-3% loss, wiring (?)

You can also add that most panels only produce 70-75% of their panel rating. Look for NOCT values.

BB. · January 2017

I use 85% efficiency. You can do better (if inverter is more heavily loaded) or do worst (lightly loaded inverter or unloaded inverter (running but no AC loads).

1,200 Watts AC * 1/0.85 inverter eff * 1/12 volts (nominal) = ~118 Amps (@ 12 VDC)

It does get a bit more complex... With AC power, you have something called "power factor". More or less, if the AC current is not 100% "in phase" with the AC Sine Wave, you have less power than you expected.

It is sort of like pulling a car with a rope. If you stand in front of the car and pull, 100% of your force goes to moving the car forward. If you stand to the side, less effort goes into moving the car forward:

100 lbs of force * Cosine of Zero Degrees = 100lbs * (1.00) = 100 lbs of force moving car forward
100 lbs of force * Cosine of 45 degrees = 100 lbs * 0.707 = 70.7 lbs moving car forward

Note, in both cases, you are still pulling 100 lbs on the rope. So--The rope does not see a reduction in force--Similar with Amps and VA (volts*amps). The AC wiring, inverter, transformers, genset, etc. still need to be sized to manage the higher current flow.

With power (and energy) it is:

Power = Voltage * Current (DC)
Power = Voltage * Current * Cos current angle
Power = Voltage * Current * Power Factor (PF)

For example, your motor... Say it draws 10 amps at 220 volts AC. The electrical rating in VA (volts*amps) would be:

VA = 220 V * 10 Amps = 2,220 VA

A typical induction motor runs around PF~0.67 to 0.80 or so... So the real power (watts):

Power = V*I*PF = 220 volts * 10 amps * 0.67 PF = 1,474 Watts

So the actual power from the battery bank will be less than you would first guess with your AC induction motor measured current because of "power factor".

-Bill

kevinjones · January 2017

Thanks very much, Bill and Photowhit

Rayson · January 2017

Hi Kevin,

I signed in today with a similar question, hope you don't mind me asking here.

I'm setting up a Solar System in Australia.
Totally off-grid.
48v DC Load (No inverter or output regulation required).

We have covered the Panel and Storage calculation.
Decided to start with 800-1000w of Solar Panels.
48v Battery Bank @200Ah.

AGM 12V Deep Cycle Batteries, have been suggested to make the 48v Bank.
Note: The physical size or weight of Batteries makes no difference In our application.

Someone else says we should "use 2v Batteries", but has not elaborated why!

My question is:
What's the differences between using 2 or 12v Batteries to make the 48v bank, (apart from needing 6x as many

.

For the same 48v 200Ah bank, is it more cost effective to use 2 or 12v Batteries.
Can anybody give me examples of 2v batteries that might be used?

Ray

mike95490 · January 2017

I will give you a suggestion to look at 6V 200ah Golf Cart batteries. 8 in series and you are all set. Cheap batteries, made by the millions for golf carts, and sturdy enough to last 2-5 years off grid.

My 2nd thought is, 48V @ 200ah is a pretty small bank, and are you sure that the size calculation is right?

BB. · January 2017

More or less, you want a "cell" that is the AH rating of the bank... If you want a 400 AH battery bank, then the cell(s) will be 400 AH.

Whether that is 24x 2 volt cells, or 8x 6 volt batteries (3 cells each), or 4x 12 volt batteries (6 cells each) @ 200 AH--It really does not matter.

What may matter is the matter is the weight of the "cell" or "battery"--I.e., a 400 AH 6 volt battery is 1/2 the weight of a 400 AH 12 volt battery. You may be able to manage a 200-300 lb battery by hand+handcart. Larger batteries (such as forklift battery) may weigh 1,000 to 2,000 lbs, and you need a flat hard surface for a pallet jack or crane--Plus someway to get it off the truck (or loading gate).

You really need to look at your loads/battery bank size--Then see what fits for your needs.

For example, a 200 AH @ 48 volt battery bank is a bit on the "small size" when trying to find a 48 VDC input AC inverter... The "typical" loads I would expect would be:

48 VDC * 200 AH * 0.85 AC inverter eff * 1/20 hour discharge rate = 408 Watts "average load" (i.e., 408 watts 5 hours a night, two nights before recharging)
48 VDC * 200 AH * 0.85 AC inverter eff * 1/8 hour discharge rate = 1,020 Watt maximum continuous load (~1,020 Watts for ~4 hours before recharging)
48 VDC * 200 AH * 0.85 AC inverter eff * 1/5 hour discharge rate = 1,632 Watt max continuous loads (minutes to an hour or so)
48 VDC * 200 AH * 0.85 AC inverter eff * 1/2.5 hour discharge rate = 3,264 Watt peak load for well pump motor, etc. (seconds to minutes)

So--More or less, you are looking at a 1,000-1,500 Watt or so maximum "useful" AC inverter for your 48 VDC at 200 AH battery bank. A larger inverter will suck the battery bank down too quickly for larger loads. And finding a 1,500 Watt 48 VDC inverter may not be easy (usually, they have higher ratings).

If you only need a ~1,200-1,500 Watt AC inverter, you can get away with an 800 AH @ 12 volt battery bank

Or possibly a 1,500 Watt inverter on a 24 volt @ 400 AH battery bank.

Figuring out if you want, as an example, 6 volt @ 200 AH "golf cart" type batteries (pretty cheap in North America) and configure your battery bank as:

2 batteries in series * 4 parallel strings (12 volt @ 800 AH)--I really suggest 3 parallel strings maximum, and 1-2 parallel strings as better
4 batteries in series * 2 parallel strings (24 volt @ 400 AH)--Not a bad configuration
8 batteries in series * 1 series string (48 volt @ 200 AH)--A single string is nice--But 48 volt inverters may be too large to be practical for this size bank

The above is just how to "think" about your battery options. You can get a 2 volt cells that run from 500 AH to > 2,000 AH and configure your own string:

https://www.solar-electric.com/residential/batteries-battery-storage/deep-cycle-batteries.html?limit=all&nav_battery_voltage=2+Volts

Or you can get "batteries" (batteries are just a few cells mechanically connected together) in 4, 6, and 12 volt (and other configurations).

A 6 volt @ 200 AH battery weighs about the same (and is the same size) as a 12 volt @ 100 AH battery (and they both store the same amount of energy).

Which is best for you--The best plan is to find batteries that are common to your area (and local suppliers) and do several configurations--See what works out best for you. Different types of batteries (forklift, golf cart, AGM, flooded cell, etc.) are all over the place in terms of pricing and available sizes (voltage and Amp*Hour).

-Bill

Rayson · January 2017

mike95490 said:

I will give you a suggestion to look at 6V 200ah Golf Cart batteries. 8 in series and you are all set. Cheap batteries, made by the millions for golf carts, and sturdy enough to last 2-5 years off grid.

My 2nd thought is, 48V @ 200ah is a pretty small bank, and are you sure that the size calculation is right?

Worth looking at, crazy how items can be so different in price based on use and sales alone.
Yes, the numbers are not that important but there are a lot of options still being considered outside the actual components.
We are looking at the difference between taking all energy during sunlight hours or all during night, different Panel/Storage percentages but it is a question for latter.

The system is being built in a module fashion in divisions of Load, so adding another 200Ah or starting with 800Ah is essentially the same

BB. said:

More or less, you want a "cell" that is the AH rating of the bank... If you want a 400 AH battery bank, then the cell(s) will be 400 AH.

Whether that is 24x 2 volt cells, or 8x 6 volt batteries (3 cells each), or 4x 12 volt batteries (6 cells each) @ 200 AH--It really does not matter.

Different types of batteries (forklift, golf cart, AGM, flooded cell, etc.) are all over the place in terms of pricing and available sizes (voltage and Amp*Hour).

What you have said fits pretty well with my thinking, basically it doesn't make any difference other then getting a good deal on product with suitable service life.

AGM distributor is talking about a replacement deal they offer after so many years, but has not replied as to what component of cost goes to this insurance type scheme.

I am not comfortable with this.
New Storage Battery Systems are being developed.
Thinking a cheaper short term solution could fit well.

You mention inverter, it doesn't matter but the system doesn't require inverter, just unregulated 48v DC direct from Batteries.

Appreciate your input.

Ray

softdown · January 2017

I have only been here a couple of years. Yet I seem to recall that a major battery break through was "just around the corner" ....a couple times/year.

I bought into that line of thinking and bought somewhat cheap batteries my first time....plus they were highly recommended by a self proclaimed solar pro. A mediocre idea though they have not been real bad. If you are going to do kind of cheap batteries.....golf carts are usually the best deal. Here in America anyway...

I do not recommend 12 volt solar batteries unless they are of very high quality. One bad cell = junk the battery.

Estragon · January 2017

BB. said:

VA = 220 V * 10 Amps = 2,220 VAA typical induction motor runs around PF~0.67 to 0.80 or so... So the real power (watts):
Power = V*I*PF = 220 volts * 10 amps * 0.67 PF = 1,474 Watts
So the actual power from the battery bank will be less than you would first guess with your AC induction motor measured current because of "power factor".

-Bill

I'm confused (not unusual). Wouldn't the power needed from the bank be the total of work done plus heating losses caused by PF<1. Maybe just wording... 1474 watts of work at motor needs 10a@220v out of inverter to get done?

Estragon · January 2017

Rayson said:

AGM distributor is talking about a replacement deal they offer after so many years, but has not replied as to what component of cost goes to this insurance type scheme.

I am not comfortable with this.
New Storage Battery Systems are being developed.
Thinking a cheaper short term solution could fit well.

Sounds like some sort of extended warranty. For most products they seem to be more of a profit center for vendors than any sort of real insurance. Obviously, prepaying for replacement runs the risk that the distributor won't be there when the time comes.

I don't know that there are "new" battery systems being developed, at least none that are likely to be in production in the lifetime of whatever battery you end up getting. What likely will happen is EV related production will scale up and drive the price of existing lithium systems down.

Rayson · January 2017

Estragon said:

Rayson said:

AGM distributor is talking about a replacement deal they offer after so many years, but has not replied as to what component of cost goes to this insurance type scheme.

I am not comfortable with this.
New Storage Battery Systems are being developed.
Thinking a cheaper short term solution could fit well.

Sounds like some sort of extended warranty. For most products they seem to be more of a profit center for vendors than any sort of real insurance. Obviously, prepaying for replacement runs the risk that the distributor won't be there when the time comes.

I don't know that there are "new" battery systems being developed, at least none that are likely to be in production in the lifetime of whatever battery you end up getting. What likely will happen is EV related production will scale up and drive the price of existing lithium systems down.

My thoughts too, Estragon

I contacted Tesla, immediately when they first announced the plans to Manufacture 7kWh lithium-ion Battery Storage "Powerwall".
Confirmed with Tesla the 7kWh system would be available for off-grid and included this criteria in the list questions for purchase.

A year later in September 2016 I am contacted by Bradford Solar under CSR Australia.
Asking where I would like the "Powerwall Grid Connection" made?

When I explain the application was made for off-grid purchase, CSR Bradfords explain:
Quote: The Tesla Powerwall is not suitable for off-grid application. Tesla have strict install provisions, one which is only certified installers are authorised to install the Tesla Powerwall.

Origin, the company that provides Electricity in Australia, has also teamed up with Tesla.
https://www.originenergy.com.au/for-home/solar/systems-batteries/tesla-powerwall.html

The subsidies and incentives to supply and install Grid Connect systems is working for the Power Generators with Government Support.

Tesla will not reply as to why they have reneged on an offer to supply.
Worst Still, Tesla are providing the 7kWh Powerwall off-grid in other countries.

Suspect there will be a few rusted on supporters of these Companies here, I will not be drawn into argument, save your energy

Ray

Rayson · January 2017

softdown said:

I have only been here a couple of years. Yet I seem to recall that a major battery break through was "just around the corner" ....a couple times/year.

I bought into that line of thinking and bought somewhat cheap batteries my first time....plus they were highly recommended by a self proclaimed solar pro. A mediocre idea though they have not been real bad. If you are going to do kind of cheap batteries.....golf carts are usually the best deal. Here in America anyway...

I do not recommend 12 volt solar batteries unless they are of very high quality. One bad cell = junk the battery.

The brands used in America could be the same as those in Australia,
Do you have any experience or hear of good experience with particular 6v brands used in Golf Carts?

BB. · January 2017

It sounds crazy--But the "work done by the motor" (voltage and current sine waves and how they 'relate' to each other by phase)--It is that Cosine/PF "fudge factor" that makes a big difference in the power supplied by the inverter (and battery bank).

Work is force * distance. If you pull very hard at 90 degrees to a car, and the car does not move at all--There is "zero" work being done.

This is the "phase relationship" between voltage and current (AC world). When they are exactly in phase with each other (zero degrees of offset)--Then 100% of the current times the voltage is "real work" being done.

If there is an offset between voltage and current, it is like pulling at an angle on the car. Only the motion "going forward" times the (reduced) force is work being done.

10 amps * 220 volts (VA) * (0.15 inverter losses + 0.03 "wiring resistance losses") = ~396 Watts in "losses" (inverter+wiring)
6.7 amps * 220 volts (VA) * (0.15 inverter losses + 0.03 "wiring resistance losses") = ~265 Watts in "losses" with PF=1.0

The above are just guesses (the inverter may have different than 15% losses between lower and higher output current levels). The loss curve is not "flat" from zero to maximum inverter output.

It can be possible to "correct" the power factor of induction motors (to PF~0.95 maximum) with a "motor run" type capacitor. But this is usually only done in companies with lots of large motors (such as an oil refinery).

Residential power meters (usually) only charge for Watts used (not VA or Power Factor Corrected draw). So power factor correction does not save a home much money (i.e., that ~1-3% wiring losses for the "extra current").

Large commercial users are charged for VA (or kVA)--Basically the Watt*Hours (or kWH) * 1/PF... So a motor running 1/0.67 PF would increase billing costs by 1/0.67 = 1.49 (almost 50% higher electric bill) for the "bad power factor".

Sort of make sense (AC power math is quite complex--This is a very simple explanation of just one element).

-Bill

softdown · January 2017

Rayson said:

softdown said:

I have only been here a couple of years. Yet I seem to recall that a major battery break through was "just around the corner" ....a couple times/year.

I bought into that line of thinking and bought somewhat cheap batteries my first time....plus they were highly recommended by a self proclaimed solar pro. A mediocre idea though they have not been real bad. If you are going to do kind of cheap batteries.....golf carts are usually the best deal. Here in America anyway...

I do not recommend 12 volt solar batteries unless they are of very high quality. One bad cell = junk the battery.

The brands used in America could be the same as those in Australia,
Do you have any experience or hear of good experience with particular 6v brands used in Golf Carts?

No I do not. Plus I do not recall posters finding one brand is better than others.

I would steer away from the "high performance" golf carts unless they weigh much more. They have this slightly underhanded trick of juicing up the specific gravity and charging more due to more deliverable Ah of energy. The price paid for this is a battery that lives a shorter life. Pretty much a gimmick in my book.

Golf cart batteries are not inferior batteries. They make millions and that drives the costs down.

Rayson · January 2017

softdown said:

I would steer away from the "high performance" golf carts unless they weigh much more. They have this slightly underhanded trick of juicing up the specific gravity and charging more due to more deliverable Ah of energy. The price paid for this is a battery that lives a shorter life. Pretty much a gimmick in my book.

Golf cart batteries are not inferior batteries. They make millions and that drives the costs down.

That is a good point that I have picked up from the forum, and not something that I was thinking in an effort to dismiss weight or size as factor. You can see where the lines have been blurred by marketers to play into the uninformed demand.

Cheers

2V and 6V battery amp-hour comparison

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