Battery amp question

[xelasnave]

My brother in law is going to purchase 6 batteries each of 2volts and rated at 420 amp hours.

The batteries will be in series to produce 12 volts.

His view is that he will have 6 x 2 volts to make up 12 volts and 6 x 420 amp hours or 2520 amp hours
with the 12 volts system.

My thoughts is that the amp hours will be 420 amp hours at 12 volts.

He is a lawyer so please cite your qualifications and reference something that sets out the science.

Thank you.
Alex

[phomer (Paul)]

Alex,
You are correct, he will end up with 12v at the same number of amp hours but if measured as Watt hours it will end up being 12x420.

The number of amp hours supplied by each "cell" does not change, only the voltage if sequential. If parallel then he will have 2v @ 6x420 amp hours.

Paul

Originally Posted by xelasnave

My brother in law is going to purchase 6 batteries each of 2volts and rated at 420 amp hours.

The batteries will be in series to produce 12 volts.

His view is that he will have 6 x 2 volts to make up 12 volts and 6 x 420 amp hours or 2520 amp hours
with the 12 volts system.

My thoughts is that the amp hours will be 420 amp hours at 12 volts.

He is a lawyer so please cite your qualifications and reference something that sets out the science.

Thank you.
Alex

[sharkbite]

Hi Alex - you are correct.

put simply -Ampere hours are the number of hours a battery will continuously supply 1 ampere.
(420AH batteries must be HUGE! a typical car battery is 70ah)
the number is used to calculate how long a battery last
based on a known discharge rate.
If you draw 2 amps out of these, they should last 210 hrs,
3 amps 140 hours, etc.)

To make 12 volts, you need to put them in series, resulting in each battery
flowing at the same current....
so if you have a 12v device that is drawing 1 amp from this 12v battery, each cell will be discharging at 1 amp.

If he wants a 2520Ah battery, the brave soul* could wire them in parallel,
but the resultant battery would only be at 2 volts.

Here is a good reference:

https://www.batterystuff.com/kb/arti...-tutorial.html

Sharkbite
AscDip Elec Eng.

the defence rests.


* i say brave, because judging by the capacity of these units, they could potentially be able to supply a fair bit of current.
If you try to connect them in parallel, and one cell is not charged as much as the others,
they could potentially all flow current into the least charged one, at a rate greater than it was designed to take...
resulting in - BOOM!

At this capacity and voltage - they must be cells for solar batteries?

In no way meaning to insult - these units are not for yer mug punter to play with...if you wire them wrong,
or rapid discharge them, due to a short , or some other thing, serious injury can result.
Please urge him to be careful - and if in doubt, call in an expert tradie.

[xelasnave]

Thank you both.
I knew I was right ...I mean the thought of me being wrong is unimaginable☺

He is always right as well.
I had to back down cause he is nit well and was starting to breath heavily....particularly after showing him Pauls winderful post.

The confusion comes from the use of watt hours for him.

Sharkbite.
Where I am at the moment haa 1100 amp hours for a 12 system...the batteries are on the way out so I am costing replacements...I have spoken with the guy supplying my brother in law and you just can not pin him down to answer one simple question " so how many amps is that?" He talks about ratings and how the draw rate gives different ratings but the way I have always approach the matter is how many amps...right or wrong I figure if 1000 amps I can use say 200 to 250 before a recharge but ...so here the daily requirement is 250 amps so hopefully 1000 amps is the size needed...as I type however I am not sure about having a correct approach...anyways I purchased 500 amp hours about 6 months ago for the shack where I was going to put my observatory...but mythinking is to get another 1000 amps of the same batteries bring the ones in the shack to the main house and all up boast a 1500 amp hour bank.
My brother in law was saying in effect six of "his" batteries would get 2000 approx amp hours, but he /they are talking watt hours it seems....anyways I said he was right simply to settle him down...but I think I will buy mine at the place I bought the earlier ones as they seem to approach things the way I do.
Alex

[mynameiscd (Andy)]

Hi Alex,
After living off grid for about 8 years I went down the12 volt path and finally came around to 48 volt.
It gives you so much more flexibility and less copper needed and that voltage drop thing with 12 volt over distance.
Most inverter chargers will take 48 volts and now most panels are 24 volt.
The 24 volt panels are so much cheaper than 12 volt ones.
The panel total voltage is 120v (3x40v in series) so a MPPT gets good voltage even when there's a little cloud.
Also i ran a step down transformer to a 12 volt system for lighting and charging.
When I had only the 12 volt system I was always running stuff at 11.8v or less because of wiring distances.
A 12 volt system will not deliver a constant 12v especially if you have distance. I wired everthing with 4mm initially and its still not enough to cover voltage drop.
Those 2v agms are good but its pretty expensive to get 24 of them.
48v at 420A is a pretty good system and at 240 volt its about 105A.
Take off 50% at least for longevity of the batteries (75% recommended) thats still a toaster on for 5 hours
By the way you are right about the volts amps thing.
Also tell him Watts=Amps X Volts
Cheers
Andy

[sharkbite]

The confusion comes from the use of watt hours for him.

Yes i can understand the confusion...

Capacity and draw rate are two different things...

the "Ampere hour" rating tells you how long your batteries will
supply a given current...this is the battery capacity

However....

batteries don't like being charged or discharged too quickly....
(too great a draw rate will reduce their lifespan)

So you need more of them in parallel to spread the load....
...and when you do this, it also increases your capacity.

(there, now i have cleared that up ;-) )

[The_bluester (Paul)]

Easiest thing to do would be to work out your average current draw on the 12VDC side, inflate that by a safety margin (Say 50% for the sake of the argument) and ask how many amp hours you can expect at that discharge rate.

To rough some figures. Say your average current draw is 6A, add 50% and round it up to 10A and ask him how many amp hours the individual batteries are good for at a 10A discharge rate.

They will talk about a C rate, which is the discharge current at which the battery will provide the rated amp hours. If for instance the C rate was 10A then they would be 420AH at 10A. If C was 5A they will be somewhat less than 420AH at 10A discharge current (2C)

[leon]

I have never heard of a 420 Amp Hour Battery, if it was available it would cost you a fortune.
What is wrong with 120 amp hours a bit of over kill Alex

Leon

[xelasnave]

Originally Posted by leon

I have never heard of a 420 Amp Hour Battery, if it was available it would cost you a fortune.
What is wrong with 120 amp hours a bit of over kill Alex

Leon

Hi Leon
Just got back from Nimbin where I purchased 6 two volt cells to give me 570 amp hours at twelve volts to add with the set I bought to go in my shack so all up will have 1140 amp hours at 12 volts.

This will run the house, with a 240 vlt fridge lights tv and when the Sun is out run my daughters gaming computer...it draws heaps.

But really it would be good to have double.

I cant remember if I mentioned it but another brother in law...I have many brother in laws she was one of 11 gave her 8 solar panels of 185 watts each to add with what is ther...4 x 300 and 7 x 100....and another inverter so I should be able to set them up with decent power...menwhile I hope to find a small caraven for my self one day


I scored a 2 kva genny at Aldi for $200 so now there is a 5 kva, 2 key start 2kva and 2 small 700 watt units ...six all up

and I have the scope set up now with a 120 amp hour battery, 120 watt solar panel and a small 200 watt inverter...

And there is a wind generator but I am yet to be convinced that contributes.


Alex

[xelasnave]

Thanks Andy, thanks SharkBite and thanks Paul.
I really appreciate your input.
My brother inlaw is a very clever chap and I thought I was right but well I am always open to being wrong...but it would mean I have been doing this stuff foer over 20 years and was wrong all that time.
I did not rub it in and backed off cause he probably does not have long with us...so now I want to stop him from making a mistake in his purchase but I don't want to cause any upset...plus everyone always thinks I am right and Mr Perfect …. and you know how that goes..you are right but they don't like you for it.

alex

[Hoges (John)]

Just to add a couple of thoughts about batteries -



You generally shouldn't draw more than 50% of the stated capacity of the batteries before recharging. The less the better. Lead acid batteries tend to give up the ghost if you discharge them too deeply. Batteries are usually rated by cycle life - the deeper and more often you discharge them, the less they will last.



Also, lead acid batteries lose 1% capacity for every degree below 25C. so if it's zero degrees in your battery shed, you've effectively lost 25% of your capacity at that time.


My batteries (I'm off grid) are 660 amp 2v cells in series for 24v - if I could have afforded twice the capacity, I would have got it - three gloomy days and I have to start the genny up.


Lithium batteries would solve the temperature/discharge restrictions but they seem to be very slow to come on to the market for off grid supply. The last time I checked, the Tesla Lithium weren't suitable for off grid although there are other technologies starting to appear.

[gary]

Not directly related to Alex's question, but an interesting statistic I learnt
today about the numbers of of solar PV installations in Australia compared to
the U.S. which some readers with a thirst for trivia might be interested
to know as well.

This 19 Dec 2018 article at the Institute of Electrical and Electronics
Engineers Spectrum magazine web site discusses a neural network system
at Stanford University that searches satellite images for solar panels
installed in the 48 contiguous U.S states, from home rooftop to utility-owned
solar plant farms.

Their conclusion, 1.47 million solar installations in the 48 contiguous U.S states,
which was higher than previous estimates of 1.02 million.

Reference :-
https://spectrum.ieee.org/view-from-...s-solar-panels

By comparison, according to the Australian PV Institute and based on
the Clean Energy Regulators database of solar PV generators, which
represents all the systems that have been installed under the Commonwealth
Government’s Renewable Energy Target (RET) scheme,
as at 30 Sept 2018, there are 1.95 million PV installations in Australia.

Reference :-
http://pv-map.apvi.org.au/analyses

[sharkbite]

Originally Posted by leon

I have never heard of a 420 Amp Hour Battery, if it was available it would cost you a fortune.
What is wrong with 120 amp hours a bit of over kill Alex

Leon

Hows this grab ya?

https://www.commodoreaustralia.com.a...420h-2v-420ah/

If you look at what is printed on the product pictured it says 800ah.

And yes you would need a lotto win to make it worthwhile....

It was not all that long ago that capacities like this were unheard of...

It astonishes me ,and I used to work in a factory that made backup supplies for telephone exchanges.

[The Mekon (John Briggs)]

Originally Posted by leon

I have never heard of a 420 Amp Hour Battery, if it was available it would cost you a fortune.
What is wrong with 120 amp hours a bit of over kill Alex

Leon

In the early 1970's I served for a time on one of the government lighthouse supply ships. At that time some of the remote lighthouses were battery powered. We would replace these batteries (no solar charging then) about once a year. This involved carrying heavy "Edison" cells in special back packs up to the light. These big black batteries were around 2v and weighed plenty. I reckon they would have been around 420 amp hour rating - but its all so long ago.

[mynameiscd (Andy)]

John is right about the 50% thing even 25% will get the full 10 or so years out of AGMs. When I was working out the battery bank you did some simple maths.
Add up all the draw in watts eg
Lighting 6x3watt 12 volt globesx10 hours =180 watt hours.
Add up everything and it might come to 1 to 2 kwh
Then multiply by 50% discharge and then cloudy days say 3
So 1.5 kwh x 2 x 3 = 9kwh.
Divide by battery voltage to get amp hours
9000/12 = 750ah of battery bank needed to run for 3 days and only discharge at 50% not including inverter loss etc.
So 4x 12 volt 220ah in series should get this.

Or you can go a compleatly different way and have a good auto start genset in the system buy an underrated battery bank and hammer the batterys and get only 4 years out of them but burn more fuel with the genset.
Fuel is a lot cheaper than batteies and if you upgrade a smaller bank every 4-5 years in the long run it may be cheaper and also technology changeover every 4-5 years is better than 10-11 years.

Then only thing is most households use up to 10kwh which needs a huge battery bank.

Hope this all makes sense.
Cheers
Andy

[xelasnave]

I figure 250 amp hours per day here.
That is fine for the new bank of 1140.
What is exciting is how much the panels charge even on an overcast day.
If in doubt run the genny.
I have all the new batteries lined up to go in the house but I am going to give each bank a spare panel charge them up the link them in parrallel drain a little and top up with a single panel to allow them to stabalise ..Just had the stiches out of my ear and leg but going back to Sydney tomorrow or Sunday so have to install them next time.
Thanks for all the help.
I have decided to not say anything to the brother in law...the fact is he will get fair value and what he gets will serve his needs...he needs to be right whereas I dont care☺ give the dog a bone and let him be happy.
But I do think the way they sell these things is a little tricky...you cant get the guy to say how many amp gours you end up with...anyways I have mine...mmm got $350 off the price so I am happy.
Have a great xmas holiday break.
Alex