# Help with Ah calculation



## Geff (Sep 25, 2014)

Hey smart folks...
please help me figure this out.

i have 38x batteries that are this type:
Voltronix 160 Ah Lithium Iron Phosphate Batteries.

need to figure out Battery capacity. Main battery’s capacity in Amphours (Ah).
so i can enter it in my Xantrex Linkpro Battery Monitor.

Thanks in advance!
Geff


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## Duncan (Dec 8, 2008)

Hi Geff

Your cells are 3.2v x 160Ah (Ampere hour)
If you connect all 38 in series you end up with 38 x 3.2 = 121V and 160Ah
If you connect them up as 19 in series x 2 in parallel you end up with 19 x 3.2 = 60v and 160Ah x 2 = 320Ah


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## Geff (Sep 25, 2014)

Thanks for clear break for for me.
You rock Duncan!!!!


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## dougingraham (Jul 26, 2011)

Geff said:


> i have 38x batteries that are this type:
> Voltronix 160 Ah Lithium Iron Phosphate Batteries.


I think Duncan got you answered for the Xantrex. I am replying because my immediate thought was you wanted to know the capacity of the battery. Battery capacity is measured in watt hours abbreviated as wh. The other common term is kilo watt hours (kwh) which is simply watt hours divided by 1000. In your case you have 38 batteries with a nominal voltage of 3.2 volts and a capacity of 160 amp hours. This gives 38*3.2*160 = 19456 watt hours or 19.5 kwh when rounded.


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## Geff (Sep 25, 2014)

That great info to have, Thanks!

I have all 38 batteries inline, connect as one battery.

I have another question for you smart people, maybe I should start another thread, but will ask here also.

When charging for the first few time, I was told to keep an eye on the voltage as it is near the end of the charge cycle...
What should the voltage be in my case?
123.0? 125.0? What if it spikes? What should I keep my eyes on? Any advise is much appreciated!
I am going to do a few full charges tomorrow.
Charge, drive a few mile, repeat cycle to test and make sure charger working properly.


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## Duncan (Dec 8, 2008)

Geff
You need to be very careful - you could easily damage or destroy your battery pack

Look up - Top Balance, or Bottom Balance

There are three ways of using lithium cells
(1) Use a "Battery Management System"
Which will measure each cell and effectively stop charging cells when they are full
BMS's can be good BUT they have also caused all sorts of problems
People either swear on them or at them

(2) You become the BMS
Top balance
You charge each cell individually up to the "top" so that they are all the same,
Then you use the car charging to the top each time
This way you won't damage any cells through overcharging

(3) You become the BMS
Bottom balance
You discharge each cell individually down to the bottom so that they are all the same,
Then when you charge the car you check the cells and STOP when the highest cell reaches the top (the other cells will be lower)
This way you won't damage any cells through over discharging

You do need to one of the three or you will probably kill some cells


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## Geff (Sep 25, 2014)

Thanks Duncan, yes I have read up on some of that You mentioned. I will read more 
So long story short, My pack was bottom balance, When I had a conversion shop put in my battery pack and xantrex battery monitor,
For some reason when going thru the functions of the xantrex, I realized they had put the setting of the pack at 130 amp hours.
I called them to find out why, they said they had done that so customers won't over charge and that setting gets set properly when the car goes back in a few months later for a checkup.
Well, They are too far for me to bring the car back so I am making sure my battery packs capacity, which you told me previously was 160 Ah, now that's been set properly, I drove the car a few miles and started to charge ,checking the xantrex for voltage reading, but decided to unplug the cord when I saw that the voltage climbing higher than I have seen before. And decide ask again before proceeding.

Does anyone know of any Ev conversion specialist with 30 miles of burbank, California?

Thanks
Geff


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## JonasFilipe9 (Jan 9, 2015)

dougingraham said:


> I think Duncan got you answered for the Xantrex. I am replying because my immediate thought was you wanted to know the capacity of the battery. Battery capacity is measured in watt hours abbreviated as wh. The other common term is kilo watt hours (kwh) which is simply watt hours divided by 1000. In your case you have 38 batteries with a nominal voltage of 3.2 volts and a capacity of 160 amp hours. This gives 38*3.2*160 = 19456 watt hours or 19.5 kwh when rounded.



Hi doug, 

I think you are not technically correct in some aspects. 

Battery capacity is usually measured in Ah. (Electric charge)
What you referred is the energy in Wh or kWh. 

This is referred in this document from MIT to understand every parameter: http://web.mit.edu/evt/summary_battery_specifications.pdf

Cheers


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## Sunking (Aug 10, 2009)

Geff said:


> i have 38x batteries that are this type:
> Voltronix 160 Ah Lithium Iron Phosphate Batteries.


Amp Hours is meaningless without a nominal voltage. Battery capacity is specified in Watt Hours, not Amp Hours. You have 19456 watt hours or 19.456 Kwh total capacity. So what voltage are you going to be running the batteries at? There are 3 possible configurations using 38 cells

19456 wh / 121.6 volts = 160 AH
19456 wh / 60.8 volts = 320 AH
19456 wh / 3.2 volts = 6080 AH

Take your pick.


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## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Amp Hours is meaningless without a nominal voltage. Battery capacity is specified in Watt Hours, not Amp Hours. You have 19456 watt hours or 19.456 Kwh total capacity. So what voltage are you going to be running the batteries at? There are 3 possible configurations using 38 cells
> 
> 19456 wh / 121.6 volts = 160 AH
> 19456 wh / 60.8 volts = 320 AH
> ...


You're battery associations are right and that what matters to our friend.

That happens because when you associate cells in paralel the total bank capacity is the sum of capacity of the cells (works like parallel current sources).
When you associate cells in series the only thing you do is rise the voltage but the capacity of the battery pack is the capacity of just one cell (the lowest).

For what I've seen in many books and literature: Q=Ixt (this is Peukert's law)
That gives you A.h units. Capacity is the current drawn from battery times the amount of time (in hours) that a battery can sustain. I think this is not wrong, but may be we are talking the same thing different ways. Anyway, what matters are the calculations


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## dougingraham (Jul 26, 2011)

JonasFilipe9 said:


> Battery capacity is usually measured in Ah. (Electric charge)
> What you referred is the energy in Wh or kWh.


The capacity of a single cell of a particular type can be measured in Amp Hours. The capacity of a battery which is a collection of cells cannot be measured this way. Amp Hours is only a portion of the calculation. And normally the Nominal cell voltage is used for the other portion of the calculation. Amp Hours times voltage gives Watt Hours or Kilo Watt Hours and is the figure of merit that must be used. As for knowing the State Of Charge you can most easily count Amp Hours in when charging and out when discharging a Lithium battery and this will tell you approximately how much energy you have remaining. I say approximately because the voltage does matter. It is watts (volts time amps) that moves your vehicle. You use less amps when the pack is full because the pack voltage is higher and you use more amps when the pack is empty because the voltage is lower. Peukert has almost no effect on Lithium type cells. So keeping track of watt hours out of the pack would give you a better idea of how much energy you have remaining in the pack it is more difficult to keep track of.


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## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> That gives you A.h units. Capacity is the current drawn from battery times the amount of time (in hours) that a battery can sustain. I think this is not wrong, but may be we are talking the same thing different ways. Anyway, what matters are the calculations


AH is meaningless because Amp Hour goes in at a higher voltage then they come out.


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## Sunking (Aug 10, 2009)

dougingraham said:


> The capacity of a single cell of a particular type can be measured in Amp Hours. The capacity of a battery which is a collection of cells cannot be measured this way. Amp Hours is only a portion of the calculation. And normally the Nominal cell voltage is used for the other portion of the calculation. Amp Hours times voltage gives Watt Hours or Kilo Watt Hours and is the figure of merit that must be used.


Correct. For those non believers answer a very simple question.

I have a single 100 AH cell. What watt hour capacity is the cell?

Trust me any answer you give is wrong and I will prove it beyond any reasonable doubt. The lesson will tell you a LFP is 100% efficient if you only count Amp Hours, But round trip efficiency is not 100%


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## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Amp Hours is meaningless without a nominal voltage. Battery capacity is specified in Watt Hours, not Amp Hours.


What is A.h then?



Sunking said:


> I have a single 100 AH cell. What watt hour capacity is the cell?
> 
> Trust me any answer you give is wrong and I will prove it beyond any reasonable doubt. The lesson will tell you a LFP is 100% efficient if you only count Amp Hours, But round trip efficiency is not 100%



You are right. It is not linear. The energy that the cell may give you depends on the current you use (time of discharging:charging).
That's the definition of capacity. If you discharge a 100Ah cell in 10 hours, you'll have a certain amount of energy. If you discharge the same cell in 1 hour, the available energy of the cell is almost half the energy you have in 10 hours discharge. (This considering the cell is C_10=100Ah)

Ps.: what do you mean by round trip?


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## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> AH is meaningless because Amp Hour goes in at a higher voltage then they come out.



You guys are right. We should look up for energy of the battery. Not capacity once Ah is different for different test conditions. 

So may should we publish a paper saying Ah is crap? Lol

Other question I have is how they do those voltage vs Ah graphs?
They need to ensure the current is constant what is very far from the truth in a real system. Any way you need to have fixed variables if you want take any conclusions... 
The best I've made so far near that is to use a resistive load (current deviation is 1A from charged to discharged pack). My graphs are then voltage(time) and not voltage(A.h) because current is changing right?


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## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> What is A.h then?


Meaningless without a nominal voltage.



JonasFilipe9 said:


> You are right. It is not linear. The energy that the cell may give you depends on the current you use (time of discharging:charging).
> That's the definition of capacity. If you discharge a 100Ah cell in 10 hours, you'll have a certain amount of energy. If you discharge the same cell in 1 hour, the available energy of the cell is almost half the energy you have in 10 hours discharge.


Hogwash Peukert Law does not apply to LFP batteries. You are stuck in a Pb mentality. I have a 100 AH cell in my hand. What is the watt hour capacity? Answer the question.


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## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Hogwash Peukert Law does not apply to LFP batteries. You are stuck in a Pb mentality. I have a 100 AH cell in my hand. What is the watt hour capacity? Answer the question.


Does not apply? But manufacturers use the same nomenclature to these cells when they refer at the specifications the cell is C_10 ou C_5 for example. 

It depends on the nominal cell voltage.


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## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Ps.: what do you mean by round trip?


Round trip in of watt hours in vs watt hours out. On any battery watt hours in will always be greater than watt hours out. On LFP AH in = AH out for 100% efficiency, but does not apply to actual energy because amps hours go in at a higher voltage than going out. So anyone who believes LFP is 100% efficient is full of beans and blowing smoke. 

In any battery system you never design based on Amp Hours because it is meaning less. Everything is designed on energy which is Watt Hours. If I want a 100 mile range and my vehicle has a efficiency of 300 wh/mile I know immediately I have to have at least a 30 Kwh battery weighing 300 Kg and cost roughly $12,000. I don't give a crap about AH until I decide on a nominal battery voltage I am going to use of say 144 volts. That would tell me I need a 45S LFP 208 AH battery.


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## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Does not apply? But manufacturers use the same nomenclature to these cells when they refer at the specifications the cell is C_10 ou C_5 for example.
> 
> It depends on the nominal cell voltage.


You are stuck in a Lead Acid battery box my friend. Get out of that box. A LFP battery has the same AH rating @ C/20 and 1C. Makes no difference a 100 AH LFP cell can deliver 5 amps for 20 hours or 100 amps for 1 hour. Peukert and is stupid law does not apply to LFP. LFP is not a chemical reaction, it is a ion exchange. That is what Doug and I are trying to tell you. You cannot apply lead acid or any other battery practices to lithium batteries. They behave differently than any other battery. Forget everything you know about Pb batteries, it does not apply.


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## Karter2 (Nov 17, 2011)

Sunking said:


> ....A LFP battery has the sa me AH rating @ C/20 and 1C.
> . a 100 AH LFP cell can deliver 5 amps for 20 hours or 100 amps for 1 hour.
> ....Peukert and is stupid law does not apply to LFP. .


I assume you are over simplifying for the sake of clarity ?
You certainly won't get the same useable capacity from the cell at 20c as you do at 1c.


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## Sunking (Aug 10, 2009)

Karter2 said:


> You certainly won't get the same useable capacity from the cell at 20c as you do at 1c.


I never said that. I said you will get the same AH rate at C/20 and 1C.


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## JonasFilipe9 (Jan 9, 2015)

Sunking said:


> Round trip in of watt hours in vs watt hours out. On any battery watt hours in will always be greater than watt hours out. On LFP AH in = AH out for 100% efficiency, but does not apply to actual energy because amps hours go in at a higher voltage than going out. So anyone who believes LFP is 100% efficient is full of beans and blowing smoke.


Yes, Energy going in is also higher than the energy going out because the cell has an internal resistance. To put the battery at 3.6V you need to dissipate a certain amount of energy in that resistance. But when you extract the energy from that 3.6V cell, it will dissipate its energy in the internal resistor so you get less energy from the cell than you give.
There is a very interesting video of MIT showing this also: https://www.youtube.com/watch?v=_oXumsWS91M



Sunking said:


> You are stuck in a Lead Acid battery box my friend. Get out of that box. A LFP battery has the same AH rating @ C/20 and 1C. Makes no difference a 100 AH LFP cell can deliver 5 amps for 20 hours or 100 amps for 1 hour. Peukert and is stupid law does not apply to LFP. LFP is not a chemical reaction, it is a ion exchange. That is what Doug and I are trying to tell you. You cannot apply lead acid or any other battery practices to lithium batteries. They behave differently than any other battery. Forget everything you know about Pb batteries, it does not apply.


Sunking, I'm not putting your hipothesis apart or maybe I'm not sure what you mean by "you will get the same AH rate at C/20 and 1C." But all sources show that the faster you charge the battery, less capacity you'll have. 
Plese see manufacturer datasheet: http://www.hipowergroup.com/uploadfile/2011/0115/20110115123901167.pdf

This website: http://www.simon.richardson.net/electric/batteries/liion.html

which cites this document http://www-scf.usc.edu/~rzhao/LFP_study.pdf which says: "With the increasing current density, LiFePO4 may be affected by capacity loss due to diffusion-controlled kinetics of the electrochemical process." citing a Journal of The Electrochemical Society document.


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## Sunking (Aug 10, 2009)

JonasFilipe9 said:


> Y
> 
> 
> 
> ...


Me thinks you need to study up on how C-Rates are expressed, and how to read graphs. For example C/20 C is the Battery AH Capacity divided by hours. So for a 100 AH battery C/20 discharge rate is 5 amps for 20 hours. 1C or C1 is the one hour rate. For the same 100 AH battery 1C = 100 amps for 1 hour.

Now go to your Hipower Graph titled "_Battery Discharge Curve In Different C Rates_". It shows AH for 1C, 2C, and 3C. So pick any battery in their lineup say 100 AH. So 1C = 100 amps discharge, 2C = 200 amps, and 3C = 300 amps. Now pick a cut-off voltage of 3 volts, and read the time

3C = 20 minutes or 300 Amps x .3333 Hours = 100 AH
2C = 30 minutes or 200 Amps x . .5 Hours = 100 AH
3C = 60 minutes or 100 Amps x 1 Hour = 100 AH

Now for my own Graph:

C/20 = 1200 minutes or 5 Amps x 20 Hours = 100 AH.

Peukert Law does not apply to LFP batteries. Peuker Law applies heavily to lead acid, NiCd, and NiMh.


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## Tomdb (Jan 28, 2013)

Sunking. Please dont bash on about things that are not true.

FACT: there is a peukert effect for ALL batteries. However for lithium its so minor you can marginalize it.

Now just to make it crystal clear. When charging the battery you put energy in, two places the energy can go; in the battery and heat generation.
When discharging the same occours.

Actual test results from a very repituble source.









http://lygte-info.dk/review/batteries2012/Sony%20US18650NC1%202900mAh%20%28Green%29%20UK.html

and a paper on the subject
www.mdpi.com/1996-1073/6/11/5625/pdf


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## major (Apr 4, 2008)

Tomdb said:


> .... there is a peukert effect for ALL batteries. However for lithium its so minor you can marginalize it.


I think there is a Peukert effect for all batteries and often, with Lithium, you can ignore it. It depends on the specific type. This Peukert effect deals with the capacity, not energy. So the graph showing Wh is a poor example. Here are a few capacity plots I picked off a google image search. Again; just examples and sorry, I didn't save the sources to reference.


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## Sunking (Aug 10, 2009)

Tomdb said:


> FACT: there is a peukert effect for ALL batteries. However for lithium its so minor you can marginalize it.


For starters the Sony you linked to is NOT LFP, so your argument is invalid and point is moot. 

For LFP 1C and less Peukert effect does not exist. Above 1C is so insignificant it can be ignored and disregarded. Cannot speak for all EV designs but with my 16S 100 AH pack at full speed pedal to the metal I am only drawing 60 to 70 amps or less than 1c on a flat level paved surface and up to 90 amps going up hills around here. The only time I ever exceed 1C is when accelerating for a few seconds. I get roughly 120 to 130 wh/mile and on a full charge on a 16S 100 AH pack have gone 35 miles on a single charge. That is right on calculations with no Peukert correction factor.

AH is a Marketing trick manufactures play with numbers. In the lead acid world deep cycle consumer grade batteries are rated at the C/20 or 20 hour discharge rate. Industrial and Commercial Grade batteries are rated from 2 to 8 hour rates. Large format LFP are rated at 1C to 3C.


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