BU-1004b: How to Calculate Battery Capacity

Know how to maintain a battery fleet and eliminate the risk of unexpected downtime.

Everyone wants a battery that is small and light, but a pack must provide critical additional attributes. Besides long runtimes, design engineer must satisfy load requirements and choose an architecture that is durable to deliver the needed cycle count in the environment deployed.

One of the best tools to calculate battery capacity is the Ragone plot. [BU-503: Determining Power Deliver by the Ragone Plot] It provides a snapshot of how specific energy in Watt/hour works hand-in-hand with power in Watts as the delivery vehicle.

Once energy and power requirements are satisfied in lab conditions, the works continues to find the limitations with a faded pack at hot and cold temperatures. These fringe areas must be defined and included in the specifications. Only by fully understanding the boundaries can battery and device deliver the anticipated performance for many years to come. 

To approve and pass a device, device manufacturers prefer picking the best battery from the lot. This satisfies the present moment but ignores real field conditions. Capacity fade occurs naturally with age and cannot be ignored.

Once a new product is released and rubber-stamped, few provisions exist to hint at what capacity a battery should be replaced to maintain reliability. Capacity is the leading health indicator that also predicts the end-of-battery-life. Measuring the internal resistance, as could be done with older battery technologies, no longer provides reliable state-of-health results. The resistance of the modern battery stays low while the capacity decreases in a predictable fashion with use and age.  

A typical capacity bandwidth of a well-managed battery fleet ranges from 80 to 100 percent. Packs below 80 percent should be replaced. In addition, the user should include 20 percent spare capacity to manage unforeseen activities. Figure 1 illustrates these requirements in the 20-20-60 capacity calculation.


Battery fade Figure 1: Calculating spare battery capacity
Reserve capacity must be calculated for a worst-case scenario. The allowable capacity range is 80-100%; a spare capacity of 20% is recommended for critical use.

The proposed 20-20-60 capacity calculation is conservative and some battery users may consider relaxing the risk factor to keep the battery longer. To do this reliably, the spare capacity after a mission or a day’s shift must be known. On the “Auto” program, the Cadex battery analyzers accomplish this by applying a discharge before charge. The first reading on the analyzer’s display relates to the spare capacity and the second is the full capacity the battery is capable of delivering with a full charge.

The spare capacity in a battery fleet should be fine-tuning to find the optimal compromise between economy and risk factor. If, for example, the lowest performing battery in the fleet comes back with still 30 percent spare capacity, then the threshold capacity can safely be lowered from, say, 80 percent to 70 percent. This enables keeping batteries in service longer without adding increased risk.

Batteries powering medical and communications devices are normally replaced when the capacity falls from the original 100 percent to 80 percent. Scanners in warehouses can often provide a full day’s work with a 60 percent battery, and starter batteries in vehicles will still crank at capacity of 40 percent. A battery analyzer calculates the required capacity for a mission and calculates at what capacity threshold the pack should be replaced. This provides optimal service life without compromising risk.

Last Updated: 20141128

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On April 23, 2011 at 3:07am


On July 4, 2011 at 4:30am
Adrian Smit wrote:

We are trying to locate the best battery for a vehicle tracking device, the battery needs to be a small as possible, have no charge source once it has been full charged our devices are not hot wired into the vehicle the device needs to live on its own battery for as long as possible. Our devices don’t have any LED lights or battery drawing components other that the GPS and GSM modules. Can you suggest the right battery for us.

Regards Adrian Smit.

On September 25, 2011 at 11:46am
satyendra wrote:

hi every one…
ageing of batteries matters for calculating the capacity. like when cells are new, it has 100% capacity but as time passes and usage cells goes on capacity decreases…so how to know exactly what is the fraction of capacity reduced while using.

On October 14, 2011 at 8:38am
mahmood wrote:

hello SM TAHIR HUSSAIN! im MBA student of PAF-KIET doing my finance elective course of financial modeling & forcasting for this i have selected atlas battery to work so i need a company contact person to have a quality output so please allow me to contact with you, your support will be a great addition to my study report!

looking forward for your kind support!

im available at email: mahmoodsofur@gmail.com

On December 13, 2011 at 8:26pm
chris king wrote:

We are trying to figure out the best size and type rechargeable battery to use in our solar light.  It currently is set up with 4 aaa’s.  it is trickle charges with a 5-6V solar panel.  Any thoughts?  Some say low mA ni-cads, others nmhi.

On March 16, 2012 at 6:35pm
Michael Mc Cann wrote:

I am designing an off-grid solar electric system for domestic use.  Based on 16.6 KW/day use, 5hours of direct sun/day and a 3.8KW PV array located 100’/30m from the battery pack. Any ideas on battery size/quantity/volts?


On March 26, 2012 at 4:04am
Deepak chaudhary wrote:

i am interested in designing battery for electric vehicle can oyu let me know design formulas?

On March 28, 2012 at 1:04am
ssempijja michael wrote:

I Thank you for the great job you’re doing in availing such credible information,are there non
lead acid batteries to a tune of 180 amp hrs that can effectively be used in MBT(Tanks).
Best regards:

On April 29, 2012 at 12:35pm
Syed Muhammad Tahir Hussain wrote:

Dear Satyendra,

I am Tahir Hussain from Karachi, Pakistan and i am working Atlas Battery Limited.
I read you comment about “aging of battery matters for calculating the capacity”.
Battery Aging and Capacity are the two different things and we can not calculate battery life from its own capacity.
(1) Battery aging mean , battery life. We always calculate battery life in cycles. One discharge plus one recharge equals one cycle . Battery cycles depend on battery size , capacity, electrolyte volume , plate surface area and etc. Some small batteries have short life (less cycles) example 800 cycle. Some batteries have large cycles 5000 cycles( its also depend on vehicle running).( You can see JIS standard )
(2) Battery capacity means the ability of a fully charged battery to deliver a specified of electricity (Amp-Hrs) at a given rate (A) and period of time (H). When both numbers multiply each other its called battery capacity and its symbol AH.  Example a battery which delivers 10 amps for 10 hrs that means 100 AH.

Best wishes,

On September 14, 2012 at 8:44am
syedkaramathali wrote:

marvelous ,great information sir.

On March 9, 2013 at 9:38am
amrendra wrote:

how many time required to discharge of 12v battery of 5ohm resistor

On June 4, 2013 at 1:05am
angelin wrote:


        I having 3.6v li-ion rechargeable battery.I also using one one charger IC,this charger IC having 600mA capacity.And my li-ion battery having 45mAh.Then how can i calculate battery capacity.Pls suggest me.

On August 17, 2013 at 6:19pm
Babul reddy wrote:

Hi This babul,

I’M Working Ship Yard Company

How to calculate Battery Capacity?

Do have any formula

Because our yard Installed   Main Engine (QSK60M, 2300BHP, 1900Rpm) = 2 No’s
                                Diesel Generator 265KW = 3 No’s
So How Much Capacity of battery’s required ( Each one Engine and Each one generator)
How much Starting Torque? 

On December 1, 2013 at 7:46am
Nitin Rao Chandavar wrote:

What is the formula for calculating Battery Capacity for a given Starter Motor Capacity.  Very kindly request you to please let me have the details by mail please…..  Thanks & Best Regards, Nitin Rao Chandavar

On December 27, 2013 at 10:58am
Victor Villasenor wrote:

Is there a way to add up the c rating (discharge rate) of a lipo battery.
I know how to increased the current of two batteries ( connected in parallel)
I also know that by placing two lipo batteries in series I add up the total voltage.
what about the discharge rate ( C ).
Very kindly request your info. by email. have a nice day.

On April 10, 2014 at 2:23am
Richard Biegel wrote:


I am struggling to identify the best battery to install in my car to run a portable fridge (draws 6A) My research indicates I need what is called a deep cycle battery and one that has a high antimony to lead ratio. Also, it must be heat “resistant” as it will be fitted inside the engine compartment.

Please give me your views

Many thanks


On April 15, 2014 at 2:04pm
Mark Dumont wrote:

Consider using the Odyssey brand battery. This uses TPPL (thin plate pure lead) technology. Very good deep cycle ability. Good temp resistance. Can fit under hood just fine. Similar types of this battery are used on military vehicles.

On July 2, 2014 at 8:01pm
Madipatla wrote:

There are various calculator for Lead Acid etc but not able to find calculator for Li-ion batteries.

Any suggestion please

On September 9, 2014 at 10:38pm
balaprasadgunana wrote:

we had 2 batteries in air compressor model CFM 300 , but some bodies theft that batteries . I don’t know how to calculate and how to take that type of capacity batteries

On April 2, 2015 at 12:05am
questech wrote:

Hello Everyone

I am prototyping an Octocopter and I have a great flying beast weighing in at 40lb with a 10lb payload.  It is designed and has the thrust to lift 90lbs.

Normal payloads are between 8lb - 15lb for this unit and it is designed to take a real punishment.

The problem, as you may guess, is keeping it in the air.

I am using a variety of test Lipos ranging from 6S 6000mAh 50C to 6S 15000mAh 30C.  These batteries came from China and I don’t trust that they were rated accurately, but the weight and size match the basic specs for Lipos in their perspective categories.

My lipo combinations are:  4X6000mAh -  6X6000mAh -  2X15000mAh -  2X15000mAh Plus 4X6000mAh.

At full throttle each motor can consume approximately 52 amps at 1150 watts at 8 motors that’s 416 amps at a whopping 9200 watts.  That’s a lot for an RC aircraft spanning 1400mm.

The motors I’m using are very efficient compared to the market leaders, but give me the power I can’t get from T-Motors or Avroto motors.

The actual problem is that I can’t get all of the mAh out of the Lipos.  Hovering with a n 11.5lb payload is at 60% throttle and consumes 19amps per motor.  No heat at all and everything functions fine.  Without the payload the Octo weights in at 28lb and it hovers at between 40% and 55% throttle consuming about 14 amps.

I can’t tell you the exact amperage because I am only getting the peak amps at the end of the flight, I have to design a better solution so that I can get real-time data, but I’m also using my motor bench test data here to give a good approximation.

During the flight, with or without the payload, I start out at 25V and over the course of the flight the voltage drops off in a nice gentle curve, no erratic behavior.  I land when my transmitter telemetry shows 21.3V.  As soon as I power off the motors the voltage goes back up to 22.5 volts and within a minute the voltage is at more than 23V.

I am getting very short flight times.  I should be getting at least 16 minutes but I’m getting 9 min. to 11 min. without the payload and only 4 minutes with the payload.

When charging the mA put back into the 15000mAh rated Lipos is only 7000.  The 6000mAh lipos only take about 4000.

I believe that I could fly longer if I landed after the Lipos reached 21.3 (or even push that a bit) and waited 3 min. - 6 min.  Although I haven’t run that test yet, I feel I could get more flight time and maybe drain some more useful current from the Lipos.

Somehow the Octo seems to be draining the Lipos faster than they can keep up, but once the demand is reduced, the lipos spring back fairly quickly.

I am no expert, but I know there has to be an explanation for what I’m experiencing… I just don’t know what it is…

Can anyone shed some light on this for me?

I much appreciate any constructive feedback you can offer!!

Thank you in advance…  Anthony