BU-501b: How to Calculate Battery Capacity

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

A battery performs well when new but the capacity begins to fade with use and time. To assure reliable service over many years, the design engineers must oversize the battery and include a reserve. The capacity of a well-managed battery ranges from 100 when new to 80 percent when aged, with 80 percent denoting retirement.

Besides allowing for the all-important capacity fade, a battery should also include spare capacity for extra activity when needed. System breakdown in an emergency is typically caused by a lack of spare capacity. Marginal batteries can hide comfortably during routine operation but will fail when longer runtimes than normal are required. These margins are often ignored by fleet users unless the batteries are maintained as part of quality control.

Figure 1 shows a battery that allocates 20 percent for fade and 20 percent for spare. This only leaves 60 percent for the actual capacity in a worst-case scenario, a ration most battery users will relax in favor of longer service life.

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.

To check the spare in your battery fleet, spot-check the capacity after a busy day. The Cadex battery analyzers provide this function on the “Prime” program by applying a discharge before charge. The first reading on the analyzer’s display is the spare capacity and the second is the full capacity after a charge.

If an older battery performing at 80 percent capacity comes back with 30 percent before charging, then the pack can be kept longer. This can be done by lowering the replacement threshold from 80 to 70 percent. Knowing the energy needs for a given application creates a sweet-spot between risk management and economics.

Batteries for medical and communications devices are typically replaced at a capacity threshold of 80 percent, but there are exceptions. Scanners in warehouses will often still provide a full day’s work with a capacity of as little as 60 percent and starter batteries in vehicles still crank the motor at a capacity of 40 percent. Even though the capacity is low, the battery still delivers full current, albeit for a reduced time.  But the moment will come when the battery will lack the needed energy to turn the engine and the driver will get stranded. (See BU-902a: How to Measure CCA (Cold Cranking Amp))

Last Updated: 2015-07-06

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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: [email protected]

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,
([email protected])

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