BU-501: Basics about Discharging

Learn how certain discharge loads will shorten battery life.

The purpose of a battery is to store energy and release it at a desired time. This section examines discharging under different C-rates and evaluates the depth of discharge to which a battery can safely go. The document also observes different discharge signatures and explores battery life under diverse loading patterns.

The electrochemical battery has the advantage over other energy storage devices in that the energy stays high during most of the charge and then drops rapidly as the charge depletes. The supercapacitor has a linear discharge, and compressed air is the inverse of the battery by delivering the highest power at the beginning. Figures 1, 2 and 3 illustrate the simulated discharge characteristics of stored energy.



Most rechargeable batteries can be overloaded briefly, but this must be kept short. Battery longevity is directly related to the level and duration of the stress inflicted, which includes charge, discharge and temperature.

Remote control (RC) hobbyists are a special breed of battery users who stretch tolerance of “frail” high-performance batteries to the maximum by discharging them at a C-rate of 30C, 30 times the rated capacity. As thrilling as an RC helicopter, race car and fast boat can be; the life expectancy of the packs will be short. RC buffs are well aware of the compromise and are willing to both pay the price and to encounter added safety risks.

To get maximum energy per weight, drone manufacturers gravitate to cells with a high capacity and choose the Energy Cell. This is in contrast to industries requiring heavy loads and long service life. These applications go for the more robust Power Cell at a reduced capacity.
 

Depth of Discharge

Lead acid discharges to 1.75V/cell; nickel-based system to 1.0V/cell; and most Li-ion to 3.0V/cell. At this level, roughly 95 percent of the energy is spent, and the voltage would drop rapidly if the discharge were to continue. To protect the battery from over-discharging, most devices prevent operation beyond the specified end-of-discharge voltage.

When removing the load after discharge, the voltage of a healthy battery gradually recovers and rises towards the nominal voltage. Differences in the affinity of metals in the electrodes produce this voltage potential even when the battery is empty. A parasitic load or high self-discharge prevents voltage recovery.

A high load current, as would be the case when drilling through concrete with a power tool, lowers the battery voltage and the end-of-discharge voltage threshold is often set lower to prevent premature cutoff. The cutoff voltage should also be lowered when discharging at very cold temperatures, as the battery voltage drops and the internal battery resistance rises. Table 4 shows typical end-of-discharge voltages of various battery chemistries.
 

End-of-discharge

               Nominal

Li-manganese

3.60V/cell

Li-phosphate

3.20V/cell

Lead acid

2.00V/cell

NiCd/NiMH

1.20V/cell

Normal load

Heavy load or
low temperature

3.00V/cell

2.70V/cell
 

2.70V/cell

2.45V/cell
 

1.75V/cell

1.40V/cell
 

1.00V/cell

0.90V/cell
 

Table 4: Nominal and recommended end-of-discharge voltages under normal and heavy load. The lower end-of-discharge voltage on a high load compensates for the greater losses.

Over-charging a lead acid battery can produce hydrogen sulfide, a colorless, poisonous and flammable gas that smells like rotten eggs. Hydrogen sulfide also occurs during the breakdown of organic matter in swamps and sewers and is present in volcanic gases and natural gas. The gas is heavier than air and accumulates at the bottom of poorly ventilated spaces. Strong at first, the sense of smell deadens with time, and the victims are unaware of the presence of the gas. (See BU-703: Health Concerns with Batteries.) 
 

What Constitutes a Discharge Cycle?

A discharge/charge cycle is commonly understood as the full discharge of a charged battery with subsequent recharge, but this is not always the case. Batteries are seldom fully discharged, and manufacturers often use the 80 percent depth-of-discharge (DoD) formula to rate a battery. This means that only 80 percent of the available energy is delivered and 20 percent remains in reserve. Cycling a battery at less than full discharge increases service life, and manufacturers argue that this is closer to a field representation than a full cycle because batteries are commonly recharged with some spare capacity left.

There is no standard definition as to what constitutes a discharge cycle. Some cycle counters add a full count when a battery is charged. A smart battery may require a 15 percent discharge after charge to qualify for a discharge cycle; anything less is not counted as a cycle. A battery in a satellite has a typical DoD of 30–40 percent before the batteries are recharged during the satellite day. A new EV battery may only charge to 80 percent and discharge to 30 percent. This bandwidth gradually widens as the battery fades to provide identical driving distances. Avoiding full charges and discharges reduces battery stress. (See also BU-1003: Electric Vehicle.)

A hybrid car only uses a fraction of the capacity during acceleration before the battery is recharged. Cranking the motor of a vehicle draws less than 5 percent energy from the starter battery, and this is also called a cycle in the automotive industry. Reference to cycle count must be done in context with the respective duty.

Last Updated 2016-04-28
 

*** Please Read Regarding Comments ***

Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.

If you have a suggestion or would like to report an error, please use the "contact us" form or email us at: BatteryU@cadex.com.  We like to hear from you but we cannot answer all inquiries. We recommend posting your question in the comment sections for the Battery University Group (BUG) to share.

Previous Lesson Next Lesson

Or Jump To A Different Article

Basics You Should Know
The Battery and You
Batteries as Power Source

Comments

On July 5, 2011 at 8:20pm
SG Hsieh wrote:

Does the long or short full charged storing period affects the battery’s life? For example, if the battery is not used for 3 months, is it better to fully charged or discharged or at what status? Many Thanks!
BR, SG

On September 6, 2011 at 9:24pm
lloyd lamb wrote:

it says discharge methods but nowhere in here does it tell you how to discharge the battery i didnt care how low i can safely discharge or the scientifical pros and cons just smiply how i can discharge the battery problem is the alternator was over charging at around 24.7volts and battery charged to 18.93volts and should be at 12.53-83volts “HOW TO???!!!!”  “WHAT TO DO” to achive discharge to get battery back to 12.53 or can it even be discharged that would be better knowllege than whats up there then you can simply say dont over discharge under 1.75volts the exactly how to do this is what most people are looking for not all the other mombogumbo specs. on how low you can discharge that has nothing to do whith the method of doing so or if it can be done like connect a light to it, a speacal charge and discharge charger, let it sit on concrete for ex amount of time scence it draws the charge for some reason

On October 17, 2011 at 3:41am
Rameshwer kumar wrote:

If 12v 150 Ah lead acid bty connected in series, How many max current drain from the bty at a time.

On November 27, 2011 at 4:49am
Markus Unread wrote:

“Does the long or short full charged storing period affects the battery’s life?” - For Lithium based batteries, high charge voltage + high battery temperature = reduced life.  Storing the battery at low temperature, but above freezing, is best.  Since you do not want the battery to self-discharge to zero, it is best to store the battery with a partial charge.  If you don’t know how long it will be stored, your safest bet is to do a full charge before storing.  It is not as good as having a partial charge, but an accidental “flatlining” of the battery will render it useless and dangerous to recharge.

On November 27, 2011 at 3:41pm
JK wrote:

lloyd lamb:

On the contrary, these specs were exactly what I was looking for.

As for how to discharge: put a load across it. If it’s at 19V you might start with, say, a 100W 24V automotive bulb, as a 12V one might blow at 19V. Consult a electrician or automotive mechanic for details and help.

On March 5, 2012 at 11:04am
Joseph wrote:

Very good resource. I lend a link on the common factors of the discharge of the batteries:
Under charging or neglect of equalizing charge - Description and treatment Of sulphated batteries http://www.amperis.com/en/products/misc/battery-dischargers/

greeting.

On April 14, 2012 at 11:54am
Sam wrote:

My battery charger (for AA, AAA & 9-Volt batteries) has a button that says discharge on it.  What is that for?  Don’t I just need to charge the drained batteries?

On April 25, 2012 at 9:04am
MARY LONGPRE wrote:

I heard once that you should never place coins near or on a Battery.
Like on a Lithium battery.  found a coin laying on top ofone ofmy Lith batterei and am concerned.

On May 10, 2012 at 12:54am
Elangom wrote:

Dear Sir/Madam,

We need to test the button cell batteries of lead acid, Li-ion, Li-polymer, Ni-Cad, NiMH, Ultra-Capacitor.

Please help me finding out the maximum charge and discharge in C-rates of each batteries,
Because it helps us to choose the type of battery tester to buy.

We have to do this experiment in 40-50 minutes.

Thanks and regards,
Elango
research associate

On July 25, 2012 at 7:55am
Louie Toehl wrote:

Bought a new Olympus TG-1 I drain the LI-90B lithium battery and charge to full took
15 photos 3 days later battery failed. Can not turn it on nothing works. Should I return
The camera back or try recharging once more?

My question why is the camera nonfunctional what is the real problem. Need help!
Thanks..

On August 8, 2012 at 2:26am
mario rossi wrote:

I kindly ask the below questions:
If I have a battery fully charged It I do not use will it be discharged after certain time?
Is it better to continuosly charge it or wait for certain level of discharge and fully charge again it?

On October 16, 2012 at 2:03pm
John Dear wrote:

this is a good source book that is online

On October 16, 2012 at 2:05pm
John Dear wrote:

can u help me with this question….

Comparing the effect of Electrical load on a fuel cell and Rechargeable Batteries

On October 16, 2012 at 2:08pm
sam and bryn wrote:

can u help us with this question….

Comparing the effect of Electrical load on a fuel cell and Rechargeable Batteries

On November 6, 2012 at 3:13am
M.Raashekar wrote:

Battery charging methods
1)low current charger 2)fast charging 3)pulse charging 4) 1 hour full current charger after low current and next pulse charging
Different battery charging methods
battery charging circuit reference and charger circuit any books

On November 11, 2012 at 3:23pm
Leobrugg wrote:

Should a Li-ion battery be recharged when it is fully discharged or when it is partially discharged ?

On January 3, 2013 at 7:14am
Gil wrote:

Markus Unread wrote:
“If you don’t know how long it will be stored, your safest bet is to do a full charge before storing.”

Hi,
it is correct for Ni-mh battery, but certainly not for Lithium battery.
The safest storage is between 40 and 60% of capacity. For example, Lithium-Polymer works between 3.0V and 4.2V with 3.7V of nominal voltage. To store it for several days, weeks or more, you have to charge/discharge it up to 3.7-3.8V per cell.

On January 15, 2013 at 8:36am
Joe Lander wrote:

Dear Mr. Raashekar;

Basically, there are two charging modes: constant current or constant voltage. Based on this you can build up many charging profiles (IUI,WSA,...) . Currently, some manufacturers, as Amperis (SMF Battery Charger), are introducing a new pulsed charging profiles.The charging curve is an enhanced version of the standard WSa (Pulsed Wa).This technology ensures a perfect mixing of the electrolyte (without using air-pumps), it reduces the water consumption and the temperature rise of the battery, and it minimizes energy consumption.

On March 30, 2013 at 7:56pm
unknown wrote:

i bought a eneloop lite battery 1.2v, 600mAh, and i need to discharge it to 0.5v, how can i do that?

On April 20, 2013 at 3:05am
suresh wrote:

COD means with respect to battery ??

On June 27, 2013 at 4:03am
rashid wrote:

if 12v 150ah two batteries are connected in series.how maximum current wiil drain out.

On July 18, 2013 at 6:57am
Paul Westfall wrote:

what is the current rate of lithium ion car batteries discharge when not in use

On July 18, 2013 at 7:03am
Paul Westfall wrote:

how long can a lithium ion car battery hold a charge when not in use?

On July 23, 2013 at 5:28pm
ed west wrote:

I’ve got a battery I suspect is bad. The cover started to come off and it made my ecig get hot. It’s an AW IMR 18650 so I know it is pretty powerful. I want to discharge it all the way for safety before I recycle it. I’ve heard to put it in salt water?

On September 20, 2013 at 6:40am
Malcolm White wrote:

How on earth do you cope with such idiotic questions as the ones asked here?

On September 20, 2013 at 1:49pm
ed west wrote:

Read number 5 in this link Macolm. apology?
http://thunderpowerrc.com/PDF/DISPOSAL-OF-LIPO-BATTERIES.pdf

On December 14, 2013 at 6:18am
jørgen eriksen wrote:

mit nye batteri til min doro 515 kan ikke lades op det melder lav Batteritemp.! hilsen jørgen eriksen.

On December 29, 2013 at 12:15am
Larry Perrin wrote:

Does a lithium ion battery need to be stored in it’s charger in order to preserve it’s life expectancy.  In other words, is it ok to leave the battery plugged into a cordless hand vacuum between uses until it runs down.
Also, what is the life expectancy of a lithium ion battery used for power tools.

On January 18, 2014 at 1:02pm
Harry Nijssen wrote:

I have a new ilithium-ion battery 36 V 11Ah for my e-bike
What are de correct instructions for charcing and recharcing’
For first use and later on for normal use

On February 12, 2014 at 2:00am
Md. Jahangir Alam wrote:

Battery discharge test set can’t maintain constant current when i start discharge test. So test set indicating error message showing “Out of Regulation” and discharge test stopped. Instruction manual of the discharge test set instructed that “during a discharge the battery volts may fall to a level such that the control circuitry can no longer maintain the current constant”. What will be the probable causes for not to maintain current constant by the test set? Note: I have connected in series 2 x 12V, 200A battery.

On February 14, 2014 at 12:20pm
Ron R wrote:

I have an extended life cell phone battery. Zero-Lemon 7,500 mah for my Samsung
Galaxy S4. I would like to buy a commercial battery pack discharger/charger to cycle these batteries..

Any manufacturer or websites as leads would be useful.

Thanks,
Ron.

On February 23, 2014 at 11:01pm
Aziz wrote:

when battery is using is it’s voltage vary or current??

On April 10, 2014 at 10:51am
Morteza wrote:

I have a question. “When removing the load after discharge, the voltage of a healthy battery gradually recovers and rises towards the nominal voltage”. can anyone explain me the reason?! thank you

On May 4, 2014 at 5:31pm
William Little wrote:

I appreciate the articles.  A lot of good information.  I use my laptop (MacBook Pro) continuously from when I rise in the morning to when I go to bed at night; therefore, I tend to keep it plugged in while in the office or at home and discharge the battery only when on the road, keeping it, most of the time nearly 100% charged.

It is not practical to remove the battery, as it is too easy to pull the magsafe chord from the socket, which instantaneously does a shut down.  My question is: While using the laptop at home, should I repeatedly allow the charge to drop to 40%, then recharge to 80%, or is it ok to keep the battery, most of the time at or near 100%?  The former would require multiple charges a day.  I’m still a little puzzled on what determines a cycle.

Thanks,  Bill.

On June 11, 2014 at 7:18pm
Lithium battery storage capacity wrote:

The safest storage is between 40 and 60% of capacity. why?

On June 15, 2014 at 8:39pm
aaron G wrote:

hi, i would appreciate your help if any one could help on telling me if there is ANY way to discharge a battery on a phone witch does not have a lid to open and get the battery out… i need my phone to discharge since it has frozen on me, thank you

On June 21, 2014 at 7:37pm
phanith wrote:

ple let my phone can be charging

On June 23, 2014 at 12:37am
raj kumar K wrote:

Hi,

I am using BQ24167 (TI) dual input charger IC & TPS2500 (TI) USB Boost converter in my charger design.

I am able to hear HUM sound from the system.(Mostly near Input section of TPS2500 )

While probing battery voltage (when Hum sound is present) i am able to see max 560mV fluctuation on bat+.

Plot looks like :-
____-‘’’’’-_____-‘’’’’-_____-‘’’’’-_____-‘’’’’-_____

Battery Signal levels for Samsung Galexy mobile load are,

Battery Spec:- 3.7V / 3Ahr Li-ion battery.

Bat Voltage = 3.59V

Load current =510mA (To mobile)

Noise Amplitude=440mV

Noise Frequency= 4.0KHz (Frequency of the signal will vary depends on Load current & battery % based on this I am able to here different hum sound ).

TO Reduce Noise :-

I tried to change TPS2500 inductor from 3.3uH to 2.2uH.—> Still noise is present.

I added 150uF bypass cap on Battery node.—> Noise amplitude reduced to half.(but still low sound noise the present)

Pls help to solve this issue.

Thanks in advance.

( If anyone interested to help pls share your mail ID i will send you the details block,Sch&plot; images)

Regards

Raj

On September 9, 2014 at 1:12am
wangchuk wrote:

i recently join ......  put up comment later

On February 20, 2015 at 6:16pm
Edgardo Suyat wrote:

Please help me create Battery Discharge form to plot the reading of battery while on discharge testing

On April 10, 2015 at 4:10am
bajaj wrote:

how

On April 10, 2015 at 10:50am
ap wrote:

please find details of dicharging

On May 8, 2015 at 11:41pm
Rj wrote:

what is ” discharge time “?
Definition Is enough

On May 12, 2015 at 9:44am
br wrote:

On the section called “Discharge Characteristics of Energy and Power Cells”, I am having trouble understanding part of the concept. I noticed that the NCR18650B by Panasonic discharge time was similar when the C-rate was changed from 0.2C to 2C. I thought that if the battery was discharged at a higher C-rate, like 2C, the voltage would drop sooner and if it has been discharged at a lower C-rate, say 0.2C, then the voltage would have a longer discharge time. I’m having trouble understanding why this battery is still discharging at similar times when the C-rate is different. Thank you for any feedback you can provide :]

On June 16, 2015 at 7:46am
Yang Li wrote:

i habe a question about the definition of the battery. What is the exactly definition of the charge/discharge cycle for the battery? For exemple if the battery charged from 60% to 61% and then dischaged from 61% to 60%. Is it a charge cycle or it can not be named a cycle? And how to count the cycles of DoD when the battery is connected to the power grid?
Thanks!

On August 18, 2015 at 7:31am
Onyebuchi Onwuka wrote:

If I decide to heat a sodium sulphur battery with a band heater for charging, how can it be discharged and cooled? and maybe using a heat exchanger in the process if i intend heating with a fluid

On August 26, 2015 at 9:31am
wadan ali wrote:

When battery is kept on earth why it discharge because earth is neutral and the plastic of battery is also insulate

On December 10, 2015 at 7:13pm
Lucamax wrote:

When you wrote “Figure 5: Discharge characteristics of NCR18650B Energy Cell by Panasonic,... the 2C discharge produces only about 2.3Ah rather than the specified 3.2Ah.”

It is not evident looking at the graph.
The 2C curve ends close to 3300 mah.
The main difference between 2C discharge and lower discharge rate is the voltage level during discharge that is significantly lower.
So finally we should consider Watts to calculate mah capacity reduction at high C discharge.
Let assume Panasonic 18650B @ 0,2C discharge has 3400mah capacity at 3,6 Volts = 12,24 Watt
At 2C discharge , average voltage is 3,25 Volts and 3300mah = 10,72 Watt .
So 14% less then 0,2C discharge
3400mah - 14% = 2982 mah capacity expected at 2C diischarge

On December 17, 2015 at 11:48am
jb wrote:

Figure 9 seems very contradictory with what I’ve seen there :
https://www.youtube.com/watch?v=pxP0Cu00sZs#t=334s

I believe different Li-ion technologies don’t behave the same at all upon this criteria of longevity versus charge/discharge C rate

On January 22, 2016 at 1:55am
prasad wrote:

what is the barrery discharge voltage

On February 11, 2016 at 10:30am
gazby wrote:

i have an old tractor with a new starter motor fitted
when battery is fully charged with the mains old fashioned straight 30 amp charger
the tractor will start well quite a few times
however the old 11 amp tractor dynamo never get enough back in and the battery soon gets flat again on short runs stop / start use.
Tractor dynamo charges ok as reflected in the tractor ammeter
i want to look after the battery next to improve the AH as it is not that old
i am sure it is sulphated, as with the engine stopper out it will turn over for less than two minutes with a full battery.The starter motor spins fast but not for long.
should i get a sulphator type charger ?
or put a seperate sulphator device direct on the battery as a permanent fixture
if above how will this affect sticking on the mains 30 amp charger when the sulphator is permanently on, will the charger destroy the desulphator ?
do desulphators work at all?
is it best for the charger desulphator to be used instead

On February 21, 2016 at 1:57pm
sam wrote:

@Gazby, you need to measure the voltage at the battery during charging, for a lead-acid 12v 6 celled batt, the fully charged voltage should be 14.3v no more, no less. if over, some different chem reaction occurs changing(reducing the capacity) the chem equilibrium. i think that’s what they call sulphuriing the plates. also foreign(crap) in the cell causes degradation as well. The best way to deal with a lead-acid batt is to slow charge up to 14.3, then measure the specific gravity with a float/bulb hydrometer

On April 13, 2016 at 1:35pm
Michael Rutland wrote:

I respectfully suggest that the text at the start of the paragraph just below the three graphs needs the following deleted to avoid duplication:
“Most rechargeable batteries can be overloaded briefly, but this should be kept brief. Battery”
Kind regards
Michael

On September 5, 2016 at 11:44pm
Naveen wrote:

It was really helpful to me thank you so much