Basics About Discharging

The purpose of a battery is to store and release energy at the desired time and in a controlled manner. This section examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. Chapter 5 also observes different discharge signatures and explores how certain patterns can affect battery life. But first, let’s look at charge and discharge rates, also known as C-rate.

Depth of Discharge

The end-of-discharge voltage for lead acid is 1.75V/cell; nickel-based system is 1.00V/cell; and most Li-ion is 3.00V/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 devises 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 metal concentration of the electrodes enable this voltage potential when the battery is empty. An aging battery with elevated self-discharge cannot recover the voltage because of the parasitic load.

A high load current lowers the battery voltage, and the end-of-discharge voltage threshold should be set lower accordingly. Internal cell resistance, wiring, protection circuits and contacts all add up to overall internal resistance. The cut-off voltage should also be lowered when discharging at very cold temperatures; this compensates for the higher-than-normal internal resistance. Table 1 shows typical end-of-discharge voltages of various battery chemistries.
 

End-of-discharge

Li-manganese

Li-phosphate

Lead acid

NiCd/NiMH

Normal load

Heavy load

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 1: Recommended end-of-discharge voltage under normal and heavy load
The lower end-of-discharge voltage on a high load compensates for the losses induced by the internal battery resistance.  

Some battery analyzers apply a secondary discharge (recondition) that drains the battery voltage of a nickel-based battery to 0.5V/cell and lower, a cut-off point that is below what manufacturers specify. These analyzers (Cadex) keep the discharge load low to stay within an allowable current while in sub-discharge range. A cell breakdown with a weak cell is possible and reconditioning would cause further deterioration in performance rather than making the battery better. This phenomenon can be compared to the experience of a patient to whom strenuous exercise is harmful.

What Constitutes a Discharge Cycle?

Most understand a discharge/charge cycle as delivering all stored energy, but this is not always the case. Rather than a 100 percent depth of discharge (DoD), manufacturers prefer rating the batteries at 80 percent DoD, meaning that only 80 percent of the available energy is being delivered and 20 percent remains in reserve. A less-than-full discharge increases service life, and manufacturers argue that this is closer to a field representation because batteries are seldom fully discharged before recharge.

There are no standard definitions of what constitutes a discharge cycle. A smart battery that keeps track of cycle count may require a depth of discharge of 70 percent to define a discharge cycle; anything less does not count as a cycle. There are many other applications that discharge the battery less. Starting a car, for example, discharges the battery by less than 5 percent, and the depth of discharge in satellites is 6 to 10 percent before the onboard batteries are being recharged during the satellite day. Furthermore, a hybrid car only uses a fraction of the capacity during acceleration before the battery is being recharged.

Discharge Signature

A classic discharge is a battery that delivers a steady load at, say, 0.2C. A flashlight is such an example. Many applications demand momentary loads at double and triple the battery’s C-rating, and GSM (Global System for Mobile Communications) of a cellular phone is such an example (Figure 2). GSM loads the battery with up to 2A at a pulse rate of 577 micro-seconds (µs). This is a large demand for a small 1,000mAh battery; however, with a high frequency the battery begins to behave like a capacitor and the characteristics change.

GSM Pulse of a cellular phone

 

Figure 2: GSM Pulse of a cellular phone

The 577 microsecond pulses adjust to field strength and can reach 2 amperes.

Courtesy of Cadex

In terms of cycle life, a moderate current at a constant discharge is better than a pulsed or momentary high load. Figure 3 shows the decreasing capacity of a NiMH battery at different load conditions and includes a gentle 0.2C DC discharge, an analog discharge and a pulsed discharge. The cycle life of other battery chemistries is similar under such load conditions.

Cycle life of NiMH under different operating conditions

Figure 3:
Cycle life of NiMH under different operating conditions

NiMH performs best with DC and analog loads; digital loads lower the cycle life. Li-ion behaves similarly.

Source: Zhang  (1998)
 

Figure 4 examines the number of full cycles a Li-ion battery with a cobalt cathode can endure when discharged at different C-rates. At a 2C discharge, the battery exhibits higher stress than at 1C, limiting the cycle count to about 450 before the capacity drops to half level.

Cycle life of Li-ion with cobalt cathode at varying discharge levels

 

Figure 4:
Cycle life of
Li-ion with cobalt cathode at varying discharge levels

The wear-and-tear of a battery increases with higher loads.

Source: Choi et al (2002)

 

For a long time, Li-ion had been considered fragile and unsuitable for high loads. This has changed, and today many lithium-based systems are more robust than the older nickel and lead chemistries. Manganese and phosphate-type Li-ion permit a continuous discharge of 30C. This means that a cell rated at 1,500mAh can provide a steady load of 45A, and this is being achieved primarily by lowering the internal resistance through optimizing the surface area between the active cell materials. Low resistance keeps the temperature down, and running at the maximum permissible discharge current, the cells heat up to about 50ºC (122ºF); the maximum temperature is limited to 60°C (140°F).

One of the unique qualities of Li-ion is the ability to deliver continuous high power. This is possible with an electrochemical recovery rate that is far superior to lead acid. The slow electrochemical reaction of lead acid can be compared to a drying felt pen than works for short marking but needs rest to replenish the ink.

Simple Guidelines for Discharging Batteries

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