BU-401: All about Chargers

The performance and longevity of rechargeable batteries are to a large extent governed by the quality of the charger. In a price-competitive world, battery chargers are often given low priority, especially as consumer products. Choosing a quality charger is important considering the cost of battery replacement and the frustration poorly performing batteries create. The charger should serve as a quintessential master and guardian angel to protect the environment and save money by extending battery life.

There are two varieties of chargers: the personal chargers and the fleet chargers. For cell phones, laptops, tablets or digital cameras, manufacturers include personal chargers. These are made for one battery type, are economically priced and perform well when used for the application intended.

The fleet charger serves employees in a team environment and often has multiple bays. The original equipment manufacturer (OEM) sells the chargers and third parties also provide them. While the OEMs meet the basic requirements, third-party manufacturers often include special features, such as a discharge function for battery conditioning and calibration.

Some manufacturers of third-party chargers have become creative and offer advanced charge methods for lead- and nickel-based batteries. While pulse charging may be beneficial for nickel-based batteries, this method is not recommended for Li-ion. The voltage peaks are too high and cause havoc with the protection circuit. Battery manufacturers do not support alternative charging methods and say that pulse charging could shorten the life of Li-ion.

There are many valuable additional features for chargers, and hot- and cold-temperature protection is one. Below freezing, the charger lowers or prevents charge depending on the type of battery. When hot, the charger only engages when the battery temperature has normalized to a safe level. Advanced lead acid chargers offer temperature-controlled voltage thresholds, as well as adjustments to optimize charging for aging batteries.

Some chargers, including Cadex chargers, feature a wake-up feature or “boost” to allow charging Li-ion batteries that have fallen asleep. This can occur if a Li-ion battery is stored in a discharged condition and self-discharge has depressed the voltage to the cut-off point. Regular chargers read these batteries as unserviceable and the packs are discarded. The boost feature applies a small charge current to activate the protection circuit to 2.20–2.90V/ cell, at which point a normal charge commences. Caution should be applied not to boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer.

There are two common charge methods, which are voltage limiting (VL) and current limiting (CL). Lead- and lithium-based chargers cap the voltage at a fixed threshold. When reaching the cut-off voltage, the battery begins to saturate and the current drops while receiving the remaining charge on its own timetable. Full charge detection occurs when the current drops to a designated level. [see BU-403: Charging Lead Acid].

Nickel-based batteries, on the other hand, charge with a controlled current and the voltage is allowed to fluctuate freely. This can be compared to lifting a weight with an elastic band. The slight voltage drop after a steady rise indicates a fully charged battery. The voltage drop method works well in terminating the fast charge, however, the charger should include other safeguards to respond to anomalies such as shorted or mismatched cells. Most batteries and chargers also include temperature sensors to end the charge if the temperature exceeds a safe level. [see BU-407: Charging Nickel-cadmium].

A temperature rise is normal, especially when nickel-based batteries move towards full-charge state. When in “ready” mode, the battery must cool down to room temperature. Heat causes stress and prolonged exposure to elevated temperature shortens battery life. If the temperature remains above ambient, the charger is not performing right and the battery should be removed when “ready” appears. Extended trickle charge also inflicts damage, and nickel-based batteries should not be left in the charger for more than a few days.

A lithium-based battery should not get warm in a charger and if this happens, the battery or charger might be faulty. Discontinue using the battery and/or charger. Li‑ion chargers do not apply a trickle charge and disconnect the battery electrically when fully charged. If these packs are left in the charger for a few weeks, a recharge may occur when the open circuit voltage drops below a set threshold. It is not necessary to remove Li-ion from the charger when full; however, if not used for a week or more, it is better to remove them and recharge before use.

A mobile phone charger draws about 2 watts on charge, while a laptop on charge takes close to 100 watts. The standby current must be low and Energy Star offers mobile phone chargers drawing 30mW or less five stars for high efficiency; 30–150mW earns four stars, 150–250mW three stars, and 250–350mW two stars. The industry average is 300mW on no-load consumption and this gets one star; higher than 500mW earns no stars. Low standby wattage is only possible with small chargers, such as the four billion mobile phone chargers that are mostly plugged in.

Simple Guidelines When Buying a Charger

Slow Charger

Also known as an “overnight charger”, the slow charger goes back to the old nickel-cadmium days and applies a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery is connected. Slow chargers are very simple; they have no full-charge detection, the charge current is always engaged, and the charge time on an empty battery is 14 to 16 hours.

When fully charged, a slow charger keeps NiCd lukewarm to the touch. Some overcharge is acceptable and the battery does not need to be removed immediately when ready. However, the pack should not stay in the charger for more than a day or two because of “memory,” also known as crystalline formation. [see Memory: Myth or Fact?].

A problem arises when charging a battery with a lower mAh rating than specified. Although the slow charger will charge the battery normally at first, higher than 0.1C current for this smaller battery will heat up the pack towards the full-charge state. Because there is no provision to lower the current or terminate the charge, excessive heat will shorten the life of this pack. Observe the battery temperature while charging and remove the battery when warm to the touch. Most slow chargers have no “ready” light.

The opposite can also occur when the slow charger charges a larger battery. In this case, the battery may never reach full charge and remains cold. Performance is poor because the battery does not receive a full charge. A nickel-based battery that is undercharged will eventually lose the ability to accept a full charge due to crystalline formation.

Slow chargers are found in cordless phones, electric toothbrushes and children’s toys. A slow charger works well for these products because the battery and charger are harmonized. Chargers servicing a broader range of batteries need some intelligence to supervise the charge, control the current when full, and provide safety if an anomaly occurs.

Rapid Charger

The rapid charger falls between the slow and fast chargers and services nickel- and lithium-based batteries. Unless specially designed, the rapid charger cannot service both nickel- and lithium-based chemistries on the same platform; it needs a designated platform.

The rapid charger is most commonly used for consumer products. The charge time of an empty pack is 3 to 6 hours (less for a partially charged battery), and when the battery is full, the charger switches to “ready.” Most rapid chargers include temperature protection to safeguard against failures. This and other features offer improved service over the slow charger, and batteries tend to perform better. Although they are more expensive to build, high-volume production makes the rapid charger available at a moderate price.

Fast Charger

The fast charger offers several advantages, and the obvious one is shorter charge times.The need for a larger power supply and more complex control circuits reserve fast chargers mostly for commercial use, such as medical, military, communications and power tools.

Faster charge times demand tighter communication between the charger and battery. At a 1C charge rate, which the fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. [see BU-402: What is C-rate?] As a battery approaches full charge, some nickel-based chargers reduce the charge current to adjust to lower charge acceptance, and when the battery is full the charger switches to trickle charge, also known as maintenance charge.

Most nickel-based fast chargers accommodate NiCd and NiMH batteries on the same algorithm, but not Li-ion. To service nickel- and Li-ion-based chemistries in the same charger, a provision is needed to select the correct charge algorithm. In many ways, Li-ion batteries are easier to charge than NiCd and NiMH. The charge to 70 percent at 1C occurs in less than an hour, the rest of the time is devoted to topping charge.

Lead acid batteries cannot be fast-charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14 hours; anything slower may be a compromise. As with all chemistries, lead acid can be charged relatively quickly to 70 percent; the all-important saturation charge takes up the remaining time. A partial charge at a high rate is fine provided the battery receives a fully saturated charge once every few weeks to prevent sulfation.

Simple Guidelines on Chargers






Charge termination

Slow charger

Lead acid




0ºC to 45ºC
(32ºF to 113ºF)

Continuous low charge or fixed timer. Subject to overcharge. Remove battery when charged.

Rapid charger

NiCd, NiMH,



10ºC to 45ºC
(50ºF to 113ºF)

Senses battery by voltage, current, temperature and time-out timer.

Fast charger

NiCd, NiMH,



10ºC to 45ºC
(50ºF to 113ºF)

Same as a rapid charger with faster service.

Ultra-fast charger

Li-ion, NiCd, NiMH


10-60 minutes

10ºC to 45ºC
(50ºF to 113ºF)

Applies ultra-fast charge to 70% SoC; limited to specialty batteries.

Table 1: Charger characteristics. Each chemistry uses a unique charge termination.


On April 15, 2011 at 12:00am
desda wrote:

I need ni-cd charger chema.

On May 16, 2011 at 6:28pm
che wrote:

Do we need to do the long hour charge for a brand new Li-Polymer battery, I mean the first charge, peoples say its up to 10 hours (some say 8 hrs, others insist it’s not applicable nor not necessary for Li-po anymore), which one is correct? what is the reason behind this?

Another things, I had another Li-ion battery as a a backup, if I’m not using it, what make a better choice for the sake of battery life, to store it on a full charged or half one since the battery will discharges itself anyway.

On July 5, 2011 at 5:44pm
Stu wrote:

After reading several of your great articles, it became clear that consumer chargers specs do not give the information required to make an informed buying decision.

Now I know more, and I’m scared more but I’m better educated. What a situation!

Thanks for the education, anyway.

On July 18, 2011 at 3:49pm
James L. McCann wrote:

can you use nicad in a nimh charger

On September 8, 2011 at 8:26pm
Tony wrote:

If a NIMH battery charger’s maximum cutoff voltage is 1.6V/cell, i don’t think it can chage NICD battery well.

On September 9, 2011 at 1:46am
Tony wrote:

Here are some schematic drawings for battery charger:

On January 13, 2012 at 10:10pm
Jimmy wrote:

I want to use a charger with the same voltage (5V) and an output of .7 A. The original charger output is 5 V and 1 A will this affect the charger, the battery or the phone ?

On February 3, 2012 at 9:51am
Bryan wrote:

I know a lead acid battery left discharged will soon be U/S, on my motorhome I have left a trickle charger connected to both engine and house batteries a total capacity of 370ah. Today I looked at the state of charge, the current was charging at 0.1amps and the voltage was 15volts across all batteries, is this high voltage at 0.1amps a problem for my batteries?

On April 30, 2012 at 8:03pm
Glenn wrote:

I have charger that will only charge up to a 5000 MaH, I have a 5800 maH pack. Will my charger hurt the battery pack ?

Sent from my iPad

On June 9, 2012 at 2:35am
Rik wrote:

I have a Nicd and Nimh charger but the battery says it takes 7 hours to charge at 40 mA but the chargers says it charges at 11 mA. How long will it take to charge the battery?

On June 25, 2012 at 7:09am
Pancy wrote:

I have a Sealed Lead Acid Battery (6V; 4.5 A ) , I want to charge it by using 5 V adaptor,what will happen?I think it is possible but , battery will charge up to 5 V ,and discharging will take longer time .Am I right ?

On June 29, 2012 at 2:22am
Pratik Halvadiya wrote:


I would like to develop a BATTERY CHARGER that charges 12 v battery form 17-18 v which i got from SOLAR PV Panel. This battery charger system with auto cut off system..

If anyone have a idea about is please share with me..!!

On July 31, 2012 at 9:20am
Ben Watson wrote:

Could anyone give more detail on the problems/dangers of using a charger that claims to charge both Li-Ion chemistries and Nickel chemistries

On September 17, 2012 at 4:05am
DAN GREEN wrote:

Does saturating a phone battery work

On October 9, 2012 at 2:16am
Sharad wrote:

Hello sir,
i want to know the rating of a charger for charging Li-ion batteries which are now a days being commonly used in mobile devices. what should be the Volt and Amp rating of the charger for say 3.7v 800mAh(3.7Wh)Li-ion battery? also if i have another battery 3.6v 6200mAh (2.3Wh) Li-ion, what would be the best common charger for charging both batteries? What all parameters should be keep in mind while calculating charger rating? what these figures (v,mAH,Wh) tell about the battery?
Also on my charger i see
INPUT : 150-300VAC 50-60Hz 0.15A
OUTPUT: 4.75V 0.55A
What these figures (0.15A) and (0.55A) in I/P and O/P are?
can u please give me some brief knowledge on it.
Best Regards

On October 9, 2012 at 2:48am
Ben Watson wrote:

for a lithium charger you need to match the voltage so you want a 1 cell 3.7V li ion charger. 

the current or charge current of your charger is important to the lifetime of your battery.  I recommend charging at 0.5C which is half the capacity of your battery, in this case 400mA. In this case your battery will charge in about 2 hours.  But your larger capacity battery will take more than 15 hrs to charge with the same charger.  You could use a higher capacity charger for instance a 1A charger then your larger capacity battery will charge in 6 hrs and your smaller capacity battery would charge in less than one hour.  But your small battery would become damaged over time.  Lithium batteries have maximum charge current limits, it would depend on the spec of your battery.

Basically there is no single charger with a fixed charge current that will charge both batteries either safely or within a reasonable amount of time.  There are universal lithium chargers available but these only charge at 800mA max. http://uk.rs-online.com/web/c/?searchTerm=vario+lithium&sra=oss

the more i read your post the more i think you mis-typed 6200mAh, im guessing you meant 620mAh as the Watt-Hours dont add up for that battery.  So a universal li-ion charger like the one in the link would be ideal for both these batteries.

with regards to your charger I/P 0.15A means your charger uses/draws 1500mA of current from the mains supply.  the O/P 0.55A means it chargers at 550mA of current.

in conclusion matching the voltage is very important and charge current should be roughly half the capacity of your battery.  So a 1Ah battery would charge with 500mA

On November 1, 2012 at 6:44am
richard wrote:


Is a charger rated at 100mAh suitable for a battery that specifies 150mAh charge rate? Same voltage of course. I’m guessing that it would take somewhat longer to charge but wouldn’t hurt the battery in the long run.


On November 4, 2012 at 4:10am
chito wrote:

is it ok to use a 40watts laptap charger to 30watts spec?
same volts only the output watts differ

On December 17, 2012 at 10:31am
Ataullah wrote:

I am looking charger i can charge my laptop battries at least 4 at a time?its avilable in market ?if not any body help me ?for design

On December 17, 2012 at 10:38am
Ataullah wrote:

Hi,How we can used Solar power for charger ?for my laptop ,

On December 26, 2012 at 2:08pm
sheryl wrote:

I have a Sharp AC adaptor battery charger for a Sharp 8Viewcam video camera. They both have some age but have continued to work well. The charger’s red Power light blinks constantly while the battery is inserted. The Charge and Refresh lights do not come on. Two new batteries I’ve tried charging are not working in my camcorder. One site suggested trying to charge again for an hour, then remove the battery and reinsert to see if it will charge.  I’ll try…appreciate any help.

On February 10, 2013 at 4:45pm
haider wrote:

hi can charge a single 12v 200ah battery on 220vac ups and 300w solar panel it same time to charge quickly thanks

On March 25, 2013 at 5:52pm
Ramamurthy magal wrote:

How to know the battery is charged

On June 27, 2013 at 11:23am
Abhishek Thapliyal wrote:

Can anyone suggest me the circuit of 1A mobile charger

On July 16, 2013 at 12:08pm
A. DasGupta wrote:

The charger that came with my Toshiba Satellite Laptop which is rated at 19V and 3.95A is not charging the battery.  I used another charger which is for a HCL laptop.  This is rated at 19V and 3.42A.  When i used this charger it seemed to be charging my laptop but the charger got extremely hot. Is it to be expected.  If so, why does it get so hot?  Is it normal? Or is it due to the fact that ithas a lower amperage than the original charger?

On August 18, 2013 at 7:23am
HelloWorld wrote:

My Original laptop charger had the following specs:
100-240v 1.5A
50/60 Hz
Output:19v 4.47A

If I use a charger with the below specs will it do harm?
100-240v 1.5A
50/60 Hz
Output:19v 3.95A

On September 27, 2013 at 2:35pm
Ian wrote:

I have a lithium battery for a single seat golf buggy and it can be connected at the golf club. The battery will, therefore, be under charge for more than 36 hours, and I am told that should the charging continue after that time there is a danger that the charger could overheat.  Someone suggested I connect the charger to a timer, set it to X hours and all should be well.  But then someone says no, because a trickle of electricity is always there and will eventually overheat the battery.
So am I safe to leave the battery on charge for more than 36 hours, or should I take the battery home to charge where I have control over time?

On October 17, 2013 at 1:08pm
Yuvaraj wrote:

Can it cause problems leaving the phone attached to the charger for hours after its fully charged?

On October 31, 2013 at 11:29pm
Stan wrote:

I purchased a EGO CE4 electronic cigarette in Thailand. It came with a charger that has a USB connection

Trying to recharge in Australia but no result

Does anyone have any hints to it it charged ??


On October 31, 2013 at 11:31pm
Stan wrote:

I purchased a EGO CE4 electronic cigarette in Thailand. It came with a charger that has a USB connection

Trying to recharge in Australia but no result

Does anyone have any hints for charging   ??


On November 17, 2013 at 5:49pm
Wei Yan wrote:

I looked for a 40 watts laptop charger but according to the manufacturer, it was already phase out and they only have 30 watts. Is it ok to use a 30 watts charger even the specs must be 40 watts? They have the same volts only the output watts differs.

On November 23, 2013 at 2:58pm
robert ray wrote:

I want to use 4 AGM batteries 12V 35AH wired in series to produce 48 volts. They will be installed in a 48 volt golf cart. What charger do I need to recharge these batteries. I don’t need specs bla bla bla. Just tell me what brand charger, and who sells it. A part number for the charger may be needed so I can make sure I get the right one.

On November 27, 2013 at 11:56am
Gerry Schreiber wrote:

So I see lots of questions on here but not one answer….is there somewhere else I should b looking?

On December 7, 2013 at 12:57am
anirudh wrote:

sir i am working on a project of electric car i need help in deciding what type of battery and how to recharge it effectively

On January 21, 2014 at 8:34am
RAJA wrote:

i want a charger to charge my lead acid batteries(12V-26Ah) within 6hours.
any options for do this, pls reply

On January 27, 2014 at 11:58am
jerson wrote:

Hi can I use my old rice cookers plug and socket to use on my ebike battery pack instead of using the anderson plug its an SLA battery. . So I can charge my ebike battery pack .

On February 10, 2014 at 10:25am
David Nelson wrote:

I’m a high school teacher working on an engineering problem for a student competition.  We allowed to put solar panels under a halogen lamp for up to 10 minutes.  We have to store the accumulated energy,and use it to power a vehicle, with a 2V electric motor (hobby size).  All we need 10-20J of usable energy.  NOTE: I’m doing this with 9th grade students who haven’t had a full physics course yet.  We’re supposed to build our system from scratch.  My local electronics store suggested AA NiCd batteries.  We connected solar panels delivering 4V and 0.3A for 10 min to three AA batteries in series.  We only got 1.1V out of the batteries at the end 10 min of chaging, barely powering the electric motor.  Any suggestions- should we try Li-ion batteries?

On June 21, 2014 at 4:20pm
melissa wrote:

Hi I have a 48 watt charger that I use on my ebike can what would happen if I use a 60 watt charger can someone please let me know thanks.

On July 8, 2014 at 11:46pm
Aniket wrote:

What will happen if I charge 6V 1600mA battery with 4.6 V 160 mA charger?

On August 6, 2014 at 10:31am

can it is possible to charge 2 set of different capacity (70 Ah 24V Ni-cad, 200 Ah 24V Led-acid)  battery’s with a single charger ( Capacity 2.5A 24V), If any Problam? Pls help me.

On August 21, 2014 at 10:57am
Ray wrote:

“A lithium-based battery should not get warm in a charger and if this happens, the battery or charger might be faulty. Discontinue using the battery and/or charger.”

Uh, unless you have 100% conversion of energy, which is not the case, you have waste heat. If your battery does NOT get warm, then your charger is dead or its not plugged in.

On October 18, 2014 at 7:04pm
maureen wrote:

what happens to your phone if it charges more than 36 hours

On October 24, 2014 at 6:34pm
David Zurick wrote:

Just got 4 10000 mAh D cell batteries 1.2 volt with a 150 mA charger. The charger does not shut off when batteries are charged. There is a calculation in the manual:  1.2V X mAh divided into charging currant mA. Doing the math this is 80 hours of charge time. Is that possible? I don’t know what the charge time would be for one battery or four. Please help if you can. Thank you very much- Dave Z

On January 9, 2015 at 3:52am
Dom wrote:

Hello. I have a question:

‘‘If four 5W lights are left switched on, how long will a 30 Amp-hr battery last before becoming discharged?’’ On a car, light vehicle.

Waiting for your reply, thank you.