BU-401: All about Chargers

Discover which charger is best for your application

A good charger provides the base for well-performing and durable batteries. In a price-competitive market chargers often receive low priority. The battery and charger must go together like horse and carriage (Figure 1), and this is not always the case. Engineers are often not fully aware of the complex power requirement of a portable device and the need to charge under adverse conditions.

Horse and Carriage Figure 1:  Battery and charger must go together like horse and carriage

One does not deliver without the other.

Chargers are divided into personal and industrial, “smart” and “dumb,” slow, fast and ultra-fast types. Consumer products come with a low-cost personal charger that performs well when used as directed. The industrial charger is often made by a third party and includes special features, such as charging at adverse temperatures. Although batteries operate below freezing, not all chemistries can be charged when cold and most Li-ion falls into this category. Lead and nickel-based batteries accept charge but at a lower rate. (See BU-410: Charging at High and Low Temperature)

Some Li-ion chargers (Cadex) include a wake-up feature, or “boost,” to allow recharging if a Li-ion battery have fallen asleep due to over-discharge. A sleep condition can occur when storing the battery in a discharged state and the self-discharge brings the voltage to the cut-off point. A regular charger treats such a battery as unserviceable and the packs are discarded. Boost applies a small charge current to raise the voltage to between 2.20 and 2.90V/cell and activate the protection circuit, at which point a normal charge commences. Caution applies if Li-ion has dwelled below 1.5V/cell for a week or longer. (See BU-803: “Can Batteries be Restored?)

Lead- and lithium-based chargers operate on Constant Current Constant Voltage (CC/CV) by which the voltage is capped when reaching a set limit. At this point of the charge cycle, the battery begins to saturate and the current drops. Full-charge occurs when the current drops to a set level. Lead acid requires a periodic full saturation to prevent sulfation.

Nickel-based batteries charge with constant current and the voltage is allowed to fluctuate freely. This can be compared to lifting a weight with an elastic band where the hand moves ahead of the load. Full charge detection occurs when observing a slight voltage drop after a steady rise. This method is known as delta Temperature over delta time, or dT/dt, and works well with rapid and fast charge. To safeguard against anomalies, such as shorted or mismatched cells, the charger should include a plateau timer to terminate charge if no voltage delta is measured, as well as a temperature sensors.

A temperature rise is normal with nickel-based batteries, especially when reaching the 70 percent charge level. The reason for this is a decrease in charge efficiency and the charge current should be lowered to limit stress. When “ready,” the battery must cool down.  If the temperature stays above ambient, then the charger is not performing right and the battery should be removed. Extended trickle charge on nickel-based batteries inflicts damage. NiCd and NiMH should not be left in the charger unattended for weeks and months. If not required, store them in a cool place and apply a charge before engagement.

Lithium-based should always stay cool on charge. Discontinue using the battery and/or charger if the battery heats up on charge. Li ion cannot absorb over-charge and therefore does not receive trickle charge when full. It is not necessary to remove Li-ion from the charger, however, if not used for a week or more, it is always best to place the pack in a cool place and recharge before use.

The most basic charger is the overnight charger, also known as slow charger. This goes back to the old nickel-cadmium days where a simple charger applied a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery was connected. Slow chargers have no full-charge detection; the charge stays engaged and a full charge of an empty battery takes 14–16 hours. When fully charged, the slow charger keeps NiCd lukewarm to the touch. Because of its reduced ability to absorb over-charge, NiMH should not be charged on a slow charger. Low-cost consumer chargers to charge C AA and AAA cells often use this charger method, so do some children’s toys.

The rapid charger falls between the slow and fast charger and is used in consumer products. The charge time of an empty pack is 3–6. When full, the charger switches to “ready.” Most rapid chargers include temperature sensing to safely charge a faulty battery.

The fast charger offers several advantages and the obvious one is shorter charge times. Short charge times demand tighter communication between the charger and battery. At a charge rate of 1C, (see BU-402:What is C-rate?) which the fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. As the battery approaches full charge, some nickel-based chargers reduce the current to adjust to the lower charge acceptance. The fully charged battery switches to trickle charge, also known as maintenance charge. Most of today’s nickel-based chargers have a reduced trickle charge to also accommodate NiMH.

Li-ion charges are most efficient and charge the battery to 70 percent in less than an hour. The extra time is devoted for the long saturation charge that is not mandatory as it is for lead acid. In fact, it is better not to fully charge Li-ion as it will last longer. Of all chargers, the Li-ion charger is the most simplistic. No trickery applies to improve battery performance and longevity. Only the CCCV method works. 

Lead acid cannot be fast-charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14–16 hours; anything slower is a compromise. Lead acid can be charged to 70 percent in about eight hours; the all-important saturation charge takes up the remaining time. A partial charge is fine provided the lead acid occasionally receives a fully saturated charge to prevent sulfation.

The standby current on a charger should be low to save energy. Energy Star assigns five stars to mobile phone and similar small charger drawing 30mW or less on standby. Four stars go to 30–150mW, three stars to 150–250mW and two stars to 250–350mW units. The average is 300mW and this gets one star. Energy Star aims to reduce current consumption of personal chargers that are mostly left plugged in.

Simple Guidelines when buying a Charger

Last Updated 3/30/2015

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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.

On February 13, 2015 at 6:08am
David Devine wrote:

Dear Sir,

I am wondering if either of these two chargers are better then the other. It seems to me that they both offer a bit different profile amd want to know which would be better for my situation. Charging wet 12 volt 232 ah batteries used as a back up power supply.

On February 13, 2015 at 5:11pm
Mark wrote:

Can I use a Lithium ion battery charger to charge a Nickel cadium battery.

On February 16, 2015 at 9:25am
Mark wrote:

If I use a Lithium Ion charger to charge a Nickel cadium battery, will it damage or shorten the life of my battery?

On March 16, 2015 at 11:58pm
tesfish kiyyaa wrote:

what are the key parameters THAT I have follow while i will go on to make charge a cyclon lead battery (12v 8AH)

On March 29, 2015 at 8:11pm
Hanson Tan wrote:

Dear Expert

Your video is very educational!

I have a question about my lithium charger.
The output voltage rating as shown on my charger brick for my hub motor shows its providing sum of of all the lithium cells in series where each cell is 3.3v.
I measured with voltmeter and indeed thats the output voltage or open voltage.

What puzzles me is that the lithium battery pack when fully charged, is at the sum of all the batteries in series where each cell is at 4.2v, the max voltage rating of each cell.

So, I am not sure why the charger is providing a lower voltage than the voltage when the battery pack is fully charged. Its logical to say the controller circuit has somehow stepped up the voltage either by PWM or other means.

I am seeking advise why the charger is designed like this and is this a standard for lithium chargers for electric motor lithium packs.

Thanks very much.