BU-405: Charging with a Power Supply

Learn how to charge a battery without a designated charger.

With technical knowledge, batteries can be charged manually with a power supply featuring user-adjustable voltage and current limiting. I stress manual because charging cannot be left unattended; charge termination is not automated. You need to observe the state-of-charge according to voltage and current behaviors. Lower the charge voltage or disconnect the charge when the battery is full. Because of difficulties in detecting full charge with nickel-based batteries, I recommend to charge only lead and lithium-based batteries manually.

Before connecting the battery, calculate the charge voltage according to the number of cells in series, and then set the desired voltage and current limit. To charge a 12-volt lead acid battery (six cells) to a voltage limit of 2.40V, set the voltage to 14.40V (6 x 2.40). Select the charge current according to battery size. For lead acid this is between 10 and 30 percent of the rated capacity. A 10Ah battery at 30 percent charges at about 3A. Starter batteries charge at lower currents, and an 80Ah pack would charge at about 10 percent of the rating, or 8A. Higher currents are possible.

Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures in a well-ventilated room. Once the battery is fully charged and the current has dropped to three percent of the rated Ah, the charge is completed. Disconnect the charge. High self-discharge (soft electrical short) may prevent the current from going to the anticipated low current level when fully charged. Disconnect the charge also when the current has bottomed out and cannot go lower. If you need float charge for operational readiness, lower the charge voltage to about 2.25V/cell.

You can also use the power supply to equalize a lead acid battery by setting the charge voltage 10 percent higher than recommended. The time in overcharge is critical and must be carefully observed. When using the power supply to perform equalizing, refer to the previous section entitled “Equalizing Charge.”

A power supply can also reverse sulfation but there is no guarantee of success. When applying a charge, a totally sulfated lead acid may draw very little current at first, and as the sulfation layer dissolves the current will gradually increase. If you must increase the charge voltage above the recommended level, set the current limiting to the lowest practical value and observe the battery voltage. If the battery does not accept a charge after 24 hours, restoration is unlikely. [BU-804, Sulfation]

Lithium-ion charges similarly to lead acid and you can use the power supply also but use extra caution. Set the voltage threshold to 4.20V/cell and make certain that none of the cells connected in series exceeds this voltage. (The protection circuit in a commercial pack does this.) Full charge is reached when the cell(s) reach 4.20V/cell voltage and the current drops to three percent of the rated current, or has bottomed out and cannot go down further. Once fully charged, disconnect the battery. Never allow a cell to dwell at 4.20V for more than a few hours. [BU-409, Charging Lithium-ion]

Please not that not all Li-ion batteries charge to 4.20V/cell. Lithium iron phosphate typically charges to 3.65V/cell and lithium-titanate to 2.85V/cell. Some high-energy cells may accept 4.30V/cell. It is important to observe these voltage limits. (See BU-205: Types of Lithium-ion.)

I do not recommend charging nickel-based batteries with a power supply. Full-charge detection is difficult to assess because the voltage signature varies with the applied charge current. If you must charge, use the temperature rise on a rapid charge as an indication for full charge. When charging at a low current, estimate the level of remaining charge and calculate the charge time. An empty 2Ah NiMH will charge in about three hours at 750–1,000mA. The trickle charge, also known as maintenance charge, must be reduced to 0.05C. [BU-407, Charging Nickel-cadmium; BU-408, Charging Nickel-metal-hydride]

Last updated 3/28/2015

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On June 30, 2011 at 10:19am
Niels-Erik Jensen wrote:

It is a bit surprising that you can charge a 2Ah(=2000mAh) battery using a 500 mA and a charging time of 3 hours.. 3h * 500mA = 1500 mAh.

The battery is heating up during charging so one would expect e.g. a 500 mA connected in e.g. 5 hours.

Please clarify. One answer could be that you should never discharge a 2Ah to more than 50% of full charge.

Please clarify. I have tried for 3 months to get an answer to this question and also how much energy (in%) is lost (to heating the battery) during decharging the battery

On July 6, 2011 at 11:06pm
Paul Peter wrote:

Give a brief idea of charging Lithium Yitrium based batteries

On July 7, 2011 at 1:20pm
Fredrick Stanley wrote:

can you send me information on how long should any cell phone charge fo

On July 9, 2011 at 2:24pm
rohit khatri wrote:

hi thanks a lot for the information you are providing.
i would like to know about dead cell and cell reversal or polarity reversal.

On August 13, 2011 at 8:56am
joel wrote:

Can you explain rapid pulse charging? I want to know how i can charge cell phones quickly in 10 mins.

On August 29, 2011 at 3:28pm
BWMichael wrote:

Joel, the faster the charge, the more damage you are doing to the battery

On March 21, 2012 at 6:50am
jamiedentims wrote:

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jamie d

On June 26, 2012 at 4:53pm
michaelnalsim wrote:

well gerry it took me ages to find it here is there link
and details,ring them for advice , mention micky n recommened you

On February 22, 2013 at 9:10am
Nikhil wrote:

I want to pose a question here….. Plz help..
If I have 2 lead acid battery packs of 24V 26Ah each, and I want to charge them. Which one of the following techniques will take lesser time????

1) I connect them in series to make a 48V 26Ah unit, and charge this pack using a suitable charger for 48Volts.

2) I charge the 2 packs of 24V 26Ah simultaneously using two suitable chargers of 24 volts.

On June 14, 2013 at 6:41am
lead acid battery wrote:


On June 25, 2013 at 6:13am
Tango wrote:

Ok, I’m confused!
My wall charger supplies 5V to the phone’s microUSB port.
Is it :
a) the charger studying the battery voltage and current to regulate the delivered voltage and current, Or
b) the phone’s circuitry and charging algorithm that is instantaneously telling the charger to deliver x Volts and y Amps?
c) the phone’s circuitry receiving constant 5V from the charger and regulating the voltage and current internally according to battery condition?

Does the phone play any role here? or is it just the battery and the charger doing their thing?
If it’s the phone, I feel safe. If it’s the charger (which i doubt), then any third party charger should damage my battery as the charger is not made with my phone’s battery specs in mind. But this does not happen, third party chargers also work great.
That said, if all Li-ion batteries charge the same way (4.2V, 3% thresholds) then it’s not an issue.

If it’s the charger doing the thinking, then it’s regulating it’s voltage from zero to 4.2V during the charge cycle, whereas it is rated at 5V DC. And so is the microUSB port - rated 5V!

Soooo confused!

On July 21, 2013 at 11:20pm
Vinay wrote:

can we use two batteries in parralell

On October 18, 2013 at 2:09am
abhishek wrote:

how much current does a 12v 26 Ah battery can provide? and please ddo provide a suitable way to recharge it..

On October 26, 2013 at 5:21am
Chris Flores wrote:

How long should I charge the battery? And how should I know when the charging is complete with sealed lead acid?

On November 5, 2013 at 12:49am
Sam wrote:

Thank you for taking the time to write this article.

On November 19, 2013 at 3:32am

What is the total heat load of 12V 26AH x 32 no’s VRLA batteries connected to a 10KVA 3 phase UPS.

On December 25, 2013 at 7:46am
Imran wrote:

Plz tell me how increase battery time

On March 21, 2014 at 4:15pm
Stephen wrote:

This article has helped me understand better the basics of what “smart” chargers are doing.  My question is how a “smart” charger might determine the correct full charge point when it “knows” battery type (I tell it that), applied voltage, charger output current and time.  Since the switch to float voltage is dependent on amp-hr capacity of the battery and charge current, can a charger reliably determine battery capacity, therefore charge current with this limited information?  Thanks in advance for your assistance. I am concerned I may have purchased the wrong charger.

On March 30, 2014 at 4:32am
manu wrote:

I have a Lithium-Ion Rechargeable Battery Pack 11.1V 1500mAh. It contains 3 X Li-ion 3.7V 1500mAh cells.  Can I recharge the battery by directly connecting it to DC Power Supply. How ?

On April 22, 2014 at 10:25pm
Ruchita wrote:

I have lead acid battery of 4 V.While charging,How can I know whether battery is fully charged or not? Please help me It’s very important for me to know.

On May 21, 2014 at 11:04am
Bill Geldy wrote:

Can I replace 3 12v 17ah batteries (connected in parallel) with a deep cycle marine battery. If so, what size marine battery would you recommend?

On May 31, 2014 at 11:39pm
Suwan Khwakhali Shrestha wrote:

why my MPS 12v series battery going heat during charging?

On June 11, 2014 at 1:03am
ajay wrote:

12v adapter with battery charger pin - 20no’s
plz contact 9949341135,9912132004

On June 24, 2014 at 7:07am
Colin Williamson wrote:

Please advise at what voltage should my charger be set in order to charge 2 x 12v batteries connected in series

On July 10, 2014 at 12:38am
Andre Van den Wyngaert wrote:

@Ruchita: 4V lead acid battery you can charge with 4.8V to do a quick charge. Float charge is 4.6V and you can apply that voltage forever. If you apply 4.6V and the current drops to a low value, the battery is full.

On July 10, 2014 at 12:42am
Andre Van den Wyngaert wrote:

@Colin Williamson:
I assume the 12V batteries are lead based batteries. Each battery takes 13.8 float charge, or 14.5 quick charge (need to terminate manually in that case). So for 2 batteries in series, you need respectively 27.6V or 29V.

On August 21, 2014 at 5:44am
Diogo Saraiva wrote:

I didn’t understand.
Can I use my radioamateur source (RPS1230SWD) to charge my 12V 1.2AH lead-acid battery?
I can only set voltage… (and noise offset)
Can you help me please?
Thank you very much

On October 19, 2014 at 10:48pm
hashan wrote:

Thanks for provinding hese information.i want to know if we want to give current and charge runng car battery(12V). what are the current and voltage we should supply.

On October 20, 2014 at 1:25am
Andre Van den Wyngaert wrote:

@ Diogo Saraiva:
RPS1230SWD is perfect for charging 12V lead acid. Set it to 13.8V (is very easy: set the slide switch o 13.8V…) and connect the battery. + of the battery to the red connector, of course. Caution: if your battery is very empty, it might draw more current the first minute(s) than is good for it. So just for being sure, you can put a resistor between the charger and the battery. I would take 10 ohm approximately, with a rating of 2 watts or higher. This will limit the current to a safe value. So the charger’s black connector goes to the - of the battery, the charger’s red goes to the resistor. The other side of the resistor goes to the + of the battery. After having it connected for, let’s keep it safe, 15 minutes, you can bypass the resistor. You can also keep it in place; charging will take some more time then. You can leave the battery connected forever to 13.8V. It will stay full and ready for use. If you switch off the power supply, better remove the battery, because it might discharge slowly through the power supply.

On October 20, 2014 at 1:28am
Andre Van den Wyngaert wrote:

@hashan: what kind of power supply will you use? Most of the time, the power supply is in danger, not the battery…

On November 23, 2014 at 3:26pm
rod wrote:

Is there a portable automotive 12VDC battery charger that will also power small 12VDC loads or a few watts like a small air pump - while NOT being hooked to a battery?

Seems using a battery charger to make DC is not possible on “automatic” battery chargers because they “sense” a dead battery?  Any work around for this?

Thanks - Rod

On November 29, 2014 at 2:30pm
1989 wrote:

Do you need to use DC current to charge batteries or is AC current ok??

On December 2, 2014 at 9:53pm
Ramesh Subramani wrote:

i have two doubts..
1) Is the charging circuit is necessary to charge a battery of 12V from a DC DC converter which gives a boosted 15V from 5V input..? will it charge if i directly connect the output of converter to the battery itself..?
2) i have a 12V battery of capacity 10Ah..can i charge it using a 250mA current which is provided by the DC DC booster circuit..?

On December 3, 2014 at 1:48am
Andre Van den Wyngaert wrote:

@ Ramesh Subramani:
yes, you can charge a 12V battery with a DC/DC converter that upconverts 5V to a higher voltage. Normally you need 13.8V. But you will need to limit the current in a certain way. If the DC/DC converter is limited, it will work without any extras. But 15V is too high to leave the battery connected. AS soon as it reaches 14.5V you need to disconnect manually. Can’t you reduce the output voltage to 13.8V? 2 1N4001 diodes in series could do the job. Then you can leave the battery connected.
From you questioning I understand that you DC/DC can deliver 250mA. That is enough to charge the battery. But it will take some time, of course…

On December 3, 2014 at 2:10am
Andre Van den Wyngaert wrote:

No, it is not allowed to connect an AC source like a mains transformer directly to the battery. Depending on the size of the transformer or adapter, you will damage it, and the battery too. If you really need to use the AC voltage, you need a bridge rectifier between the adapter and the battery. And some current limiting, depending on the voltage of the AC source. After the bridge rectifier is pulsing DC, which is no problem.

On December 3, 2014 at 4:23am
Ramesh Subramani wrote:

thank you very much for your reply sir..
i have another doubt, as i read it in an article, the battery draws more current from the supply when it is empty and if it is full the current input to the battery is zero..if so then if i connect the DC DC converter directly which can deliver only 250mA, whether it will damage the converter.? or what happen to the current output from the DC DC converter..?

On December 4, 2014 at 2:00am
Andre Van den Wyngaert wrote:

@Ramesh Subramani:
that is exactly what I mean when I said you need some current limiting circuit. If you upconverter has no current limiting, it can be overloaded.
So there are 2 possibilities:
1. your upconverter is current limited. Then there is no problem: you can connect it directly to the battery (eventuelly through 2 diodes to lower the 15V to 13.8V, but it is better if you change the circuit so that the output voltage becomes 13.8V. Most of the time only one resistor need to change value to do this)
2. your upconverter is not current limited. You could add a small circuit to limit the current. Example: http://www.eeweb.com/blog/circuit_projects/additional-current-limiter-for-power-supply   (the leftmost circuit will do. Use 2.7 ohm in the emitter of the transistor to limit to 250mA)
Good luck!

On December 6, 2014 at 7:47pm
Ramesh Subramani wrote:

thank you sir..

On December 18, 2014 at 5:19pm
John Creed wrote:

I have a three wheeled electric scooter witht a 36v electrical system powered by 3 -12v SLA batteriues. I would like to replace the SLA batteries by makeing a 36v Lithium-Ion battery by soldering 30 - AA 1.2v batteries in series. I see no problem making the battery, what I don’t know is how to charge it. Can I use the lead acid charger that came with the scooter or do I need a 36v charger designed for Lithium-Ion batteries?
Thanks for any input,

On January 6, 2015 at 3:30am
Andre Van den Wyngaert wrote:

@ John Creed:
Are you sure your cells are 1.2V Lithium-Ion? Some well-known Chinese websites e.g. Alibaba state “1.2V Lithium cell” but when you read on, it appears to be Ni-MH or even Ni-Cd. Imho, 1.2V Li-ion does not exist, unless maybe someone placed some circuitry inside the battery to reduce the output voltage.
Anyway, for charging li-ion, you cannot use the lead battery charger anymore. Li-ion requires a very accurately regulated output voltage and current limiting.
Li-ion “native” voltage is 3.6V…3.8V So you could use 10 of those in series to get 36V.

On January 20, 2015 at 12:24pm
Samuel wrote:

I have a macrotel rechargeable 9v battery @290mah, and my charger delivers 15mah, and the charge time is eternal, so here is the matter, i don’t know if you have heard of those power supplies for guitar pedals, well anyway, this power supply delivers 9v at 100mah, so calculating it would take less than 3 hours to charge, so i want to know if i can do it, and if there are risks on doing it, hope you can help me, regards!

On January 21, 2015 at 1:25am
Andre Van den Wyngaert wrote:

@ Samuel:
9V 290mAh - I assume it is NiMH since Lithium has higher capacity. 9V NiMH is actually 8.4V (7x 1.2V cells). You need at least 1.5V…1.6V per cell to charge, so 7x1.5V= 10.5V. If your guitar pedal supply delivers 9V exactly, you cannot charge the battery with it. If you connect it directly, you will see some current but quickly it will lower down to almost nothing.
A second issue would be the max current allowable. Those 9V cells, unless it is a fast charge type, prefer 1/10 of the capacity as charge current, so that would be 29mA, for 15 hours.
And a third one: you can never connect a standard power supply directly to a NiMH battery, unless it has a good current limitation circuit, and you can terminate the charge manually.

On January 23, 2015 at 3:12pm
Samuel wrote:

Hey Andre, Thank you for your answer i really aprecciate it. So if you say that my option isn’t as much good, where can i get a charger that is more efficient?. I’m not from USA so i would really thank if you can give a web page or some dealers that ship worldwide and be trustful obviously, thank you again Andrew, Regards smile

On January 23, 2015 at 5:21pm
Glen Kinder wrote:

Hi - I have a 36v 1.6 amp.  scooter battery (3-12’s)  Would it do harm to charge them with a 36v -1.8 amp charger?

On January 26, 2015 at 2:12am
Andre Van den Wyngaert wrote:

@ Samuel:
you can find lots of chargers at Ebay but they are slow (13hours for 175mA so over 20 hours for your battery). One that might solve your problem is called “Intelligent Battery Charger for 1-4 PP3 8.4V (9V) NiCd/NiMH batteries. UK seller” It states it fills a 280mAh battery in 3 hours…

On January 26, 2015 at 2:23am
Andre Van den Wyngaert wrote:

@Glen Kinder:
in order to answer that question, some more detail would be welcome… e.g. battery type number, and charger type number. But I assume it is a scooter battery charger for the right battery chemistry. So imho the 12.5% more current will normally do no harm. But that’s only my opinion.

On March 25, 2015 at 10:21pm
zam wrote:

It helped me a lot but my question is that while we are discharging the lead acid battery we usually concern only with DC voltage level. So is it that AC voltage is not recommended?