BU-404: What is Equalizing Charge?

Know how to apply an equalize charge and not damage the battery.

Stationary batteries are almost exclusively lead acid and some maintenance is required, one of which is equalizing charge. Applying a periodic equalizing charge brings all cells to similar levels by increasing the voltage to 2.50V/cell, or 10 percent higher than the recommended charge voltage. 

An equalizing charge is nothing more than a deliberate overcharge to remove sulfate crystals that build up on the plates over time. Left unchecked, sulfation can reduce the overall capacity of the battery and render the battery unserviceable in extreme cases. An equalizing charge also reverses acid stratification, a condition where acid concentration is greater at the bottom of the battery than at the top.

Experts recommend equalizing services once a month to once or twice a year. A better method is to apply a fully saturated charge and then compare the specific gravity readings (SG) on the individual cells of a flooded lead acid battery with a hydrometer. Only apply equalization if the SG difference between the cells is 0.030.

During equalizing charge, check the changes in the SG reading every hour and disconnect the charge when the gravity no longer rises. This is the time when no further improvement is possible and a continued charge would have a negative effect on the battery.

The battery must be kept cool and under close observation for unusual heat rise and excessive venting. Some venting is normal and the hydrogen emitted is highly flammable. The battery room must have good ventilation as the hydrogen gas becomes explosive at a concentration of 4 percent.

Equalizing VRLA and other sealed batteries involves guesswork. Observing the differences in cell voltage does not give a conclusive solution and good judgment plays a pivotal role when estimating the frequency and duration of the service. Some manufacturers recommend monthly equalizations for 2–16 hours. Most VRLAs vent at 34kPa (5psi), and repeated venting leads to the depletion of the electrolyte, which can lead to a dry-out condition.

Not all chargers feature equalizing charge. If not available, the service should be performed with a dedicated device.

Last updated 2016-02-23

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On March 17, 2011 at 9:51am
Jane Olivier wrote:

Would like to know more about equalization - dangers, gasses, effect of the gasses if inhaled - how does one recogise that inhalation has occurred, etc.  This is very important especially for people living on boats where the batteries are normally in inaccessible places with little ventilation.

On July 20, 2011 at 4:57am
Mick Bignell wrote:

Should the equalizing charge be 2.5v/cell or 10%? As plus 10% of 2.4v = 2.64v/cell.
Thanks for a very informative web site.

On January 27, 2012 at 9:31pm
nitin solke wrote:

very very informative site. gained immensly from the material. thanks


On May 23, 2012 at 5:44am
Kashif wrote:

This is a very useful web site and I understood among those questions which will confuse me on my Job.

On July 9, 2012 at 6:34am
Dustin Kick wrote:

Is current any concern at all, here?  That is, should it be limited to an extremely low current?

On September 26, 2012 at 6:38pm
Lenly Jane Angue wrote:

tanong lang p…gaano po katagal bago maubos ung charge ng HR1221W na sealed lead acid battery kung tuloy tuloy ang paggamit??pls po,,pakisagot..

On September 26, 2012 at 6:43pm
Lenly Jane Angue wrote:

tanong lang p…gaano po katagal bago maubos ung charge ng HR1221W na sealed lead acid battery kung tuloy tuloy ang paggamit??mga ilang oras po kaya??? pls po,,pakisagot.. thanks po!

On January 21, 2013 at 12:43pm
GEOFF wrote:

Should the batteries be discharged before equalizing? Does this act as a charge cycle too, or should this serve as a low amp/high voltage charge when the batteries are at capacity?

On February 3, 2013 at 8:47pm
m.ramakrishnan wrote:

For equalizing the batteries ,instead of waiting for six months we may measure individual cell voltages and if a non uniformity among the cell voltage values is noted then immediately it can be recommended for equalizing until all the cells show a almost uniform voltage values.

On March 4, 2013 at 6:39am
Batte Ryman wrote:

To m.ramakrishnan, February 3, 2013 at 8:47 pm

If one is able to individually measure the cell voltages, then the cells can also be charged individually, and overcharging in order to equalise the cells (luckily) isn’t needed anymore.

One could even connect equalisers (small power electronic converters which move charge from or to the individual cells) on the battery. This makes separate equalisation unnecessary.

On March 14, 2013 at 11:42pm
JIn wrote:

I understant the voltage criteria but what about the current, what current should be used for the equalization . Is 5 amps alright???

On April 11, 2013 at 7:33am
peter wrote:

Lenly Jane Angue wrote:

tanong lang p…gaano po katagal bago maubos ung charge ng HR1221W na sealed lead acid battery kung tuloy tuloy ang paggamit??mga ilang oras po kaya??? pls po,,pakisagot.. thanks po!

Peter replies,
depende yan sa ampere-hours na specs ng battery. kun sa battery mo 48 amp-hrs. ibig sabihin kun my bulb ka na 12 volts at 48 watts ang rating yung kaya ng battery mo ay 1 hr ang tagal nya. nominal lang pero sa actual medyo mababa.. hpoe this helps… peter

On May 27, 2013 at 8:06pm
Robert H. wrote:

Jln: I start an equalization on the SLA batteries (sizes: ~75 amp hours or more; 6 cell) I service @ 1.75 amps @ 14.4 volts and monitor s.g.  If needed I have increased the amperage as high as a 2.5 amp limit and gone as high as 15.3 volts, but I don’t leave the battery very long between s.g. checks at these higher settings, and I shut the equalization down as soon as no further improvement is noted, or if the electrolyte starts to redden/darken at all as I don’t want to corrode the battery to death either.  So far this has saved a number of batteries for me. I recommend a good understanding of lead acid batteries and proper charging and assessment prior to any equalization. I’m certain deep cycle lead acid batteries are a little more forgiving to the higher overall energy inputs.  (this information represents my opinion only.  use only at your own risk and full responsibility as I am not).

On December 9, 2013 at 10:25pm

To know about the non uniformity among the cells in mono-block lead acid batteries, two number of similar reference electrodes may be inserted into the adjacent cells ,the voltages across the ref electrodes( ICV) may be measured with a voltmeter in open circuit or closed circuit condition. These readings showing any alarming variations indicate the defective plates presence and non uniformity among battery cells.Now the battery may be recom mended for equalisation until an uniformity exist in the INTER-CELL VOLTAGE measurements. The reference electrodes recomended for this purpose can be Ag/Ag2So4 /H2So4 electrodes.

On December 24, 2013 at 2:39am
akhilesh wrote:

plz lemme which ic will be better for charging 12V lead acid battery.And please temme how to configure the Lead acid battery charger,how to design and point to be remember.

On April 28, 2014 at 10:29am
Willie... wrote:

An excellent battery charging IC is the UC3906N
It requires external resistors and an FET to create a “smart” charger for Lead/Acid, Gel, and AGM batteries. This IC also has a built-in temperature sensor, so ideally, it should be located physically close to the battery compartment for best effect.

Here is a link to one supplier:

On June 13, 2014 at 12:19pm
Amarnath Datta wrote:

What can be ideal for max. SG when equalizing charge is complete?

On October 29, 2014 at 10:21am
glenn webb wrote:

if you have two 12v bateries in series will one always degrade quicker than the other?if so why and which one..

On November 24, 2014 at 6:09am
Deda Poke wrote:

I had 4 vrla 100ah batteries in series and one of them was allways degrading quicker than others. I have that problem for years now and I can not find solution… Recently, I switched to gel batteries to see will that help, and have no results of that test yet.

On December 5, 2014 at 12:43am
ann wrote:

If one of the batteries in my 4 battery bank is weaker (lower SG in all cells) is it OK to hook just that one battery up to my solar array as long as I watch the voltage and monitor the SG and electrolyte level?  I have a 24V array that usually gets to 32V on a sunny day and the batteries are 6V each.  Or maybe hook 2 or 3 batteries at once in series?  I used to be able to equalize with the solar array but with the weak battery the voltage doesn’t get high enough to equalize the battery bank

On March 30, 2015 at 3:17pm
andy jackson wrote:

I do equalization on dead batterys starting at under 4v and very low specific gravity readings.  If the battery is completely dead it helps often to reverse the charge leads so the + charges the negative post and vice versa.
Once I start seeing small bubbles forming in the cells I monitor the heat and length of time charging.  If the battery gets hot, I stop and wait untill it cools off.  Switch to lower level 13.5 volt charging and then occaisionally up it to 16 + volts.  The amperage will come in at high levels (over 20 amps) if the bulk charging is needed or if heavy sulfation seems to be present.  After a day maybe of this high and low voltage charging (more of which is low) the battery usually recovers to a mid capacity of new and maybe to 80 or 90% of new.  I am a great believer in Equalization as a restoring technique for lead acid batterys.
The negative aspects of overcharging is corroding the plates and shortening the life of the battery, but a battery with no capacity is worthless, so restoring its capacity to hold a charge is worth it IMHO. 
As the battery recorvers and gets into the mid 12 volt area the high voltage charge may only come in at 2 or less amps, so a lot less heat accompanies the lower amp rate.
As stated in Trojan batteries page equalization should be done when the hydrometer readings are .030 different from one another in a battery.  I would include having batterys with noticably less capacity also is a great reason to equalize them..

On May 20, 2015 at 3:45am
Arnab Banerjee wrote:

We have 60 cells (@2 V / cell) VRLA battery bank.
We discovered battery bank is defective.
On checking per cell voltage we observed one cell voltage is 8 V while others are 2.1 V. When we removed the cell from the bank and measured the cell voltage separately we found it to be 2.1 V. WHY IS IT SO?

My understanding is: Since the cell developed higher internal resistance, it’s voltage during charging goes high.
But I am not able to understand why voltage remains high when bank is disconnected from charger / load.

On October 17, 2015 at 10:19am
Gregg wrote:

I have a Vanner 60A battery equalizer, but the main cover is missing. The only electrical connections are the 24 V connection, the 12 V connection, and the ground connection. From the discussion here and on Wikipedia, it sounds like you would have to be able to connect the equalizer from a battery charger to the individual cells of the battery. How do you do that? Or is that not required? I don’t see how it could not be required, since the terminals on a lead-acid battery always connect to all the cells, not to any individual cell. Surely I’m not supposed to pry the lid off the battery and stick jumper wires down into the acid!? Even if I did that, I would still be electrically connecting to all the positive terminals at once, or all the negative terminals at once, because they are all connected to each other. Maybe if you pry the lid off, that’s no longer true, but wow. Who would take such risky action just to extend the life of a car or boat battery? How long can you extend the life of the battery through equalization, anyway? And people are talking about doing this every month? I must have a serious misunderstanding of the procedure, because this all sounds like far too much trouble for the benefits gained.

On January 22, 2016 at 1:18pm
David T. wrote:

I have a large motorhome with two battery banks -  two 12v maintenance free (sealed) batteries called “Chassis Batteries” and six 6v golf cart (unsealed) batteries called “House Batteries” - plus a Xantrex Freedom SW 3000 KW inverter/charger.  After three years of use I think it’s time to “equalize” the batteries.  Note: the two battery banks are “tied” together with a Big Boy/BIRD system.  If I start an equalization procedure, then both battery banks will be involved.

1) Can and should the two 12v maintenance free (sealed) batteries be equalized?  If not, then I will need to unhook the two 12v batteries - yes?

On February 6, 2016 at 9:27pm
Jed wrote:

I have a 24v 30A MPPT Tracer Solar Charge Controller with 2 x 12v 75Ah SLA batteries in series ... by default, the controller carries out a 2-hour long Equalization charge on the 28th of every month at 29.2v (14.6v for a 12v system) with quite a low current limit to minimize heat. The normal float voltage for a 24v system is 27.6v (13.8v for a 12v system) and the idea of the higher voltage for equalization (for anyone that’s curious as to how Equalization works) is simply to ensure that if one or two cells happen to be increasing in resistance because of sulfation, the applied voltage potential, when distributed amongst all cells, needs to be sufficiently high as to clean up the plates in the higher resistance cells despite slightly “over-charging” the ones with normal resistance levels. Small amounts of controlled over-charging at low current levels to minimize heat is potentially less harmful than allowing sulfation to degrade performance to a point beyond which it becomes irreversible wink

On February 12, 2016 at 4:37pm
Batteryguy wrote:

14.6V EQV is ok for batteries in good condition. However, if a string is not maintained properly EQV can extend up to 16V for a 12V (nominal) battery. Temperature controlled of course. The additional voltage is required to overcome resistance losses.

I have recovered batteries with up to 20V EQ which have been dead flat for over a year.

On April 24, 2016 at 1:00pm
Phillip Haley wrote:

Hi Guys.n.Girls..
Nasty Smell factor is missing from lead acid batteries when charging…just add stink bomb capsules.

On July 16, 2016 at 4:12pm
David M. wrote:

I had a sulfated 12V 17Ah lead acid battery from UPS. I guess real capacity was less than 4Ah. When i connected device called “desulfator” (NE555 timer + mosfet + coils + capacitors) + 13,5V charging adapter, peak voltage on battery terminals was up to 30V. Is it safe to charge battery with short (10us) pulses of high voltage high current?

On September 1, 2016 at 12:31pm
Gabriel K wrote:

@David M.

of course it is , because it works. If you do it right. Comparator is better. NE555 is a bit fidly. I doubt people will have a clue what you are talking about. So…

On September 1, 2016 at 5:43pm
ProtoF wrote:

From a small UPS means AGM probably., Pop the sealed lid and the rubber caps and add distilled water with a droppper until it is full. Then pour out the excess. Then desulfate.
Brought back a 36 V (12v x 3) lawnmower battery like that. Saved $200 USD.

On November 10, 2016 at 4:26pm
george gonzalez wrote:

Years ago while driving in a very remote part of Mexico the battery in my pickup died.
Some local guys removed the battery and dumped all of the acid into a bucket.  When the battery was empty they proceeded to fill it with coca cola…shake it around foe a while, then emptied the coke…re-filled battery with acid.  Said battery lived for another couple of years.

On January 16, 2017 at 8:51am
Hugh5 wrote:

I have a D8 flooded lead acid battery 1400CCA and 255Ah capacity. it is 6 years old on a boat with a 3 stage 12 amp charger which is on all the time when the boat is not in use.
battery is now very low in specific Gravity 1175 less than 40% capacity. it’s voltage 3 days after a full charge is 13.7v.  this is far too high and we suspected sulfation to possibly be the cause of the higher voltage.(we read this somewhere)
we then overcharged it at 16 volts and it only drew 3 amps . After about 17 hours the SG had gone up to above 1200 about a 50% charge. a remarkable improvement.
my question to you all is why is the battery voltage still high on open circuit and why is there only 3 amps of charge at 16volts. should we continue to overcharge it ?