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Equalization of flooded batteries

Tuesday, September 22nd, 2009

August, 1995 (rev B), ©2000 – 2010 Xantrex Technology Inc.

Equalizing or conditioning batteries refers to a method of charging deep cycle wet- cell batteries and is intended to restore battery capacity, revive battery efficiency, and extend battery life.
The process involves periodic application of a controlled overcharge cycle to batteries. This type of charge cycle requires that certain procedures and precautions be followed.

While a battery is being discharged, sulfuric acid in the electrolyte reacts with the lead plates in a chemical reaction that produces electricity and lead sulfate. When the battery is re-charged, electricity flows back into the battery and causes a reverse chemical reaction that turns the lead sulfate back into lead and sulfuric acid.
However, with each discharge and recharge cycle, a small amount of lead sulfate will remain on the plates. Using a three-stage charger, the amount of residual sulfate left on the plates will be less but some will still accumulate with each discharge and recharge cycle.
If this sulfate is left in place for very long, it will harden or crystallize and eventually reduce the battery’s capacity, increase its internal resistance and destroy the battery’s ability to produce an adequate amount of power.
When this occurs, even an equalize charge can not remove the sulfate and the battery becomes useless except as a recyclable item. (Recycling batteries is recommended to reclaim the lead and prevent contamination of the environment.)

Over time, the electrolyte tends to stratify into layers of acid and water with higher concentrations of acid near the bottom of each cell and more diluted electrolyte near the top. This causes uneven specific gravity within a cell and further reduces its capacity and efficiency.

An equalizing charge is a controlled overcharge cycle that performs several actions within the battery and provides certain benefits. During equalization, the voltage is raised to approximately 2.7 volts per cell, or about 16.2 volts for a 12-volt battery.
The current output of the charger should be limited to about 5% of the battery’s capacity. In other words, a 200-amp hour battery should be allowed to accept no more than about 10 amps of current. This will help prevent overheating.
The equalize cycle is timed to be between 4 and 8 hours depending on the features of the charging source, but the cycle can always be terminated early if necessary.
The particular battery manufacturer’s recommendations for equalization time should be followed.
This elevated voltage results in a vigorous charging action to take place within each cell that has several effects on the battery. First, much of the residual sulfate is forced to re-combine with the electrolyte in the form of sulfuric acid. Crystallized sulfate that will not re-combine is broken loose from the plates and falls harmlessly to the bottom of the battery.
Deep cycle batteries have additional space beneath the plates intended to collect this material. This action cleans the plates exposing fresh lead to the electrolyte and restores battery capacity.

The vigorous bubbling action that occurs during equalization stirs up the electrolyte and restores it to a consistent mixture of acid and water. The equalizing process also causes all cells in a battery to reach their maximum idle potential of 2.1 volts.

It is best to check with the battery manufacturer’s recommendations before equalizing since each manufacturer has slightly different suggestions on how often and how long to equalize their batteries. But, as a general rule, it is a good practice to equalize batteries after every 10 or 12 deep discharge and re-charge cycles. For batteries in constant discharge and recharge usage, this would mean about every two weeks. For periodic users, it would mean about two or three times a year.
For seasonal users, this could mean at the beginning and end of the season.

Again, check with the battery manufacturer’s recommendations, but as a general rule, the following steps should be observed:
1. Only attempt to equalize wet cell deep cycle batteries. Never equalize gel batteries or maintenance free batteries.
2. The batteries should be fully charged and near ambient temperature before beginning an equalize charging cycle.
3. There should be a sufficient amount of electrolyte in each cell to cover the plates, but do not top-off each cell until after equalizing. Since there is some heating of the cells during equalization, the electrolyte will expand and could overflow the cells if they were topped-off before equalizing. This would not only make a mess, but force you to terminate the equalize cycle too early to gain maximum benefit and would result in diluted electrolyte when it came time to add water.
4. Leave the caps on each cell. The caps are vented, and when left in place will prevent splattering of electrolyte onto the top of the battery when the bubbles pop. It is a good idea to lay a paper towel over the caps. This makes it easy to spot a cell that may start spitting electrolyte, and will soak up the liquid when this does happen.
5. Since the batteries will give off significant quantities of explosive hydrogen and oxygen gas during equalization, and produce moisture that will contain some amount of corrosive sulphuric acid, it is imperative that sufficient ventilation be provided.
Avoid smoking or generating sparks or flame near the batteries during this charge cycle.
6. All DC loads on the batteries should be turned OFF and disconnected. Since the battery voltage will be higher than normal during the charge cycle, some DC equipment could be damaged if left ON. These loads would draw current from the charger that should be available to the battery instead.
7. Equalize only one bank of batteries at a time.
8. During the equalize cycle, periodically check the batteries for any spitting cells and if this begins to happen, terminate the equalize cycle early. Do not start an equalize cycle and then leave the batteries unattended.
9. After equalizing, turn OFF the charging source, and allow the batteries to cool to ambient temperature before resuming normal float charging. After the batteries cool, it is a good time to check the specific gravity in each cell. They should all be 1.265 +/- .050 at 80 degrees F.
11. Refill each cell with distilled water up to the FULL indicator.

End ©2000 – 2010 Xantrex Technology Inc.

Equalization charging can be done with your PV system if your array is large enough, or with an engine generator.
Most PV Solar charge controllers and inverter-chargers have battery equalization functions.

(quoted from article below by Windy Dankoff, well worth a read on this link)
© 2006 by Windy Dankoff – Home Power Magazine

although it sounds technically difficult, equalization is quite simple to do
Just remember to check your electrolyte (distilled water) level is topped up to the correct level before & after the equalization.  If your levels are low, wait with the EQ until you have reached the full level of distilled water but do this over 2 to 3 times. It is not recommended to fill a low battery all the way to the top level in one go. Wait a couple of charge cycles (days) before commencing the process. This is specially important in the hot summer months, as the equalization process will drop the water level a bit.

We don’t recommend Equalization in the south of Spain during the hot summer months. Ambient battery room temperature should be below 25C. If you have serious battery problems call us on 6299 47701 for direct advice.

Outback FM 60/80

This unit supports equalization and you can set the desired time. The Outback also supports an automatic setting for doing the EQ in intervals of your choice but we don’t recommend using this feature. It is far better that you are in full control of when you want to do the over charge. For one thing the auto setting could initiate an EQ charge late in the afternoon, if your previous night consumption was heavy. That could mean you won’t achieve the full length of your recommended EQ time. If the batteries goes into another discharge cycle after a short/inferior EQ charge it can have adverse effects.

Make sure you are up around the 85-90% state of charge with at least 4 hours of direct sun on your panels left before you initiate the eq.


Midnite solar Classic

This unit also supports EQ voltage settings, time and has an auto feature. Please read the statement above under the Outback entry for the auto eq settings.


Charge……………….Specific Gravity

When you check the gravity meter, you should position the top of the fluid level with your eyes. Take the reading from the lowest part of the level (the acid will make a slight depression in the Center of the cylinder, clinging a bit higher on the sides, creating something that looks like two levels).