Equalizing batteries can improve performance and extend their lifespan.

Lead acid house batteries aboard boats that spend most of their time in a slip and connected to shorepower—they’ve got it easy. But the same batteries aboard boats that spend a lot of time disconnected from shorepower get discharged and then only partially recharged by solar panels and wind generators—they lead a hard life and often die prematurely. Sure, charging current from an engine-driven alternator or dedicated generator helps, but typically either is run only an hour or two each day, if that, which brings the batteries to only 80 to 90 percent of full charge. The performance of traditional lead acid batteries will soon begin to decline if this cycle is repeated often enough.

That performance decline shows in batteries that appear to charge and discharge much more quickly than when they were new. But the good news is that there is something you can do about it, a process called equalization.

Lead acid battery chemistry

All lead acid batteries, whether flooded, AGM, or gel batteries, depend on a chemical reaction between sulfuric acid and lead to produce an electrical potential or voltage. When a load is placed on a charged battery, the lead dioxide plate reacts with the acid, leaving a lead sulfate crystal on the plate, producing a hydrogen and an oxygen ion (which makes a water molecule) and releasing an electron. As the electrons leave the battery to power an LED reading lamp or a water pump, the lead sulfate crystals continue to accumulate on the plate, and the sulfuric acid becomes diluted with water.

Charging the battery reverses the process, forcing electrons back into the mix, and converting the lead sulfate crystals back into sulfuric acid. The battery is fully charged when all the lead sulfate crystals and water molecules have been converted back into their two original components: sulfuric acid and lead dioxide. This is ideal. And it rarely happens.

Instead, batteries aren’t typically returned to a fully charged state, and some of the lead sulfate crystals remain on the lead plates, where they harden with time. Once the crystals are hardened, the charging current can no longer convert the crystals back to sulfuric acid. As this cycle is repeated—discharge, followed by a partial recharge—more and more of the lead sulfate crystals build up, reducing the effective capacity of the battery. This is called sulfation.

As the amount of sulfation increases, the battery will begin to discharge more quickly and the battery voltage will increase much more quickly during the charging state than when the battery was new. If sulfation ever builds up enough to bridge the gap between any two lead plates, the plates will short and the battery will be a goner.

Another problem that occurs with flooded batteries is stratification. Stratification happens because the acid is heavier than water and settles to the bottom, or perhaps more accurately, the water floats to the top. In a perfect world, when batteries are fully recharged each day, the water is fully converted back to acid and the mixture remains homogeneous. In a battery that is consistently undercharged, the water and less concentrated acid has a tendency to stratify and rise to the top of the solution, separating from the more concentrated acid. The same is true of a battery that isn’t used for a long period of time. The symptoms are the same as a battery with sulfation. Additionally, stratification promotes sulfation.

Sulfation can sometimes be removed via a process called equalization (some battery manufacturers refer to the equalization process as conditioning). In this process the batteries are brought to full charge, then the charge voltage is increased to a much higher level than usual, typically between 15 and 16 volts, to overcome the higher internal resistance caused by the sulfation. To prevent cooking the batteries, the current is limited to between 5 and 10 percent of the battery’s amp-hour capacity. This voltage is then held for several hours.


Sulfation over time

As sulfation increases over time, more of the battery plate is covered in non-conductive crystals, reducing the conductive surface area of the plate thus, the battery’s capacity.

Charging a battery at a higher voltage causes the electrolyte to produce bubbling that agitates the liquid and helps to remix it. In addition to dissolving sulfate crystals on the plates, equalization causes the water molecules that dilute the acid and lead to stratification to reform into their original components of the acid mixture.

Most flooded cell-type batteries can be equalized, as well as most AGM batteries. It’s imperative, first and foremost, to make sure your batteries can be equalized. Check with the manufacturer first to be sure.

Most manufacturers provide information on how often batteries should be equalized and the procedure for doing so. Some recommend monthly equalization while others recommend it only when the battery becomes symptomatic.

Equalization Methods

The are several ways to equalize your batteries. Most of these methods sound good in theory, but I’ve found some of them impractical in practice.

There are many battery chargers on the market that incorporate an equalization mode, but it’s important to check the specs. Some manufacturers claim their charger has an equalization mode, but in reality, the charger does a poor job of equalizing batteries. My previous batteries were Lifeline batteries, which require, depending on the battery temperature, between 15.2 and 16.1 volts over a period of 8 to 10 hours to properly equalize the cells. My old battery charger was a “smart” three-stage charger. Its equalization mode, however, only provided a voltage of 14.8 volts and would only allow the battery to be equalized for three hours. My new Magnum inverter/charger allows me to set the equalization time and voltage to match my battery specifications.

Some alternator and voltage regulator combinations provide an equalization mode, but as with battery chargers, not all equalization modes are the same. The Balmar regulator we used aboard Nine of Cups was quite versatile. It not only had an equalization mode, but it allowed me to set the voltage, maximum current, and time. Another option is to use a manually adjustable alternator controller and continually monitor it for the optimal equalization voltage.

If you have a wind generator and/or solar panels, you may be able to equalize the batteries on a windy and/or sunny day. Most of the newer MPPT (maximum power point tracking) solar controllers have an equalization mode, but it is important to check the specifications to make sure the controller will, indeed, provide the correct equalization voltage and current required for your batteries.

Another equalization option is to use a variable DC power supply, which allows precise setting of the maximum current and voltage applied to the battery. This was my preferred method for equalizing batteries before I upgraded to the latest in solar controller and inverter/charger technologies. Once the batteries are fully charged, the steps for using a variable DC power supply to equalize the batteries are as follows:

  • Adjust max current of power supply to the recommended equalization current.
  • Adjust power supply voltage to the recommended equalization voltage.
  • Connect to the battery and monitor, adjusting the voltage and current as necessary.

If you decide to purchase a DC power supply, you will want to make sure it is rated for at least 20 volts and 15 amps. It should also be output protected. If you connect a DC power supply to a battery and it is not output protected, you will quite likely fry the output section of the power supply. I purchased a Volteq model HY3020EX, which is a 30-VDC (volts DC), 20-amp variable power supply. Currently, the cost of this power supply is about $230 from a couple of online sources. It is the least expensive option I know of for obtaining a tool that will optimally equalize any battery that can be equalized.

Some manufacturers recommend equalizing the batteries as often as once a month, while others suggest waiting until the batteries become symptomatic. On Nine of Cups, our first batteries were flooded batteries. I equalized them once a year, and they barely lasted three years. Our second set were Lifeline 4D AGM batteries, and I managed to equalize them about once every three months (before they began to show signs of sulfation). Those batteries lasted seven years. I know that’s comparing apples and oranges, but for a cruising boat that rarely saw a marina, seven years of service from any battery type was pretty good. Given the high cost of replacing batteries, getting a few more years out of that battery bank was well worth the investment of time and the cost of a DC power supply.


The Devil in the Details – DL

Even though it’s good practice to equalize batteries (assuming your batteries are the types that can and should be equalized), it’s a process that requires care and caution. Before you do it, be aware of the following:

Battery type: Not all batteries can be equalized. In general, flooded cell batteries can usually be equalized, as well as most AGM batteries. Gel batteries usually cannot. Check with your battery manufacturer.

Ventilation: Equalizing batteries produces hydrogen gas. Even our sealed Lifeline batteries released enough hydrogen to set off our bilge gas detector if we equalized them without venting the battery compartment. Hydrogen is lighter than air and will quickly dissipate if the battery compartment is open, but if it is confined in a closed battery compartment, it can be extremely explosive.

Equalization parameters: Most battery manufacturers will provide the optimal voltage and current that should be applied to the battery, as well as the length of time necessary to properly equalize their batteries. Make sure whatever method you use to equalize the batteries can be adjusted to meet the manufacturer’s recommendations.

Equalization voltage: Equalization voltages are considerably higher than normal charging voltages, and some equipment cannot tolerate these higher voltages. When in doubt, disconnect any equipment, especially electronics, from the battery during the equalization process.

Battery temperature: The optimal equalization voltage varies with battery temperature. The better charging systems, whether solar, wind, alternator, or inverter/charger, utilize a temperature sensor mounted on the battery to adjust the equalization voltage accordingly. If you are using a DC power supply, it is best if you can monitor the battery temperature and tweak the output voltage as needed.