Battery Management 101. Agm battery equalization

Deep Cycle Battery FAQ

Flooded lead acid (wet cell) batteries are one of the most common types of battery for deep cycle applications. The major difference between the wet type of battery and the sealed kind is that the end user is able to replace water lost through gassing via the vent caps in the top of the battery. The flooded battery has been around for a very long time but remains popular because it has a good balance between price and performance. A gel or AGM cell battery is generally somewhere around twice the price per amp hour that you would find in a flooded battery. Flooded batteries do require a lot more maintenance than their sealed lead acid counterparts because the batteries are not recombinant the water is lost through the top of the vent caps while charging. Replacing this lost water can sometimes be an onerous task, but it is a necessary one to get the most out of your battery.

What is a Sealed Lead Acid battery (VRLA’s)

Sealed Lead Acid batteries are often referred to as being maintenance free or valve regulated. In a sealed lead acid battery the unit is sealed, meaning that water loss is kept to a bare minimum and also that you needn’t put any water into the battery. Most of the sealed VRLA batteries are recombinant which essentially means that the water lost through the venting of hydrogen and oxygen in a flooded battery is the most common types of Sealed Lead Acid or SLA batteries that we carry are the AGM (Absorbed Glass Mat) batteries; another is the GEL type of battery.

AGM batteries have a very fine Boron-Silicate glass mat in between the plates inside the battery. The mats are about 95% filled up with electrolytes rather than completely saturated. The considerable advantage of this type of battery is that they can be operated in almost any orientation and will even function underwater. They will also never leak, even if punctured.

GEL batteries have the electrolyte stored in a gel form; this also won’t spill out even if the batteries are broken and also prevents the stratification that other battery types can have. Gel cells, however, do have some limitations; they take a slower charge than a comparable sized AGM battery. If they are overcharged voids can form in the gel which permanently reduces the battery capacity.

For deep cycle applications however the Sealed Lead Acid type of battery can be a great deal more expensive than a comparable Flooded Lead Acid battery. However, there are times when a sealed battery will better meet your needs than a flooded battery would. Particularly marine applications where stability isn’t always a given, and ventilated spaces are hard to come by are often well tailored to a sealed lead acid battery. One other advantage that AGM and Gel batteries have over flooded types is the low self discharge rate. Where a Flooded battery will lose about 13% of its charge in a month, a Gel or AGM will lose 1%.3%.

What are the differences between a starting and deep cycle battery?

Generally speaking, there are two different types of lead acid batteries, Starting and Deep Cycle. If a starting battery is routinely deep cycled (discharged below 20%-50% of max capacity), it will generally fail after 30-150 cycles. The same starting battery will last for thousands of cycles if it is just used normally (2%. 5% discharge).

  • Starting batteries are generally designed to start some form of an internal combustion engine (car, truck, boat, etc). In a starting battery, you will find more lead plates, thinner and often made of a lead sponge similar looking to a foam sponge. This sort of arrangement means that the plates have much more surface area in the solution than a Deep Cycle battery and allow them to draw larger currents much quicker than a Deep Cycle battery.
  • Deep cycle battery, on the other hand, have much thicker plates and, they are solid, not sponge. These thicker plates have less surface area and thus less of the instant power that a starting battery needs. They are designed to be discharged down to 20% of their maximum charge repetitively. The thicker lead plates allow for this as they are much sturdier than their sponge counterparts.
  • Cold Cranking Amps (CCA) is an indicator of the amount of current a battery can deliver for 30 seconds at zero degrees Fahrenheit without dropping below a specified cutoff voltage (normally 10.5 volts). The cranking amps a battery can produce changes with temperature. The warmer it is the more Cranking Amps a battery will produce.

You can use a Deep Cycle battery as a starting battery provided that you take into account the lower CCA of a Deep Cycle battery. As a rule of thumb, it’s a good idea to upsize the battery by about 20% to deliver the same amount of cranking amps from a deep cycle battery. Also, the self-discharge rate of Sealed batteries is a lot less than flooded lead acid types.

How long will my battery last?

How long a battery will last depends hugely upon the way it is used and how well the battery is maintained. Both overcharging and undercharging will have serious adverse effects on the lifespan of a deep cycle battery. In particular, you can seriously shorten the lifespan of a battery if it is used in a deep cycle application that it was not designed for. An example of this would if you were to use an automotive starting battery as a deep cycle battery.

General expectations for batteries if deep cycled (these are just approximate guidelines):

  • Starting battery (Automotive battery etc) : 3-13 months
  • Marine Battery : 1-6 years
  • AGM Deep cycle: 4-7 years
  • Gel Cell Deep Cycle: 2-5 years
  • Flooded Lead Acid Deep Cycle Battery (L16-RE etc): 4-8 years

The main things that you can do to ensure you get the maximum value out of your deep cycle batteries are to keep them maintained. This means keeping them watered to the appropriate level, trying to prevent them from discharging them more than 50% of their total capacity and having appropriate charging systems in place. The charging is of special importance because both over and undercharging will severely limit the life of your batteries, also if your batteries will see an extended period without being used you should ensure they are routinely checked, cleaned and fully charged before being stored. Also in some cases, it can be a good idea to put your batteries on a maintenance charge over long periods of disuse.

Battery Cycles vs. Battery Lifetime?

Often the lifespan you can expect to get out of your battery is referred to in terms of cycles. A battery cycle is one complete discharge and recharge cycle. The discharge state of a battery is often measured in Depth of Discharge (DOD). This refers to how far down the battery has been taken, for instance, a battery that has 25% of its capacity remaining would be said to be at 75% DOD. The lifetime of a battery is directly related to the depth of the discharge that it regularly experiences. Lead acid batteries are fickle things. If you subject a deep cycle battery to 80% DOD on a regular basis you will get roughly half the life out of your battery than if you were to cycle it to 50% DOD. While this doesn’t mean that you can’t go down to 80% DOD you should generally try to design your battery banks to allow for cycling at around 50%. Conversely there is also an upper limit on the DOD of a battery, usually, a battery that is only regularly cycled down to 5% or less will not last as long as a battery cycled to 10% or more. This is because on smaller cycles the Lead Dioxide can clump up around the positive plates. On heavier discharges, this would be more of an even film.

What does Equalizing my batteries entail and when should I do it?

Equalizing a lead acid battery is the practice of applying a controlled overcharge in order to prolong the battery life, restore lost capacity and to make the battery more efficient. Batteries require equalization because as a battery is cycled (discharged and recharged) a small amount of lead sulfate remains on the lead plates. When you use a three stage charge the degree of sulfation is lessened, but not entirely eliminated. If this sulfate is left on the lead plates it will crystallize, once enough of these crystals have built up on the lead plates they can lead to lowered capacity and lower the amount of power the battery can produce. Also over the lifetime of a battery, the electrolyte solution can tend to stratify, forming layers inside the battery. This can cause the acid near the top of the cell to be more dilute than the acid at the bottom. An equalizing charge is essentially a controlled overcharging of the batteries which will allow the sulfate on the lead plates to recombine into sulfuric acid, it will also break loose any crystallized sulfate on the plates which will then fall to the bottom of the battery. It’s always a good idea to not fill your batteries before you equalize, the electrolyte will expand during equalization and if the batteries are too full they could overflow. There should be enough liquid to cover the plates and not much more.

Below is a table from Trojan batteries website detailing their recommended charge levels and durations:

Trojan Batteries recommended charger voltage settings:
System Voltage

Important things to note before equalizing your batteries:

  • Any DC loads on the batteries should be turned off, the greater than the normal voltage that occurs during equalization can cause damage to anything attached.
  • Leave the caps on the cells, these caps are vented and will prevent your electrolyte splattering onto the top of the battery. Sometimes laying a paper towel over the tops of the batteries can be a wise precaution also.
  • If a battery starts to spit acid stop the charging process immediately
  • Batteries should be fully charged and at room temperature before you equalize

Equalizing steps:

  • Make sure the batteries are fully charged, at an even temperature and are flooded acid.
  • Take off any loads from the batteries
  • Connect your charger
  • Set the charger for the correct voltage
  • Start to charge your batteries
  • The batteries should begin gassing and bubbling
  • Take specific gravity readings every hour or so
  • When the gravity values no longer rise during the gassing stage the equalization is complete

How often should I refill my batteries (or do I even need to)?

Flooded batteries have to be watered periodically to maintain their performance and increase their longevity. As the chemical reactions take place some of the water is lost as gas from the vent caps. This means that the acid in the battery will eventually get below the level of the lead plates. Luckily as it is just water that is being lost (hydrogen and oxygen) all that has to go back in is pure water.

Battery Management 101

Many RV owners may not have the technical background to understand the operation of their battery charger. Sometimes they forget about the need to maintain the battery water level and to keep it fully charged when their RV is in storage.RV owners do however remember the consequences of improper battery charging and maintenance…a dead battery that is expensive to replace!

battery, management, equalization

The following analogy uses the flow of water from a Charger Tank (battery charger) to fill (recharge) a Battery Tank (battery) to simulate a battery recharging cycle. The goal is to fill (recharge) the Battery Tank as fast as possible, while keeping water loss to a minimum. The same goal you have when recharging your RV battery. This analogy explains the need to constantly monitor and adjust the flow rate to prevent water loss. Battery chargers also require voltage adjustments during the recharge cycle to reduce water loss. The small leak at the bottom of Battery Tank represents the small internal current leak present in all lead acid batteries. This internal current leak self-discharges your RV battery approximately 4% per week, thus the need to charge your battery periodically when in storage.

At the start, both tanks shown below are at the same height, 12.6 feet; therefore, water (amps) in the Charger Tank will not flow into Battery Tank. Water will only flow if the height of Charger Tank is increased. Remember when recharging your RV batteries the charger voltage must also be higher than battery voltage to allow charging current to flow.Typical converters/chargers used in the RV Industry have a fixed output voltage of 13.6 volts.

As Battery Tank begins to approach the 90% Full Level the Charger Tank must be lowered to 13.6-Ft. (figure # 4) to prevent excess splashing and water loss over the top. The Battery Tank would then complete the fill at this slower rate.

In figure the Battery Tank is now full, but the Charger Tank continues to remain at 13.6-Ft. This causes more water to flow than required to maintain a full level in the tank. This excess water is lost as it splashes out the top of the Battery Tank.

To reduce water loss even more, once the Battery Tank is fully charged the height of Charger Tank must lowered to 13.2-Ft. This lower height reduces the flow from the Charger Tank so that it just equals water lost due to the leak at the bottom of Battery Tank.

As you have seen filling a Battery Tank as fast as possible with minimum loss of water, requires several adjustments to the height of the Charger Tank. First, it has to be raised to 14.4 feet to refill as fast as possible. Then it must be lowered to 13.6 feet to finish the fill with minimum water loss. Once the tank is full, you have to lower it again to 13.2 feet to reduce water loss even further.

Proper recharging of RV and Marine batteries also requires charger voltage adjustments during charging cycle to accomplish these same benefits. Unfortunately, most battery converter/chargers on the market today have a fixed output voltage of 13.6 volts and can not be adjusted.

It would be nice if someone created a robot called the “Battery Butler” that automatically came around every week to check and adjust your battery system. Well someone has, Progressive Dynamics developed a microprocessor based programmer-controller for our electronic power converters. This device proved to be so intelligent, we called it the Charge Wizard, because it automatically adjusts the charging voltage as required by the battery to ensure fast recharge cycles, with minimum water loss.

battery, management, equalization

Now that you understand the requirements for properly filling a tank with water, it is time to learn more about the construction operation and recharge requirements of your RV and Marine lead acid batteries.

Since a single lead-acid cell only produces 2.1; volts, six (6) cells must be connected in series to create the typical RV or marine battery (see Figure below). Six cells connected in series will provide a total voltage output of 12.6-volts. The standard RV battery (shown below) has an output voltage of 12.6 volts, when fully charged!

The Battery Discharge Cycle

In figure below, the battery is partially discharged and Lead Sulfate (sulfation) has begun to coat both lead plates. The output voltage of the battery has dropped to 12.3 volts.

In figure below, the battery has now been discharged to 50% of its total capacity. The Lead Sulfate (sulfation) is now covering more of the surface area of the plates. The battery voltage has dropped to 12.1 volts.

Figure #11 below illustrates a fully discharged battery and the voltage has dropped to 10.5 volts. Battery Management 101 now completely covers the surface of all plates. Initially the Lead Sulfate coating is soft, thin and easily reconverted into lead and sulfuric acid when battery is recharged. It is important to remember, The longer your battery remains discharged, the more it will begin to form hard crystals of Lead Sulfate…RECHARGE YOUR BATTERY AS SOON AS POSSIBLE! Once these hard crystals form, they are impossible to remove during a standard fixed voltage (13.6 volts) charging process.

Now that the battery is completely discharged it is time to discuss the recharge cycle. Remember, when recharging your RV battery the goal is to recharge it as fast as possible without excess water loss.

Recharging Lead Acid Batteries with the Charge Wizard

In figure below, the battery is in the process of being recharged. The Charge Wizard has detected the battery is very low and has automatically selected BOOST MODE (14.4 volts), of operation to return battery to 90% of full charge in 2-3 hours.

In figure # 13, the battery has reached the 90% of full charge level. The Charge Wizard has reduced charging voltage to 13.6 volts to prevent excess battery gassing and water loss. The remaining 10% of charge cycle will be completed at this lower voltage.

When the battery reaches full charge (figure # 14), charging voltage must be reduced to 13.2 volts. This lower voltage reduces charging current to 20 to 25 milliamps, enough to replace current lost through the internal leakage present in all batteries. The Charge Wizard knows this and automatically selects STORAGE MODE of operation and reduces charger voltage to 13.2 volts.


When electrical current flows through water during the charging cycle, it breaks the water down into its original components, a mixture of Hydrogen Oxygen. These two gasses are extremely flammable and can cause an explosion if the battery is not properly vented to the outside of the RV. This normal conversion of water into hydrogen and oxygen is part of the battery recharge cycle and is another reason you should check the battery water level at least once a month.

Other Battery Maintenance Problems Solved by the Charge Wizard

Even after receiving a full charge, notice that near the bottom of the battery, some of the Lead Sulfate has not converted back into lead and sulfuric acid. This remaining Lead Sulfate has formed hard crystals that can not be easily reconverted. These hard crystals are the beginning of BATTERY SULFATION, the leading cause of battery failure.

To remove the remaining Lead Sulfate, the battery must receive an EQUALIZING CHARGE (i.e. increase the charging voltage to 14.4 volts or higher periodically for a short time). This equalizing charge will eventually convert this crystallized Lead Sulfate into its components (lead and sulfuric acid).

Problem solved the Charge Wizard automatically provides an EQUALIZING CHARGE that increases charging voltage from 13.2 to 14.4 volts for 15 minutes every 21 hours, when operating in the STORAGE MODE. The Charge Wizard’s EQUALIZATION MODE automatically eliminates BATTERY SULFATION BUILD-UP before it becomes a problem! Remember that the rate of SULFATION increases rapidly as a battery discharges…so keep your battery fully charged when not in use!

Now that you have seen what the Charge Wizard can do to improve battery recharge time, adjust charge voltages to reduce water loss, and eliminate sulfation, its time to eliminate the last of the battery plagues…BATTERY STRATIFICATION!

What is BATTERY STRATIFICATION? You learned previously that the electrolyte inside a battery is a mixture of water and sulfuric acid and like all mixtures, one component is heavier than the other is. In this case Sulfuric Acid is the heavy component and will eventually begin to settle to the bottom of the battery. This process is called STRATIFICATION!Stratification will increase build-up of lead sulfate and reduce battery capacity.

The way to prevent BATTERY STRATIFICATION is to apply an EQUALIZING CHARGE (i.e. increase charging voltage to 14.4 volts) to your fully charged battery for a short period at least once a month. This EQUALIZING CHARGE will cause heavy gassing. This heavy gassing mixes up the electrolyte and equalizes the water/sulfuric acid mix. Equalizing also breaks down lead sulfate crystals that may have begun to form. The Charge Wizard automatically Equalizes your battery for 15 minutes every 21 hours, when in the STORAGE MODE to prevent BATTERY STRATIFICATION.

As you have learned, the Charge Wizard is a very valuable addition to your battery charging system and it will eliminate battery problems. Charge Wizards are available at leading RV Dealers and Distributors, or you may contact Progressive Dynamics at 269 781 4241. ORDER YOUR CHARGE WIZARD TODAY AND ELIMINATE BATTERY PROBLEMS.

Answers to Common Questions about Batteries

Question ? Use the search above to find an answer. this column is hidden in CSS options


Perform this test when installing.

Equalizing Batteries, The Reality

Fully charging your batteries after each discharge on a live-aboard cruising sailboat is simply not practical. Instead, most of us will cycle our batteries between 50 and 80% of their capacity. The bad news is that this will ruin your lead-acid batteries (regardless of type) in a distressingly short time due to sulphation.

The Solution

But there is an answer to this problem: regular equalization, a process where you deliberately overcharge your batteries for a specified time and voltage to remove the sulphation.

To continue reading:

From John

I’m truly dumb when it comes to electricity, but I’m going to ask this – What about solar panels windvanes? Won’t these top up the batteries? Solar panels windvanes can be used at a marina – no need for shore power. My windvane is connected to a regulator to prevent overcharging of the batteries. My hard solar panel is small is put out on deck when I’m away.

PS – The solar panel the windvane terminate in clips (as in alligator clips, or jumper cables) which I can move from one bank of batteries to the other as needed. A brilliant idea suggested by an electrician friend.

You are not missing anything at all. Solar and wind certainly have their place. And for a boat with light electricity use they can, properly installed, solve most of the problems we are discussing here.

battery, management, equalization

However, for a situation like ours, where we typically use 200-250 amp hours a day, mainly due to computer usage to support this site and photography, as well as refrigeration, they would only make a real difference if we had a huge array of solar, or a really big wind generator(s). We don’t really have room for either, at least if they were installed in a seaman like way.

Also, neither will effectively equalize a battery since this requires a constant voltage without the variation that solar and wind are subject to.

Colin, AAC European Correspondent, uses solar and wind generation installed on the arch of his Ovni. Any Комментарии и мнения владельцев Colin?

Isn’t equalizing the batteries a completely separate issue from the regular charge/discharge cycle that we all put the batteries through regardless of the number and kinds of charging capabilities we may have?

Now I feel like I’m missing something in this ‘conversation’.

On the topic of equalization; what happens when the batteries have to be left aboard for the winter while the boat is laid up? We have a couple of solar panels that will keep them topped up but there is no way to equalize them every month. Will we just suffer the loss of life at some point?

You only need to equalize monthly if you are discharging and then not fully charging daily. In fact if you have AGM batteries there is no need to keep the solar panels charging them since if they are not being used, they won’t self discharge over a winter.

All this should be clear, I hope, if you read the full Online Book. I know it is a lot of mind numbing detail but I have struggled mightily to cover a complex subject as completely as possible. Let me know if anything is fuzzy in my writing anywhere in the book.

I have built one 24v 40ah LIFEpo4 lithium battery, and just bought a pre-made (DIY CLASS) 200ah 12v battery, which arrived today. I think these will be the standard batteries for many cruisers not too many years from now. Some highlights:

-little to no Pekert effect. 1c charging and 3c discharging allowed1/2 of the weight.5x the cycling gassing, and no with most charging sources. Since they are designed to be charged up higher than most marine charging sources, current stops flowing before they reach full charge, but the % lost not charging fully is minusculeBulk charging extends up to 90%, and you can safely discharge 80% without grossly effecting the # of cycles. The cells I have in the Torqueedo pack are rated to 2000 cycles at 80% discharge, the Hipower brand cells in the 200ah 12v pack are rated to 1000 cycles at 100% discharge, I have not found their rating at 80% online yet. Thundersky brand cells are rated even higher.

Cost? You can put together what I bought, a 200ah 12v pack with a Battery management system to protect both the cells and your alternator, for about 1200 cell cost and 450 BMS cost. This one weighs about 80lbs in a 4d battery box, and is set to replace 4, t-105 wet cells at 445ah capacity.

Re. equalizing on solar/wind: With typical setups, I’d agree that equalizing from solar or wind power wouldn’t work. But I can’t think of any reason why a good buck/boost MPPT controller couldn’t be programmed to provide an equalization voltage from a solar or wind source. It should be just a matter of programming the unit’s microcontroller to allow a few different operating modes.

Re. LiFePO4: It’s interesting to see those costs, Chris- they seem to be coming down dramatically. The current interest in electric cars is causing a lot of money to be poured into battery development, and I would consider it quite reasonable to expect the option of choosing between a wide range of traction batteries using half a dozen different chemistries within six or eight years’ time.

Thanks for the comment. I would really like the idea of equalizing with solar or wind. My worry was that the voltage would drop repeatedly below optimal as clouds or lulls in winds came though. Could you explain a bit more about these buck/boost controllers? Also, could you link to a brand that you like?

I assume that they attempt to provide a constant voltage, although I would have still thought that the volts will drop if the total watts drop below that required to supply say 3 amps at 15.5 volts or 46.5 watts? Am I missing something?

Perhaps solar would be better than wind in that I have often seen wind power go to zero multiple times in an hour as lulls come through?

To make this work, I think you would need to split the battery bank in two and switch the charging feed back and forth so you could equalize one while using the other. But then we need to do that with our method, so nothing is lost there.

I’m also thinking you would need to have a bank that was equal to four times your daily use to make this work. That is if you wanted to be able to live as normal during the equalization cycle.

Justin again from Lifeline Batteries. Interesting posts. John, your article is right on about equalizing. Re: Equalizing using solar and Wind. This can be done but it just needs to remain constant for the full 6-8 hours. This is usually the down fall to equalizing this way. If you can find a way to have it remain constant and still push good amperage then by all means use that source.

Re: LIFEpo4 or and Lithium Batteries for that matter. We have built and have been testing Lithium batteries for quite some time. You can see this link here where we have built one for the US Navy and got the contract for it: You can also find a full sheet of the advantages and disadvantages here: In summary here is the limitations to the technology that we have found. Requires protection circuit to maintain voltage and current within safe limits. Protection circuitry involves both additional hardware and software. On aircraft battery monitoring and alarms will be required for safe operation. Subject to aging, even if not in use – storage in a cool place at 40% charge reduces the aging effect. Transportation restrictions – shipment of larger batteries may be subject to regulatory control. Expensive to manufacture – about 40 percent higher in cost than nickel-cadmium. Not a fully mature chemistry – metals and chemicals are changing on a continuing basis. Extremely flammable electrolyte.

Although we are working very hard to provide top quality products we are also very concerned about safety. Right now the lithium cells are very dangerous and very volitale. If they get the wrong scenario you don’t want to be anywhere around that battery. Lithium Ion cells if ignited can and will burn at 1,100 degrees F. Not only do they burn at that temperature they also create their own oxygen at the battery plates, which means you cannot put out the fire. The cargo plane that just crashed in Dubai caught on fire because of the Lithium batteries aboard. Lithium batteries can spontaneously ignite if the air gets hot enough. That’s what they think happened. They think when it was sitting on the runway the cargo hold got up to 135 – 140 degrees and they self ignited. We have come up with a non oxygen producing Lithium Ion but the electrolyte is still very, very flammable. There are new shipping regulations going into effect January 2011 for Lithium Batteries and they are expected to cost the Lithium industry 1 billion dollars more annually. This means the price of Lithium is going up next year, not down. Not to mention they are now considered Hazardous Cargo even when shipping ground. That still gives AGM batteries the best advantage.

Types of Lead Acid Batteries How to Charge Them

Morningstar controllers have been designed for Lead Acid batteries which were the first rechargeable battery ever built and are still the most common rechargeable battery on the market to this day. Due to the low cost and high power-to-weight ratio lead-acid batteries remain in high demand as starter batteries. However, because they also have low energy-to-weight and energy-to-volume ratios they are often chosen for stationary applications rather than mobile and portable applications like electric vehicles or hand tools. Lead-acid battery technologies should continue to be used extensively for off-grid solar applications for years to come.

The two categories of lead-acid batteries available on the market are Sealed and Flooded.


Flooded batteries allow fluid in the form of hydrogen and oxygen gas to escape during charging and require more maintenance than sealed batteries. Flooded batteries also can be overly charged with less risk of damage than sealed batteries. Taking advantage of this feature, the flooded batteries can receive excessive charging periodically to better “equalize” the battery cells so that the cells that might not be doing well can be brought to a full state of charge on a regular basis. Maintaining flooded lead-acid batteries enables them to last longer than sealed batteries.


Sealed batteries are synonymous with VRLA because typically the battery is sealed but if the charging or discharging is high enough pressure will build up and the valve will allow gas to escape some. AGM or Gel keeps the batteries from evaporating so they do not need water and are therefore also referred to as “maintenance-free.” In addition, the AGM and Gel keep the electrolyte mixed with the water so it does not settle to the bottom which is referred to as stratification. This eliminates the need for vigorous equalization charging which helps mix the electrolyte with the water in flooded batteries.

Both AGM and Gel batteries are good for off-grid applications where several days of autonomy is preferred and charge rates are more likely to remain low. The water inside the battery is less likely to freeze since it does not separate from the electrolyte which means sealed batteries are preferred in colder temperatures. Also, Gel batteries are a little better equipped than AGM to perform in both extremely hot and extremely cold temperatures.

Morningstar MPPT and PWM controllers use a 4-stage battery charging algorithm for Rapid, efficient, and safe battery charging. The following graph shows the sequence of stages.

Bulk Charge Stage

During Bulk charging, the battery is not at 100% state of charge and battery voltage has not yet charged to the Absorption voltage set-point. The controller will deliver 100% of available solar power to recharge the battery.

Absorption Stage

When the battery has recharged to the Absorption voltage set-point, constant-voltage regulation is used to maintain battery voltage at the Absorption set-point. This prevents heating and excessive battery gassing. The battery is allowed to come to a full state of charge at the Absorption voltage set-point. The green SOC LED will blink once per second during Absorption charging.

The battery must remain in the Absorption charging stage for a cumulative 120 – 150 minutes, depending on battery type, before the transition to the Float stage will occur. However, Absorption time will be extended by 30 minutes if the battery discharges below 50 Volts the previous night.

The Absorption set-point is temperature compensated if there is an internal temperature sensor or an RTS is connected.

Float Stage

After the battery is fully charged in the Absorption stage, the ProStar MPPT reduces the battery voltage to the Float voltage set-point. When the battery is fully recharged, there can be no more chemical reactions and all the charging current is turned into heat and gassing. The float stage provides a very low rate of maintenance charging while reducing the heating and gassing of a fully charged battery. The purpose of float is to protect the battery from long-term overcharge. The green SOC LED will blink once every two (2) seconds during Float charging.

Once in the Float stage, loads can continue to draw power from the battery. In the event that the system load(s) exceed the solar charge current, the controller will no longer be able to maintain the battery at the Float set-point. Should the battery voltage remain below the Float set-point for a cumulative 60 minute period, the controller will exit the Float stage and return to Bulk charging.

The Float set-point is temperature compensated if there is an internal temperature sensor or an RTS is connected.

Equalization Stage

Certain battery types benefit from a periodic boost charge to stir the electrolyte, level the cell voltages, and complete the chemical reactions. Equalization charging raises the battery voltage above the standard absorption voltage so that the electrolyte gases. The green SOC LED will blink rapidly two (2) times per second during equalization charging. The duration of the equalized charge is determined by the selected battery type for the controller being used. Equalization Time is defined as time spent at the equalization set-point. If there is insufficient charge current to reach the equalization voltage, the equalization will terminate after a certain period of time with battery voltages above the Absorption voltage setpoint. This is done to avoid over-gassing or heating the battery. If the battery requires more time in equalization, an equalize can be requested using the TriStar Meter or push-button to continue for one or more additional equalization cycles.

The Equalization set-point is temperature compensated if there is an internal temperature sensor or an RTS is connected.

Why Equalize?

Routine equalization cycles are often vital to the performance and life of a battery – particularly in a solar system. During battery discharge, sulfuric acid is consumed and soft lead sulfate crystals form on the plates. If the battery remains in a partially discharged condition, the soft crystals will turn into hard crystals over time. This process, called “lead sulfation”, causes the crystals to become harder over time and more difficult to convert back to soft active materials. Sulfation from chronic undercharging of the battery is the leading cause of battery failures in solar systems. In addition to reducing the battery capacity, sulfate build-up is the most common cause of buckling plates and cracked grids. Deep cycle batteries are particularly susceptible to lead sulfation.

Normal charging of the battery can convert the sulfate back to the soft active material if the battery is fully recharged. However, a solar battery is seldom completely recharged, so the soft lead sulfate crystals harden over a period of time. Only a long controlled overcharge, or equalization, at a higher voltage can reverse the hardening of sulfate crystals.

When to Equalize?

The ideal frequency of equalizations depends on the battery type (lead-calcium, lead-antimony, etc.), the depth of discharging, battery age, temperature, and other factors. One very broad guide is to equalize flooded batteries every 1 to 3 months or every 5 to 10 deep discharges. Some batteries, such as the L-16 group, will need more frequent equalizations.

The difference between the highest cell and lowest cell in a battery can also indicate the need for equalization. Either the specific gravity or the cell voltage can be measured. The battery manufacturer can recommend the specific gravity or voltage values for your particular battery.

Preparation for Equalization: First, confirm that all of the system loads are rated for the equalization voltage. Consider that at 0°C (32°F) the equalization voltage will reach 16.75 volts for L-16 batteries with a temperature sensor installed. Disconnect any loads at risk of damage due to the high input voltage.

If Hydrocaps are used, be sure to remove them before starting an equalization. Replace the Hydrocaps with standard battery cell caps. The Hydrocaps can get very hot during an equalization. Also, if Hydrocaps are used, the equalization should be set for manual only (many controllers have a switch that enables automatic or manual equalization).

After the equalization is finished, add distilled water to each cell to replace gassing losses. Check that the battery plates are covered.

Equalizing a Sealed Lead-Acid Battery?

Some Morningstar controllers include factory pre-set sealed battery settings with an Equalization cycle. These minimal “boost” cycles to level individual cells are not equalization, and will not vent gas from sealed batteries that require up to 14.4V charging (12V battery). Many VRLA batteries, including AGM and gel, have charging requirements up to 14.4V (12V battery). Depending on the battery manufacturer’s recommendation, the “boost” cycle for sealed cells can also be disabled using an equalize setting switch to manual, if required included with some Morningstar controllers. It is also possible to disable equalization completely with controllers that have custom programming capability.

Custom Programming Options for Lead Acid Batteries

Morningstar controllers will have up to seven factory presets and many can be custom programmed. Typically one of the seven presets of the TriStar, TriStar MPPT (150V 600V), and ProStar MPPT will work perfectly fine for a specific Lead-Acid battery. Some battery manufacturers provide voltage regulation set-points that closely match Morningstar presets.

Morningstar’s default temperature compensation is based on.5mV/°C per cell or.30mV/°C per 12V battery bank. This is generally the accepted temperature compensation in the industry but can be modified if it differs from the battery manufacturer’s specifications.

battery, management, equalization

We receive questions sometimes regarding Morningstar’s Float regulation voltage which can be a bit higher than indicated by the battery manufacturer. It should be noted that in solar applications these higher Float settings work better since the battery can only maintain Float during the daytime. The Float settings indicated by the manufacturer are lower due to situations where the battery will sit for multiple days or weeks at a time without ever getting discharged or dropping out of Float. In these continuous Float or zero discharge situations, it is better that the trickle charging is minimized. Battery backup is one such condition where the inverter sell mode is set lower than the controller but during backup periods the higher Float setting of Morningstar controllers will take over and it will be fine since the batteries will get discharged at night during a backup situation.

In general, a battery that gets discharged more needs more charging. That is why Morningstar includes the following custom programmable options which can increase or decrease the amount of charging that a battery receives.

  • Absorption extension increases the absorption time if the battery voltage is low the previous day.
  • Float cancel for a full day of Absorption charge recovery if battery voltage gets very low.
  • Increased Float regulation voltage so the batteries can continue charging at a lower rate through the afternoon.
  • Equalization can be more frequent if batteries will experience deeper discharge.

Installers and end-users can make small adjustments based on many factors including battery bank size and type, average daily loads, array size and temperatures. Morningstar’s defaults will work well with most situations but making small changes can improve performance for many systems. It is not necessary to make modifications but there could be times where battery charging needs to be increased or decreased and there are several ways to do this with the custom settings.

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