Types of Lead Acid Batteries & How to Charge Them

Types of Lead Acid Batteries (PbSO4)

  • Flooded
  • Sealed or VRLA (Valve Regulated Lead–Acid)
    • AGM (Absorbed Glass Mat)
    • Gel (Gelled Electrolytes)

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.

4-Stage Charging for Lead-Acid Batteries:

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.

4-Stage Charging for Lead-Acid Batteries diagram

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.

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.