This article will provide detailed instructions relating to proper charging and system programming for Rolls flooded lead-acid batteries.  For additional information, please refer to Rolls Battery User Manual.

Improper charge settings and failure to make system adjustments are the most common cause of battery failure.  Failure to program the manufacturer-recommended charge voltages, charge time and/or adjust for changing charge conditions will result in possible under/overcharge, sulfation buildup, capacity loss and eventual battery failure.   

In a renewable energy application, seasonal charges often require adjustment in charge voltage and time.  Despite the use of a battery temperature sensor (BTS), adjustments to the programmed charge setting are commonly required 2-3 times per year.  For example, longer sun hours and reduced usage in summer months (with the exception of Air Conditioning where applicable) will see different charge requirements than winter months where loads may increase as the end-user is home more often and the charge must be completed during shortened daylight hours.

Most inverters and charge controllers are pre-programmed with default charge voltages and times.  More often than not, these settings do not align with the recommended charge voltages and times provided by the battery manufacturer.   These are unique to each battery manufacturer based on cell construction and will also vary by battery type. Charging at an improper voltage or insufficient time will quickly result in capacity loss and/or failure which is considered improper use and would not be covered under the manufacturer warranty.

To protect your investment and ensure you're properly charging your Rolls batteries, review the Rolls-recommended charging requirements for your specific battery model before setting up the system. When installing, you must fully understand the system voltage, battery type, AH rating & quantity, battery state-of-charge (testing specific gravity & voltage) and size of the renewable charge source (and backup source(s) where applicable).

This guide will help determine your necessary charge settings quickly and efficiently.

Nominal Battery Bank Voltage

Most battery banks are set up in 12, 24, 32, 36 or 48-volt series strings.  Renewable Energy applications are most commonly set up in 12, 24 or 48-volt configurations.

Lead-acid batteries are made up of individual 2-volt cells.  The manufacture-recommended charge voltage is often provided in a "voltage per cell" range.  A 12V system is made up of 6 x 2-volt cells, 24V system = 12 x 2-volt cells, 48V system = 24 x 2-volt cells.

For example, if charge voltage is noted at 2.5VPC, a 12-volt battery having 6 cells would then require 6 x 2.5VPC or 15V.

24V = 12 x 2.5VPC = 30V

48V = 24 x 2.5VPC = 60V

Recommended charge settings assume the batteries are installed in a cool, dry location and the provided battery temperature sensor (BTS) connected to the charge controller is properly installed.  Charge voltage requirements will increase or decrease based on the temperature of the battery bank.  This sensor will adjust the voltage in programmed increments based on the temperature of a test battery cell.  For an accurate reading, the sensor must be properly installed on the side of the battery case, approximately 1/2 way down the side, below the electrolyte level. The temperature sensor should not be mounted to the terminal posts or the top of the battery as the actual cell temperature is typically 10-20C higher than these areas.   Improper installation of the battery temperature sensor (BTS) will result is an under/overcharge condition causing premature failure of the battery bank.

Rolls Battery Flooded Charging Parameters.

Regular Cycling or Partial State of Charge Recovery

The below chart (Table 2a) is to be used in situations of full-time regular daily cycling (ex. off-grid applications) or recovery where the battery bank has experienced repeated partial state-of-charge operation.

When a battery temperature sensor (BTS) is used, the values in the highlighted red column should be programmed.

The controller will adjust the actual charge voltage based on the measured temperature of the battery bank.  

If you do not have a battery temperature sensor (BTS) installed, then you must test and actively adjust the charging voltages based on the temperatures of the batteries, not the ambient temperatures of the battery bank.

The flowing chart (Table 2b) is to be used where the battery bank experiences infrequently cycling or rest at full state-of-charge for extended periods - ex. a backup application.

These two charts work for most battery installations in the field.  However, there will be times that there are deviations based on climate, system & load size, charge efficiency and how the end-user is using the system.

For flooded lead-acid batteries, testing specific gravity on a regular basis is the best method to confirm proper charging, battery health and current state-of-charge.

Rolls-recommended charging parameters for flooded lead-acid models:

Bulk/Absorption Voltage: 2.45 to 2.5 VPC

Float Voltage: 2.25 VPC

Equalization Voltage: 2.6-2.65 VPC

Absorption Time:

Absorption charge time must be adequately programmed.  Settings such as pre-set battery AH capacity or End Amps settings which override the Absorption Time must be properly programmed to prevent the charge from prematurely ending the Absorption charge phase.

To fully charge the battery bank to 100% SOC, the Absorption charge must be completed.  Failing to fully charge the battery bank will lead to sulfation buildup, capacity loss and eventual failure of the battery bank.

When the initial Bulk charge has completed the charge controller will enter into Absorption charge.  At this phase, the battery bank has reached approximately 80% state-of-charge.  Ex. 1000 AH battery bank entering Absorption Charge will still have a remaining 200 AH (+20%) remaining to reach full state-of-charge.  When the Absorption charge stage is reached, the charge current to the battery bank from the controller will begin to drop significantly as the internal resistance of the battery bank increases.  To complete the charge, it is imperative that the charger continues to push current to the battery bank, forcing it to reach full state-of-charge.  This is done by forcing the charger to continue the Absorption charge voltage for a set period of time.

To determine the required Absorption charge time for flooded models, the provided formula is used. To calculate, you must know the available charge current (or max charge current output of the charger) and battery bank AH capacity.

.42 X C/20 / charge current

.42 (Assumed current losses while in Absorption charge)

C/20 = C/20 or 20 Hr AH rating of the battery bank

C = charger current

Charge current value is the peak charging Amp output to the battery bank in the Bulk charge.  If the charge source generates more current than the charge controller is capable of outputting, the max charge output of the controller is used.  It is common to de-rate this value 20-30% from the peak current in some applications due to increased resistance and a gradual decrease in charge output  - ex current output decrease as current generation drops in solar applications.


Charge source (PV array) output is 70 Amps at peak

Charge controller max output is 100 Amps

70 Amps is used.

Charge source (PV array) output is 120 Amps at peak

Charge controller max output is 100 Amps

100 Amps is used.

System Settings:

The provided example outlines a common system setup and programming requirements.

48-volt System:

- Sixteen (16) x S6 L16-HC (445 AH) batteries, configured into two parallel series strings at 48-volts.  This is a total battery bank capacity of 890 AH @ 48-volts.

- A Single 6000-Watt Inverter/Charger capable of 120 Amps of DC battery charging.

- A 4500-watt Solar Array Configured to charge a 48-volt battery bank.  This array is connected to an 80 Amp charge controller that will peak at 80 Amps of solar power.  This value will be de-rated by 20% due to typical, solar array in-efficiencies caused by shading, heat, improper angle and soiling. (ex. adjusted to 65 Amps)

Charge Controller Settings:

The solar array should never be connected directly to the battery bank as this will result in a severe overvoltage situation on the battery bank as there is no charge voltage regulation.  The array must be connected to a charge controller to regulate and provide the proper charge voltage and Amp output to the battery bank.

In most situations, your average amount of “good” sun is between 3.5 to 5.2 hours daily.  This will vary by region, temperature, angle of the sun, season and position/angle of the PV array.  When determining charge settings, losses due to these conditions must be accounted for.  Assuming the best-case scenario and maximum charge output will result in severe undercharging.

Initial Charge Controller settings:

Bulk/Absorption voltage: 2.45 to 2.5 VPC (58.8 to 60-volts)

Absorption time: = .42 X 890 AH/ 65 Amps = 5.75 Hours

Float voltage: 2.25VPC (54-volts)

Float time: 1 Hour

Equalization voltage: 2.6VPC (62.4-volts)

Equalization time: generally 3-4 hours or 50-75% of the Absorption charge time.  

Note: Rolls recommends testing and completing a corrective equalization only as necessary.  Equalizations are performed to remove sulfation buildup and improve charge balance.  It is unnecessary to equalize a balanced and healthy battery bank as this overcharge will burn oxide paste from the plates, reducing capacity and cycle life.

End Amps: this programmed setting triggers the charge controller to end the Absorption charge and begin the Float voltage phase.  This is typically set at 2% of the C/20 or 20 Hr AH rating of the battery bank for 60 minutes for flooded models (2% of 890 AH = 18 Amps).  If the value is set higher this may trigger the charge controller to prematurely end the Absorption charge before the batteries reach full state-of-charge.

Battery Efficiency Percentage: 80% for flooded lead-acid models

Temperature Compensation: -5mv per Degree C for flooded models, multiplied by the number of cells. (+/- 120mv)

In this example, the solar array is undersized for adequate charging.  The charge source should be capable of a current output between 10% to 20% of the C/20 rate of the battery bank.  A rate of 10% will require 4.2 hours of Absorption charge which is considered a typical daily charge time using a PV array.  Undersizing the system and charging at a rate lower than 10% will result in undercharging due to limited charge time (solar).

A system using an 890 AH battery bank you should have a charge source capable of at least 89 to 178 Amps of charge current. 

In the example, the array will output an average of 65 Amps at peak, meaning it will not sustain charge current long enough to complete the charge.

Most off-grid systems are designed for a 25-40% daily depth of discharge.  To reach full state-of-charge during limited charge times (solar), the adequate charge current must be generated.  If the charge current is lower than 10% of the C/20 rate of the battery bank and Absorption charge time calculation results in a time of 5.5 hours or more, this will result in consist daily undercharge and deficit cycling.

In the given example, a backup charge source such as a generator is required to supplement the inadequate solar charge. It would likely be necessary to run the generator 2-4 times a week year-round to get the battery bank to a full 100% charge.

Inverter Charger Settings

The Inverter/Charger is of course meant to provide power from your battery bank to your loads. If sized properly you can also use it as a charger to be a backup to the solar system as you are not always going to have enough sun to keep the batteries charged unless you have a lot of solar.

It is often recommended to start the inverter/charger in the morning before the full sun is achieved to bring the battery bank into the Absorption charge phase as quickly as possible, allowing the PV array to bring the battery bank to full state-of-charge as efficiently as possible.  Weather conditions will also determine charge efficiency.  It may be necessary to continue running the inverter/charger throughout the afternoon to reach full state-of-charge and to prevent deficit cycling. 

Programming with an adequately sized PV array with an average peak output of 100 Amps:

Bulk/Absorption voltage: 2.45 to 2.5vpc (58.8 to 60-volts)

Absorption Time: .42 X 890 AH / 100 Amps = 3.74 hours

Float Voltage: 2.25vpc (54 Volts)

Float Time: 1 hour

Equalization Voltage: 2.6VPC (62.4 Volts)

Equalization Time: generally 3-4 hours or 50-75% of the Absorption charge time.  

End Amps: 2% of the C/20 or 20 Hr AH rating of the battery bank for 60 minutes (18 Amps)

Battery Efficiency Percentage: 80% for flooded lead-acid models

Temperature Compensation: -5mv per Degree C for flooded models, multiplied by the number of cells. (+/- 120mv)

You’re Finished Right?

The short answer…. no.

Once you’ve commissioned the system, now it’s up to the end-user or Installer to check the specific gravity of each battery cell regularly.  Testing should be done while the batteries are resting at Float charge, confirming they've reached full state-of-charge (1.265 to 1.275 specific gravity).  Varying specific gravity readings indicate charge imbalance, sulfation buildup and/or cell failure.

If you notice the specific gravity of cells begins to decrease after the first 3-8 weeks of usage (<1.255), we recommend increasing the Bulk and Absorption charge voltage and/or Absorption time in small increments.   

In the above example, if the Absorption charge voltage is set at 58.8-59.6V volts on the charge controller and specific gravity readings are gradually decreasing, it may be necessary to increase the Absorption charge voltage in .4 volt increments to compensate for significant changes in temperature or charge resistance.  Do not exceed 60.0 volts when using a battery temperature sensor as this will cause an overcharge.  Allow the system to cycle for 2-3 weeks and then recheck the specific gravity to note improvements or any other changes.  If the readings show no change, add 30-60 mins to the Absorption charge time and repeat testing in 2-3 weeks.

If the specific gravity readings are higher than normal at a Float charge (1.280+), Absorption voltage settings may be decreased in a similar manner.  Be sure to test all cells as high specific gravity may indicate a failure in the battery bank causing overcharge on the remaining cells.

It is expected that adjustments in voltage and Absorption time will be necessary a few times per year based on changes in temperature and usage conditions.  Voltage must be adjusted based on cell temperatures.  If a battery temperature sensor (BTS) is not used, you must make the necessary adjustments.  Rolls Battery strongly recommends using a BTS with any device that charges your battery bank.  If a sensor is supplied with the charger it must be used.  If disconnected, this will cause the charger to improperly adjust charge voltage.  Failure to use the supplied BTS will result in under/overcharge and poor battery performance or failure which is not covered under the terms of our manufacturer warranty.

To prevent sulfation buildup in flooded lead-acid batteries, it is essential that at least one full Bulk & Absorption charge be completed every 7-10 days.  However, it is recommended that the system be sized to bring the batteries to a full state-of-charge on a daily basis.  Full state-of-charge requirements will depend on cycle frequency and depth of discharge.