Detailed Settings Information for Rolls/Surrette Flooded Lead Acid Batteries.


The following guide is meant to be used as an addendum to the Rolls Battery Engineering manual.

All too often, we at Rolls battery receives requests daily on how to program your products to properly charge Rolls Flooded Lead Acid batteries.

One of the major issues when determining what settings for a solar system should be is the system in general, and the user dictate where settings should be, there are no “magic” works for all settings.   With most systems settings will change 2-3 times per year based on how the system is used, changes should be made based on weather and usage.  For Example: In the summer you get more sun, less usage (unless you are running Air Conditioning as a load), while in the winter you have less sun, and more usage.

Another issue is many end users or installers contact battery manufactures, or for that matter the inverter or charger controller manufactures without all the data needed to determine where the settings should be or how they need to be adjusted.   To really make an informed decision on how to program a system you must know, Current Battery Specific Gravity’s, current charging settings, battery type and quantity, size of renewable sources, and all renewable charging sources.

Hopefully this guide will help you in determining your necessary settings quickly and efficiently.


Nominal Battery Bank Voltage


To start, most battery bank voltages can be 12v, 24v, 32v, 36v, and 48v, this just depends on the inverter/controller product that you’ve chosen to install.   When dealing with Flooded Lead Acid batteries from Rolls Battery, our “cells” are made up of 2volt cells.    This is important because battery manufactures when suggesting charging voltages refer to those voltages as “Voltage per cell”.  

So, a 12v Battery bank has “6” cells, a 24v battery bank has “12” cells, a 48v battery bank has “24” cells.  If you are going to charge the battery at 2.45vpc, they you multiply 2.45 X Number of Cells = 58.8volts, this will give you the required voltage.  

All the Following System settings are assuming that the batteries are installed in a cool dry location and battery temperature compensation is properly installed below the level of the plates. This is the long Side of the battery about ½ down the side.  Battery temp sensors should never be mounted to the terminal posts, or the top of the battery as the sides are typically 10-20 degrees Celsius warmer than the post or the top of the battery.   Improper installation of the BTS will result is an overcharged condition and will cause 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 if you are living full time off the battery bank, or if you are trying to recover a battery bank that has been abused from partial state of charge operation, or what we call deficit cycling.




Please notice the red column, if you are using a battery temp sensor this is where your settings should be programmed.   The BTS will be adjusting the actual charge voltages based on the measured temperature of the battery bank.  If you do not have a BTS, then you must actively adjust the charging voltages based on the actual temperatures of the batteries, not the ambient temperatures of the battery bank.



The flowing chart (Table 2b) is to be used when you are infrequently cycling the battery bank, or you are using the battery bank for a backup application.  

These two charts work for about 90% of the Rolls Battery installations in the field but there will be times that there are deviations based on how the end user is using these systems.





This is where checking your specific gravity measurements on a regular basis will help you determine when adjusting settings is required.  



What does this all mean?


What this mean is your settings in general will be between the following.

Bulk/Absorb Voltage

2.40 to 2.50 Volts Per Cell (14.4 to 15 Volts(12V), 28.8 to 30 Volts(24V), 57.6 to 60 Volts(48v)

Float Voltage

2.20 to 2.3 Volts Per Cell (13.2 to 13.8 Volts(12V), 26.4 to 27.6 Volts(24V), 52.8 to 55.2 Volts(48v)

EQ Voltage

2.60 to 2.65 Volts Per Cell (15.6 to 15.9 Volts(12V), 31.2 to 31.8 Volts(24V), 62.4 to 63.6 Volts(48v)




Absorb Timers:


Another setting that many get confused are where the absorb timers need to be set. Unfortunately, this stems from a misunderstanding of what is happening during the absorb stage.   You want to be careful as there are settings such as battery capacity, or end amp settings that override absorb timers, you will need to ensure that you don’t program “competing” settings.

If you never complete a full Absorb timer, you will never get the battery bank to a full 100% state of charge and this will lead to sulfation, and eventual failure of the battery bank.

First thing that many need to know is when you finish the bulk stage, and you enter the absorb stage your batteries are only at about 80% State of charge.  This means of you have a 1000 ah battery bank you still have 200 ah (+20%) to go before the battery bank is full.  

The second thing is when you reach the absorb stage, the current flowing into the battery bank drops off steeply.  This is due to the increased internal resistance of the battery bank and the level of the voltage at the battery bank.  It’s imperative that you let the current flow into the battery bank, if you don’t then again, the bank will not become fully charged, and of course this will cause deficit cycling issues.

At Rolls Battery we have put together a formula to determine this time based on the peak current that is flowing into the battery while in the bulk stage.

The Formula is:

.42 X C/20 / Charger Current in Bulk.

.42 = Assumed Current Losses While in Absorb

C/20 = C/20 Rate of the Battery Bank.

Charger Current = Current at which the chargers are charging the battery bank in the Bulk mode, this is best measured.    I will typically de-rate this 20-30 from peak… If the charger is capable of 100 amps of current I will usually use 70-80 amps.


System Settings:


Just throwing settings at you without an example system would be of no good… so in the following examples we are going to use the following system parameters.

16 S550 428 Ah Batteries configured into two parallel strings of batteries.   This is a total battery bank capacity of 856ah @ 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 but will be de-rated by 30% due to typical, solar array in-efficiencies due to shading, heat, and soiling.   This means you will an average of 55 to 65 amps of charge current coming from this charger.


Solar System Settings.


The Solar System should be connected to a solar charge controller, the panels should never be connected directly to the batteries, this will result in a severe over voltage situation on the battery bank as there is not charge voltage regulation.

Once the solar array is purchased and installed, it becomes “free” energy, you want to maximize the amount of power you are getting from this charging source.  In most homes your average amount of “good” sun is between 3.5 to 5.2 hours, of course these changes depending on the season course and where you live these sun hours’ time can be severely affected even by the angle to the sun, and the temperature of the solar panels.   Keep in mind that if you will see variances in power from that array depending on the position of the sun and the temperature of panels.    When determining charging settings, you must account for these losses.  Assuming that you are always getting full power out of your array is a recipe for severe undercharging problems from your solar array.  

For the Solar Charge Controllers Here is where I would start with these settings.

Bulk/Absorb Voltage 2.48 to 2.5vpc (59.5 to 60 Volts)

Absorb Time .42 X 856 / 65 = 5.53 Hours.

Float Voltage 2.25vpc (54 Volts)

Float Times 1 Hours.

EQ Voltage 2.56vpc (61.5 Volts)

EQ Times Generally 50-75% of the Absorb Timers (3-4 Hours)

End Amps When Current Falls below 2% of the C/20 Capacity for 60 mins.  (17 Amps)

Battery Efficiency Percentage: 80%

Temperature Compensation. 5mv Per Degree C, multiplied by the number of cells. (+/- 120mv)

In this example, the solar array is undersized, typically you want at least one charging source that is capable of at least 10% to 20% of the C/20 rate of the battery bank.  

In this case if you have an 856ah battery bank you should have a charger capable of at least 85 to 170 amps of charge current.  This array is going to average about 65 to 70 amps while it may peak around 80, it will never be able to sustain that kind of charging current.

With the Solar array having an absorb timer of over 5.5 hours if your customer ever discharges the battery bank below 90% State of charge the batteries will never be taken to a full 100% charge with the Solar alone.   Many of these systems are designed for a 25 to 40% daily depth of discharge, so the customer will need to start the generator and use the inverter/charger to supplement solar charge between 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.  

Often, I advise to start the inverter/charger in the morning before you achieve full sun on the solar to get the battery bank into absorb with the inverter/charger so that the solar can finish the charge during the day.  Of course, you must keep in mind that if you don’t have a good solar day, you may have to run the inverter charger in the afternoon to finish the charge.  If you don’t do this deficit cycling will become an issue and could cause premature failure of the battery bank.

For the Inverter/Charger settings here is where I would start with these settings.

Bulk/Absorb Voltage 2.45 to 2.48vpc (58.8 to 59.5 Volts)

Absorb Time .42 X 856 / 100 = 3.59 Hours.

Float Voltage 2.25vpc (54 Volts)

Float Times 1 Hours.

EQ Voltage 2.56vpc (61.5 Volts)

EQ Times Generally 50-75% of the Absorb Timers (2-3 Hours)

End Amps When Current Falls below 2% of the C/20 Capacity for 60 mins.  (17 Amps)

Battery Efficiency Percentage: 80%

Temperature Compensation. 5mv Per Degree C, multiplied by the number of cells. (+/- 120mv)


You’re Finished Right?


The short answer…. NO, not a chance!

Once you’ve commissioned the system now it’s up to the end user, or installer to check the Specific Gravity’s and compare the readings to your settings.

If you notice the specific gravity of cells begin to decrease after the first 3-8 weeks of usage we recommend increasing Bulk and Absorption voltage or Absorption time in small increments.   In the above example, say you’ve set your absorb voltage to 59.6 on your controller and you notice the specific gravity falling off.  I would add .4 volts to absorb voltage to start, wait 2-3 weeks and then recheck specific gravities.  They should have risen a bit, if they didn’t rise enough, then I would add 30-60 mins to the absorb timer since now your bulk/absorb voltage is at the max of 60volts.  

If you record higher than normal specific gravity readings at a full charge (resting in Float), these voltage settings may be decreased slightly.

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.

If you are not using temp compensation, you need to make sure battery temperature compensation is used, if not regardless of the battery manufacture you use a BTS… Most manufactures will not cover your batteries under warranty if you don’t use a BTS.   The BTS changes the charging voltage based on the actual temperature of your batteries, because this temperature changes rapidly, specifically when charging your batteries, we strongly recommend using a BTS with any device that charges your battery bank.

For Flooded Batteries to prevent sulfation, it's essential that you complete at least 1 full absorb/bulk cycle at least every 7-10 days if the bank is being cycled daily.   The frequency a full charge depends on how hard you work them, the harder they work the more often they need a (1.265 to 1.275 Specific Gravity) full 100%.