One of the most common questions we’re asked as a battery manufacturer…


“What charge settings should be programmed on my solar system?”


A knowledgeable supplier or system installer will know that the answer to this question varies and is dependent on system design, individual usage, location as well as other unique factors.  System setup and programming of equipment will vary from one installation to the next and will often require adjustment from season to season.  An identical system may be installed in 100 different homes, but power generation, load requirements and usage may vary requiring 50-60 different programming profiles.  Each Installation should be considered unique and the system sized accordingly.


Most battery manufactures will provide recommended guidelines or a range of voltage settings for charging your battery bank. These are often written or tested in scenarios where the batteries see 10-14 hours of charging on a daily basis.  Many Renewable Energy systems are limited to much shorter charge periods.  At best, systems with a solar array tracker may see 6-8 hours of charging with an adequately-sized PV array.  Most PV systems see 3-5 hours of good charge time per day.  In situations where charge time is limited, system settings must be more aggressive to attain a full charge on the battery bank.

Of course, charge current will also be dependent on weather and climate.  It is important for system installers and customers to understand that weather conditions may have a dramatic effect on charging and system performance.  For example, current will be dramatically reduced where heavy atmospheric disturbances occur, such as heavy cloud cover or smoke.  While skies were filled with smoke in Western Washington state earlier this year, testing showed the amount of actual amount of charge current generated on a 4000W PV array was reduced to 300-400W /m2.


    As an example, we will assume the following system setup:

  • 850 AH flooded lead acid battery bank connected at 48V
  • 4000W solar array connected to an 80 Amp MPPT Charge Controller
  • 4000W Inverter/Charger connected to a 6000 W generator with a rated charge of 60 Amps DC


Solar Controller Settings


As solar is considered “free energy”, it is logical to take advantage of as much charge as possible from the PV array during available hours.  With off-grid systems cycling regularly, we will often recommend using charge voltages on the higher end of the manufacturer’s recommended range, slightly higher than the programmed voltages on the inverter/charger, to maximize charge efficiency during limited sun hours.


Bulk & Absorption Voltage

The solar charge controller should be programmed to charge at the higher end of the manufacturer’s recommended charge range.  Where 2.45-2.50 VPC (58.8-60.0V for 48V systems) is the range, it is recommended that the Bulk & Absorption voltage be set at 59.6-60.0V when the system is cycling regularly.


Absorption Time

The Absorption phase of the charge cycle brings the batteries from 80% to 100% state of charge.  The amount of time required to complete the remaining 20% charge will depend on the depth of discharge and the charge current.  Typically, when the battery bank reaches 80% state of charge the current has dropped to 40-50% of peak charge current produced in the Bulk stage.


In this example, a 4000W PV array connected to an 80 Amp MPPT controller will produce a peak current of 80 Amps if the panels are operating at 25°C (77°F).  Factoring in losses such as resistance, panel efficiency and less than ideal conditions, a realistic average will be 60-70 Amps charge current during the Bulk charge phase.  With 850 Ah total capacity, the battery bank will still require approximately 170 Amps of current during the Absorption charge phase to bring to a full charge.  If the charge current has dropped to 40 Amps when the Absorption charge phase begins this will require 4-5 hours to bring the batteries to 100% state of charge.


Water usage is also a consideration when using flooded lead acid batteries.  Extended Absorption charge times will result in heavier off-gassing for longer periods of time, requiring more water.  To minimize water usage, measure and monitor specific gravity and fine tune Absorption charge time accordingly when the battery bank has reached full state of charge.


Float Voltage 

Float voltage maintains the battery bank at 100% after completing the Bulk & Absorption charge phases.  Float voltage is generally programmed at 2.2-2.3 VPC (52.8-55.2V for a 48 V system) depending on the battery type and manufacturer’s recommendations.  In an off-grid system this phase may not be reached on a daily basis.  It is important to size a system with the intention of reaching full state of charge as often as possible to prevent deficit cycling and sulfation which will reduce capacity and shorten the life of the battery bank.


Equalization (EQ) Voltage 

Equalization is a maintenance step used to balance and remove sulfation buildup from the battery bank.  Voltage settings for Equalization range from 2.54 – 2.65 VPC depending on the battery manufacturer.   This procedure should only be performed as necessary, where specific gravity measurements note a difference greater than 0.025 to .030 between cells, as equalizing too often may also cause plate burning and active material loss, shortening the life of the battery.


End Amps 

End Amps is a trigger used by the charge controller to determine that the battery bank is fully charged and no longer accepting current.  It is important to properly program the End Amps setting at the time of install.  If this is programmed incorrectly or incompatible with the battery type sulfation will develop quickly.


As a battery reaches full state of charge its charge acceptance drops considerably.  Typically, when the charge current falls below 2-4% of the battery capacity for 60 minutes the battery bank is no longer accepting charge and the Absorption phase should be terminated.  Flooded batteries have a higher internal resistance, so we recommend setting End Amps at 2%.  VRLA batteries, such as AGM & GEL, have a lower internal resistance.  For these models it is recommended that End Amps be programmed closer to 3-4%.


Be cautious as some charge controller manufacturers program End Amps by entering in a set time and others using the entered battery bank capacity, setting automatically at 2% of this value.  The concern is the time set at 2% is often only 1 minute, which is insufficient and will often result in a build-up of sulfation.


Looking at the system example provided above, we would program End Amps at 17 Amps (2% of 850 AH) for 60 minutes where a time variable is used.  If there is no time variable required, we would program End Amps at 8-12 Amps to prolong the charge slightly allowing them to sit at full charge a bit longer.



Inverter / Charger Settings


When charging the battery bank, the inverter/charger voltage settings will determine the charge efficiency and cost of fossil fuels when running a backup generator.  While it is a good idea to try to maximize these setting to take advantage of your PV charge source, you must also look at charge efficiency.  Charge efficiency is at its highest during the Bulk charge phase and is decreases as the batteries are charged in Absorption and Float.  We often advise clients with a sufficiently-sized PV array or good sunny day to run the generator to partially charge the bank in the morning, allowing the PV array to complete the charge, reducing the amount of time the generator will run.  Some inverter/charger allow auto start programming to trigger the generator during these times.


Bulk & Absorption Voltage

Bulk & Absorption voltage settings should be programmed in the inverter/charger at the low – mid range of the manufacturer’s recommended voltage settings. Where 2.45-2.50 VPC (58.8-60.0V for 48V systems) is the range, it is recommended that the Bulk & Absorption voltage for the inverter/charger be set at 58.8-59.6V when the system is cycling regularly.


Absorption Time

As explained earlier, the Absorption time is set to complete the remaining 20% of charge, the programmed time will depend on the depth of discharge and charge current.


In the example, a 4000W inverter/charger rated at a max current of 60 Amps will see 50-60 Amps of current during the Bulk charge phase, dropping to 20-30 Amps during the Absorption phase.  Completing the charge on a 850 AH    battery bank will still require 170 Amps of current at this point.  At 20-30 Amps this will require 6-8 hours Absorption time to bring the battery bank to 100%.


Float Voltage 

The recommended Float voltage setting is 2.2-2.3 VPC (52.8- 55.2V for 48V systems) depending on the battery type and manufacturer’s recommendations.  This voltage setting will be the same for the solar controller and inverter/charger.


Equalization (EQ) Voltage 

Equalization voltage setting will range from 2.54-2.65 VPC depending on the battery manufacturer’s recommendations.  As mentioned earlier, this procedure should only be performed as a maintenance step when necessary, where specific gravity measurements note a difference greater than 0.025 to .030 between cells.  Equalizing too often may cause problems such as active material loss, shortening the life of the battery.


End Amps 

The End Amps settings and considerations will be the same for the solar controller and inverter/charger.


Follow-up

With these settings programmed, many system Installers and customers will assume the system is properly set up and make the common mistake not to confirm the battery bank is fully charging and the system is operating as efficiently as possible. Most systems will require adjustment.  Flooded lead acid batteries offer an advantage as they may be checked to accurately confirm state of charge.


After 2-4 weeks of use, we recommend checking specific gravity in each cell when the battery bank is resting in Float charge. If the specific gravity readings are higher or lower than recommended, charge adjustments should be made accordingly. Specific gravity at 100% state of charge should read 1.265-1.270 at 25°C (77°F).


Just like driving down a freeway, if you find you’re going too quickly you need to let off the accelerator to maintain speed.  If you’re slowing down, you need to apply more pressure to the accelerator.


Higher than recommended specific gravity will increase capacity in the battery bank.  However, this will also lead to early plate degradation and failure, often due to plate corrosion.  Low specific gravity over time will cause capacity loss due to sulfation and premature cell failure.  When the specific gravity readings are high the charge voltage or Absorption time should be reduced.  When low, the charge voltage and Absorption time should be increased.  These steps should be done in small increments, allowing 2-3 weeks of cycling between adjustment, and specific gravity readings rechecked to monitor improvements.


Completing these steps for the first 6-12 months of use and keeping records of these changes will indicate the necessary adjustments going forward from season to season.  By doing so, many users will then know what adjustments should be made and can confidently reduce specific gravity checks from monthly to quarterly.  Doing so will ensure the battery bank offers reliable, long-lasting capacity.