Charging Characteristics: Flooded Lead Acid and Sealed Lead Acid Deep Cycle Batteries

Charging Characteristics:
Flooded Lead Acid and Sealed Lead Acid Deep Cycle Batteries

By K. Fred Wehmeyer, Vice President, Product & Process Engineering,
U. S. Battery Manufacturing Company, Inc

Proper charging is a critical factor in optimizing the cycle life characteristics of deep cycle lead acid batteries and can have a profound effect on both the rate of ‘cycle-up’ and the overall cycle life of batteries.  U.S. Battery Manufacturing Company, Inc. recommends the use of ‘opportunity charging’ or charging batteries and battery packs at every opportunity while in storage or service.  Following this recommendation will assure that batteries are always at the highest possible State of Charge (SOC) to maximize performance and range and to minimize the battery’s Depth of Discharge (DOD) to optimize performance and life.

The charging process is intended to fulfill several objectives.  First, the charging process should replace the capacity (in amp-hours) removed from the battery during previous discharges.  Second, the charging process should return additional capacity (in amp-hours) to offset the thermodynamic inefficiencies inherent in the charging process.  This additional capacity can be measured as a charge factor calculated by:  charge Ah in / discharge Ah out.  The charge factor varies with temperature, condition and age of the battery but is usually in the range of 105 - 150%.  Third, the charging process should charge the battery at a voltage and/or charge rate at the end of charge that will result in controlled gassing of the electrolyte.  This gassing is required to mix the electrolyte to prevent stratification.  Without proper mixing of the electrolyte, the heavier acid generated during charging can sink to the bottom of the cell and will adversely affect performance and life of the battery.  Finally, the charging process should result in a fully charged battery with electrolyte specific gravity that is constant over several end-of-charge readings, consistent between and among the cells of the battery pack, and within the proper range for the battery type per the battery manufacturer’s specifications.

(Note: Opportunity charging is defined as recharging whenever there is a significant discharge and there is a convenient opportunity for recharging.  Unlike other battery chemistries, lead-acid batteries do not suffer from ‘cyclic memory ^{See Footnote}’ caused by shallow discharge/recharge cycles.  Also, lead-acid batteries are more resistant to the effects of overcharge than they are to over-discharge effects.  Lead-acid batteries should always be maintained at the highest possible state of charge and should never be stored in a discharged state.  Opportunity charging not only maximizes available run-time, but also maintains the battery in a healthy, fully charged state.)

A fundamental technique for determining differences in charging methods is to measure the percentage of amp-hours returned on charge as a function of the amp-hours discharged on the previous discharge (% Ah Return or Charge Factor).  Battery chargers have historically been designed to recharge batteries to 110% to 125% Ah Return.  With better control over the charge profile, a charger can be designed to control the amount of overcharge such that optimum performance over the life of the battery can be obtained.  For example, a battery recharged to 120-125% Ah Return can deliver 100% of rated capacity within 50-100 cycles and peak capacity at over 100% of rated capacity within 150 cycles.  When the overcharge is reduced to 110-115% Ah Return, rated and peak capacity are achieved later in life, but the overall cycle life can be extended by 10% or more.  Recharging below 110% Ah Return on a frequent basis will tend to undercharge batteries without regular ‘equalization charging’.  However, with precise control over Ah Return and regularly scheduled equalization charging, battery life can be extended by an additional 50% or more with minimal effect on peak performance.  This type of charge profile typically requires a programmable (smart) charger using any of a number of user selectable or controlled charge algorithms.

(Note: Equalization charging is defined as an increased overcharge done on an infrequent but regular basis to overcome the effects of progressive undercharge and/or sulfation resulting from severe undercharge or prolonged inactivity.)

U.S. Battery is active in the development of new charging methods and regularly tests and evaluates new charger technologies.  As part of U.S. Battery’s charging recommendations, charging methods are categorized into three basic methodologies based on the number of charge stages used in the charging process.  It should be noted that the basic charge stages should result in a fully charged battery at the end of the final charge stage.  Using this criterion; float charging, maintenance charging, and equalization charging are not considered to be one of the basic charge stages.  These basic charge stage methodologies can be defined as follows:

• Three-Stage Charging – Charging using bulk charge, absorption charge, and finish charge (usually constant current - constant voltage – constant current) and often referred to as IUI charging.
• Two-Stage Charging – Charging using bulk charge and absorption charge only

(usually constant current - constant voltage).

• Single-Stage (Ferroresonant) Charging – Charging using a single-stage charge with tapering current and voltage.

U.S. Battery’s charging recommendations for deep cycle flooded lead-acid (FLA) and sealed absorptive glass mat (AGM) batteries are shown in Appendix A & B below.

APPENDIX A: Flooded Deep Cycle Battery Charging Recommendations

APPENDIX B: AGM Battery Charging Recommendations

Note that the charging parameters recommended for each of these depend on both the battery type and charger type.  These charging parameters are often controlled by specific charge algorithms that can be selected or programmed by the user.  Users should consult the charger manufacturer and/or U.S. Battery for proper selection or programming of algorithm controlled chargers.  U.S. Battery prefers the use of Three-Stage Charging with dV/dt charge termination to minimize the charge time required for full charge and to reduce the risk of abusive undercharging or overcharging of batteries and battery packs.

Following is a description of some of the common charger technologies used to charge deep cycle lead acid batteries and some common definitions of charge control parameters used by chargers to control both overcharge and undercharge over the life of the battery.

Charging Technologies and Charger Definitions
Charger Definitions:

• Ferroresonant Charger – A charger employing a saturable core reactor to provide non-linear, tapering DC current vs voltage.
• High Frequency (Switch-Mode) Charger - A battery charger employing high-frequency switching in the rectifying circuit to provide DC constant current or constant voltage.
• Charger Algorithm – Designed-in or programmed set-points used by a charger to control transition from one charge phase to another or to initiate and/or terminate charge.
• Charge Time – The time required for the charger to complete all charge phases and reach the final charge termination criteria (not including float or maintenance charge phases).
• Bulk Charge – High rate charge phase at the start of charge designed to quickly bring battery SOC to 75-90%; may be constant current, stepped current, constant power, taper charge or ferroresonant charge.
• Absorption Charge – Secondary charge phase designed to bring battery SOC to 90-100% (usually controlled by charger algorithm).
• Finish Charge – Final charge phase designed to return an additional    5-20% of battery capacity to fully charge battery, to offset charge inefficiency, and to mix battery electrolyte through gassing.
• Float or Maintenance Charge – Optional charge phase after finish charge to maintain battery at 100% SOC and to offset self-discharge; may be a continuous charge at float voltage or programmed pulse charge.
• Equalization Charge – A periodic fixed overcharge designed to overcome the effects of prolonged undercharge and to fully mix electrolyte.
• Charge termination – The method employed by the charger to sense full charge and to terminate charging, for example:
• Finish Rate:  Low constant current or taper charge at or near the end of charge (must be limited if constant current)
• Time Limit:  Timer controlled for entire charge period or a pre-determined portion of the charge period
• dV/dt Limit:  Rate of change of charge voltage over time at a constant current charge rate
• *Max V Limit:  Maximum voltage termination at finish rate
• * Min I Limit:  Minimum current termination at fixed voltage
• *% OC Limit:  Fixed % overcharge at constant current (usually after 100% Ah return indication is reached)
• * -dV Limit:  Net decrease in voltage at finish current rate
• *Pulse Limit:  On/Off pulse charging using a High Voltage Set-point and a Low Voltage Set-point (can be used for both bulk charge termination and maintenance charging)

*Requires USB charge voltage temperature compensation.

Note:  Programmable chargers often use combinations of charge termination methods depending on conditions.

 

About the Author:


Fred Wehmeyer is Senior Vice President of Product and Process Engineering for U. S. Battery Manufacturing Company, Inc. in Augusta, Georgia. Fred is a seasoned battery professional with over 35 years experience in the design, manufacture, and quality assurance/testing of rechargeable batteries including lead-acid, nickel-cadmium, nickel-metal hydride, and lithium based chemistries. Fred’s previous experience was as Vice President of Quality Assurance for EnerSys and Director of Product and Process Engineering for C&D Technologies. Fred has a Bachelor of Science Degree in Electrochemistry and has done graduate work in Engineering Management and Six Sigma/Lean Engineering.