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Genset Starting Education Module #6

Battery Charging Basics 

William F Kaewert | SENS – Stored Energy Systems LLC 

Battery charging fundamentals 

This section discusses basic battery charging concepts and explains the relationship of battery  characteristics of self-discharge and internal resistance to the charger’s float and boost charging modes,  respectively. 

As with any reservoir, storage batteries can be drained (discharged), filled again (charged) and kept full  despite leakage (float charged to offset self-discharge).  

The illustration below makes an analogy of the different operating modes of a storage battery to a  reservoir of fluid (a water tower). The force of gravity represents the battery’s electrical potential (volts).  The battery’s capacity is represented by how full the water tower is. A leaky outlet valve represents the  battery’s tendency to discharge itself. The flow of water up into the tower represents battery charging  current.  

Figure D illustrates the fundamental concept of battery charging. When pressure, the electrical  equivalent of which is voltage, is sufficiently high the battery will accept current from the charger. The  rate at which current flows into the battery can be adjusted changing the charger’s voltage. The charger  does not force current into the battery; rather the battery accepts current from the charger when the  charger’s voltage is higher than battery voltage. A large difference in voltage between charger and  battery enables the battery to accept more current, subject to the charger’s current limit.1 Conversely, a  small difference in voltage between charger and battery enables the battery to accept only a small  amount of current. 
Battery chargers are current-limited to prevent excess current flow from damaging the charger or operating its  protective fuses or circuit breakers

Illustration: Battery & water tower analogy; Storage reservoir = battery Water main = charger



Relationship of battery self-discharge to float voltage and current

Self-discharge is the tendency for all batteries to discharge by themselves over time, depicted above in  Figure A. Lead-acid and Ni-Cd batteries self-discharge quickly compared to consumer type dry cells. Self discharge limits lead-acid batteries to a maximum of six months’ shelf life after last charge before  irreversible sulfation occurs. Storage in hot climates can halve this duration. Ni-Cd batteries also suffer  damage if left off charge too long, but often can be resurrected through a process of many cycles of  aggressive charging and discharging.  

Float charging, depicted in Figure B, counters battery self-discharge. When voltage applied by the  charger is correct, current flow from charger into battery will slightly exceed self-discharge current. This  net excess current2 flow into the battery dissociates water-based electrolyte into hydrogen and oxygen  gases that, in a flooded battery, escape to the atmosphere from the battery’s vent caps. This slow loss creates the need for periodic maintenance and water addition. In an AGM/VRLA type battery, the net excess current gets absorbed into the oxygen recombination reaction3 in which hydrogen and oxygen  gases are combined back into water inside each battery cell under slight pressure.  

The above description is accurate only when the charging voltage is ideal. When charging voltage is  higher than the ideal value (overcharging), the excess current accelerates the rate at which hydrogen and  oxygen gases are generated. In a flooded battery this faster water use shortens maintenance intervals.

Float current into the battery minus self-discharge current.  Also known as “recombinant lead-acid”.  long as a flooded battery is kept filled, the risk of catastrophic failure under overcharge conditions is little  greater than when properly charged. This is not true with an AGM/VRLA battery. When a VRLA battery is  overcharged, gas is generated faster than the recombination reaction can convert it back into water.  When excess gas pressure inside a battery cell exceeds a pre-set release value (about 5 psi), gas is vented  to the atmosphere through a pressure relief valve. Each time the relief valve operates, the battery dries  out a little, and the battery’s conductivity and performance drop. This is a problem because once lost,  electrolyte cannot be replaced in an AGM/VRLA battery. AGM/VRLA batteries contain no extra  electrolyte, so correct charging of AGM/VRLA batteries is thus essential to achieving the battery’s  intended life. 

“Boost charging” shortens recharge time by enabling the charger to spend more time delivering its maximum current 

All batteries suffer from internal resistance, represented below as a resistor connected in series with an  ideal battery. Attempting to charge a battery faster by attaching a huge charger operating at the float  voltage does not deliver hoped-for increase in recharge speed because battery internal resistance  consumes a greater portion of charging current as charge current increases.  

Illustration: Battery internal resistance and effective charging voltage comparison


Instead, the most effective way to reduce charging time is to temporarily increase charging voltage  above the normal float setting during battery recharge. This is called “boost charging”. Excess voltage  applied to the battery compensates for voltage lost to the battery’s internal resistance. Operating at the  higher boost voltage allows the battery to accept the charger’s maximum current longer than it would at  float voltage. When the battery reaches full charge, the charger’s voltage must be reduced to the correct  float voltage. If this transition is not made, or is made long after the battery reaches full charge, the  battery will be overcharged and may be damaged.

Alternatives for controlling when the charger operates in boost or float mode

The charging performance gain enabled by boost charging is accompanied by risk of overcharging.  Several alternatives are available to control the charger’s operating mode so that the charger reverts to  float mode once the battery is charged. The strengths and weaknesses of these are compared below in  the table below. Fully automatic boost control systems are strongly recommended for genset battery  charging. 

Table: Comparison of boost mode control methods

Boost mode control Advantage Disadvantage Comment
Manual boost  mode switch
Easy to forget and leave  charger in boost mode.  
Requires user judgment to  decide when to start and  stop boost charging.
Poor choice for unattended sites.  High risk of overcharging the battery.  Not recommended. 
initiated boost  timer
Timer limits overcharge  damage potential;  
enables user to  
manually initiate  
equalize4,5 charge.
Requires user judgment to  decide when boost charging  is needed. Requires user  judgment to decide how  long to set timer.
Timer function is better than a  switch. Acceptable solution at  attended sites. Not recommended  for unattended sites with lead-acid  batteries. Recommended for sites  with Ni-Cd batteries.
boost on  
Automatically enters  and exits boost based on  battery discharge.  
Operates on pre 
determined rules.
Users accustomed to  
controlling boost of the  
charger may feel lack of  control with a fully  
automated system.
Recommended for unattended sites.
boost to  
equalize the  
Meets the need of Ni-Cd batteries for periodic  boost charging to  
maintain full capacity.
More complex system that  requires a microprocessor to  keep time.
Function should be used only with  Ni-Cd batteries. 

Summary of key points

  1. The charger does not force current into the battery; rather the battery accepts current from the  charger when the charger’s voltage is higher than battery voltage.
  2. Float charging counters battery self-discharge. Float is the voltage at which a fully charged  battery is maintained at a state of high charge. During float charging, current into the battery  very slightly exceeds the battery’s self-discharge rate.
  3. “Boost” charge is an elevated voltage mode that shortens recharge time by enabling the charger  to spend more time delivering its maximum current.
  4. The most effective way to reduce charging time is to temporarily increase charging voltage above  the normal float setting during battery recharge.
  5. Fully automatic boost charging mode control systems are strongly recommended for genset  battery charging. 

“Equalize” charging is the application of the boost charge voltage to an already charged battery. This deliberately  overcharges the battery for the purpose of increasing capacity of the weakest cells in the battery string. 5 “Equalizing is an overcharge performed on flooded lead acid batteries after they have been fully charged. It  reverses the buildup of negative chemical effects like stratification, a condition where acid concentration is greater  at the bottom of the battery than at the top. Equalizing also helps to remove sulfate crystals that might have built  up on the plates. If left unchecked, this condition, called sulfation, will reduce the overall capacity of the battery. Many experts recommend that batteries be equalized periodically, ranging anywhere from once a month to once or  twice per year. However, Trojan only recommends equalizing when low or wide ranging specific gravity (+/- 0.015)  are detected after fully charging a battery.” Source Trojan Battery Company