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Module 6

Genset Starting Education Module #6:
Battery Charging Basics

William F Kaewert
SENS – Stored Energy Systems LLC

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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.

Battery charging fundamentals
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.

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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. As 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.

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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.

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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.

Footnotes

  1. Battery chargers are current-limited to prevent excess current flow from damaging the charger or operating its protective fuses or circuit breakers.
  2. Float current into the battery minus self-discharge current.
  3. Also known as “recombinant lead-acid”.
  4. “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

Copyright © 2012 William F Kaewert, SENS – Stored Energy Systems LLC

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