Parallelling VRLA Batteries

Last Updated on 10 April 2015 Hits: 2944

Lead-Acid Batteries

 Lead-acid batteries are one of the most cost effective ways to store electrical energy, provided that they are used in a controlled temperature environment and the weight is not an issue.  The really bad part is that they contain hazardous materials: lead, which is a health hazard, and dilute sulfuric acid, which is corrosive and can cause eye injury.  During charging, they can give off hydrogen, which is flammable and corrosive.  Carbon monoxide detectors are susceptible to damage from hydrogen and may issue alarms due to small amounts of hydrogen.  VRLA (valve regulated sealed lead acid) batteries are designed to recombine the hydrogen with oxygen and are the only type that should ever be used indoors.  Never use automotive or marine type batteries indoors, even though you would love to have deep cycle rated batteries.  VRLA batteries come in two flavors:  AGM (absorbed glass mat) and gelled electrolyte.  AGM batteries are by far the more common, but either is fine for operating electronic equipment when commercial power is not available.  These batteries are used in most UPS applications and worn batteries that still have some life left are often available for reasonable prices.  New batteries can be purchased from a number of sources including Batteries Plus and Interstate Batteries.   Note that it is illegal to dispose of batteries containing lead; they must be recycled.  I have had luck taking discards to Interstate Batteries and WalMart. They may even be willing to pay a small amount (“core charge”) for larger sized batteries.

 Paralleling Batteries:

 It is absolutely okay to parallel lead acid batteries as long as they have the same voltage rating and are of the same construction. (Do not mix automotive type batteries and AGM type batteries, for example).  A concern is if one battery has an internal fault that causes self-discharge, it will indeed discharge the parallel battery, too.  There is an easy test to detect this:  with nothing connected to either battery, check the terminal voltage.  A week later, check the terminal voltage, again.  If the voltage on one battery has declined more than a couple of tenths of a volt, it is ready to be recycled.  If the open circuit terminal voltage on any stored "12 volt" lead acid battery drops below 12.4 volts, it needs to be recycled.

 I strongly urge the use of a fuse on each battery to protect the wiring.  A battery can deliver more than enough current to ignite the wire and anything nearby. Locate the fuse close to the battery terminal. Automotive fuses are inexpensive, readily available, and work well for nominal 12 volt applications.  Fuse sizing:  30 A for 10 gauge wire or larger, 20 A for 12 gauge wire, 15 A for 14 gauge wire, 10 A for 16 gauge wire.

 What the numbers mean:

 Voltage is the electrical potential, measured in volts (V),  that causes current to flow in a circuit.  Current, measured in amperes (A), is the movement of charge that actually transports the energy from one point in a circuit to another.  Energy, measured in joules (J), is the ability to do work; for example, making a lamp glow for ten minutes or pumping a gallon water ten feet higher than it was before.  Power, measured in watts (W), is the rate of energy delivery.  One watt is the delivery of one joule per second.  For direct current, power is simply calculated as volts multiplied by amperes.

 Hybrid units have often been used to measure energy.  For example, commercial electric energy is measured in “kilowatt-hours” (kWH).  One kWH is 3600000 J.  The energy capacity of a battery is usually rated in watt-hours.  Since batteries come in various voltage ratings, battery companies like to rate them relative to the cell sizes.  To remove the voltage from the energy rating, they rate batteries using “ampere hours” (AH).   Multiply AH by the voltage rating and you get the energy rating of the battery in WH, more or less.  Complicating all of this is that the actual voltage of the battery depends on load current and on state of charge.  Also, the AH rating of the battery varies with the current.  By law, the manufacturer must state the discharge rate used to specify the AH rating.  Usually, this is the current that would deplete the battery in 20 hours, expressed as “20 H”.  For example, a 33 AH battery can provide 1.65 A for 20 hours – 1.65 A X 20 H = 33 AH.  Discharging at a higher rate (more than 1.65 A) will result in fewer available ampere-hours; that is, less energy can be extracted.  Slower rates yield greater capacity.  This is why paralleling batteries will yield more energy than using one battery at a time.  The down-side of extracting more energy is that a deeper discharge will shorten the life of the battery. Never discharge a battery too deeply—for a “12 V” battery, remove the load when the voltage dips to 10.5 V and recharge the battery as soon as possible.

 The end of life for a lead-acid battery is defined as when the energy capacity of the battery drops below 80% of the rated energy capacity.  Often, a battery continues to be useful beyond its rated lifetime; for example, an old battery may still start your car as long as it does not get too cold outside.  This loss of capacity is caused by physical and chemical changes inside the battery and cannot be reversed.  Conditions that cause loss of capacity include: number and depth of discharge cycles, time spent in a discharged state, overcharging, storage at elevated temperatures, and the passage of time.

 The number one rule for keeping lead-acid batteries healthy is to keep them charged.  It is okay to use an automotive type charger to recharge, but do not leave the charger connected to the battery as most automotive type chargers do not have regulated voltage output and will overcharge the battery.  An ideal charger for most applications is a regulated power supply that puts out 13.6 Vdc.  This type can be left connected all of the time; this is called “float charging”.  If float charging is not used, the battery should be stored with nothing connected.  The charge should be topped every couple of months.

 Some good battery related web sites:

 Discussion of many battery chemistries:  

DE Robert Baker N8ADO

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