Lots of info about batteries and self discharging. I picked this up from a Google Post. http://groups.google.com/groups?q=we...al.com&rnum=10
2. WHY BOTHER?
Because only the rich can afford cheap batteries.....
A good quality deep cycle lead acid battery will cost between $50 and $200 and,if properly maintained, will give you at least 150 deep discharge
The purpose of a deep cycle battery is to provide power for trolling motors, golf carts, fork lift trucks, uninterruptible power supplies (UPS), and other accessories for marine and recreational vehicle (RV) and commercial applications.
A twelve volt deep cycle battery is made up of six cells, each producing 2.1 volts that are connected in series positive to negative. Each cell
is made up of an element containing positive plates that are all connected together and negative plates, which are also all connected together. They are individually separated with thin sheets of electrically insulating, porous material that are used as
spacers between the positive and negative plates to keep them from shorting to each other. The plates, within a cell
, alternate with a positive plate, a negative plate and so on. A plate is made up of a metal grid that serves as the supporting framework for the active material which is "pasted" on it. The cells are inserted into a polypropylene case, connected to the terminals, covered, filled with a dilute sulfuric acid electrolyte and formed (initially charged). The electrolyte is replaced and the battery is given a finishing charge.
A battery is created when two different metals such as Lead Dioxide (Pb O2), the positive plates, and Sponger lead (Pb), the negative plates, are immersed in Sulfuric Acid (H2SO4), the electrolyte. A typical lead-acid battery produces approximately 2.1 volts per cell
. The chemical action between the metals and the electrolyte creates the electrical energy, that flows from the battery as soon as there is a load, such a light bulb, between the positive and negative terminals.
The electrical current flows as charged portions of acid (ions) between the battery plates and as electrons through the external circuit. The action of the lead-acid storage battery is determined by chemicals used, temperature, and load.
Normally a battery "ages" as the active positive plate material sheds (or flakes off) due to the expansion and contraction that occurs during the discharge
and recharge cycles, the positive grid metal corrodes in the electrolyte,
or the negative grid shrinks. Deep discharges, heat, vibration, over charging, or non-use accelerate this "aging" process. Sediment or "mud" builds up in the bottom of the case and the can short the cell
out. Another major cause of premature battery failure is sulfation. When batteries are stored discharged or stored fully charged for over three months without being recharged, the plates slow discharge
and become coated with hard and dense layer of
lead sulfate. Over time, the battery can not be recharged.
Using tap water to refill batteries can produce calcium sulfate, which also will coat the plates. Recharging a sulfated battery is like trying to wash your hands with
gloves on. When the active material in the plates can no longer sustain a discharge
current and the battery "dies".
Most of the "defective" batteries that are returned to the manufacturer during free placement warranty periods are good. This suggests that most sellers of new batteries do not know how or take the time to properly load test or
3. HOW DO I TEST A BATTERY?
There are six simple steps in testing a deep cycle battery--inspect, recharge, remove surface charge, measure the state-of-charge, load test, and recharge.
If you have a non-sealed battery, it is highly RECOMMENDED that you use a temperature compensated hydrometer, which can be purchased, at an auto parts
store between $5 and $20. A hydrometer is a float type device used to determine the state-of-charge by measuring the specific gravity of the electrolyte in each cell
. It is a very accurate way of determining a battery's state-of-charge and weak or dead cells.
To troubleshoot charging or electrical systems or if you have a sealed battery, you will need a digital voltmeter with .5% (or better) accuracy. A digital voltmeter can be purchased at an electronics store,for example, Radio Shack, between $20 and $200. Analog voltmeters are not accurate enough to measure the millivolt differences of the battery's state-of-charge or the output of the charging system. A battery load tester is optional.
Visually inspect for obvious problems, for example, loose of broken alternator belt,electrolyte levels BELOW the top of the plates, corroded or swollen cables,corroded terminal clamps loose hold-down clamps, loose cable terminals, or leaking or damaged battery case.
If the electrolyte levels are low in non-sealed batteries, allow the battery to cool and add DISTILLED water to the level indicated by the battery manufacturer or to 1/4 inch BELOW the bottom of the plastic filler tube (vent wells). The plates need to be covered at all times and avoid OVERFILLING, especially in hot climates, because the heat will cause the electrolyte to expand and overflow.
Recharge the battery to 100% state-of-charge. If the battery has a difference of .03 specific gravity reading between the lowest and highest cell
, then you should equalize it. (Please see Section 6.)
3.3. REMOVE SURFACE CHARGE
Surface charge is the uneven mixture of sulfuric acid and water within the surface of the plates as a result of charging or discharging. It will make a weak battery
appear good or a good battery appear bad. You need eliminate the surface charge by one of the following methods:
3.3.1. Allow the battery to sit for four to twelve hours to allow for the surface charge to dissipate. [RECOMMENDED].
3.3.2. Apply a load that is 33% of the ampere hour capacity for five minutes and wait five to ten minutes.
3.3.3. With a battery load tester, apply a load at one half the battery's CCA rating for 15 seconds and wait five to ten minutes.
3.4. MEASURE THE STATE-OF-CHARGE
If the battery's electrolyte is above 100 degrees, allow it to cool. To determine the battery's state-of-charge with the battery's electrolyte temperature at 80 degrees, use the following table, which assumes that 1.265 specific gravity reading is as fully charged battery:
Open Circuit Approximate Average Cell
Voltage State-of-charge Specific Gravity
12.65 100% 1.265
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 Discharged 1.120
[Source: Battery Council International]
[Electrolyte temperature compensation, depending on the battery manufacturer's recommendations, will vary.
If you are using a NON-temperature compensated HYDROMETER, make the following adjustments, and if the temperature of the electrolyte is BELOW 80 degrees, SUBTRACT .004 specific gravity per 10 degrees below 80 degrees FROM the Specific Gravity indicated in the table above. For example, at 30 degrees, the specific gravity reading would be 1.245 for a 100% state-of-charge. If the temperature of the electrolyte is ABOVE 80 degrees, then ADD .004 specific gravity per 10 degrees above 80 degrees TO the Specific Gravity. For example, at 100 degrees,
the specific gravity would be 1.273 for 100% state-of-charge. This is why using a temperature compensated hydrometer is highly RECOMMENDED and more accurate.
If you are using a digital VOLTMETER, then from the table above make the following adjustments, if the temperature of the electrolyte is BELOW 80 degrees, SUBTRACT
.0012 to .028 volts (1.2 to 28 millivolts) per 10 degrees below 80 degrees from the Open Circuit Voltage indicated. For example, at 30 degrees and using 28 millivolts, the reading would be 12.51 VDC at 100% state-of-charge. If the temperature of the electrolyte is ABOVE 80 degrees, then ADD .0012 to .028 volts (1.2 to 28 millivolts) per 10 degrees above 80 degrees. For example, at 100 degrees and using 28 millivolts, the Open Circuit Voltage would be 12.71 VDC.
For non-sealed batteries, check the specific gravity in each cell
with a hydrometer and average the readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate digital voltmeter. It is the only way you can determine the state-of-charge.
Some batteries have a built-in hydrometer, which only measures the state-of-charge in ONE of its six cells. If the built-in indicator is clear or light yellow, then the battery has a low electrolyte level and should be refilled and
recharged before proceeding. If sealed, the battery is toast.
If the state-of-charge is BELOW 75% using either the specific gravity or voltage test or the built-in hydrometer indicates "bad" (usually dark), then the battery needs to be recharged BEFORE proceeding. You should replace the battery, if one or more of the following conditions occur:
3.4.1. If there is a .05 or more difference in the specific gravity reading between the highest and lowest cell
, you have a weak or dead cell
(s). If you are really lucky, applying an EQUALIZING charge may correct this condition. (Please see Section 6.)
3.4.2. If the battery will not recharge to a 75% or more state-of-charge level
or if the built-in hydrometer still does not indicate "good" (usually green, which is 65% state-of-charge or better).
[If you know that a battery has spilled or "bubbled over" and the electrolyte has been replaced with water, you can replace the old electrolyte with new electrolyte
and go back to Step 3.2 above. Battery electrolyte is a mixture of 25% sulfuric acid and distilled water. It is cheaper to replace the electrolyte than to buy a new battery.]
3.4.3 If digital voltmeter indicates 0 volts, you have an open cell
3.4.4. If the digital voltmeter indicates 10.45 to 10.65 volts, you probably have a shorted cell
or a severely discharged battery. [A shorted cell
is caused by plates touching, sediment "mud" build-up or "treeing" between the plates.]
3.5. LOAD TEST
If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of the battery by applying known load and measure
the time it take to discharge
the battery until 20% capacity is remaining.
Normally a discharge
rate that will discharge
a battery in 20 hours can be used [C/20]. For example, if you have a 80 ampere hour rated battery, then a load of four amps would discharge
the battery in approximately 20 hours
(or 16 hours down to the 20% level). New batteries can take up to 50 charge/discharge
cycles before they reach their rated capacity. Depending on your application, batteries with 80% or less of their original capacity
are considered history.
If the battery passes the load test, you should recharge it as soon as possible to restore it to peak performance and to prevent lead sulfation.
4. WHAT DO I LOOK FOR IN BUYING A NEW BATTERY?
4.1. Ampere Hour (or Reserve Capacity) Rating
The most important consideration in buying a deep cycle battery is the Ampere Hour (AH) or Reserve Capacity rating that will meet or exceed your requirements and how much weight you can carry. [Most deep cycle batteries
are rated in discharge rates
of 100 amps, 20 amps, or 8 amps.
Higher the discharge
, the lower the capacity due to the Peukert Effect and the internal resistance of the battery. Reserve Capacity (RC) is the number of minutes a fully charged battery at 80 degrees is discharged at 25 amps
before the voltage falls below 10.5 volts.] To convert Reserve Capacity to Ampere Hours, multiple RC by .6. For example, a battery with 120 minute RCwill have approximately 72 Ampere Hours. This means that the battery should produce one amp for 72 hours of continuous use. More ampere hours (or RC) are better in every case.
Within a BCI group size, the battery with larger ampere hours (or RC) will tend to weigh more because it contains more lead.
[If more ampere hours are required, two new and identical six volt batteries can be connected in series (positive terminal of Battery One to the negative terminal of Battery Two). Two (or more) new and identical 12 volt
can be connected in parallel (positive terminals to positive terminals and negative terminals to negative terminals with identical cables). When connecting in series or parallel and to prevent recharging problems, do NOT mix old and new batteries or ones with different types. Cable lengths
should be kept short and cable sized large enough to prevent significant voltage drop [.2 volts (200 millivolts) or less] between batteries.]