Battery Maintenance: Testing and Charging
Jul 1, 2014 9:00 AM, Doug Irwin, CPBE DRB AMD
How many large, lead-acid batteries are you depending upon right now? Do you test them?
Testing a generator is one common scenario. Hopefully, you''ll discover a dead battery when you do the test, and not when you need it. That can make for a very bad day.
So let''s say that you find a dead battery during a test, and discover that the real problem is not so much the battery, but the charger. Let''s also say that the charger is 40 years old and uses selenium rectifiers. It''s time for a new one. How does one pick a new battery charger? http://www.batterystuff.com/kb/articles/charging-articles/how-do-i-pick-a-battery-charger.html
That depends on the size of the battery. A good rule-of-thumb is this: take the ampere-hour rating of the battery, divide by the charger rating, and then add 10%. For example, you have a 100 AH battery. If you were to buy a charger rated at 15A, it would take just over 7 hours to charge the battery, assuming it was completely discharged. Another thing to consider: during the absorption period, the charger output should be between 14.4 and 14.9 volts. After the battery is charged, the �float� voltage should be between 13.1 and 13.4 volts.
Now let''s say that it''s a weekend, and you can''t get a new battery charger right away. Can you use a big power supply to do the charge? Yes, with careful attention. http://batteryuniversity.com/learn/article/equalizing_charge
- -Calculate the voltage necessary. For lead-acid, use 2.40 V per cell. For a 12 V cell then, the power supply output would be 14.4 volts.
- -Set current limiting between 10 and 30% of the battery''s AH rating.
- -You should observe the battery temperature, voltage, and current during the charge period. Charge only in a well-ventilated space.
- -As the battery charges, the current will diminish. When it �bottoms out�, disconnect the charging source.
- -The battery can be maintained in a �ready� state by keeping a �float� voltage across it, again, between 13.1 and 13.4 volts.
It''s also possible that prior to the battery charger failure, it was not maintaining the charge level correctly. One possible symptom of under-charging is sulfation. Recently, I was doing some research for an article on the design of alternative power systems. http://radiomagonline.com/infrastructure/power/trends_alt_power_0601/
Jeremy Preece of EMF in Southern California told me the following about his station''s photo-voltaic system battery bank:
�On our old 3000AH bank, we were only able to charge at roughly half the C/20 rate, and that''s why the batteries died young. They needed more current to boil off the sulfate build-up than what the (solar) panels could provide under peak performance.�
I''ve been using flooded batteries for years and admittedly, I was not familiar with this issue. Let''s take a look at just what this means. �Sulfation occurs when a lead acid battery is deprived of a full charge. Solar cells and wind turbines do not always provide sufficient charge, and lead acid banks succumb to sulfation. What is sulfation? During use, small sulfate crystals form, but these are normal and are not harmful. During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline layer deposited on the negative plates. This in turn leads to the development of larger crystals, which reduce the surface area of the battery''s active material. Sulfation also lowers charge acceptance, and makes charging take longer because of elevated internal resistance.� You may be able to reverse the effects of sulfation�I recommend you go directly to my reference should you want to attempt this.
The Battery University page also has basic recommendations for maintaining lead-acid batteries:
- -Always keep lead acid batteries charged. Avoid storage below 2.10 V/cell, or at a specific gravity level below 1.190.
- -Avoid deep discharges. The deeper the discharge, the shorter the battery life will be. A brief charge on a 1 to 2 hour break during heavy use prolongs battery life.
- -Never allow the electrolyte to drop below the tops of the plates. Exposed plates sulfate and become inactive. When low, add only enough water to cover the exposed plates before charging; fill to the correct level after charge. Use distilled or ionized water. Never add acid. This would raise the specific gravity too high and cause excessive corrosion.
Irwin is RF engineer/project manager for Clear Channel, Los Angeles. Contact him at firstname.lastname@example.org.
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