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The Battery Capacity Discharge Test

Written by William C. Shumay Jr. for Arcon Equipment Inc., published in the Material Handling Wholesaler
copyright © William C. Shumay Jr.
For more articles, please visit http://www.arconequipment.com

Uncertainty about the capacity of an electric lift truck battery is bad for business- both yours (if you are a material handling equipment dealer) and your customer’s. New batteries are clearly marked with an ampere–hour rating, but that value cannot be used as much of a guideline as the battery ages and is exposed to years of on the job abuse and neglect.

Even the best industrial batteries lose running capacity with age, though it is clear that some hold up better than others. As someone who depends on knowing enough about your equipment to certify its suitability for a given job, you have a big stake in knowing just where your electric lift truck batteries stand. The key to this is an individual battery evaluation that includes an "industry standard" 6-hour discharge test.

The results of a test will tell you the percentage of capacity remaining in the battery (as compared to the "nominal" or new rating) as well as several other important things:

1. The test readings will show how a battery behaves on charge: whether it will tend to heat up (and use excessive amounts of water), or cause charger controls to terminate the charge cycle too late, for example.

2. An evaluation by an experienced battery serviceman will give indications about the advisability of investing in repairs that may be required. Although raw test data may show that replacement of several cells is needed, it is the judgment of the evaluator (who considers battery age, evidence of abuse, and other factors) that helps you make the critical decision to undertake the expense of a major repair.

3. The battery evaluation can give clues about the history of the battery in service- chronic overwatering, overcharging, improper storage, or rough handling, for example, – that can help you in future transactions with the supplier of the battery (or operator of the fleet).

4. A capacity discharge test and evaluation can provide you with the information you need to make any warranty claims for which you may qualify.

5. Even if you do not undertake major repairs, a battery that has been professionally tested will come back to you "tuned up" and in improved condition because, among other things, it has had extended "equalize" charging and electrolyte (acid) strength adjustment.

6. A battery evaluator will also discuss proper battery charging with you, and make certain that you are using a charger with the correct rating. Battery test results may point back to battery charger problems that can subsequently be remedied.

What happens during a battery test?

As a battery owner or lift truck service manager, you have just made the decision to send a battery out for an evaluation. You may need to know the value (or performance capabilities) of a battery recently purchased for resale, or you may have had reports of unusual battery behavior. Poor battery voltage may have been the source of high-current damage to your rental units (burned motors and controls). Perhaps the trucker’s job assignment has changed, and now you need to know if it can "go the distance". These are all important reasons for sending a battery out for a test.

Once in the battery shop, your battery is checked for electrolyte level and any dead or shorted cells. All connections are checked, and the battery’s ability to accept a normal charge rate is verified.

If a normal charging behavior is indicated, the battery is given a full equalizing charge (11-12 hours) to bring all cells up to full charge. If the battery does not take full charging current, sulfation of the cell plates (caused by long uncharged storage or other factors) is investigated as a possible cause. In order to reverse sulfation, a special charging protocol can be used, which may take days of constant current (at higher than normal voltage) before the battery is ready for testing.

After the battery has received a full equalizing charge, the specific gravity (an indicator of acid concentration) of each cell’s electrolyte is measured. Measured amounts of concentrated acid are added (in a carefully controlled manner) to cells that have lost acid due to overflow or leakage. (Under normal conditions, acid additions are not needed by a battery because only water is lost in the evaporation process).

It is only after the above steps have been taken that a meaningful capacity test can take place. After one final measurement – the reading and recording of each cell’s finish rate voltage – the battery is plugged into a constant–current load bank to begin the six-hour capacity test. The load used depends on the battery capacity: the load bank must maintain a current equal to 1/6 the battery’s ampere-hour rating for the duration of the test. Individual cell voltage readings are recorded at hourly intervals with more frequent readings toward the end of the test.

An in–depth evaluation and discussion of the results wraps up the procedure. Expect to end up with a battery capacity figure (expressed as a percentage of a new battery’s capacity), and solid advice about the advisability of investing money in any indicated repairs. Remember; no short–term test that involves less than several monitored hours of high–current load can give the battery owner as reliable a source of information as the above procedure. When one considers tat a battery test helps determine the future of an expensive piece of equipment, it is clear that a good test is good business.

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