DISCHARGING INTERVALS FOR DEEP CYCLE BATTERIES
The correct discharging interval, or "cycle," for a set of deep cycle batteries is a topic with much misunderstanding. Deep cycle batteries used in golf cars will last anywhere from two to six years depending upon many factors. One of the most important being how far down they are discharged before being recharged, also known as cycled. The other main factors are temperature, load and charging rate. For efficiency sake we will assume that all the aforementioned variables are constant. So let's focus on the charging cycle.
Battery capacity is the usable electrical energy stored in a battery. The manufacturer's rated capacity is given for 100% discharge of the battery. However, the actual usable capacity is only 80% of the manufactures rated capacity. That capacity as stated in ampere-hours, would mean a battery rated at 1200 ampere-hours would actually have a 960 ampere-hour usable capacity. That is if battery life is of any concern. Another limitation as to how deeply a battery can be discharged is today's automatic chargers. They require a 70% nominal voltage rating to start the charging cycle. For both battery life and practical reasons 80% is considered the maximum that a battery pack should be discharged or cycled.
Now that we have our maximum lets find a minimum. Battery life is related to how deep the battery is cycled each time. If a battery is discharged to 50% between charges, it will last about twice as long as if cycled to 80% of capacity. If cycled only 10%, it will last about 5 times as long as one cycled to 50% capacity. The limits on how far the batteries are cycled will have a lot to do with application. A round of golf in most golf cars will use somewhere between 25% and 40% of capacity. It would be impractical to change the batteries after nine holes to prolong the life of the battery by not discharging below a certain point. Also a battery that is continually cycled to 5% of capacity or less will have a shorter life than one cycled to 10% of capacity. That's because with a very short cycle, the lead dioxide tends to build up in clumps on the positive plates rather than in an even film. A minimum is dictated more by practicality than anything else. It would be an ideal number at any rate. We will use 25% as our minimum.
We have a minimum of 25% capacity and a maximum of 80% capacity. Those are the upper and lower limits that we would like to discharge batteries before we charge them. That would make our average discharge 50% of capacity. This is the optimum or target amount that you want to discharge before charging. This is not to say that you can't run them down to 80% capacity when needed. Or charge them at only 25% capacity because you know you will need a full charge the next day. If you follow this formula you will not overcharge, short cycle, or cause low voltage damage to your batteries.
In order to be able to even get anywhere even close to the 50% capacity goal on a regular basis, you will need to have a state-of-charge meter on the golf car. I consider it the most important tool for maintaining your batteries. This advice is from many sources and may not be the same given in the OEM service manual, or seen in other reliable sources. Check with your battery supplier if you are looking for more information on the correlation between battery lifespan and cycles.
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Reprinted from Golf Car News Magazine, 2002 May /June