Originally Posted by kmeindin
I was told by a local battery Distributer that it best to charge the batteries and then just leave them as long as there is no draw on them. Then just before you go camping charge them. He said by leaving the charger on them all the time you are causing them to wear out quicker. He said they are only good for so many cycles. By discharging a small amount and then charging them to full capacity that counts as one cycle.
Kmeindin, as a retired EE I too must disagree with the advice you were given. I found the following at batteriesinaflash.com and it agrees 100% with my 20+ years of experience in designing battery operated equipment which is field mounted and charged by a solar panel. I LOL in agreement when the author stated lead acid batteries are fickle things. I emphasized some statements in italics.
What are the differences between a starting and deep cycle battery?
Generally speaking, there are two different types of lead acid batteries, Starting and Deep Cycle. If a starting battery is routinely deep cycled (discharged below 20%-50% of max capacity), it will generally fail after 30-150 cycles. The same starting battery will last for thousands of cycles if it is just used normally (2% - 5% discharge). Where a Flooded battery will lose about 13% of its charge in a month, a Gel or AGM will lose 1% -3%.
Starting batteries are generally designed to start some form of an internal combustion engine (car, truck, boat, etc). In a starting battery, you will find more lead plates, thinner and often made of a lead "sponge" similar looking to a foam sponge. This sort of arrangement means that the plates have much more surface area in the solution than a Deep Cycle battery and allow them to draw larger currents much quicker than a Deep Cycle battery.
Deep cycle battery, on the other hand, have much thicker plates and, they are solid, not sponge. These thicker plates have less surface area and thus less of the instant power that a starting battery needs. They are designed to be discharged down to 20% of their maximum charge repetitively. The thicker lead plates allow for this as they are much sturdier than their sponge counterparts.
Cold Cranking Amps (CCA) is an indicator of the amount of current a battery can deliver for 30 seconds at zero degrees Fahrenheit without dropping below a specified cutoff voltage (normally 10.5 volts). The cranking amps a battery can produce changes with temperature. The warmer it is the more Cranking Amps a battery will produce.
You can use a Deep Cycle battery as a starting battery provided that you take into account the lower CCA of a Deep Cycle battery. As a rule of thumb, it's a good idea to upsize the battery by about 20% to deliver the same amount of cranking amps from a deep cycle battery. Also, the self-discharge rate of Sealed batteries is a lot less than flooded lead acid types.
How long will my battery last?
How long a battery will last depends hugely upon the way it is used and how well the battery is maintained. Both overcharging and undercharging will have serious adverse effects on the lifespan of a deep cycle battery. In particular, you can seriously shorten the lifespan of a battery if it is used in a deep cycle application that it was not designed for. An example of this would if you were to use an automotive starting battery as a deep cycle battery.
General expectations for batteries if deep cycled (these are just approximate guidelines):
Starting battery (Automotive battery etc) : 3-13 months
Marine Battery : 1-6 years
AGM Deep cycle: 4-7 years
Gel Cell Deep Cycle: 2-5 years
Flooded Lead Acid Deep Cycle Battery (L16-RE etc): 4-8 years
The main things that you can do to ensure you get the maximum value out of your deep cycle batteries are to keep them maintained. This means keeping them watered to the appropriate level, trying to prevent them from discharging them more than 50% of their total capacity and having appropriate charging systems in place. The charging is of special importance because both over and undercharging will severely limit the life of your batteries, also if your batteries will see an extended period without being used you should ensure they are routinely checked, cleaned and fully charged before being stored. Also in some cases, it can be a good idea to put your batteries on a maintenance charge over long periods of disuse.
Battery Cycles vs. Battery Lifetime?
Often the lifespan you can expect to get out of your battery is referred to in terms of "cycles". A battery cycle is one complete discharge and recharge cycle. The discharge state of a battery is often measured in Depth of Discharge (DOD). This refers to how far down the battery has been taken, for instance, a battery that has 25% of its capacity remaining would be said to be at 75% DOD. The lifetime of a battery is directly related to the depth of the discharge that it regularly experiences. Lead acid batteries are fickle things. If you subject a deep cycle battery to 80% DOD on a regular basis you will get roughly half the life out of your battery than if you were to cycle it to 50% DOD. While this doesn't mean that you can't go down to 80% DOD you should generally try to design your battery banks to allow for cycling at around 50%. Conversely there is also an upper limit on the DOD of a battery, usually, a battery that is only regularly cycled down to 5% or less will not last as long as a battery cycled to 10% or more. This is because on smaller cycles the Lead Dioxide can clump up around the positive plates. On heavier discharges, this would be more of an even film.