SLA Batteries Demystified E-mail
Written by Allen Sansano   
Monday, 29 October 2012 17:47

Genesis7AH The basic design of a lead acid battery has been around since the mid-1800's. Today, they account for over half of all battery sales. In the 1970's, a form of lead acid battery using a gel as an electrolyte was developed. This allowed the Sealed Lead Acid (SLA) battery to be used in different positions without leakage. The low cost of SLA batteries combined with the ease of maintenance have made them a popular source of power on fishing kayaks, used to run everything from fishfinders to bait tank pumps. Knowing some basic information about SLA batteries will help you get the most out of them. How a battery is charged, used, selected and stored will affect the lifespan of the battery.

Charging

Your typical 12V SLA battery is composed of 6 cells. These cells need a minimum voltage to charge. However, too much voltage and the water in the electrolyte breaks down into hydrogen and oxygen. This process is known as gassing. Gassing has many issues associated with it. Since you're unable to replace the water in a SLA battery, your battery will lose its ability to hold a change. The lifespan of the battery will effectively be reduced. Even more dangerous is that the discharge of hydrogen and oxygen can cause an explosion. Note that a "sealed" lead acid batter is not really sealed, rather it is valve regulated.

The minimum charging voltage and the gassing voltage vary with temperature. A typical charging voltage is between 12.9V and 14.1 V, while the gassing voltage at 25 degrees C (77 degrees F) is 14.34V.

There are many types of chargers on the market. For simplicity we bin them into unregulated charges and regulated charges. The absolute cheapest charges on the market are unregulated charges. Unfortunately, they also have the ability to do the most damage. An unregulated charger is a basic transformer designed to deliver the typical charging voltage, approximately 13-14V when in full on charge mode. However, as the battery charges, the current consumption of the battery will decrease. When this happens, the unregulated charger's voltage increases in excess of the gassing voltage. You must disconnect this charger when the battery is fully charged to prevent damage due to gassing.

Regulated chargers are slightly more expensive than unregulated chargers. Their design is such an appropriate voltage is applied during the charging stage. The real benefit occurs when the battery is charged. At this point, the voltage is reduced to 12.8-13.2V . This is referred to as a float charge or a trickle charge. It maintains the battery from discharging without overcharging it and causing gassing issues.

Use

How deeply you discharge a battery will also affect the battery's lifespan. A battery that is routinely discharged 80% will last half as long as a battery routinely discharged to 50%. This means that you don't want to be running your battery dead every time you use it. Rather, select a somewhat larger battery size. This will also allow you some leeway in case you run a longer day on the water, or forget to charge your battery before a second day of use

Selection

SLA batteries are typically rated proportional to how long a battery can supply a specific current for 20 hours. So what does a battery SLA rating of say 7AH mean? A 7AH battery can supply a 0.350 ampere (or 350 milliampere) current for 20 hours when the battery is in the new condition. In this case 0.350*20=7. However, this does not scale proportionally for higher currents (or lower currents) and it also reduces over time. Selecting a battery for 20 hours of use will allow for reduction in capacity over time and also satisfy the "select a somwhat larger battery size" stated above.

Storage

Keep in mind that a battery will also discharge when not in use. The typical SLA battery will have a self-discharge rate of about 5% per month. If you store your battery in a depleted state after a day of use without charging it, it will continue to discharge. This can cause extra strain on the battery, affecting the lifespan. Storing at a low charge will also induce sulfation, a physical process affecting the battery that will reduce capacity. It is best to store your battery in a fully charged state in a cool (15 degrees C or 59 degrees F) dry location.

Myth

You've probably heard about not storing SLA batteries on a cement floor. That is a myth. It stems from the early days when battery cases where made of materials that would leak. The battery would then discharge through the conductive cement floor. This no longer applies to today's modern SLA batteries.

 

 

 

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