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MM FAQ

  1. What are lead acid batteries?
  2. What is an AGM Battery?
  3. What is a GEL Battery?
  4. How do I select a lead-acid battery?
  5. What are amp hours?
  6. How is required battery capacity calculated?
  7. Do physical dimensions matter?
  8. What are the terminal types?
  9. Do lead acid batteries discharge when not in use?
  10. Do lead acid batteries develop a memory?
  11. Do I need to completely discharge my lead acid battery before recharging it?
  12. What is an equalizing charge?
  13. When do I need to perform an equalization charge?
  14. Can my batteries freeze?
  15. What are the most common mistakes made by owners of lead acid batteries?




  1. What are lead acid batteries?
  2. Lead acid batteries are the oldest and still the most common type of rechargeable battery on the market. Although not used for applications like consumer electronics where small batteries are required, their high discharge capacity makes them popular for larger-scale applications, such as vehicle starting and emergency backup power.

    Lead acid batteries fall into two categories: vented and sealed. Vented batteries allow gas to escape without any positive pressure buildup in the battery. This ability makes them less susceptible to problems caused by high ambient temperatures and overcharging. Sealed batteries are used in applications where vented batteries are impractical, either due to a lack of ventilation or the mounting position of the battery. They are also more compact, allowing them to be used in some applications where vented batteries may not be practical. While vented batteries must be mounted upright to prevent the acid in them from escaping, sealed lead-acid batteries can be mounted in any orientation.

    Sealed lead-acid batteries are used in applications where a high discharge capacity may be needed, such as uninterruptible power supplies (UPS), emergency lighting, and backup generator starting. While the energy density of sealed lead-acid batteries is much lower than that of other rechargeables, their ability to release a large amount of energy quickly makes them ideal for these applications.

  3. What is an AGM Battery
  4. The term AGM is short for Absorbed Glass Mat, which is a specific manufacturing process utilized in the production of sealed lead acid batteries (SLAs). In the AGM manufacturing process, extremely thin glass fibers are woven together to form a glass mat separator that is placed between each lead plate. The glass mat plates allow for the even distribution of electrolyte over the entire surface area of its adjacent lead plate(s) due to its ability to hold acid. This maximization of electrolyte to plate coverage enables these batteries to far exceed the performance of conventional flooded lead acid batteries.

    Every battery that is produced using AGM technology will fall into the VRLA category, or valve regulated lead acid. Through the use of AGM and GEL manufacturing processes, manufacturers are able to make a sealed maintenance free battery. These batteries have several advantages when compared to the typical flooded lead acid battery. The first of which is the maintenance free nature of the battery - you will not have the need (or the ability) to add any electrolyte to the battery. Due to the VRLA design, the battery is recombinant and will vent less gas than flooded batteries. These batteries can also be mounted in any position and function without leaking any electrolyte.

  5. What is a GEL Battery?
  6. Two of the most common terms that we hear from customers are AGM and GEL, which are often mistaken for the other more than half the time. We have already explained what exactly an AGM battery is, now we will be taking a look into what sets a true GEL battery apart from the rest of the sealed lead acid pack.

    Okay, first and foremost, the term GEL is short for gelified electrolyte lead acid. In other words, a true GEL battery will actually contain a gelatin in place of the typical free flowing electrolyte (battery acid). A special gelling agent is added to the electrolyte to reduce the movement of acid inside of the battery – the process literally turns the acid mixture into a gooey Jell-O like substance. Due to this mixture of Jell-O like acid, the GEL battery is designed with special one way safety valves in place of open vents typically found on wet-cell batteries (non-sealed), this method of venting allows the internal gasses to recombine back into water in the battery, greatly reducing gassing.

    The GEL battery is a sealed non-spillable type. If the casing of the battery were to develop cracks or otherwise be compromised, there will be no free flowing acid leaking out the of the battery. The GEL battery is also a maintenance free type. The term battery maintenance typically refers to the need to check on the electrolyte level of the battery about every thirty to sixty days depending upon usage. The valve system used in both AGM and GEL applications minimizes the amount of electrolyte released from the battery during the charging period.

    The GEL style battery is used in wide variety of deep-cycle applications due to their ability to recover from deep discharge cycles. Where the GEL battery will excel is in situations that require a slow and steady discharge. A prime example of a good application for a GEL battery would be an electric mobility chair.

  7. How do I select a lead-acid battery?
  8. When selecting lead-acid batteries, it is important to match the battery to the application. More than anything, this means selecting a battery that has enough capacity for the application. If the battery is a replacement for an existing battery, then it needs to have at least as high a capacity, as measured in amp hours (Ah), as the original.

  9. What are amp hours?
  10. Amp hours is the normal measurement for any battery’s capacity, whether rechargeable or not. The term refers to how many amps of current the battery can provide in an hour’s time. Please note that this is a purely theoretical number, as a fast discharge will reduce the available capacity by as much as 40 percent. Another way of looking at this rating is to see it as how many hours the battery could power a device if one amp of power was required from it.

  11. How is required battery capacity calculated?
  12. While it would seem that calculating the necessary battery capacity would merely consist of multiplying the expected load current by the number of hours of power needed, this is not so. There is some tolerance built into the battery’s rating. In addition, there are a number of factors which will reduce the battery’s efficiency and ability to transfer current for use. All of these need to be calculated in when determining the needed battery capacity.

    To properly calculate the battery capacity needed, follow these steps:

    1. Multiply the average load by the number of backup hours of operation needed.
    2. Add 15 percent to cover the loss of capacity from tolerances and un-cycled batteries.
    3. For every 10 degrees Celsius of ambient temperature where the battery will be used, add another 10 percent (use the worst-case temperature expected).
    4. For expected discharge times that are less than 20 hours, add 10 percent for every time that the backup time needs to be doubled to reach 20 hours. In other words, if the expected discharge time is 5 hours, 5 needs to be doubled twice to reach 20. Thus, 20 percent should be added for the reduction in battery inefficiency.
    5. Add a final 40 percent to account for an efficient battery life cycle. Any battery that has had its capacity reduced to 60 percent is considered worn out.

    Taking these steps, a battery backup for a device that needs to run 12 hours at 200 mA (0.20 amps) in a 0-degree climate would need to be calculated as 12 x 0.20 = 2.40Ah. Plus 15 percent for tolerances is another 0.36Ah, which equals 2.76Ah. Plus 20 percent for temperature is 0.55Ah, which equals 3.31Ah. Plus 10 percent for fast discharging is another 0.33Ah, which brings the total 3.64Ah. Add 40 percent for an efficient life cycle to get 1.46Ah. This brings the total needed capacity to 5.10Ah.

    If a 5.1Ah battery is not available, the next larger size should be selected. Using a larger battery than what is needed will increase the battery’s life expectancy.

  13. Do physical dimensions matter?
  14. Regardless of the manufacturer, sealed lead-acid batteries for a particular voltage and capacity are generally the same size, plus or minus a small amount. Therefore, they are readily interchangeable. However, differences in the physical configurations of batteries can affect their ability to be used as direct replacements. Sealed lead-acid batteries can be manufactured for vertical or horizontal mounting and can have the terminals either on the top or on one end. While top mounting of terminals is the most common, one should never assume that the battery follows any particular configuration.

    When space is at a premium, such as when replacing batteries in a UPS or emergency light, the actual dimensions and locations of the battery terminals should be verified. Some common types of lead-acid batteries are identified by specific numbers. In these cases, the numbers will be the same industry-wide, regardless of the manufacturer.

  15. What are the terminal types?
  16. The most common terminal configuration for lead-acid batteries is two 0.25-inch-wide tabs mounted on the top of the battery. These are designed so that slip-on crimp terminals can be used with the battery, for ease of replacement. However, some may have two-pin Molex connectors or threaded studs. It is important to match the terminal type and position to the existing battery. If the terminals are located in different positions, it is possible that the battery lead wires will not be long enough to reach the terminals.

  17. Do lead acid batteries discharge when not in use?
  18. All batteries, regardless of their chemistry, will self-discharge. The rate of self-discharge for lead acid batteries depends on the storage or operating temperature. At a temperature of 80 degrees F. a lead acid battery will self-discharge at a rate of approximately 4% a week. A battery with a 125-amp hour rating would self-discharge at a rate of approximately five amps per week. Keeping this in mind if a 125 AH battery is stored for four months (16 weeks) winter without being charged, it will loose 80 amps of its 125-amp capacity. It will also have severe sulfation, which causes additional loss of capacity. Keep your batteries charged while not in use!

  19. Do lead acid batteries develop a memory?
  20. Lead acid batteries do not develop any type of memory.

  21. Do I need to completely discharge my lead acid battery before recharging it?
  22. No, in fact you should never discharge your lead acid battery below 80% of its rated capacity. Discharging it below this point or 10.5 volts can damage it.

  23. What is an equalizing charge?
  24. An equalizing charge for a 12 volt battery requires that it be charged with a voltage of at least 14.4 volts for a period of at least one hour once a month, or every 10 discharge cycles. An equalizing charge prevents battery stratification and reduces sulfation, the leading cause of battery failure.

  25. When do I need to perform an equalization charge?
  26. Equalizing should be performed when a battery is first purchased (called a freshening charge) and on a regular basis (every 10 discharge cycles or at least once a month). Reduced performance can also be an indicator that an equalizing charge is needed.

  27. Can my batteries freeze?
  28. If your battery is partially discharged, the electrolyte in a lead acid battery may freeze. At a 40% state of charge, electrolyte will freeze if the temperature drops to approximately -16 degrees F. When a battery is fully charged the electrolyte will not freeze until the temperature drops to approximately -92 degrees F.

  29. What are the most common mistakes made by owners of lead acid batteries?
  30. Undercharging - Generally caused by not allowing the charger to restore the battery to full charge after use. Continuously operating a battery in a partial state of charge, or storing the battery in the discharged state results in the formation of lead sulfate (sulfation) on the plates. Sulfation reduces the performance of the battery and may cause premature battery failure.

    Overcharging - Continuous-charging causes accelerated corrosion of the positive plates, excessive water consumption and in some cases, damaging temperatures within the battery. Lead acid batteries should be charged after each discharge of more the 50% of its rated capacity and during or after prolonged storage of 30 days or more.