Flooded (FLA) or Sealed (SLA) Lead Acid Batteries?

During the mid 1970s, researchers developed a maintenance-free lead-acid battery that can operate in any position. The liquid electrolyte is gelled into moistened separators and the enclosure is sealed. Safety valves allow venting during charge, discharge and atmospheric pressure changes.

Driven by different market needs, two lead-acid systems emerged: The small sealed lead-acid (SLA), also known under the brand name of Gelcell, and the larger Valve-regulated-lead-acid (VRLA). Both batteries are similar. Engineers may argue that the word 'sealed lead-acid' is a misnomer because no rechargeable battery can be totally sealed. 

Unlike the flooded lead-acid battery, both SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge because excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential. To reduce dry-out, sealed lead-acid batteries use lead-calcium instead of the lead-antimony.

The optimum operating temperature for the lead-acid battery is 25*C (77*F). Elevated temperature reduces longevity. As a guideline, every 8°C (15°F) rise in temperature cuts the battery life in half. A VRLA, which would last for 10 years at 25°C (77°F), would only be good for 5 years if operated at 33°C (92°F). The same battery would desist after 2½ years if kept at a constant desert temperature of 41°C (106°F).

Figure 1: Sealed lead-acid battery

The sealed lead-acid battery is rated at a 5-hour (0.2) and 20-hour (0.05C) discharge. Longer discharge times produce higher capacity readings because of lower losses. The lead-acid performs well on high load currents.

Absorbed Glass Mat Batteries (AGM)

The AGM is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. Nearly all AGM batteries are recombinant, meaning they can recombine 99% of the oxygen and hydrogen. There is almost no water is loss.

The charging voltages are the same as for other lead-acid batteries. Even under severe overcharge conditions, hydrogen emission is below the 4% specified for aircraft and enclosed spaces. The low self-discharge of 1-3% per month allows long storage before recharging. The AGM costs twice that of the flooded version of the same capacity. Because of durability, German high performance cars use AGM batteries in favor of the flooded type.

Advantages

  • Inexpensive and simple to manufacture. 
  • Mature, reliable and well-understood technology - when used correctly, lead-acid is durable and provides dependable service.
  • The self-discharge is among the lowest of rechargeable battery systems.
  • Capable of high discharge rates.

Limitations

  • Low energy density - poor weight-to-energy ratio limits use to stationary and wheeled applications.
  • Cannot be stored in a discharged condition - the cell voltage should never drop below 2.10V.
  • Allows only a limited number of full discharge cycles - well suited for standby applications that require only occasional deep discharges.
  • lead content and electrolyte make the battery environmentally unfriendly. 
  • Transportation restrictions on flooded lead acid - there are environmental concerns regarding spillage.
  • Thermal runaway can occur if improperly charged.