Battery Direct logo

New Zealand's premium Battery Specialists

0800 12volt   0800 128 658


Sealed Lead Acid and Silicon Batteries


Sealed Lead Acid batteries come in a variety of technologies. Each technology has its attributes, advantages and disadvantages in any given application - however, they all remain 'Lead Acid' batteries even the lead carbon battery except of course the Silicon battery.

They are known as;

  • SLA (Sealed Lead Acid) 
  • VRLA (Valve Regulated Lead Acid) 
  • AGM (Absorbed Glass Mat) 
  • Gel (Gelled electrolyte) 
  • Hybrid (Gel/AGM Hybrid)


Where a regular flooded battery has the electrolyte in a 'wet' fluid state, also referred to as 'free electrolyte', the essence of a sealed battery is to immobilise the electrolyte. However, it is necessary that the gasses generated during charging are recombined in a so called 'oxygen cycle' returning to the active plates as H2O once again. Should the gasses escape, a gradual drying out would occur. To reduce the formation of the water's constituents, oxygen and hydrogen, the sealed case is held in a positive pressure of 2 or 3lbs above atmospheric pressure.
In the case of rapid generation of oxygen gas exceeding the oxygen recombination absorbing capacity, the pressure relief valve will open to release the excessive gasses.


SLA (Sealed Lead Acid) batteries otherwise known as VRLA (Valve Regulated Lead Acid) were originally known as "Dry Batteries". They were introduced in the 1950's and at that time, the only sealed battery untilised a Gel electrolyte. The otherwise free acid was immobilised with a fine silica powder and formed a gel substance.

The Gel battery is primarily a deep cycle battery and not a good start battery due to the slower defusion rate of the gelled electrolyte.

In the 1970's technology moved to AGM (Absorbed Glass Mat) where the separators between the plates are made of a micro-fine glass fiber matting absorbing and immobilising the electrolyte. The quantity in the seperator is 95% saturation, starvation saturation, hence the term 'Dry Cell battery'.

 The AGM, still using a liquid elecytolyte, by plate designed is available in batteries suited to Starting, Deep Cycle or standby. Where the start and deep cycle are perhaps obvious, the 'stand by' battery is used for alarms, emergency signage, UPS systems.  They are specifically designed to sit on a float voltage in preparation for that evential power cut and replace the lost mains power.

Battery failure

We find multiple methods of killing our batteries but there are a few which are dominant.

Suphation: A natural chemical process of a discharging battery is to form Lead Sulphate (PbSo4) on both positive and negative plates but mainly the negative plate. During the recharge process, the PbSo4 is disolved back into the electrolyte. However, if the battery sits in a partial or complete discharged state, the PbSo4 crystalises and will not disolve. The plates are now insulated from further chemical processes and the battery is non responsive - Dead.  The fix, keep the battery fully charged at all times.

Dry out. Wet batteries require electrolitic maintenance where plates must be covered with the electrolyte at all times. However, AGM and Gel batteries can also suffer from dry out. Batteries experiencing 30 deg plus can gas earlier from charging and heavy discharging. Exhausting the water content through the pressure relief valves will dry the battery out. Of course, this cannot be rectified. The hot enviroments are experienced in engine rooms of launches or vessels sitting in the tropics for some months. A typical life expectancy in these conditions would be halved. The fix, Use the most appropriate type for the enviroment

 New battery types

New batteries are introduced from time to time. Some go and some stay. One that went was the Gel / AGM hybrid. A good concept on paper but repetitive failures within the one or two years of duty saw them exit.

To avoid the sulphation issue experienced in batteries sitting in a partial state of charge, battery developers have used one of two methods to avoid the problem. One school of thought was to replace the negative Lead plate with a plate containing carbon. Lead carbon batteries have been around for some years and the most experienced would arguably be Sonnenschien of Germany (Exide) and now also featured in the Endurant range.

The other school of thought was to partially remove the sulphuric acid and replace it with a Silicon based electrolyte. Both of these methods have reduced or eliminated the production of Lead sulphate.

Unfortunately, to maintain the Lead Crystal battery's integrity, it is to be chargerd at 30% of the battery capacity every month or so. It is impractical to find this current from a solar array and a sizable battery charger is required as part of the system. If the battery is not charged correctly, the extended life expectancy will be reduced to that of a stardard AGM battery. The advatage of the high priced Silicon, Lead Crystal, battery would be lost.

 Consquently, Lead carbon technology has become the preferred option. 

View Sealed Lead Acid Battery Models and Pricing


The stackable PBG26 26AH battery being put through real time discharge simulation in Battery Technology's BatLab



Battery Direct NZ Ltd  Terms of Trade
Telephone:   0800 12volt   (0800 128 658)   |   FAX:   09 424 8179
Physical address:   Gulf Harbour Marina, Hibiscus Coast
Postal address:   BatteryDirect, PO Box 907, Whangaparaoa 0943
E-mail:   General Information:   |   Sales:   |   Customer Support:
Send mail to with questions or comments about this web site.

Disclaimer and condition of purchase: BatteryDirect or Gulf Harbour Electronics Ltd or employees or advisors cannot be held responsible for any damage to property or person through mis-applied or mis-fitment of any electrical appliance or battery or accessory supplied by us or any other associate supplier whatsoever.

Copyright © 2006 - Battery Direct NZ Ltd | website by pixelmagic v2