Energy Unlimited by Victron

2.3.6. The sealed (VRLA) gel battery

Here the electrolyte is immobilised as gel. Familiar as the Sonnenschein Dryfit A200, Sportline or Exide Prevailer battery.

2.3.7. The sealed (VRLA) AGM battery

AGM stands for Absorbed Glass Mat. In these batteries the electrolyte is absorbed (“sucked up”) into a glass-fibre mat between the plates by capillary action. In an AGM battery the charge carriers, hydrogen ions (H 2 ) and sulphate ions (SO 4 ), move more easily between the plates than in a gel battery. This makes an AGM battery more suitable for short-time delivery of very high currents than a gel battery. Examples of AGM batteries are the Concorde Lifeline and the Northstar battery.

2.3.8. The sealed (VRLA) spiral cell battery

Known as the Optima battery (Exide now has a similar product), this is a variant of the VRLA AGM battery. Each cell consists of 1 negative and 1 positive plate that are spiralled, thereby achieving higher mechanical rigidity and extremely low internal resistance. The spiral cell battery can deliver very high discharge currents, accepts very high recharge currents without overheating and is also, for a VRLA battery, very tolerant regarding charge voltage.

2.4. Function and use of the battery

In an autonomous energy system the battery acts as buffer between the current sources (DC generator, charger, solar panel, wind generator, alternator) and the consumers. In practice this means cyclic use, but in fact a quite special “irregular” variation of cyclic use. This contrasts with the forklift truck example where the duty cycle is very predictable. As boats are often also left unused for long periods of time, so are their batteries.

For instance on a sailing yacht the following situations can arise:

- The yacht is under sail or at anchor in a pleasant bay. Those aboard would not want any noise, so all electricity comes from the battery. The main engine or a diesel generator is used once or twice a day for a few hours to charge the house battery sufficiently to ride through the next generator-free period. This is cyclic use, where, significantly, the charging time is too brief to fully charge the battery.

- The yacht is travelling under power for several hours. The alternators on the main engine then have the time to charge the battery properly.

- The yacht is moored at the quayside. The battery chargers are connected to shore power supply and the battery is under float charge 24 hours a day. If the DC concept is used (section 8.2) several shallow discharges may occur every day.

- The yacht is out of service during wintertime. The batteries are either left disconnected for several months, left under float charge from a battery charger, or are kept charged by a solar panel or wind generator.

The number of cycles per year, the ambient temperature and many other factors influencing a battery’s service life will vary user by user. The following briefly discusses all of these factors.