What are the main differences between AGM and GEL batteries?

Both types are sealed lead-acid (VRLA) batteries that need no topping up. In AGM batteries the electrolyte is absorbed in glass-fibre separators, while in GEL batteries it is in gel form. This difference explains how they behave.

The low internal resistance of AGM batteries lets them deliver high current for a few minutes. That is why they are the usual choice for UPS units, where the battery sits on continuous float charge and must power a large load during short outages. GEL batteries tolerate repeated deep discharges and higher temperatures better, so they suit solar, off-grid and other daily-cycling applications.

For example, in an office UPS with ten minutes of backup, AGM is the logical choice, while in a holiday home that runs on solar panels and batteries every evening, GEL is usually the better option.

Note: GEL batteries usually need a different, slightly lower charging voltage. Do not replace AGM with GEL or vice versa unless the charger or UPS has a setting for that type, and never mix the two types in the same battery string.