AGM, gel and flooded: charging profiles that fit
All three chemistries are lead-acid, and all three are damaged by the wrong voltage. The differences are in how the electrolyte is held and how much over-voltage each can survive. That single fact decides which charger and which profile you need.
The three chemistries
| Flooded (wet) | AGM | Gel | |
|---|---|---|---|
| Electrolyte | Liquid, vents gas | Absorbed in glass mat | Suspended in silica gel |
| Maintenance | Water top-up | None | None |
| Absorption (12 V) | 14.4 – 14.8 V | 14.4 – 14.7 V | 14.1 – 14.4 V |
| Equalization | Yes, sometimes | No | No |
| Best for | Cycling, cost per Ah | Vibration, marine, cycling | Deep cycle, standby |
The gel ceiling is the one people get wrong. Push a gel battery to 14.8 V and you can bubble the electrolyte permanently — the damage does not recover.
What sulfation is, and when it starts
During discharge, lead sulfate forms on the plates. Recharge normally returns it to the electrolyte. Leave a battery partly discharged and the sulfate hardens into a crystalline layer that will not come off during a normal charge — capacity drops, internal resistance rises, and eventually the battery will not accept a charge at all.
It begins below roughly 12.4 V (12 V battery) or 1.225 specific gravity. A significant share of lead-acid batteries die from this rather than from wear.
Preventing it beats fixing it
- Recharge promptly after discharge — the same day is ideal.
- Keep it charged in storage. A float charger or a maintainer costs less than a battery.
- Do not leave it at 50% for weeks. Self-discharge does the rest of the work for you.
Sealed lead-acid self-discharges far more slowly than flooded — roughly 2–10% per month versus 8–40% — which is why AGM and gel survive unattended equipment better.
Pulse desulfation
Pulse desulfators apply a controlled waveform that breaks down hardened sulfate and returns it to the electrolyte as active material. It works on genuinely sulfated batteries, not on worn-out ones, and it is not a substitute for charging.
Two families: passive units that connect to the pack and pulse using its own power (milliamp draw, negligible), and active units that charge and desulfate together. Passive units are the right choice for stored equipment where AC is unavailable; active units suit shop and maintenance use.
Depth of discharge and cycle life
Shallower cycles last longer. A battery cycled to 50% will deliver roughly twice the total cycles of one cycled to 80%, and at 25% depth of discharge the count climbs again. If you are choosing a bank size, this is the lever that matters more than brand.
Temperature cuts both ways: capacity rises with warmth, but life falls sharply above about 25 °C. A battery kept cool and shallow-cycled can outlast a "better" one kept hot and deep-cycled.
Parts mentioned in this guide
Common questions
Can I use a flooded charger on an AGM battery?
Sometimes — AGM tolerates similar absorption voltages — but never on gel. The safest answer is a charger with a selectable AGM/gel profile.
Does desulfation restore a dead battery?
It restores capacity lost to sulfation. It cannot repair a shorted cell, a physically damaged plate, or a battery that has simply reached end of life.
AGM or gel for a boat?
AGM for most marine use: better charge acceptance and vibration tolerance. Gel suits deep-cycle standby where charging is gentle and infrequent.