How 3-stage battery charging works
A modern “smart” charger is a small computer that watches the battery and changes what it feeds it. The sequence is always the same — bulk, absorption, float — and understanding it explains why a cheap trickle charger cooks batteries while a microprocessor unit can stay connected indefinitely.
The three stages, in order
- Bulk — constant current. The charger pushes everything it can, up to its rated output, while the battery voltage climbs. This delivers roughly the first 80% of the charge. A properly sized charger will not push a flooded battery past 125 °F, or an AGM/gel (valve-regulated) battery past 100 °F.
- Absorption — constant voltage. The charger holds the absorption set point and lets current taper off as the battery fills. This is the last ~20% and it is where most chargers cut corners. If current never drops, or the battery will not hold voltage, you are usually looking at sulphation.
- Float — maintenance. Voltage drops to a level that simply offsets self-discharge. Current falls below about 1% of capacity, which is why a float stage is safe to leave connected for weeks.
Some vendors call the absorption stage “equalization.” It is not — see equalization below for what that term should mean.
Target voltages by chemistry
These are the set points we look for on a 12 volt system. Get the chemistry wrong and you either never reach a full charge or you damage the battery — gel is the one that punishes an over-voltage.
| Chemistry | Absorption | Float | Notes |
|---|---|---|---|
| Flooded (wet) | 14.4 – 14.8 V | 13.2 – 13.8 V | Tolerates equalization; check water |
| AGM | 14.4 – 14.7 V | 13.2 – 13.6 V | Sealed; no watering, no stratification |
| Gel | 14.1 – 14.4 V | 13.2 – 13.6 V | Lower ceiling — over-voltage bubbles the gel permanently |
| Lithium (LiFePO₄) | 14.2 – 14.6 V | 13.4 – 13.6 V | Needs a lithium-specific profile |
Double these for 24 volt, triple for 36 volt, quadruple for 48 volt. Always follow the battery manufacturer's published numbers over any table, including this one.
Sizing the charger
Most manufacturers suggest a charger rated near 25% of the battery's amp-hour capacity. A 100 Ah battery therefore wants roughly a 25 amp charger; a 200 Ah pack wants about 50 amps. Going bigger shortens recharge time but costs battery life. Going smaller is fine for maintenance — a 1–2 amp maintainer is exactly what you want for a battery that sits between uses.
Recharge time can be estimated with a little arithmetic:
Example: 10 Ah to replace on a 5 amp charger → 10 ÷ (0.9 × 5) ≈ 2.2 hours.
Heavily discharged batteries are slower than the formula predicts — the last stretch of absorption takes disproportionately long. See sizing a battery charger for the full walkthrough with volt/amp matching.
Equalization is not a stage
Equalization is a deliberate, controlled over-charge — around 15–16 volts on a 12 volt system, held for a limited period, well into gassing. It stirs stratified electrolyte in tall flooded batteries and knocks loose sulphation off the plates.
It is not appropriate for AGM or gel. Their construction largely eliminates stratification and most manufacturers explicitly advise against it. If you equalize flooded cells, follow the battery maker's voltage and time limits — both are critical.
Batteries in series and parallel
Parallel batteries (positive to positive) look like one big battery to the charger: three 12 V 100 Ah batteries become a single 12 V 300 Ah bank. One charger with the sum of the recommended amps will do it.
Series strings (positive to negative) raise voltage: the same three batteries become 36 V at 100 Ah, charged across the pack. Either approach works — unless one battery in the string is tapped at a lower voltage for a radio, lights or a logger. That imbalances the pack, and a single system-voltage charger cannot correct it. A multi-bank charger that connects to each battery individually fixes the imbalance on every cycle.
Testing state of charge
Remove the surface charge first — let a freshly charged battery rest about 12 hours, or draw it down with a 20 amp load for a few minutes. Then measure voltage, or specific gravity if the cells are accessible.
| State of charge | 12 V | 6 V | Specific gravity |
|---|---|---|---|
| 100% | 12.7 | 6.3 | 1.265 |
| 75% | 12.4 | 6.2 | 1.225 |
| 50% | 12.2 | 6.1 | 1.190 |
| 25% | 12.0 | 6.0 | 1.155 |
| Discharged | 11.9 | 6.0 | 1.120 |
Sulphation begins below roughly 12.4 V (12 V battery) or 1.225 specific gravity. Left long enough it hardens and permanently costs capacity — the single biggest reason lead-acid batteries die before their chemical life is up. Proper charging and a desulfation strategy prevent it.
Parts mentioned in this guide
Common questions
Can I leave a smart charger connected all the time?
Yes, if it has a true float stage. In float the charger only replaces self-discharge, so it is safe for weeks or months. A trickle charger without float control will overcharge and dry out a flooded battery.
Why does my charger never reach the float stage?
Usually sulphation, a bad cell, or a charger sized too small for the pack. If current does not taper during absorption, the battery is no longer accepting charge normally.
What is the difference between absorption and equalization?
Absorption is the normal constant-voltage stage at 14.1–14.8 V on a 12 volt battery. Equalization is a deliberate over-charge to 15–16 V for a limited time, only on flooded batteries.