The 3-Stage Charging Process
- Bulk Stage (0-80% SOC): The charger pushes maximum current into the battery at a constant rate. This is the fast part. A 10A charger charging 80Ah of depleted capacity takes about 8-9 hours in this stage. The battery voltage rises steadily throughout.
- Absorption Stage (80-95% SOC): The charger holds voltage constant while current gradually tapers. The battery is almost full and can no longer accept charge as fast. Current drops from the full 10A to perhaps 2-3A. This stage takes 1-3 hours for lead-acid and 30-60 minutes for LiFePO4.
- Float Stage (95-100% SOC): A low maintenance voltage is applied to keep the battery topped off without overcharging. This stage is mostly relevant for lead-acid batteries. LiFePO4 batteries should not be float-charged continuously — disconnect them at full charge.
Why the Last 20% Takes So Long
Think of it like filling a glass of water. The first 80% goes in fast because the glass accepts the stream easily. The final 20% requires you to slow the pour to avoid overflow — and if you keep pouring at full speed, you damage the glass (or in battery terms, you generate excess heat and gas).
Lead-acid batteries are especially slow in the absorption phase because their internal resistance increases sharply as they approach full charge. The charger must reduce current to avoid exceeding the absorption voltage (typically 14.4-14.8V for a 12V lead-acid battery).
LiFePO4 batteries are faster throughout the entire charging process. Their lower internal resistance means they accept higher currents for longer, and the absorption phase is much shorter. A 100Ah LiFePO4 with a 20A charger often reaches 100% in under 5 hours total. The same battery in lead-acid chemistry might take 8-12 hours.
Worked Examples
Charging a Depleted 100Ah LiFePO4 with a 20A Charger
Context
Your 100Ah LiFePO4 was discharged to 20% remaining (80% DoD). You plug in a 20A charger. LiFePO4 accepts bulk charge efficiently — about 95% of the current goes to charging.
Calculation
Ah to replace: 100 x 0.80 = 80 Ah
At 95% efficiency: 80 / 0.95 = 84.2 Ah input needed
Time: 84.2 / 20 = 4.2 hours
Interpretation
About 4 hours and 12 minutes from 20% to 100%. LiFePO4 charges faster than lead-acid because it accepts full current almost to the end — no long absorption phase.
Takeaway
If you are charging from solar instead of shore power, the charge current varies with sunlight. Size your panels with our solar panel size calculator to match your charging needs.
Why a 200Ah Lead-Acid Takes So Long with a 10A Charger
Context
Your 200Ah flooded lead-acid battery is at 50% DoD. A 10A charger is your only option. Lead-acid charging efficiency drops to 50-60% during the absorption phase.
Calculation
Ah to replace: 200 x 0.50 = 100 Ah
At 85% efficiency: 100 / 0.85 = 117.6 Ah input
Bulk phase (~80% of charge at full current): 94 Ah / 10A = 9.4 hrs
Absorption phase (remaining 20% at tapering current): ~4-6 hrs
Total: approximately 13-15 hours
Interpretation
The last 20% of charge takes almost as long as the first 80%. Lead-acid batteries taper the charge current during absorption to prevent gassing, which stretches the total time dramatically.
Takeaway
To understand how long this battery actually lasts under load once charged, try our lead-acid runtime calculator — it accounts for the Peukert effect that hits lead-acid at high discharge rates.
Frequently Asked Questions
Glossary
Three-Stage Charging
The standard method for charging lead-acid batteries: Bulk (full current until ~80%), Absorption (voltage held constant, current tapers), Float (low voltage maintains full charge). See Battery University. LiFePO4 skips the absorption taper, which is why it charges faster.
Charge Efficiency
The ratio of energy stored vs energy delivered by the charger. Lead-acid wastes 15-20% as heat during charging. LiFePO4 wastes only 3-5%. This directly affects charge time — lower efficiency means more input is needed.
Absorption Phase
The final 20% of a lead-acid charge cycle where the charger holds voltage constant and current gradually tapers to near zero. This phase can take 2-6 hours and is the main reason lead-acid batteries charge slowly.
Charging from solar panels? The <a href="/solar/solar-battery-charge-time-calculator">solar battery charge time calculator</a> accounts for variable panel output throughout the day.
Related calculators
12V Battery Runtime Calculator
Calculate how long a 12 volt battery lasts under any load. Pre-tuned for 12V deep cycle, AGM, and LiFePO4 systems.
Battery
AGM Battery Runtime Calculator
Estimate AGM battery discharge time with chemistry-specific defaults. Compare AGM vs flooded lead-acid for your use case.
Battery
Battery Capacity (Amp Hours) Calculator
Calculate battery capacity in amp-hours from watt-hours and voltage. Convert between Wh, Ah, and watts easily.
Battery
The calculator above gives you the bulk-phase estimate — the fastest, simplest part of the charge. For planning purposes, add 15-30% to the result for lead-acid batteries to account for the absorption phase, or 5-10% for LiFePO4. If you are charging overnight, that extra time rarely matters. If you are charging between uses during the day, it is the difference between a full battery and a 90% battery. Our depth of discharge guide explains why the DoD you start charging from affects both charge time and long-term battery health.
Last updated:
Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.