Charge Time for 100Ah Battery by Source
| Charging Source | Typical Amps | Time from 20% to 100% (LiFePO4) | Time from 20% to 100% (Lead-Acid) |
|---|---|---|---|
| 5A trickle charger | 5A | ~17 hours | ~20 hours |
| 10A smart charger | 10A | ~8.5 hours | ~11 hours |
| 20A battery charger | 20A | ~4.3 hours | ~6 hours |
| 40A DC-DC charger | 40A | ~2.2 hours | ~3.5 hours |
| 100W solar panel (avg) | 5-6A | ~15 hours (3-4 days) | ~18 hours (4-5 days) |
| 200W solar panel (avg) | 10-12A | ~7.5 hours (1.5 days) | ~10 hours (2 days) |
| 400W solar array (avg) | 20-25A | ~3.5 hours (1 day) | ~5 hours (1.5 days) |
| Vehicle alternator (smart) | 20-60A | ~1.5-4.5 hours | ~2-6 hours |
Solar times assume 4-5 peak sun hours per day. "Days" reflects that solar only charges during daylight. Lead-acid times are longer because of lower charging efficiency (80-90%) and the extended absorption phase.
Example: Charging a 100Ah LiFePO4 with Solar
You have a 200W solar panel and an MPPT charge controller connected to a 100Ah 12V LiFePO4 battery that is 80% depleted (20% SOC remaining). You need to replace 80Ah of capacity.
A 200W panel produces roughly 200W x 0.85 (panel derating) = 170W in peak sun. At 12V charging voltage (about 14V actual), that is approximately 12A.
Bulk charging takes the battery from 20% to 80%: 60Ah to replace at 12A = 5 hours. The absorption phase (80% to 100%) is fast on LiFePO4 — roughly 30-60 minutes as the BMS tapers current.
Total: about 5.5-6 hours of good sun. With 5 peak sun hours on a clear day, you can nearly fully recharge in a single day. On a partly cloudy day with 3 peak sun hours, you will get to about 60% recovery — the battery reaches roughly 56% SOC and finishes the next morning.
Add a 100W second panel and the math changes dramatically: 24A charge current means full recharge in about 3.5 hours of good sun, with room to spare even on cloudy days.
Worked Examples
Charging 100Ah LiFePO4 from a 50A Alternator-Powered Charger
Context
Your vehicle alternator feeds a 50A DC-DC charger connected to a 100Ah LiFePO4 at 80% DoD. You want to know how long to drive to fully recharge.
Calculation
Ah to replace: 100 x 0.80 = 80 Ah
At 95% charge efficiency: 80 / 0.95 = 84.2 Ah input
Time: 84.2 / 50 = 1.68 hours = 1 hour 41 minutes
Interpretation
Under 2 hours of driving fully recharges from 20% to 100%. LiFePO4 accepts the full 50A right up to the last few percent, unlike lead-acid which tapers.
Takeaway
For trips where you also charge from solar while parked, figure out what size panel you need with our solar panel size calculator to model daily charging capacity.
How Long Does a Single 100W Solar Panel Take?
Context
A 100W solar panel produces about 6A in full sun through an MPPT controller. Your 100Ah LiFePO4 is at 20% remaining (80% DoD). You get 5 peak sun hours per day.
Calculation
Ah to replace: 80 Ah
Daily charge: 6A x 5 hrs x 0.90 eff = 27 Ah per day
Days to full: 80 / 27 = 2.96 days
Interpretation
Nearly 3 days of sun to fully recharge from a single 100W panel. That works for light daily usage (20-25 Ah per night), but not if you drain below 50% each day.
Takeaway
For faster recharging, size a larger solar array. Our solar panel and battery sizing calculator determines exactly how many panels match your daily energy use.
Frequently Asked Questions
Glossary
DC-DC Charger
A device that charges a secondary battery from the vehicle's alternator while isolating the two electrical systems. It boosts or regulates the alternator output to the correct charging profile for the house battery chemistry.
MPPT Controller
A Maximum Power Point Tracking charge controller that optimizes the voltage-current relationship from solar panels to extract the maximum possible energy. MPPT controllers are 15-30% more efficient than older PWM controllers, especially when panel voltage differs from battery voltage.
Charge Acceptance
The rate at which a battery can absorb energy from a charger. LiFePO4 accepts full charge current until nearly full. Lead-acid begins tapering current at about 80% SOC, which dramatically slows the final 20% of charging.
Sizing your solar array to fully recharge daily? Our <a href="/solar/solar-battery-charge-time-calculator">solar battery charge time calculator</a> models panel output throughout the day.
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Charging speed for a 100Ah battery ranges from under 2 hours (high-current DC-DC charger) to multiple days (small solar panel in bad weather). Pick the charging source that matches your daily energy deficit, and size it with enough margin that cloudy days do not leave you short. The charge time gap between LiFePO4 and lead-acid is even wider than the numbers suggest once you factor in the absorption phase — our LiFePO4 vs lead-acid comparison covers both charge and discharge differences.
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.