How does this calculation work?
The charging time calculator models battery charging behavior based on the charge profile for each battery type. Lithium batteries (LiFePO4) charge nearly linearly up to about 90% SoC, then switch to a taper phase. Lead-acid batteries (AGM/gel) have three phases: bulk (to 80%), absorption (80-95%) and float (95-100%).
The tool calculates how many watt-hours (Wh) need to go into the battery. Formula: capacity (Ah) x voltage (V) x SoC difference (%). Example: 200Ah x 12V x 80% = 1920 Wh.
Each battery type has a specific charge profile. During the bulk phase, the battery charges at full power. In later phases, charging power decreases, making the last percentages take longer. The tool applies the correct current fraction and efficiency per phase.
For each phase, the tool divides the required energy by the effective charging power (accounting for charge efficiency). Phase times are summed to give the total estimated charging time.
Charging profiles based on manufacturer specifications from Victron Energy (LiFePO4, AGM, gel) and Battle Born Batteries (LiFePO4). Efficiency factors per phase from IEEE 485 (recommended practice for sizing lead-acid batteries) and IEC 61427 (secondary cells for renewable energy). C-rate limits per chemistry: lithium 0.5C continuous (manufacturer recommendation), AGM/gel 0.2-0.25C (IEC 61056-1). Multi-phase charge model (bulk/absorption/float) per IEC 61427-1.
Frequently Asked Questions
Why does charging above 80% take so much longer with AGM batteries?
AGM and gel batteries switch to the absorption phase above 80% SoC, where charging current drops significantly. This protects the battery from overheating and gassing. The last 20% can take as long as the first 80%. With lithium, this effect is much smaller.
Is the charging time accurate for solar energy?
The calculated time assumes continuous peak power. In practice, a 200Wp panel delivers an average of 4-6 effective peak sun hours per day (in summer in Western Europe). Spread the calculated time over multiple days if you're charging on solar only.
What's the difference between a DC-DC charger and an alternator?
A DC-DC charger (also called B2B charger or booster) sits between the starter battery and your leisure battery. It precisely controls the charge profile, which is especially important for lithium. Charging directly from the alternator (via a split-charge relay) provides a less controlled charge profile and is only suitable for lead-acid batteries.
What is C-rate and why does it matter?
C-rate indicates how fast you're charging relative to battery capacity. 1C means charging a 100Ah battery at 100A (theoretically full in 1 hour). Most lithium camper batteries accept 0.5C continuous. AGM/gel batteries max 0.2-0.25C. Charging too fast shortens battery life and can be dangerous.